ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2018

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1708

Enhancing the performance of theDigital Cherenkov Viewing Device

Detecting partial defects in irradiated nuclear fuelassemblies using Cherenkov light

ERIK BRANGER

ISSN 1651-6214ISBN 978-91-513-0415-1urn:nbn:se:uu:diva-357578

Dissertation presented at Uppsala University to be publicly examined in Room 2005,Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Friday, 12 October 2018 at 13:00 forthe degree of Doctor of Philosophy. The examination will be conducted in English. Facultyexaminer: Dr. Christopher Orton (Pacific Northwest National Laboratory, PNNL).

AbstractBranger, E. 2018. Enhancing the performance of the Digital Cherenkov Viewing Device.Detecting partial defects in irradiated nuclear fuel assemblies using Cherenkov light. DigitalComprehensive Summaries of Uppsala Dissertations from the Faculty of Science andTechnology 1708. 97 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0415-1.

The Digital Cherenkov Viewing Device (DCVD) is an instrument used by authority safeguardsinspectors to verify irradiated nuclear fuel assemblies in wet storage based on Cherenkov lightemission. It is frequently used to verify that parts of an assembly have not been diverted, whichis done by comparing the measured Cherenkov light intensity to a predicted one.

This thesis presents work done to further enhance the verification capability of the DCVD,and has focused on developing a second-generation prediction model (2GM), used to predictthe Cherenkov light intensity of an assembly. The 2GM was developed to take into account theirradiation history, assembly type and beta decays, while still being usable to an inspector in-field. The 2GM also introduces a method to correct for the Cherenkov light intensity emanatingfrom neighbouring assemblies. Additionally, a method to simulate DCVD images has beenseamlessly incorporated into the 2GM.

The capabilities of the 2GM has been demonstrated on experimental data. In one verificationcampaign on fuel assemblies with short cooling time, the first-generation model showed a RootMean Square error of 15.2% when comparing predictions and measurements. This was reducedby the 2GM to 7.8% and 8.1%, for predictions with and without near-neighbour corrections. Asimplified version of the 2GM for single assemblies will be included in the next version of theofficial DCVD software, which will be available to inspectors shortly. The inclusion of the 2GMallows the DCVD to be used to verify short-cooled assemblies and assemblies with unusualirradiation history, with increased accuracy.

Experimental measurements show that there are situations when the intensity contributiondue to neighbours is significant, and should be included in the intensity predictions. The imagesimulation method has been demonstrated to also allow the effect of structural differences inthe assemblies to be considered in the predictions, allowing assemblies of different designs tobe compared with enhanced accuracy.

Keywords: DCVD, Nuclear safeguards, Cherenkov light, Nuclear fuel assembly, Partial defectverification

Erik Branger, Department of Physics and Astronomy, Applied Nuclear Physics, Box 516,Uppsala University, SE-751 20 Uppsala, Sweden.

© Erik Branger 2018

ISSN 1651-6214ISBN 978-91-513-0415-1urn:nbn:se:uu:diva-357578 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-357578)

There can be no doubt that the usefulness of this radiation[Cherenkov light] will in the future be rapidly extended.

-Pavel C̆erenkov [1]

List of papers

This thesis is based on the following papers, which are referred to in the textby their Roman numerals.

I E. Branger, S. Grape, S. Jacobsson Svärd, P. Jansson, E. AnderssonSundén, On Cherenkov light production by irradiated nuclear fuelrods. Journal of Instrumentation, June 2017. DOI:10.1088/1748-0221/12/06/T06001My contribution: I made the simulations and analysed the results. Iam the main author of the paper.

II E. Branger, S. Grape, S. Jacobsson Svärd, P. Jansson, E. AnderssonSundén, Comparison of prediction models for Cherenkov lightemissions from nuclear fuel assemblies. Journal of Instrumentation,June 2017. DOI: 10.1088/1748-0221/12/06/P06007My contribution: I made the simulations and analysed the results. Iam the main author of the paper.

III E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, Improving theprediction model for Cherenkov light generation by irradiated nuclearfuel assemblies in wet storage for enhanced partial-defect verificationcapability. The ESARDA Bulletin issue no. 53, June 2016.My contribution: I made the simulations and proposed the predictionmethod. I am the main author of the paper.

IV E. Branger, S. Grape, P. Jansson, E. Andersson Sundén, S. JacobssonSvärd, Investigating the Cherenkov light production due to cross-talk inclosely stored nuclear fuel assemblies in wet storage. Paper presentedat the 39th ESARDA Annual Meeting, 16-18 May 2017, Düsseldorf,Germany. Accepted for publication in the ESARDA Bulletin.My contribution: I made the simulations and proposed the predictionmethod. I am the main author of the paper.

V E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, Experimentalevaluation of models for predicting Cherenkov light intensities fromshort-cooled nuclear fuel assemblies. Journal of Instrumentation,February 2018. DOI: 10.1088/1748-0221/13/02/P02022My contribution: I made the analyses of the results and thesimulations. I am the main author of the paper.

VI E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, On the inclusionof light transport in prediction tools for Cherenkov light intensityassessment of irradiated nuclear fuel assemblies. Manuscript.My contribution: I made the simulations and the analyses. I am themain author of the paper.

VII E. Branger, S. Grape, P. Jansson, S. Jacobsson Svärd, Experimentalstudy of background subtraction in Digital Cherenkov Viewing Devicemeasurements. Journal of Instrumentation, August 2018. DOI:10.1088/1748-0221/13/08/T08008My contribution: I did the measurements and the analyses. I am themain author of the paper.

Reprints were made with permission from the publishers.

Additional papers part of this work, but not included in the thesis:

i E. Branger, E. L. G. Wernersson, S. Grape, S. Jacobsson Svärd, Imageanalysis as a tool for improved use of the Digital Cherenkov ViewingDevice for inspection of irradiated PWR fuel assemblies. Report, June2014. Available in DiVA: diva2:3A766776

My contribution: I assisted in writing the report.

ii E. Branger, S. Grape, S. Jacobsson Svärd, E. L. G. Wernersson, Im-proved DCVD assessments of irradiated nuclear fuel using image anal-ysis techniques. Paper presented at the 55th INMM Annual Meeting,Atlanta, USA, 2014.

My contribution: I wrote and presented the paper. I am the main authorof the paper.

iii E. Branger, S. Grape, S. Jacobsson Svärd, E. L. G. Wernersson, To-wards unattended partial-defect verification of irradiated nuclear fuelassemblies using the DCVD. Paper presented at the IAEA Symposiumon International Safeguards, Vienna, Austria, 2014.

My contribution: I wrote and presented the paper. I am the main authorof the paper.

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.1 The need for nuclear safeguards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.2 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2 Nuclear safeguards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.1 The legal framework for nuclear safeguards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.2 Material and facilities under safeguards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3 Safeguards verification of nuclear material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3 Nuclear fuel assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.1 Physical design of nuclear fuel assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2 Fuel usage in a reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3 Safeguards verification of irradiated nuclear fuel assemblies . . . . . 24

4 Cherenkov light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.1 The physics of Cherenkov light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2 Cherenkov light from irradiated nuclear fuel assemblies . . . . . . . . . . . . 28

5 The Digital Cherenkov Viewing Device, DCVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.2 Measuring fuel assemblies with a DCVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.3 Detecting partial defects using a DCVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.3.1 Partial defect intensity limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395.4 First-generation method (1GM) for predicting Cherenkov light

intensities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.5 Limitations addressed developing the second-generation

prediction method (2GM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.6 Practical aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.1 Simulation tools used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

6.1.1 Simulating sources of ionizing radiation . . . . . . . . . . . . . . . . . . . . . . . 456.1.2 Simulating radiation transport and Cherenkov light

production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.1.3 Simulating light transport to the DCVD and image

creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486.2 Simulated light production by single fuel rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6.2.1 Contributions from different types of radiation . . . . . . . . . . . . . 49

6.2.2 Effect of source distribution in a rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516.2.3 Anisotropy of produced light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516.2.4 Dependencies of light production on fuel rod

dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546.3 Simulated light production in complete fuel assemblies . . . . . . . . . . . . . 54

6.3.1 Systematic differences between assemblies of differenttypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.3.2 Contribution from beta emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566.4 Simulated light production in neighbouring assemblies . . . . . . . . . . . . . . 566.5 Including light transport and image creation in simulations . . . . . . . 596.6 Speeding up the simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7 Predicting Cherenkov light intensities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647.1 Suggested second-generation prediction method (2GM) . . . . . . . . . . . . 64

7.1.1 General methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647.1.2 Single assembly predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657.1.3 Neighbourhood predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687.1.4 Predictions adjusted for top plate designs . . . . . . . . . . . . . . . . . . . . . . 69

