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D R A F T

DESIGN AND SAFETY DOCUMENT OF THE NPDGAMMA LIQUID HYDROGEN TARGET

20 October 2010

Design and Safety Document of the NPDGamma Liquid Hydrogen Target

Table of Contents................................................................................................1List of Figures......................................................................................................5List of Tables.......................................................................................................6Update/Revision Log...........................................................................................7List of design documents, calculations, and drawings .....................................7 1.0 INTRODUCTION...........................................................................................121.1 Scope of the document ................................................................................121.2 Overview of the NPDGamma experiment and the liquid hydrogen target....121.3 History of the target: Reviews and the run at LANSCE …………………...…151.4 Code compliance plan; codes and standards...............................................161.5 Passed reviews at SNS...............................................................................171.6 Unreviewed Safety Issue Determination (USID)...........................................181.7 The 2nd hydrogen safety review at SNS........................................................181.8 Target Design Review..................................................................................181.9 Plan for completion of the target construction and installation and responsibilities.....................................................................................................18

2.0 DESCRIBTION OF THE NPDGAMMA LH2 TARGET SYSTEM ……………………...192.1 Introduction of target components and their functions..................................192.2 Overview of safety aspects of the target.......................................................24

3.0 DESIG OF THE LH2 TARGET SYSTEM ………………………………………………………243.1 Hydrogen gas handling...............................................................................26 3.1.1 H2 supply manifold...................................................................................26 3.1.2 Hydrogen manifold shed.............................................................................273.2 Gas handling system.....................................................................................273.2 LH2 vessel.....................................................................................................303.3.1 Bi-metallic joints..........................................................................................303.4 Fill/vent line....................................................................................................303.4.1 Bellows.......................................................................................................333.5 Cryocooler and electrical feedthroughs.........................................................343.6 Precooling of H2...........................................................................................343.7 Cooling of the LH2 vessel.............................................................................353.8 Liquefacation chamber and ortho-para conveter..........................................353.9 Cryostat – vacuum chamber.........................................................................363.9.1 Vacuum chamber.......................................................................................36

Design and Safety Document of the NPDGamma Liquid Hydrogen Target 3

3.9.2 Beam windows ..........................................................................................363.9.3 Radiation shielding and superinsulation ....................................................373.9.4 He channels................................................................................................37................................................................................................................................3.10 Vacuum system and pumps.......................................................................373.10.1 Vacuum system and pumps....................................................................373.10.2 RGA..........................................................................................................383.12 Relief/isolation chamber..............................................................................383.13 Vent stack....................................................................................................383.14 Relief of overpressures from LH2 vessel and from isolation vacuum.........383.15 Deflagration and explosion in fill/vent line; ..................................................423.16 Cabinet ventilation line...............................................................................42

4.0 Instrumentation………………………………………………………………………………………434.1 Pressures/flows/thermometers/heaters........................................................444.2 Target monitoring system..............................................................................44

5.0 TAREGT OPERATION……………………..………………………………………………………445.1 Target signal processing...............................................................................445.2 Operation of LH2 target.................................................................................46

6.0 QUALITYMANAGEMENT……………………………………..…………………………………44

7.0 FALURE and hazard analysis…………………….…………………………………………44

REFERENCES

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List of Figures

Figure 1. Conceptual view of the BL13 with the NPDGamma experiment..…... 13 Figure 2. An overall view of the NPDGamma experiment in BL13.. ………...…..14Figure 3. A closer view of the Experimental apparatus ……………………... 15Figure 4. Outside the cave is the relief/isolation cabinet.……………………

15Figure 5. Schematic of the LH2 target ……………………………………………..

21Figure 6. The LH2 target piping and flow diagram ………………………………….

23Figure 7. Diagram of the hydrogen supply manifold ……………………………….

26Figure 8. Diagram of gas handling manifold ………………………………….

29Figure 9. Assembly drawing of the coaxial fill/vent line. …………………………….

32Figure 10. Assembly of the gas precooling cryo-refrigerator ……………………….

