overview of sr-can; Äspö task force meeting jan 17 th 2007 overview of the safety assessment...

Overview of SR-Can; Äspö Task Force meeting Jan 17th 2007

Overview of the safety assessment SR-Can

Allan Hedin, SKB


The SR-Can team• Kastriot Spahiu, fuel

• Lars Werme, canister

• Patrik Sellin, buffer and backfill

• Ignasi Puigdomenech, geochemistry

• Raymond Munier, geology

• Jan-Olof Selroos, flow and transport geosphere

• Ulrik Kautsky, biosphere

• Lena Morén, FHA issues

• Jens-Ove Näslund, future climate

• Fredrik Vahlund, radionuclide transport and dose calculations, input data

• Karin Pers (Kemakta), Initial state engineered barriers

• Kristina Skagius (Kemakta), FEP database, methodology, link to SDM Forsmark

• Johan Andersson (JA Streamflow AB), methodology, coordination with site investigations and design, coordination of mechanics issues, input data

• Lisa Wedin, project administration

• Allan Hedin, project leader, editor main report, methodology


Context• Application for Encapsulation plant filed

November 8:th

• Site investigations at Forsmark and Oskarshamn progressing according to plan– SR-Can is based on data from the initial site

investigation stage

• SR-Can is a preparatory step for the SR-Site assessment. SR-Site will support SKB’s application for a final repository, planned for 2009.

• SR-Can is not part of any application

• Interim SR-Can report 2004, reviewed 2005– Appendix C: SKB’s handling of main review

findings


Purposes of SR-Can

1. To make a first assessment of the safety of potential KBS-3 repositories at Forsmark and Laxemar to dispose of canisters as specified in the application to build the encapsulation plant;

2. To provide feedback to design development, to SKB’s R&D programme, to further site investigations and to future safety assessment projects;

3. To foster a dialogue with the authorities that oversee SKB’s activities, i.e. the Swedish Nuclear Power Inspectorate, SKI, and the Swedish Radiation Protection Authority, SSI, regarding interpretation of applicable regulations, as a preparation for the SR-Site project.


The KBS-3 concept

Primary safety function: Complete isolation

Secondary safety function: Retardation


Relation to other projects


Applicable regulations

• The Swedish Nuclear Power Inspectorate’s regulations concerning safety in final disposal of nuclear waste, SKIFS 2002:1– … and General guidance to SKIFS 2002:1

• Regulations for final disposal of spent nuclear fuel, SSI FS 1998:1– … and General advice to SSI FS 1998:1

• Regulations reproduced in Appendix A in the main report, with references to “implementation” in main text.

• Risk criterion: – The annual risk of harmful effects after closure must not exceed 10−6 for a

representative individual in the group exposed to the greatest risk. – “Harmful effects” refer to cancer and hereditary effects. – Limit corresponds to effective dose limit ≈1.4∙10−5 Sv/yr ≈ 1% of natural

background radiation in Sweden.– Applicable 100,000 years after closure

• Time scale for the assessment: One million years


Report hierarchy

Main references

FEP report Initial state report

Fuel and canister process report

Buffer & backfill process report

Geosphere process report

Climate report FHA report Model summary report

Data report

Additional references

SR-Can

Main report

• All available @ www.skb.se ; Main report TR-06-09

• Site descriptive model Forsmark v. 1.2 + layout D1

• Site descriptive model Laxemar v. 1.2 + layout D1


Methodology in ten steps


Safety functions

• A safety function is a role through which a repository component contributes to safety– Example: The canister should withstand isostatic load

• A safety function indicator is a measurable or calculable property of a repository component that indicates the extent to which a safety function is fulfilled– Example: Isostatic stress in canister

• A safety function indicator criterion is a quantitative limit such that if the safety function indicator to which it relates fulfils the criterion, the corresponding safety function is maintained– Example: Isostatic stress < isostatic collapse load


Safety functions, canister and buffer

Buffer

Bu1. Limit advective transport a) Hydraulic conductivity < 10−12 m/s b) Swelling pressure > 1 MPa

Bu2. Filter colloids Density > 1,650 kg/m3

Bu3. Eliminate microbes Swelling pressure > 2 MPa

Bu4. Damp rock shear Density < 2,050 kg/m3

Bu6. Prevent canister sinking Swelling pressure > 0.2 MPa

Bu 7. Limit pressure on canister and rock Temperature > −5 °C

Bu5. Resist transformation Temperature < 100 °C

Canister

C2. Withstand isostatic load Strength > isostatic load

C3. Withstand shear load Rupture limit > shear stress

C1. Provide corrosion barrier Copper thickness > 0


Safety functions, deposition tunnel backfill and geosphere

Deposition tunnel backfill

BF1. Limit advective transport a) Hydraulic conductivity < 10−10 m/s b) Swelling pressure > 0.1 MPa c) Temperature > 0 °C

Geosphere

R1. Provide chemically favourable conditions a) Reducing conditions; Eh limited b) Salinity; TDS limited c) Ionic strength; [M2+] > 1 mM d) Concentrations of K, HS−, Fe; limited e) pH; pH < 11 f) Avoid chloride corrosion; pH > 4 or [Cl−] < 3M

R3. Provide mechanically stable conditions a) Shear movements at deposition holes < 0.1 m . b) GW pressure; limited .