7.2 Experimental evaluations of the 2GM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707.2.1 Performance of near-neighbour predictions . . . . . . . . . . . . . . . . . . . 717.2.2 Performance on short-cooled fuel assemblies . . . . . . . . . . . . . . . 727.2.3 Performance of predictions adjusted for top plate

design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

8 Background in DCVD measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788.1 Intensity components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788.2 Background subtraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

8.2.1 Currently used background subtraction . . . . . . . . . . . . . . . . . . . . . . . . . 788.2.2 Alternative dark-frame subtraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 798.2.3 Experimental evaluations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

8.3 Background light sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

9 Conclusions and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849.1 Results of simulation studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849.2 Development and evaluation of prediction tools . . . . . . . . . . . . . . . . . . . . . . . . . . 859.3 Improving the background subtraction routines . . . . . . . . . . . . . . . . . . . . . . . . . . 869.4 Implementation of new prediction tools in IAEA safeguards . . . . . 87

10 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8810.1 Modelling partial defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8810.2 Analysing image properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8810.3 Enhancing the quality of the measured data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8910.4 Combining data from different instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

11 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

12 Sammanfattning på Svenska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

List of abbreviations

1GM First-generation prediction model2GM Second-generation prediction modelAP Additional ProtocolBU BurnupBWR Boiling Water ReactorClab Swedish Central Interim Storage Facility for Spent Nuclear FuelCT Cooling timeDA Destructive AssayDCVD Digital Cherenkov Viewing DeviceIAEA International Atomic Energy AgencyICVD Improved Cherenkov Viewing DeviceIE Initial enrichmentNDA Non-Destructive AssayNN Near-NeighbourNNWS Non Nuclear Weapons StateNPT Non-Proliferation TreatyNWS Nuclear Weapons StatePWR Pressurized Water ReactorROI Region Of InterestSQ Significant Quantity

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1. Introduction

1.1 The need for nuclear safeguardsShortly after the discovery of nuclear fission [2], i.e. the splitting of atomicnuclei, researchers realized that vast amounts of energy could be released bycreating a fission chain reaction. This energy has found peaceful applicationsin terms of electricity production in nuclear power plants, but also destructiveuse in terms of nuclear weapons. Whether used peacefully or for militarypurposes, nuclear energy requires nuclear material, i.e. material that is fissileor can be converted to fissile material through nuclear reactions. A fissilematerial contains nuclei that can fission following the absorption of a neutron,while emitting one or several neutrons following the fission, thus allowing achain reaction to take place.

To promote peaceful use of nuclear energy and nuclear technology, andto inhibit its use for nuclear weapons or other military purposes, the Inter-national Atomic Energy Agency (IAEA) was founded in 1957 [3]. Abouta decade later, the work of the IAEA was significantly expanded when theTreaty on Non-Proliferation of Nuclear Weapons (NPT) opened up for sig-natures in 1968. To ensure that nuclear materials and technologies are usedpeacefully, the NPT signatory states are required to sign a nuclear safeguardsagreement with the IAEA. The safeguards agreement give the IAEA the rightto inspect and verify a state’s nuclear facilities, and to verify a state’s posses-sion of nuclear material. The IAEA also verifies that no undeclared nuclearactivities take place within the state. Through these inspections the IAEA canprovide credible, independent confirmation that states are using their nucleartechnologies and materials only for peaceful purposes, and that no nuclear ma-terial is diverted to any clandestine nuclear weapons program, or for any othernon-peaceful purposes.

The civilian nuclear fuel cycle contains vast amounts of nuclear materialthat is placed under safeguards. As of 2018, there are about 450 commercialelectricity-producing nuclear reactors in operation worldwide, with another60 under construction [4]. The nuclear material under safeguards is moni-tored and verified throughout the entire nuclear fuel cycle, i.e. from when it ismined, through its conversion to and usage as nuclear fuel, as well as thoughany reprocessing to make new fuel from used fuel, and until its disposal in afinal repository. Verifying that all nuclear material is accounted for and onlyused peacefully in each step of the nuclear fuel cycle is a massive undertaking,and consequently the inspections executed by the IAEA need to be efficient,accurate and comprehensive.

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To aid the authority inspectors in verifying nuclear material in its variousforms, a multitude of instruments have been developed to independently ver-ify, describe, quantify or characterize nuclear material [5]. This thesis cov-ers developments of analysis tools related to one such instrument, the Digi-tal Cherenkov Viewing Device (DCVD). The DCVD is used to measure theCherenkov light produced by spent nuclear fuel assemblies in wet storage, i.e.stored in water pools. Based on the presence, characteristics and intensity ofthe Cherenkov light, the properties of the nuclear fuel assembly can be veri-fied. Considering the vast amounts of nuclear material existing in the form ofspent nuclear fuel assemblies in this type of storages worldwide, it is importantthat the verification tools are both accurate and time-efficient. Accordingly, at-tention is paid to both the practical aspects of the developed tools and to theaccuracy and precision in comparison to the currently used tools.

1.2 Outline of the thesisThis thesis is based on seven scientific papers, which can be found at the endof this thesis. The key findings of all the papers are presented in the compre-hensive summary, of which you are now reading the first chapter.

Chapter 2 presents the fundamentals of nuclear safeguards, in terms of his-tory, aims, and techniques and methods used. It also presents which nuclearmaterials are under safeguards, how the materials are verified, and providesthe context in which the work summarized in this thesis should be seen.

Chapter 3 introduces nuclear fuel assemblies, their design, and safeguardsaspects that should be considered when verifying spent nuclear fuel assem-blies. Important parameters describing the assemblies are presented, and dif-ferences in physical design for assemblies of different reactor types are dis-cussed.

Chapter 4 presents Cherenkov light and the physics behind its occurrence,and discusses how the Cherenkov light can be used to verify nuclear fuel as-semblies.

Chapter 5 introduces the DCVD and the measurement methodology used,and details how the DCVD data is used to verify spent nuclear fuel assem-blies in wet storage. The chapter also presents some earlier research that hasbeen done on safeguards verification with the DCVD prior to this thesis, andpresents how this work allows for the capabilities of the instrument and asso-ciated analyses to be extended.

Chapter 6 summarizes the Monte-Carlo simulations that have been per-formed as part of this work. The code used for the simulations is presented,and its capabilities are shown. Simulation results are presented for nuclearfuel rods, assemblies, and assemblies stored close to other neighbouring as-semblies in wet storage. These results are primarily based on papers I and II,but also includes papers IV and VI.

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Chapter 7 presents the Cherenkov light prediction tools that have been de-veloped in this work, and how these tools extend the capabilities of the DCVDverification methodology compared to the previously used one. The tools de-veloped can be used to predict the Cherenkov light production in isolated as-semblies, to predict the light contribution due to nearby neighbouring assem-blies, and to predict the effect on the measured light intensity due to variousstructural components of the assemblies. The performance of the predictiontools have been evaluated based on experimental data. The results presentedin this chapter are based on papers III, IV, V and VI.

Chapter 8 presents work done on improving the background-subtractionmethod used in DCVD measurements. An improved method is proposed andevaluated using experimental data. This chapter is based on paper VII.

Finally, chapter 9 provides some concluding remarks on what have beenconsidered the most important outcomes of this work, and chapter 10 discussespossible future work that can be done to further improve the performance ofthe DCVD and its associated analysis, based on the key findings of this thesis.

For formal reasons, it should be noted that parts of chapter 2 and 4 are basedon the author’s licentiate thesis [6]. The material has been adapted to betterfit into this work, but some portions of the text and some figures may haveremained identical to [6].

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2. Nuclear safeguards

2.1 The legal framework for nuclear safeguardsThe Nuclear Non-Proliferation Treaty (NPT) opened for signatures in 1968and entered into force in 1970. To date, the treaty has 191 signatory states [7],making it one of the most adhered to arms limitation and disarmament treatiesin history. Currently, only India, Israel, Pakistan and South Sudan have notsigned the treaty, and North Korea signed the treaty in 1985 but withdrew in2003. The NPT serves three main purposes:

• To prevent the proliferation of nuclear weapons.The NPT prohibits the five nuclear weapon states (NWS) recognized bythe treaty (China, France, Russia, the United Kingdom and the UnitedStates) from transferring nuclear weapons to other states. The treatyalso prohibits the NWS from transferring equipment that can be usedto produce nuclear weapons, or to encourage other states to obtain nu-clear weapons. The signatory non-nuclear weapons states (NNWS) areobliged to refrain from receiving assistance in or trying to develop nu-clear weapons.