33Figure 11. Assembly of a polyethylene box ………………….……………………….

35Figure 12. Relief/isolation cabinet and vent stack on platform. ……………………… 41Figure 13. A view of the relief/isolation chamber ……………………………………

42Figure 14. Cross section view of the relief/isolation chamber …………………43

…


List of Tables

Table 1. National consensus codes and standards …………………………..16

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Design and Safety Document of the SNS Liquid Hydrogen TargetUpdate/Revision Log

Revision Number

Date Description of Changes Pages Changed

2.00 06-05-2008 Base line for the document n/a


The design documents, calculations, and drawings of the target system. These are available at the web sitehttp://battlestar.phys.utk.edu/mediawiki/index.php/Hydrogen_Target_Documents

Document Name Page ContentLH2TargetModifications.pdf 15 Description of modifications to LH2

target since LANSCE operationReportof1stSafetyReview.pdf 16 Report of the 1st SNS target safety

reviewSNS 102030102-ES0029-R01 Unreviewed Safety Issue

Determination (USID) analysis of the target

LH2TgtEngStudy.pdf 17 Verification of dimensional calculationsFUND13NPDG-24-DE0001-R00.pdf

18 Target design review presentations and comments

MOUDraft.pdf 18 MOU draftLH2TgtOperatingProcedure.pdf 25 Target User GuideHembrittlement.pdf Analysis of hydrogen embrittlement

effects in relief valves and rupture disksLH2EngDoc2.00.pdf Target engineering document for the

LANSCE targetKH2_FEA_Luttrel.pdf Window and vessel FEAAbilityVesselReport.pdf 24 Design/QA/testing report on the target

vesselQAvesselwindows.pdf 25 Documentation of quality assurance for

target vessel and vacuum window testsThermalCyclingBimetalJoint.pdf 25 Thermal cycling tests of aluminum-

stainless bimetallic jointsVentlineassemblyplan.pdf Plan for sequential assembly of fill/vent

line FailureHazardAnalysis.pdf 29 Discussion of safety issues and

mitigation specificsHlinebellowscertificationinfo.pdf 30 Bellows certification and specificationsLH2filltimeestimate.pdf Cooling calculations for the fill/vent lineRefrigerator3coolingtest.pdf Cooling test to verify that fill/vent line

refrigerator stays above 21KLH2TargetInstrumentation.pdf Instrumentation and specifications for

the hydrogen target and gas handling system

AnalysisConflatsUnderPressure.pdf

FEA analysis of stresses in Conflat flanges under internal gas pressure

DraftExemptioncomponentlistV1.2.pdf

Compilation of fill/vent line component properties

Vacuumvesselreport.pdf Design/QA/testing document on main vacuum vessel

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BeamWindowDesignSafety.doc Design, FEA, and safety analysis for the vacuum windows

thickAldomesreport.pdf Design/QA/testing document on main vacuum windows

TargetThermometerConnections.pdf

Wiring diagram for target thermometer connections

ComputerDAQ.pdf Description of the LH2 target data acquisition computer and safety issues

Designreliefvent.pdf Calculations of H2 gas venting in accident scenarios

TargetPlumbingOPC.pdf Description and tests of the OPC vessel

LH2targetinstrumentation.pdf Description of thermometry, heaters and wiring for LH2 target

TritiumProduction.pdf Calculations of rate of production of tritium in LH2 target

Ventlinestress.pdf Calculations of hoop stress for fill/vent line tubing

Tdistributioninreliefventline.pdf Estimated temperature distribution during target venting

SNSBL13CROSSEVALUATION.pdf

FEA analysis of the 6-way cross on the fill/vent line

SNSBL13WELDING.pdf Outline of ORNL welding requirementsTargetplumbingOPC.pdf Description of internal liquid hydrogen

plumbing and the OPCVentisolationchamberreport.pdf Description of vent isolation chamber

design and vent calculationsAccidentscenariotesting.pdf Description of tests of target venting

rates in normal venting and under accident scenarios

EarthquakeAnalysisvessel.pdf Analysis of response of the target vessel to a 0.5g acceleration from an earthquake