R2. Provide favourable hydrologic and transport conditions a) Transport resistance; high b) Fracture transmissivity; limited c) Hydraulic gradients; limited d) Kd, De; high e) Colloid concentration; low

R4. Provide thermally favourable conditions Temperature > Buffer freezing temperature


Reference evolution

• Definition– Reference initial state conditions (IS report + SDMs + layouts D1)– Processes handled according to Process report “recipes”; data from Data report– External: Assumed repetitions of the last 120,000 year glacial cycle– Variant of external: Increased greenhouse effect

• Repository evolution analysed in four time frames; focusing on isolation– Excavation/operation period– Initial temperate period– Initial glacial cycle– Remaining seven glacial cycles

• Modelling of T, H, M, C, R, B processes in each time frame

• Evaluation of all safety functions after each time frame– Detailed evaluation after initial temperate period and initial glacial cycle

• Analysis of greenhouse variant essentially as comparison to base variant

• Quantification of radiological consequences as subsequent task


Definition of the main scenario

• Essentially the reference evolution…

• …with more precise definitions of handling of uncertainties identified in reference evolution


Selection of additional scenarios based on safety functions

• Consider every safety function: How can it be lost?

• Example: The canister should withstand isostatic load

• Define scenario “Canister failure due to isostatic load”

• Look for all possible ways this could occur– Evaluate uncertainties not considered in the reference evolution/main

scenario– Higher than reference buffer density leading to high buffer swelling

pressures on canister?– Severe design flaws in canister insert, weakening the structure?– More massive ice sheets in future glaciations than assumed in reference

evolution?– I.e. evaluate uncertainties related to all FEPs of relevance for this safety

function


Scenarios, cont.

• Bottom line: Could it happen?

• If yes: – estimate probability or assume pessimistically p=1– calculate consequences and include in risk summation

• If no:– consider as residual scenario not to be included in risk summation– In some cases: calculate consequences for illustrational purposes

• Not all safety functions used to generate a scenario– Lumping necessary since functions are connected and not fully

independent


Selected scenarios

• Main scenario– Base variant– Greenhouse variant

• Buffer advection

• Buffer freezing

• Buffer transformation

Results of buffer scenarios propagated to analyses of

• Canister failure due to corrosion

• Canister failure due to isostatic load

• Canister failure due to shear load

In addition:

• Scenarios related to future human actions


Conclusions; Compliance with regulatory risk criterion

• No canisters are assessed to fail during the initial temperate period, expected to last several thousand years

• A repository at Forsmark is assessed to comply with the regulatory risk criterion

• A repository at Laxemar is preliminarily assessed to comply with the regulatory risk criterion – but more representative data is required


Conclusions;Issues related to glacial conditions

• Freezing of an intact buffer is assessed as ruled out – even for very pessimistically chosen climate conditions

• Canister failure due to isostatic load is assessed as ruled out – even for very pessimistically chosen climate conditions

• Oxygen penetration is preliminarily assessed as ruled out – even for very pessimistically chosen conditions

• The risk contribution from earthquakes is assessed to be small

• Loss of buffer may occur from exposure to glacial melt waters but the extent is uncertain – further studies are required

• Substantial loss of buffer may lead to canister failures in very long time perspectives

• An prolonged period of warm climate (increased greenhouse effect) before the next glacial period is assessed to be primarily beneficial for repository safety


Conclusions; Other issues related to barrier performance and design

• Crucial to avoid deposition positions intersected by large or highly water conductive fractures – further studies are required

• The heat from the canister may fracture the rock in the deposition hole wall, which may enhance the in- and outward transport of dissolved substances – further studies are required

• The importance of the backfilled deposition tunnels as a transport path for radionuclides is limited

• The importance of the excavation damaged zone in the rock around the deposition tunnels as a transport path for radionuclides is limited


Risk summation

• Several pessimistic assumptions; e.g.– Hydraulic interpretation of the Forsmark site– Occurrence of buffer erosion– Geochemical conditions (sulphide concentrations)

• Data for Laxemar are not representative– Recently obtained data of intended repository volume

indicate more favourable conditions

• Risk criterion, applicable for ≈100,000 years, fulfilled

• Higher calculated risk for one million year period


Risk summation

overview of sr-can; Äspö task force meeting jan 17 th 2007 overview of the safety assessment...

Documents