• To promote nuclear disarmament.The NPT states that the NWS shall pursue negotiations in good faithfor nuclear disarmament, though the NPT does not impose any nucleardisarmament agreements itself, and it does not set any time limit onwhen the disarmament should be completed.

• To promote peaceful use of nuclear technology.The NPT acknowledges the right of all parties to the treaty to developnuclear technology for peaceful purposes. The treaty also encourages in-ternational cooperation on nuclear development, provided that the statescan demonstrate that their nuclear programs are not being used for thedevelopment or production of nuclear weapons.

As part of the NPT, the signatory NNWS are required to sign a safeguardsagreement with the International Atomic Energy Agency. Under this agree-ment, all nuclear material and nuclear activities shall have safeguards appliedto them, to verify that no nuclear material is diverted for production of nuclearweapons, and that the nuclear facilities are used only for peaceful purposes.The objectives of nuclear safeguards is the timely detection of diversion of nu-clear material for the manufacture of nuclear weapons, or for other unknownpurposes, and the deterrence of diversion by the risk of early detection.

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The safeguards agreement signed by the NNWS is called the Comprehen-sive Safeguards Agreement [8], and under this agreement the IAEA has theright and obligation to ensure that safeguards measures are applied to all nu-clear material in the state, and to verify that no material diversion takes place.Consequently, the IAEA can and shall provide credible and independent assur-ances that states are honouring their obligations and do not pursue obtainingnuclear weapons.

In addition to the comprehensive safeguards agreement, a total of 146 stateshave also signed the Additional Protocol (AP), further extending the rights ofthe IAEA to inspect facilities suspected of being used for nuclear activities.More information on the IAEA safeguards legal framework can be found in[9].

In addition to the IAEA, other organizations are also part of the internationalsafeguards work. For example, Euratom is a European organization foundedunder the Euratom treaty, with the purpose of creating a specialist market fornuclear power in Europe, and developing nuclear energy in Europe. As part oftheir work, Euratom perform inspections at nuclear facilities, and the goals ofthe inspections includes verifying that no nuclear material has been diverted,and that no nuclear facility is used for other purposes than intended. Euratomfrequently performs safeguards inspections together with the IAEA in the Eu-ropean States.

There are also national organizations working with domestic safeguards;one example is the Swedish Radiation Safety Authority, or Strålsäkerhetsmyn-digheten (SSM). SSM has a mandate to work proactively and preventivelywith nuclear safety, radiation protection and nuclear non-proliferation in Swe-den [10]. Within nuclear non-proliferation the authority works with exportcontrol, safeguards as well as illicit trafficking of nuclear material. SSM hasalso been appointed the task by the Swedish parliament of providing the IAEAwith a support program concerning safeguards, where research and training,specifically for spent nuclear fuel verification, plays a major role. Further-more, SSM supports scientific research of value for the work of the authority,which provides a scientific foundation to its recommendations and regulations.

2.2 Material and facilities under safeguardsThe foundation for verifying that no diversion of nuclear material has occurredlies in material accountancy. Under a safeguards agreement, a State must es-tablish a bookkeeping system containing all nuclear material present in theState, and any material entering or exiting the state. The IAEA performs in-spections to verify that the bookkeeping is correct and complete, and that allmaterial is accounted for, thus verifying that no material has been diverted.During 2016, the IAEA collected and evaluated over 1 million nuclear mate-rials reports [11].

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A central concept in IAEA safeguards is the "Significant Quantity" (SQ),which is "the approximate amount of nuclear material for which the possibilityof manufacturing a nuclear explosive device cannot be excluded" [12]. Theconcept of a SQ takes into account unavoidable losses in the conversion andmanufacturing processes required to produce a nuclear weapon. A SQ shouldnot be confused with the critical mass of a nuclear material, which is lower.

Two elements of particular importance for nuclear safeguards are uraniumand plutonium. For uranium, there is one naturally occurring fissile isotope,235U. This isotope can be used in nuclear weapons, but it must first be sepa-rated from the much more abundant isotope 238U, since natural uranium con-tains about 99.3% 238U and only 0.7% 235U. The process of removing 238U toincrease the fraction of 235U is called enrichment, and for a nuclear weaponthe uranium is typically enriched to more than about 90% 235U. Plutonium,on the other hand, does not occur in nature, but can be produced in a nuclearreactor. If 238U absorbs a neutron, it will turn into 239U, which will beta decaytwice, at a half-life of in the order of a few days, turning it into 239Pu, i.e.uranium is transmuted to plutonium. Once plutonium has been produced in areactor, it can be chemically separated from the fuel, which is less complicatedthan uranium enrichment.

The mass of one SQ depends on the type of material, and the SQ values for235U and plutonium are given in table 2.1. Note that for plutonium, certain iso-topes are undesirable in a nuclear weapon for practical reasons, but within thesafeguards framework all Pu isotopes are conservatively considered equallyuseful for weapons manufacturing [13]. By the end of 2016, there were a totalof 204 000 SQs of nuclear material under IAEA safeguards [11].

Table 2.1. Significant quantities for fissile materials of relevance in the context ofnuclear fuel [12].

Material Significant QuantityPu 8 kgHigh-enriched U (235U ≥ 20%) 25 kg 235ULow-enriched U (235U < 20%) 75 kg 235U

The IAEA has also set up detection timeliness goals, to ensure that the di-version of any nuclear material can be detected before the diverter has hadenough time to convert it to a nuclear weapon. Different materials have dif-ferent timeliness goals, reflecting the estimated time required to convert thematerial to a weapons-useable form, and these timeliness goals determine thefrequency of inspections for these materials. Nuclear materials that can beused in the manufacturing of nuclear weapons without further enrichment ortransmutation are referred to as direct-use materials, and are associated withshort timeliness goals and frequent inspections. Materials falling in this cat-egory are e.g. Pu in fresh nuclear fuel assemblies of mixed-oxide type (seesection 3), or high-enriched uranium (235U ≥ 20%). Materials requiring addi-

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tional enrichment or transmutation before being usable in a nuclear weapon,such as low-enriched U (235U < 20%), have longer timeliness goals. Irradiatednuclear material such as spent nuclear fuel is also associated with longer time-liness goals, due to the difficulty of handling the strongly radioactive fissionproducts present, and the difficulty of separating the nuclear material from allother fission products. The timeliness goals specified by the IAEA are pre-sented in table 2.2.

Table 2.2. Timeliness goals defined by the IAEA for detecting diversion of one or moreSQ of nuclear material [12]. Direct use material refers to material that can be usedto manufacture a nuclear weapon without transmutation or further enrichment.

Material Example of material Timeliness goalUnirradiated direct- MOX fuel 1 monthuse material High-enriched U (235U ≥ 20%)Irradiated direct- Spent fuel 3 monthsuse materialIndirect- Low-enriched U (235U < 20%) 12 monthsuse material

In addition to material accountancy, design schematics of all nuclear facil-ities must be provided to the IAEA by the signatory States, and the physicaldesign of the facilities is verified through inspections. This allows the IAEAto verify that the facilities are being used as declared. The inspections are alsoaimed at verifying the absence of undeclared activities, and with the introduc-tion of the AP, the IAEA has gained extended rights to inspect also undeclaredfacilities in a State, further strengthening this capacity.

2.3 Safeguards verification of nuclear materialSeveral instruments have been developed to assess nuclear material, in order toallow inspectors to independently characterize and verify it. A comprehensivesurvey of the safeguards techniques and equipment used by the IAEA can befound in [5].

Two general types of methods are commonly used when verifying nuclearmaterial, Destructive Assay (DA) and Non-Destructive Assay (NDA). DAmeasurements are typically performed on samples of nuclear materials, whichare sent to laboratories for analysis. Parts of, or the full sample, is consumedin the analysis, since DA methods requires that the samples are altered chem-ically or physically. DA usually offers superior precision compared to NDA,being able to identify a material and its isotopic composition with a high levelof detail. Some downsides are that DA can only be used when it is possibleto extract samples for analysis, such as when the nuclear material is handled

19

in bulk, and that sending a sample to a laboratory and analysing it is time-consuming and sometimes difficult.

NDA measurements typically make use of the gamma or neutron radiationemissions from a nuclear material. This radiation carries information aboutthe material, and since it can penetrate relatively thick layers of materials, suchas some storage containers, the radiation can be detected from the outside ofthe container. Thus, NDA measurements can verify and characterize nuclearmaterial without altering the material itself, and there is often no need to openup a container to verify its contents. NDA measurements are typically quick,and may give immediate information about the measurement results, but theyare rarely as precise as DA measurements.