Hembrittlement.pdf Analysis of hydrogen embrittlement effects in relief valves and rupture disks

SignalProcessing.pdf Details of target warnings and interlocks


1. INTRODUCTION

1.1 Scope of the Document

The scope of the Design and Safety Document of the NPDGamma Liquid Hydrogen Target is to give an introduction to the experiment and especially to the hydrogen target and then summarize design, operation, and safety of the target system that is going to be operated on the beamline 13 (BL13) (knownalso as FNPB) at SNS by the NPDGamma experiment. The Dewsign and Safety Document lists most of the documents, calculations, drawings produced by the design of the hydrogen target system. In addition, the document presents procedures for safe operation of the target, and defines qualifications and training for target specialists who will be responsible to operate the target.

1.2 Overview of the NPDGamma experiment and the hydrogen target

The NPDGamma experiment searches for parity violation in the angular distribution of 2.2-MeV gamma rays produced by polarized cold neutron capture in unpolarized liquid parahydrogen target. The only way that the experiment can reach the physics goal is to use a 16 liter liquid parahydrogen target. The 2.2-MeV gamma-rays from the neutron capture on hydrogen are detected by CsI detector surrounding the hydrogen target cryostat. A full discussion of the experiment can be found in [1]. For the purposes of this document we will define the “target” broadly to include (1) the target cryostat and its isolation vacuum system inside the BL13 shielding enclosure (cave), where the neutron captures take place, (2) the gas supply, handling, and target control system external to the cave, and (3) the safety system, including the relief devices and vent lines. A conceptual view of the experiment and beamline is shown in figure 1. Figure 2 shows an overview of the BL13 setup given by the model. Figure 3 shows a closer view of the apparatus as is shown in the model.

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Fig. 1. Conceptual view of the BL13 withthe NPDGamma experiment.The BL13 is a curved beamline with two choppers. Neutrons from the guide are polarized for the experiment by a supermirror polarizer. All the shielding around the beamline and experiment is not shown in the figure. The details of the shielding can be found from the BL13 radiological shielding report draft. Distance from the moderator to the end of the guide is about 50 ft and from the guide window to the center of the detector about 8ft.


Fig. 2. An overall view of the NPDGamma experiment and radiological shielding structure in the BL13 given by the model when the roof shielding has been removed for this view.

11CsI gamma detectorRF spin flipper

Neutron guide

Vent stack

Cave access door

Vacuum fill/vent line from target

Polarizer/guide shielding

Beam stop

Detector

Relief/isolation cabinet


Fig. 3. A closer view of the NPDGamma apparatus taken from the model.

Fig. 4. Outside the cave is the relief/isolation cabinet on platform, vent stack, cabinet ventilation pipe, and gas handling cabinet.

1.3 History of the target: Reviews and the run at LANSCE

In 2006 the NPDGamma experiment with the liquid parahydrogen target was operated for 3 months on FP12 at LANSCE. Before running, the hydrogen target had to pass an extensive review process. In addition to the target

Spin Flipper

CryostatDetector

Beam stop

Lead wall #2

Fill/vent line

Vent stack

Ventilation pipe

Relief/isolation cabinet

Gas handling cabinet

HEVACsystem


operation for the experiment, several target tests were performed to verify the target design parameters. These tests included for instance a measurement of the target boil-off rate by opening the isolation vacuum to air when the target was full of liquid hydrogen During the run the target was filled three times,, A conclusion of the LANSCE run was thattargetperformed as expected. The LANSCE target is described in detail in the literature [2]. The LANSCE target safety, review reports, and design documentation can be found in web site http://www.iucf.indiana.edu/U/lh2target/export-files/. Results and experiences for operating of the target system at LANSCE have been used to further improve the target for the SNS run where the target has to meet more stringent physics and safety requirements. The plans for the target improvements are described in document LH2TargetModifications.pdf which is as also the other documents, is available at http://battlestar.phys.utk.edu/mediawiki/index.php/Hydrogen_Target_Documents.

1.4 Code compliance plan; codes and standards

The goal in the design of the target was to meet the national codes and SNS requirements. But if a component of the target could not be designed to be in compliance with the code without compromising the physics goals, equivalent measures were taken which included for instance FEA analyses or special tests. The target also has some legacy issues; some of the components were designed and used in the target that was operated at LANSCE. These components were reanalyzed and produced documents filed as required for the new design. Table 1 lists the main codes and standards used in the design.

Table 1. Codes and Standards

- Section VIII of ASME - Boiler and Pressure Vessel- ASME B 31.3 - Process Piping- ASME B 31.12-2008 - Hydrogen Piping and Pipelines- CGA S-1.3 - Pressure Relief Device Standards-Part 3 – Compressed

Gas Stationary Storage Containers- CGA G-5.5 – 2004 - Hydrogen Vent Systems- Expansion Joint Manufacture Association (EJMA) - division 1, of the

ASME Boiler and Pressure Vessel Code, and standards- DOE-STD-1020DOE-STD-1021NFPA 50A – Standard for Gaseous

Hydrogen Systems at Consumer Sites – 1999 edition - NFPA 50B – Standard for Liquefied Hydrogen Systems at Consumer

Sites – 1999 edition- NFPA 55 – Standard for Storage, Use, and Handling of Compressed

Gases and Cryogenic Fluids in Portable and Stationary Containers, Cylinders, and Tanks – 2005 edition

- SNSBL13WELDING.doc describes ORNL welding requirements.- SNS Cryogenic Manual

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1.5 Passed reviews at SNS

The size of the liquid hydrogen target (approximately 16 liters) coupled with its location inside the BL13 shielding structure of the experiment (cave) and the presence of several electrical systems inside the cave and in parts of the apparatus, dictate stringent safety requirements. A preliminary assessment of the safety requirements for this target was performed in 2007 at ORNL by the Liquid Hydrogen Safety Committee. The report of this safety assessment and recommendations of the Hydrogen Safety Committee, which evaluated a preliminary conceptual design of the target, is summarized in Reportof1stSafetyReview.pdf. Due partly to the physics goals at SNS, partly to changes in DOE requirements after the 2006 run at LANSCE, and partly due to different level of safety at ORNL, some major changes in the target were required for operation at SNS (LH2TargetModifications.pdf). The most significant changes were as follows:

(a) The aluminum target vessel must be manufactured in accordance with Section VIII of the ASME Boiler and Pressure Vessel Code and U stamped. The goal here was to have a pressure stamped LH2 vessel with thinner neutron entry window. A significant improvement of the safety of the hydrogen components in the target.

(b) To reduce the signal backgrounds, the thickness of the neutron entry windows on the isolation vacuum chamber have to be decreased. This improves of the physics performance of the experiment without decreasing the safety.

(c) A new ~6 m long horizontal section of the coaxial fill/vent/vacuum line is needed to connect the cryostat to the relief/isolation chamber, to the vacuum system, and to the gas handling system located outside the cave downstream from the cave entry. The goal with the new coaxial structure of the fill/vent line is to minimize the number of non-welded demountable joints in the hydrogen system inside the cave. In the coaxial fill/vent line the hydrogen line is inside the vacuum line which serves as a buffer against air. The hydrogen line itself was to possess only one demountable joint between the entrance to the cave and the entrance to the target cryostat. In the cave all demountable joints on the isolation vacuum chamber and on the fill/vent line must be surrounded by a helium atmosphere. Outside the cave all the demountable joints must be inside the two ventilated cabinets; gas handling cabinet and relief/isolation cabinet.

(d) Liquid helium cooling of the initial ortho-to-para convertor has been replaced by a mechanical refrigerator coupled to the fill/vent line to cool the incoming room temperature hydrogen gas in order to reduce the target filling time. The target is designed to meet DOE-STD-1020, PC-2 requirements.


(e) The target has to be designed to meet mostly DOE-STD-1020, PC-2 requirements.