Verification of nuclear material and its accountancy can be done at differentlevels of precision, depending on the inspection goals, the material propertiesand on which instruments are available to assess them. The three main levelsof verification used by the IAEA are [12]:

• Gross defect verification, where the inspector verifies the presence orabsence of nuclear material in an inspected item. This level of verifi-cation is often performed using NDA techniques, since low precision isrequired and fast measurements are preferred.

• Partial defect verification, where the inspector verifies that a fraction ofthe nuclear material has not been diverted from an inspected item. Thecurrent IAEA requirement on instruments used to perform this level ofverification is that they must be able to reliably detect a 50% removal orsubstitution of nuclear material from the item.

• Bias defect verification, where the inspector verifies that small portionsof the nuclear material has not been diverted from an inspected item.This requires instruments and measurements with a high degree of pre-cision.

Once the nuclear material has been successfully verified, the IAEA deployscontainment and surveillance (C/S) techniques to verify that no material is di-verted at a later stage. Commonly used C/S techniques involve seals used toverify that containers remain sealed, or video surveillance to monitor that nodiversion activities occurs at a site. The seals and surveillance ensure that theauthorities maintain Continuity of Knowledge (CoK), by knowing that the ma-terial has not been diverted or altered since the last inspection. By maintainingCoK, the absence of diversion can be verified by inspecting the applied C/Sdevices, without having to re-verify the nuclear material. However, should theCoK be lost, all material affected will need to be re-verified, to ensure that nomaterial has been diverted during the time that the CoK was lost.

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3. Nuclear fuel assemblies

The nuclear material used in a civilian power-producing reactor is in the formof Nuclear Fuel Assemblies. The design of these assemblies ensures that acontrolled fission chain reaction can take place, and that the heat produced inthe fuel material can be transported away, to be used to produce electricity.The design also ensures that radioactive fission products produced by fissionevents stay inside the assembly, and that no radioactive isotopes leak out intothe reactor. A nuclear fuel assembly is normally the smallest unit of nuclearmaterial handled at a reactor site, and is often referred to as an item in a safe-guards inspection (see section 2.3).

The two most common types of commercial reactors in the world are Boil-ing Water Reactor (BWR) and Pressurized Water Reactor (PWR). In a BWR,the energy released by the nuclear chain reaction produces steam directly in-side the reactor vessel, which is led to a turbine where the steam is used toproduce electricity. In a PWR, the high pressure in the reactor vessel keepsthe water from boiling. Instead, the hot water is led to a steam generator,where it produces steam in a secondary loop. Due to the different modesof operation of these facilities, BWR and PWR nuclear fuel assemblies havenoticeably different designs. Still, many of their properties are similar, andsafeguards verification procedures do not differ significantly between fuel as-sembly types.

3.1 Physical design of nuclear fuel assembliesMost of the commercial nuclear reactors in the world use uranium dioxide(UO2) as fuel, enriched to about 4-5% of 235U, in order to increase the fissilecontent. A few reactors use a mixture of UO2 and plutonium dioxide (PuO2),which is referred to as Mixed-Oxide Fuel (MOX). In MOX fuel, 239Pu is gen-erally the dominant fissile isotope.

To manufacture a nuclear fuel assembly, the fuel material is first turned intocylindrical pellets, with a height and diameter of typically about 1 cm. UO2and MOX are ceramic materials, which can withstand high temperatures with-out melting, and which can trap fission products inside the fuel material toensure that they do not leak out. Next, several hundred pellets are stackedinside a metal tube, forming a fuel rod or pin, which is typically around 4 min length. The tube, or cladding, is generally made of zircaloy, a metal al-loy consisting primarily of zirconium, with a thickness of about 1 mm. Since

21

zirconium has a small neutron capture cross-section, few neutrons are lost bybeing absorbed by the zirconium, which is beneficial for sustaining the fissionchain reaction. The fuel rods are assembled into a fuel assembly, with a BWRassembly typically containing 60-100 rods and a PWR assembly typically con-taining 200-300 rods. The fuel rods are held in position by spacers, which aremanufactured from zircaloy or stainless steel (typically Inconel steel), and bysteel top and bottom plates. At the top plate there is also a lifting handle, usedwhen moving the assembly. In total, the length of an assembly is on the orderof 4 m, and the width is around 10-25 cm.

As a gross means to control the neutron flux in the reactor, a BWR usescontrol rods (sometimes called control blades for a BWR), containing a neu-tron absorbing material, which can be inserted or removed from the reactorcore during operation. The main use of the control rods are for stopping andstarting the reactor. The control rods are inserted in between the assemblies,consequently the outer dimensions of all BWR assemblies in one reactor coremust be similar to provide space for the control rods. However, the configura-tion of fuel rods inside the assembly can be chosen more freely. Thus, BWRassemblies of noticeably different designs can be present in the same reactor,and the number of fuel rods and their dimension may vary from assembly toassembly. In addition to the fuel rods, which are typically arranged in a squarelattice in the assembly, there may also be water channels present to provide ahigher flow of water in desired parts of the assembly. Some rod positions maycontain part-length rods, to ensure that there is more space between the fuelrods at the top of the assembly as water turns into steam, which requires morespace. BWR assemblies also feature a fuel channel, i.e. the entire assembly isplaced inside a zircaloy containment box, to ensure that water does not escapethe assembly when boiling occurs. In total, a fresh BWR assembly typicallycontains 200-300 kg of UO2 or MOX.

A PWR also uses control rods, but these are inserted into the assembliesrather than in between them. As for the BWR, their main use is for starting andstopping the reactor. Since the control rods are inserted into the assemblies, thePWR assemblies feature guide tubes into which the control rods are inserted.For all assemblies accepting the same control rod type, the guide tubes mustbe placed in the same position, and because of optimizations, the fuel rodplacement and dimensions will vary little for these assemblies. However, thereare still some variations. One notable difference in the context of this work isthat the top plate designs may vary noticeably, which affect the transport ofCherenkov light (see section 7.2.3). PWR assemblies also tend to be biggerthan BWR ones, as illustrated in figure 3.1, and they typically contain around400-600 kg of UO2 or MOX.

22

Figure 3.1. Left: Example of the fuel rod placement in a PWR 17x17 assemblyRight: Example of the fuel rod placement in a BWR 8x8 assembly. The Cherenkovlight production in these two assembly types have been studied in chapter 6. TheBWR assembly has one rod position functioning as a water channel, and is enclosedby a fuel channel, illustrated schematically in the figure. The PWR assembly featuresa central instrumentation tube, and 24 control rod guide tubes. The BWR assembly is13.6 cm wide, and the PWR assembly is 21.4 cm wide. The assembly height is around4 m.

3.2 Fuel usage in a reactorNuclear power plants typically operate in cycles, with a long period of runningthe reactor, and a short period of downtime for replacing spent fuel assembliesand performing maintenance work. For Swedish reactors, the running time istypically about 11 months, with one month of downtime. The downtime isgenerally scheduled in the summer when the Swedish electricity consumptionis lower. For countries with less pronounced seasonal electricity usage differ-ences, longer cycles are often used, and 18 or even 24 months are common.Significantly longer running times are however not possible in most commer-cial reactors, since the reactor core cannot be loaded with too much fissilematerial, and the fuel must be replaced once it is used up.

As the fuel material undergoes fission in the reactor, fission products arecreated, which build up as the fuel is irradiated. Many of the fission prod-ucts are radioactive, and will decay until they have turned into stable isotopes.Several fission products are relatively long-lived, and consequently the fuel as-semblies will emit radiation also after being discharged from the reactor. Theactivity is high enough that a noticeable amount of decay heat is produced.To shield the environment from the radiation, and to cool the residual heat,the assemblies are often stored in water. For this reason, reactors generallyhave a storage pool next to the reactor to store recently discharged assemblies.A civilian nuclear reactor, such as the ones in Sweden, will typically replace20-25% of its fuel assembly inventory each year, corresponding to 40-140 as-semblies, or 20-40 significant quantities of Pu. These assemblies are storedat the reactor for one to two years, after which their radioactivity has decayed

23

to a level low enough that they can be moved to the central Swedish interimstorage for spent nuclear fuel, Clab. Worldwide, dry storages are also com-mon, where the spent fuel assemblies are put in massive canisters to shield thesurrounding environment from the radiation.

Other than undergoing nuclear fission, the elements in the nuclear fuel mayinstead absorb neutrons, and turn into heavier isotopes. Thus, plutonium isproduced, as described in section 2.2, as well as even heavier isotopes. Theseheavy metals are radioactive, and some have very long half-lives resulting in aneed for long-term storage of the used fuel material. When a UO2 fuel assem-bly is discharged, it contains about 1% 235U, 1% Pu, 3-4% fission products,and the rest is 238U, with some trace amounts of elements heavier than pluto-nium present.

Two parameters used to describe a spent nuclear fuel assembly are its bur-nup (BU) and cooling time (CT). The BU is a measure of the amount of energythat has been released from the fuel material through fission, and it is givenin units of MWd/kgU (Mega-Watt days per kilogram of uranium) or GWd/tU(Giga-Watt days per ton of uranium). Typical BU values of discharged com-mercial nuclear fuel assemblies are in the order of 40-50 MWd/kgU, or equiv-alently around 1 million kWh per kg uranium. CT is the time since the assem-bly was discharged from the reactor. High BU results in a large production ofradioactive isotopes in the fuel, while long CT implies that a relatively largerfraction of the activity has decayed away.

Once a fuel assembly has been discharged from a reactor following its finalcycle, it is often referred to as either a spent or a used fuel assembly. In thiswork, assemblies are generally referred to as irradiated, since assemblies thatwill be further irradiated in the reactor are also of interest.

Since the nuclear power plants optimize the use of the nuclear fuel, mostassemblies are used in a similar way. Depending on the reactor type, the fuelassemblies are used for 3-6 years, until they have reached their design BU,after which they are discharged. Occasionally, assemblies will be used in adifferent way from this standard usage. As an example, a fuel assembly mayrequire reparation after suffering damage, which will result in it spending oneor a few cycles outside the reactor, before being used again. Furthermore,some assemblies from the first core loading of a reactor may need to be re-placed more quickly, resulting in a low burnup at discharge.

3.3 Safeguards verification of irradiated nuclear fuelassemblies

Depending on the design of a nuclear fuel assembly, one or a few irradiated as-semblies will contain one SQ of nuclear material. Accordingly, an importanttask carried out by safeguards inspectors is verifying that no nuclear mate-rial is diverted from the irradiated fuel assemblies. Of particular importance

24

is plutonium, since it is abundant in spent fuel, and can relatively easily bechemically separated from the other elements in the fuel assembly.

Verifying the nuclear material in an irradiated fuel assembly is a challeng-ing task, since the material is not accessible, preventing DA techniques frombeing used, and since the intense gamma and neutron radiation emitted by thefission products and minor actinides present interferes with direct NDA mea-surements of the fissile material. For this reason, safeguards verification ofirradiated nuclear fuel assemblies often aim at verifying the BU and CT ofthe assemblies. This is often done by measuring the radiation emitted by thefission products, to verify that the abundance of fission products is consistentwith irradiated nuclear fuel. While such indirect measurements do not assessthe quantity of nuclear material, which is what is of interest to safeguards,the measurements can indicate that the nuclear fuel assembly has been usedas declared, and that it has not been tampered with. The measurements canthus give an indication that no diversion has taken place, even if the nuclearmaterial is not measured. An additional complication is that the intense radi-ation emitted by the assembly necessitates that it is stored in strong radiationshielding, which may make it difficult to place a detector close to the assem-bly. In the case of assemblies in wet storage, the measuring equipment mayhave to be submerged in the water to get close to the assembly, which presentsadditional technical challenges.

As mentioned in section 2.3, different diversion scenarios are considered,requiring different instruments and measurement methodologies to detect di-version in irradiated nuclear fuel assemblies:

• For gross defects, diverting one or a few irradiated assemblies is suffi-cient to divert one SQ of Pu. To detect this type of diversion, the entireinventory needs to be verified to find if any assemblies are missing orreplaced with non-radioactive substitutes. Consequently, the measure-ments must be fast to be able to cover a large assembly inventory, butthey do not need to be very precise, since they only have to determineif an item under study is radioactive or not. Out of the safeguards in-struments used by the IAEA to verify irradiated fuel assemblies [5], amajority are used for gross defect verification.

• For partial defects on the 50% level, diverting fuel rods from about 4-10irradiated assemblies is sufficient to divert one SQ of Pu. This is still rel-atively few assemblies, and to ensure that these assemblies are coveredin a verification campaign, a large part of the fuel assembly inventoryneeds to be measured. Thus, detecting this type of diversion calls fora fast measurement technique, which must also be sensitive enough todetect if 50% or more of the fuel rods have been removed or replacedwith non-radioactive ones. The DCVD is suited for this scenario, sincemeasurements are fast and since it can be used for partial defect verifi-cation [14], as further detailed in chapter 5. Other than the DCVD, the

25

FORK detector is occasionally used for partial defect verification at thislevel [15].

• For bias defects, diverting single rods from 200-300 irradiated assem-blies is required to divert one SQ of Pu. This is a substantial numberof assemblies, and thus a sampling of a relatively small fraction of theassembly inventory is sufficient to detect with high probability if thistype of diversion has occurred. Consequently, measurement techniquesfor detecting this type of diversions can be more time-consuming, butthey have to be highly precise to detect a single removed or substitutedrod in an assembly. At present, gamma tomography is the only methodapproved by the IAEA to perform this type of verification [16].

One may also identify diversion scenarios with defect levels in between thethree presented levels. This opens up for additional considerations with respectto time consumption and precision of the techniques used. In this context, onemay note that enhancing the precision of the DCVD assessments would openup for additional segments of the defect levels to be covered, which is onemotivation for the work of this thesis.

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4. Cherenkov light

4.1 The physics of Cherenkov lightIn 1934 the Soviet scientist Pavel C̆erenkov observed that when water wassubjected to ionizing radiation, it emitted blue light. While initially thought tobe caused by fluorescence, through careful observation he concluded that thislight was produced by other means, and between 1934 and 1937 he publishedhis investigations. In 1937 Ilya Frank and Igor Tamm provided a theoreticalexplanation of this light [17], explaining the mechanism behind its productionand characterizing its properties. For this discovery and explanation, the threewere awarded the Nobel Prize in physics in 1958 [18].

Cherenkov light is produced when a charged particle moves faster than thespeed of light in a medium. While nothing can move faster than the speedof light in vacuum (c), the effective speed of light in a medium is lower. Fora medium with refractive index n it is vl = c/n, and for example in watervl ≈ 0.75c for visible light. Thus, any charged particle with velocity vp in therange 0.75c < vp < c will radiate visible Cherenkov light in water. For anelectron, this corresponds to a threshold kinetic energy of about 250 keV.

When a charged particle propagates in a dielectric medium, it will disruptthe local electromagnetic field, polarizing the medium. If the particle movesslowly, this disruption will elastically relax back to an equilibrium state, andno photons are radiated. However, if the particle is moving faster than thespeed of light in the medium, a disturbance is left in the wake of the particle,which will emit its energy in the form of a coherent shockwave, i.e. Cherenkovlight is emitted. A common analogy to this phenomenon is the sonic boom ofa supersonic aircraft.

When a charged particle emits Cherenkov light, the radiated photons forman angle θ to the particle propagation direction. For a particle with velocityβ = vp/c travelling in a medium with refractive index n, this angle will followthe relation given by equation 4.1, as illustrated in figure 4.1.

cosθ =1

βn(4.1)

The spectral characteristic of the Cherenkov light is given by the Frank-Tamm formula [17], presented in equation 4.2 [19]. This equation gives thenumber of Cherenkov photons N emitted in a wavelength range dλ , for acharged particle with electric charge z traversing a distance dx in the medium.In equation 4.2, α is the so-called fine-structure constant (which is approxi-mately 1/137). Equation 4.2 is valid as long as the expression in parenthesis is

27

θ

vl · t = cn · t

vp · t = β · c · t

Figure 4.1. Cherenkov light is produced at an angle θ to the charged particle propa-gation direction, determined by equation 4.1. Consequently, the produced Cherenkovlight forms a cone. The length of the sides of the triangles are marked in the fig-ure as a function of the particle and light velocity. Note that the refractive index nof a medium typically depends on photon wavelength, and as a result the angle θ iswavelength-specific.

larger than zero, which corresponds to particles moving fast enough to radiateCherenkov light, i.e. vp ≥ vl .

d2Ndxdλ

=2παz2

λ 2

(1− 1

β 2n2(λ )

)(4.2)

Equation 4.2 also show that the spectral intensity is proportional to 1/λ 2.As a result, the number of radiated photons of a short wavelength (such asblue) is higher than that of longer wavelengths (such as red). Consequently,Cherenkov light appears blue to the naked eye, although the Cherenkov lightintensity can be even higher at shorter wavelengths, such as ultraviolet (UV).For Cherenkov light in water, one should note that at wavelengths shorter thanUV, water is no longer transparent, and radiated Cherenkov photons of suchwavelengths are immediately absorbed. No matter the medium, Cherenkovlight cannot be produced with energy above roughly that of x-rays.

4.2 Cherenkov light from irradiated nuclear fuelassemblies

As mentioned in chapter 3, irradiated nuclear fuel assemblies are often storedin water, both for radiation protection and for decay heat removal. The in-tense radiation emitted by these assemblies cause high-energy electrons to be

28

200 250 300 350 400 450 500 550 600 6500

0.2

0.4

0.6

0.8

1

Wavelength [nm]

Rel

ativ

eC

here

nkov

inte

nsity

[arb

.uni

t]

10−4

10−3

10−2

10−1

100

101

Atte

nuat

ion

coef

ficie

nt[c

m−1

]

Cherenkov spectrum (linear scale)Attenuation (log scale)

Figure 4.2. The green line show the attenuation coefficient of water for soft-UV andvisible light (on the log scale on the right axis) based on data from [20] and [21]. Theblue line show the Cherenkov light spectrum measurable by the safeguards instrumentconsidered in this work (on the left axis). The spectrum was calculated using equation4.2 including the refractive index of water, and assuming a 480 keV electron, corre-sponding to the maximum energy of an electron after Compton-scattering of a 662 keV137Cs gamma. It was also assumed that the light must traverse 10 m of water, whichattenuates the light, corresponding to the depth at which irradiated fuel assemblies aretypically stored.

released in the water surrounding the fuel assembly. These electrons will inturn radiate Cherenkov light as they propagate through the water. Thus, thepresence of Cherenkov light surrounding an item indicates that the item is ra-dioactive, and the quantity of the Cherenkov light depend on the quantity andenergy of the radiation emitted by the item. Several safeguards instrumentshave been developed to measure the Cherenkov light emitted by a nuclear fuelassembly assembly, to verify that it is a strongly radioactive item and to quan-tify the Cherenkov light emitted. One such instrument, the DCVD, which isthe subject of this thesis, is further described in chapter 5.

As shown in figure 4.2, the Cherenkov light intensity peaks in the soft-UVrange in water. For this reason, the safeguards instrument considered in thiswork was designed to be sensitive to the UV-light component of the Cherenkovlight, where the intensity is the highest. By using a UV filter, visible light com-ponents are also excluded from the measurements, making them less sensitiveto background light such as facility lighting.

In irradiated nuclear fuel, several sources of ionizing radiation are presentthat cause Cherenkov light to be produced. One significant source of radiationis electrons produced in beta decays. These electrons frequently have morethan 250 keV of kinetic energy [22], allowing them to produce Cherenkovlight in water. However, electrons are effectively stopped in materials as they

29

continuously lose energy when interacting with electrons in the material. Elec-tron ranges tend to be on the order of 1 mm per MeV in dense material, and onthe order of 2 mm per MeV in low-density materials [23]. Since the claddingthickness is on the order of 1 mm and typically beta energies are lower than1 MeV, electrons produced in the fuel material through beta decays may beexpected to contribute negligibly to the Cherenkov light intensity in the watersurrounding the fuel assembly. However, such contributions to the Cherenkovlight production may still require attention.

Another major source of ionizing radiation is gamma rays, which are fre-quently emitted in radioactive decays. Since gamma rays are high-energyphotons, they carry no charge and do not directly emit Cherenkov light, butthey can interact with matter, causing secondary high-energy electrons to bereleased. The gamma rays can also penetrate the fuel material and claddingrelatively easily, hence they can interact in the water. The interactions of rele-vance to this work, i.e. those that can produce electrons with a kinetic energyabove the 250 keV threshold for Cherenkov light production in water, are [23]:

• Photoelectric absorption. An atom absorbs the gamma-ray photon, andas a result it ejects one of its electrons. Any energy carried by the initialphoton above the binding energy of the ejected electron becomes kineticenergy. Since electrons are relatively loosely bound in light materialssuch as oxygen and hydrogen, a gamma ray with only slightly moreenergy than the threshold energy can cause production of Cherenkovlight.

• Compton Scattering. The gamma ray scatters on an electron, transfer-ring parts of its energy to the electron. A gamma ray must have aboveroughly 420 keV of kinetic energy for a Compton-scattered electron tobe able to recieve kinetic energy above the threshold for Cherenkov lightproduction. Note that the cross section for scattering (as given by theKlein-Nishina formula [19]) gives at hand that the most likely scatteringmodes change the gamma-ray direction little, and consequently transfervery little energy to the electron. Instead the lower-probability, high-angle scattering is what can produce electrons of sufficient energy toradiate Cherenkov light.

• Pair production. If a gamma ray has energy above twice the rest mass ofan electron (i.e. above 1.02 MeV) it can be converted into an electron-positron pair, and any excess energy is converted into kinetic energy ofthe newly produced particles. A gamma ray with energy above roughly1.5 MeV can then create an electron-positron pair having sufficient ki-netic energy that they can radiate Cherenkov light in water. The positronwill also annihilate on an electron in the material after it has lost mostof its kinetic energy, which produces two 511 keV annihilation photonsthat may in turn result in the production of additional Cherenkov light.

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Figure 4.3. A schematic of the dominant path of Cherenkov light production causedby gamma decays in nuclear fuel assembly. (1) A gamma ray is emitted following aradioactive decay. (2) The gamma ray Compton-scatters on an electron in the water,transferring its energy to the electron. (3) The electron radiates Cherenkov light.

It has been shown in paper I that Compton scattering is the dominant typeof interaction in water, when considering the radiation emitted by the fuelmaterial with sufficient energy to produce Cherenkov light in water. This pro-cedure is illustrated in figure 4.3. In the fuel material, photoelectric absorptionwill dominate up to about 800 keV, which is low enough that the photoelec-tron is not expected to escape the fuel rod with sufficient energy to produceCherenkov light in the water. At energies above 800 keV, Compton scatteringbecomes dominant, and such high-energy electrons could potentially escapethe fuel with more than 250 keV of kinetic energy. Pair production is notexpected to contribute significantly to the interactions for the gamma-ray en-ergies encountered in decays of fission product isotopes.

Heavier isotopes present in the fuel material may spontaneously fission,and will emit neutrons while doing so. These neutrons cannot directly pro-duce Cherenkov light due to their lack of charge, but they can cause othernuclear reactions to occur. The most likely neutron interaction of relevance toCherenkov light production is absorption in a nuclei in the fuel material, whichmay produce new nuclei that decay with beta and/or gamma emissions. How-ever, the intensity of neutron emission is expected to be much lower comparedto beta and gamma emissions, and consequently the neutrons are not expectedto contribute significantly to the Cherenkov light production. Heavier isotopescan also alpha decay, but due to the low intensity and since the alpha par-ticles are easily stopped, they are not expected to contribute significantly toCherenkov light production.

For the produced Cherenkov light to be detected, it must escape the fuelassembly and reach the detector. However, due to oxidation in the harsh re-actor environment, and due to material depositions (CRUD), the surfaces ofany fuel rods or other structural components are typically dark, and thus thesurfaces absorb any incoming visible or UV light to a large extent. SinceCherenkov light measurements are done from above the assembly, only the

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vertical Cherenkov light can escape the assembly and be detected, as morehorizontally directed light is likely to encounter a fuel assembly surface andbe absorbed. As an additional consequence, the Cherenkov light that can bedetected will be highly collimated along the direction of the fuel rods, andthe intensity that can be measured depends strongly upon how the instrumentis aligned above the assembly. Thus, when investigating the Cherenkov lightproduced in an assembly and its propagation to a detector, the vertical lightcomponent is of primary interest.

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5. The Digital Cherenkov Viewing Device,DCVD

5.1 HistorySeveral instruments have been developed to assess irradiated nuclear fuel inwet storage based on the Cherenkov light produced, and the two instrumentscurrently in use by the IAEA are the Improved Cherenkov Viewing Device(ICVD) and the Digital Cherenkov Viewing Device (DCVD). The ICVD isan analogue instrument, which converts UV light to visible light, allowing aninspector to visually inspect the Cherenkov light emitted by an assembly. TheICVD does not allow for storage of images for further processing or documen-tation. Furthermore, due to the modest efficiency of the light conversion, theICVD is not sensitive enough to allow an inspector to verify assemblies withweak Cherenkov light emission, such as assemblies with long CT and low BU.The ICVD currently used (as of 2018) by the inspectors is the Mark IV CVD,which has been in use for close to thirty years [24].

The DCVD was originally developed to enable verification also of low-intensity assemblies using Cherenkov light. The initial target was the abilityto reliably verify assemblies with a cooling time of 40 years and a burnup of10 MWd/kgU, corresponding to low-BU assemblies from the first core loadingof a reactor. In field tests, the initial DCVD prototype could not only measurethe Cherenkov light from such assemblies, but it was also able to measure thelight from assemblies with cooling times of 30 years and a burnup of only1.1 MWd/kgU [25].

It was later realized that the DCVD could not only be used to qualitativelyprovide an image of the Cherenkov light emitted by an assembly, but it couldalso be used for quantitative measurements of the emitted Cherenkov light in-tensity. The recorded intensity is used to detect partial defects in an assembly,based on the change in Cherenkov light intensity caused by removing radioac-tive rods from an assembly. The DCVD is most frequently used for partialdefect verification, as the ICVD is easier to use for gross defect verification.

5.2 Measuring fuel assemblies with a DCVDDuring a measurement with the DCVD, the instrument is normally mountedonto the railing of a fuel handling machine, as shown in figure 5.1. The DCVDis looking down into the fuel assembly storage pool, where the assemblies are

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typically covered by around 10 meters of water, as schematically shown to theleft in figure 5.2. The result of a measurement is an image of the Cherenkovlight emitted by an assembly, an example of which is shown to the right infigure 5.2. The digital image obtained from the measurement can then beanalysed, as further detailed in section 5.3.

When qualitatively measuring the Cherenkov light emission from an as-sembly, using either an ICVD or a DCVD, the instrument is positioned aboveand then moved across an assembly, to confirm the presence of Cherenkovlight, and to verify that the light is collimated, as discussed in section 4.2. Thepresence and characteristics of the Cherenkov light then confirms whether ornot the item under study is an irradiated nuclear fuel assembly, as opposed toa non-radioactive item.

When quantitatively measuring an assembly with the DCVD, the inspectorwill first align the DCVD above the centre of the assembly, along the directionof the fuel rods (which is close the vertical direction, but the assemblies maybe slightly tilted). Next, the inspector manually selects a Region Of Interest(ROI) in the image, containing the fuel assembly and excluding its surround-ings. After that, the measurements are performed, including typically threeto five measurements per assembly. For each measurement, a background-subtraction is performed, aimed at removing an electronics-induced offset inthe pixel values (further described in section 8). The pixel values inside theROI are then summed to provide a total emitted intensity value of the assem-bly. The reported assembly intensity value is the average of the three to fivemeasurements. The reason for performing multiple measurements of each as-sembly is to reduce the effect of noise and changing conditions over time, suchas the effect of ripples on the water surface that can slightly distort an image.Note that the pixel values are expected to be proportional to the measured lightintensity (further discussed in section 8), but no calibration is done to convertthe pixel values to a photon flux.

The DCVD detector consists of a Charged-Coupled Device (CCD) chipsensitive to UV-light. The DCVD optics consists primarily of a motorizedzoom lens, which allows different zoom levels to be set, and for the focusto be adjusted to ensure that the top of the fuel assembly is in focus. Theoptics also contains a UV-filter for selecting relevant wavelengths, ensuringthat visible light will not be detected. The detector setup can be extended ortilted, to allow the detector to be positioned vertically above a fuel assembly.The DCVD can be powered either by two hot-swappable batteries, or by apower cable if a wall-socket is available.

The DCVD contains an on-board computer, which handles the data outputfrom the detector. The user interacts with the DCVD software through a touch-sensitive LCD screen. Customised software performs image processing andcalculates the total Cherenkov light intensity based on the recorded imagesprovided by the detector. The software also keeps track of which measurement

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Figure 5.1. Top: A DCVD instrument. To the right is the CCD detector and optics,mounted on a yoke that can be extended and tilted. The computer and electronics arehoused inside the casing, and the batteries are seen to the left. On top is an LCD screenproviding the user interface. Bottom: A DCVD in use. Images courtesy of DennisParcey and Channel Systems Inc.

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Figure 5.2. Left: A schematic illustration of the measurement situation when usingthe DCVD. Note that the assemblies are typically around 4 m in length, and coveredby about 10 m of water. Right: An example of an image obtained from a measurementof a PWR fuel assembly with the DCVD. The bright, circular regions are the guidetubes of the PWR assembly, the smaller bright spots are the regions in between fuelrods.

corresponds to which assembly, if the user provides information about theassemblies verified in a measurement campaign.

The DCVD has several advantages and disadvantages compared to othersafeguards instruments, which are considered when selecting an instrumentfor a verification campaign. Advantages include:

• Non-intrusiveness:The fuel assemblies are measured where they are stored, and there is noneed to move the assemblies to a dedicated measurement station. Thereis neither any need to insert the device or any associated equipment intothe water to get close to an assembly, which reduces the risk of contam-ination.

• Speed:Since there is no need for moving any assemblies, measurements arequick, typically requiring approximately 10-30 seconds for one assem-bly, depending on the assembly intensity and storage conditions. Thisspeed makes it feasible to verify a large assembly inventory using theDCVD.

The DCVD also has some limitations, which must be considered when ver-ifying a fuel assembly inventory:

• Limited information:The currently used methodology analyses the total Cherenkov light in-tensity, which depends on the intensity of the ionizing radiation emittedby the assembly. It does not provide any information about the sourceof the radiation, such as the presence or abundance of fission productisotopes, nor of the fissile contents of the assembly.

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• Limited sensitivity:In order to detect a partial defect, sufficiently many rods must have beendiverted to affect the total Cherenkov light intensity of an assembly no-ticeably. Furthermore, some rods may be hidden under the top plateor lifting handle and contribute little to the measured Cherenkov lightintensity, making the diversion of such rods difficult to detect usingCherenkov light.

As a consequence of these characteristics, the DCVD is well suited fordetecting partial defects (as described in section 3.3) in scenarios where a largefraction of the fuel rods in an assembly have been removed or replaced withnon-radioactive ones. Previous work has shown that the DCVD is sensitiveenough to detect diversions on the order of 50-100% removed or substitutedrods [14]. Furthermore, the measurements are fast enough that the DCVDcan be used to verify a large assembly inventory, to find the relatively fewassemblies where such diversion may have taken place.

5.3 Detecting partial defects using a DCVDThere are two methods in use to detect partial defects with the DCVD. Thefirst method uses image analysis to identify removed rods, which is based onthe identification of bright regions in the images which should be dark dueto the expected presence of a fuel rod in that position. This method can thusdetect removed rods in visible positions, and it is frequently used for BWR fuelassemblies since individual rods are often visible in BWR assembly designs.An example of an intact BWR assembly and one with two removed rods isshown in figure 5.3.

Figure 5.3. Left: A DCVD measurement of a complete BWR assembly. Right:A BWR assembly with two removed rods. Images courtesy of Dennis Parcey andChannel Systems Inc.

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The second method, which is the focus of this thesis, is based on quanti-tatively measuring the total Cherenkov light intensity from an assembly, andcomparing it to a predicted intensity, which is calculated based on the declaredassembly information [26]. This method is capable of detecting diversion sce-narios where fuel rods have been substituted with non-radioactive replace-ments. Previously performed simulations [14] have shown that a 50% substi-tution of irradiated fuel rods with non-radioactive steel rods will decrease theCherenkov light intensity by at least 30%. Thus, if a measured intensity is30% or more below the predicted intensity, the fuel is flagged by the DCVDsoftware as an outlier requiring additional investigation, as illustrated in fig-ure 5.4. Key to this procedure is a prediction method with high accuracy, andimproving the prediction model is a major part of this work. Furthermore, thepredictions must be quickly executed on modest hardware, to allow for in-fielduse by an inspector.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

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0.4

0.6

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Predicted intenisty [arb. unit]

Mea

sure

din

teni

sty

[arb

.uni

t]

Figure 5.4. Example of the analysis performed as part of quantitative Cherenkov lightverification. Once the measurements and predictions are available, a least-square fitis performed to find the multiplier that relates the two. The solid line indicates theexpected agreement between predictions and measurement, after adjustment with themultiplier. The dashed lines indicate a ±30% deviation. The data point marked with ared circle has a measured intensity more than 30% below expected, and is flagged asan outlier requiring further investigation.

An inspector in the field performs quantitative intensity verification accord-ing to the following steps:

1. The inspector obtains information about the fuel assemblies present atthe facility, including parameters such as assembly type, BU, and CT.These parameters are used to predict the Cherenkov light intensity ofthe assembly, as further detailed in section 5.4. Based on the declara-

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tions, the assemblies will be grouped according to their type, or physicaldesign.

2. The inspector measures the Cherenkov light emissions from the assem-blies in the inventory. If there are not too many assemblies, all of themcan be measured; otherwise the inspector will make a random samplingof the assemblies present.

3. For each group of assemblies, a least-square fit is made to find the mul-tiplier relating the predictions to the measurements, corresponding to anassembly-type specific calibration factor. After adjusting the predictionswith this multiplier, the measured intensities are compared to the predic-tions, and any assembly with an intensity deviating more than 30% fromexpected is flagged as an outlier requiring further investigation.

Due to the least-square fitting, the predicted intensity of an assembly needonly to correspond to the relative intensity of the assembly, as compared toall other assemblies in the group. While it is in principle possible to predictthe absolute intensity, such predictions must take into account the assemblydesign, the storage conditions, the water quality and the instrument response,information that is not always available with sufficient detail to be included ina prediction. However, in a verification campaign using the same instrumenton fuel assemblies of the same type, all stored under the same conditions,these parameters will affect all measurements equally, and thus the relativeintensity of the assemblies can be used instead of the absolute intensity. As aconsequence of this procedure, analyses can only be made for a certain fuelassembly type if a sufficiently large number of assemblies are available for thecalibration and comparison to be relevant.

5.3.1 Partial defect intensity limitsThe currently used partial defect intensity limits are based on the work of[14], which showed that a 50% substitution of radioactive fuel rods with non-radioactive steel substitute rods will lower the Cherenkov light intensity of theassembly by at least 30%. These results are based on simulations of differentsubstitution scenarios in a BWR 8x8 and a PWR 17x17 assembly, and the 30%limit comes from the diversion cases most difficult to detect using Cherenkovlight. Note that currently, fuel assemblies are tested against a global 30% limit,although there are diversion scenarios on the level of 50% that are expected tolower the Cherenkov light intensity by more than 30%.

Simulations of assemblies with partial defects require that DCVD imagesare simulated, and these simulations are performed in three steps [27]: 1)Simulating the gamma emission spectra of an assembly with a typical BU andCT 2) Simulating the gamma transport and Cherenkov light production in anassembly geometry, and 3) Simulating the propagation of the Cherenkov light

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in an assembly geometry, and its detection in a camera model. Simulationsof assemblies with partial defects are necessary, since very few documentedassemblies are subject to a significant removal or replacement of rods. Thus,simulations of DCVD measurements are required to assess the performanceof the instrument to detect partial defects in assemblies, and the three-stepmethodology of [27] is capable of simulating DCVD images for assemblieswith partial defects.

5.4 First-generation method (1GM) for predictingCherenkov light intensities

The method that has been used previously by the IAEA for predicting thetotal Cherenkov light intensity from a fuel assembly is based on [28], and willbe referred to as the first-generation method (1GM) in this work. This methodtakes into account the BU and CT of the assembly, which are the two dominantparameters describing the intensity of the Cherenkov light. Furthermore, theseparameters are fundamental enough that an inspector should be able to obtainthem for any assembly.

The predictions rely on pre-calculated Cherenkov light intensities for BWRassemblies of varying BU and CT, which were used to obtain intensity curvesrelating the BU and CT to the Cherenkov light intensity, as shown in figure 5.5.To obtain a prediction, the operator-declared BU and CT of the assembly understudy are used to interpolate the Cherenkov light intensity from the curves infigure 5.5. In later unpublished work, the curves in figure 5.5 were extendedto consider longer cooling times, and lower burnups than the ones consideredin [28].

To calculate the Cherenkov light intensity curves in figure 5.5, the followingsimulation procedure was used:

1. The abundance of gamma- and beta-emitting isotopes were calculatedusing the nuclear fuel depletion code ORIGEN [29], for BWR assem-blies with various BU and CT. The six most abundant isotopes wereconsidered, namely 106Ru, 134Cs, 137Cs, 144Ce, 154Eu and 90Sr. The firstfive of these isotopes undergo gamma decay, and 90Sr and its daugh-ter 90Y undergo beta decay. All of these isotopes, or their short liveddaughters, decay with energy high enough that Cherenkov light can beproduced. A standard irradiation history was assumed when calculatingthe fission product inventory, which matches the typical irradiation ex-perienced by a fuel assembly in a commercial reactor, with a 12-monthcycle.

2. The calculated gamma spectrum was used as a source in a Monte-Carloradiation transport code, simulating the radiation and its interactions ina BWR 8x8 assembly. The simulations included the interactions of the

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0 5 10 15 20 25 30 35 40

105

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Cooling time [years]

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renk

ovlig

htin

tens

ity[a

rb.u

nit]

10 MWd/kgU20 MWd/kgU30 MWd/kgU40 MWd/kgU50 MWd/kgU

Figure 5.5. Intensity of the Cherenkov light as a function of fuel assembly BU andCT, from [28]. Given the BU and CT of an assembly, the expected Cherenkov lightintensity can be interpolated from these curves, forming the first-generation predictionmodel (1GM).

gamma rays with the fuel and water, the production of Cherenkov light,and the transport of the Cherenkov light to a detector position. The resultof the simulations was an estimate of the detectable Cherenkov lightintensity for an assembly with the simulated BU and CT.

This prediction method has proven itself to work well at long cooling times,when the most important gamma-emitting isotope is 137Cs. This isotope buildsup linearly with burnup, and it has a relatively long half-life. Consequently,knowing the BU and CT is sufficient for a good estimate of the abundance of137Cs, and thus of the Cherenkov light intensity at long CT. This predictionmethod does however include several simplifying assumptions, which limitthe applicability of the method. The limitations are further discussed in thenext section.

5.5 Limitations addressed developing thesecond-generation prediction method (2GM)

While the DCVD has been used successfully for partial defect verification be-fore this work, it is possible to improve the verification procedure and method-ology further. Improvements are primarily aimed at developing and enhancingthe procedure for predicting the expected Cherenkov light intensity of an as-sembly. The procedure described in section 5.4 has limitations, some due tothe procedure itself, and some due to approximations and simplifications thatwere necessary at the time it was developed. The primary limitations that are

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addressed in this work to develop the second-generation prediction method(2GM) are:

• The 1GM considers gamma emissions from the fuel material caused bya limited set of six fission product isotopes, which were found to cause amajority of the Cherenkov light after a few years of cooling. The accu-racy of the predictions can be improved if all gamma-emitting isotopesare included, especially for short cooling times when additional short-lived isotopes are present.

• The 1GM assumes that all beta-decay electrons are stopped in the fuelmaterial, emitting bremsstrahlung as they are stopped. The simulationsperformed as part of this work, presented in chapter 6, however showthat some beta electrons may escape the fuel rods and directly produceCherenkov light in the surrounding water. Including this contributioncan further improve the precision of the predictions.

• The 1GM assumes a standard irradiation history, matching the typicalirradiation an assembly experiences in the reactor. However, for as-semblies with a more irregular irradiation history, this assumption is nolonger valid. The prediction model could thus be improved to includealso the irradiation history. This is particularly important for assemblieswith a short CT or an unusual irradiation history.

• The 1GM assumes that all assemblies behave as a BWR 8x8 assembly.Thus, any systematic effects caused by differences in physical assemblydesign is ignored. Consequently, the prediction model may be improvedif also the assembly design is taken into account.

• Assemblies are often stored close together, and radiation from one as-sembly can enter a neighbour to create Cherenkov light there. This so-called near-neighbour effect is not included in the 1GM predictions, andlimits the accuracy of the verification, in particular for situations where alow-intensity assembly is surrounded by high-intensity neighbours. Thisproblem can be remedied by developing a near-neighbour intensity pre-diction model.

• The current analysis groups assemblies according to their type, to makecomparisons of assemblies with similar physical design. Should a fuelinventory contain a single assembly of a certain type, there will be noother assemblies to group it with. To solve this problem, correction fac-tors based on assembly design could be introduced, allowing assembliesof different designs to be compared.

• The 1GM simulations include light transport to a detector position, butno image creation calculations. Later image creation simulations used adifferent level of detail in the Monte-Carlo radiation transport step and

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the optical photon propagation step, and required the use of licensedsoftware for the optical photon step. The image creation simulationscould be improved if these two steps were seamlessly integrated, andusing one code rather than two simplifies the simulation workflow.

To gain more knowledge regarding Cherenkov light and nuclear fuel assem-blies, much of this work has focused on developing software to simulate theproduction of Cherenkov light in nuclear fuel assemblies (described in chapter6), and its propagation to a detector position and subsequent image creation.In a second step, these simulations were used to develop the 2GM (describedin chapter 7). The aim of the 2GM was to enable better accuracy in the pre-dictions, which may enable a higher sensitivity of the instrument in detectingpartial defects. A goal of developing the 2GM was also to extend the range offuel assemblies that can be verified with the instrument, by overcoming limi-tations in the 1GM, making DCVD measurements viable in more situations.

5.6 Practical aspectsWhile the aim of this project was to improve the