(f) In its role as a part of the “one-time event boundary” of the volume defined as the venting volume, the isolation vacuum volume of the target cryostat, the vacuum volume of the fill/vent line, and relief/isolation chamber were required to withstand an internal pressure event of 150 psi as required by CGA G-5.5-2004 to confine a possible detonation/deflagration of the hydrogen in the system. These design goals mean that also vacuum components attached to the vacuum such as vacuum gauges, cryo-cooler couplings, penetrations etc. have to tolerate without failure a one-time 150-psi pressure event. The vent stack also has to be designed to the 150 psi as required by CGA G-5.5 – 2004 - Hydrogen Vent Systems.

1.6 Unreviewed Safety Issue Determination (USID)

The USID for the target system has to be performed to see whether the installation and operation of the NPDGamma hydrogen target in the SNS instrument hall constitutes an unreviewed safety issue. The emphasis was on whether the hydrogen target can significantly affect the previously reviewed analyses of the safety of the mercury spallation target. The negative result of the analysis showed that the hydrogen target doesn’t constituent a USI. The results of the analysis is documented in SNS 102030102-ES0029-R01.

1.7 The 2nd hydrogen safety review

The 2nd Hydrogen Safety review was held at SNS on September 2008. One of the main conclusions of this review was that “There is enough concern (lack of expertise) from the committee that we (committee) would like the Hydrogen System design to be peer reviewed and further presented to the SNS Cryogenic Safety Committee“. The summary of the comments can be found in 2ndSafetyReview.pdf. Following the advice of the Committee the results of the sizing and safety calculations of the target were verified and findings written down in the report “Study of Sizing of Components of the NPDGamma Liquid Hydrogen Target with Respect to Hydrogen Safety” by Michael MacDonald in 2009 (LH2TgtEngStudy.pdf). In addition, Phil Buchanan from National Research Management was charged to perform a thorough peer review of the overall target system design. His results are summarized in the Design Analysis and Calculations FUND13NPDG-20-DA0001-R00 ”NPDGAMMA – Beam Line 13 Liquid Hydrogen Target System Structural Analysis Calculation”.

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1.8 Target Design Review

In April 2010 the Hydrogen Supply to Cryogenic Target, Relief and Vent System Design Review was held at SNS to obtain additional feedback from the SNS experts for the proposed target design. File FUND13NPDG-24-DE0001-R00.pdf gives presentations and comments to the design.

1.9 Completion of the target construction and installation and responsibilities

After this 3rd hydrogen target safety review organized by ICSS for October 22, 2010, we expect that target construction and installation will be completed in July 2011 and then the IRR for the running of the Hydrogen target will be held in September 2011. The earlier IRR of the experiment which will be held in November 2010 will deal the radiological issues of the experiment except for the hydrogen target

The Physics Division and SNS are jointly operating the BL13 user program according to the MOU “The Operation of the Fundamental Neutron Beam Physics Beamline”. This MOU defines the roles and responsibilities of the both parties and the NPDGamma experiment uses the MOU as a guidance for the operation. The draft of the MOU can be found in the web site.

Internal roles and responsibilities in the NPDGamma experiment are carried by the spokesmen, project manager, project engineer, project ES&H officer and work package leaders. The responsibility of the target construction and installation is shared by the Indiana University group and ORNL.

2.0 DESCRIPTION OF THE NPDGAMMA LH2 TARGET SYSTEM

2.1 Introduction of target components and their functions

The NPDGamma liquid hydrogen target and the components of the hydrogen gas handling system (LH2 vessel-fill/vent line-relief system) are shown schematically in figure 4. The 16 liquid liter hydrogen target is contained in an isolation vacuum chamber (IVC). The hydrogen gas is conducted into the vessel through a single combined fill/vent line and condensed into the target vessel using three mechanical refrigerators. This fill/vent line connects to a relief system in the relief/isolation chamber (RIC) outside the experimental cave and to a hydrogen gas handling system (GHS).The three hydrogen gas supply cylinders and manifold are housed in a Chemical Storage Locker located on the Northeastern end of the slab outside of the Target Building, After passing through passive flow restrictors and an a solenoid actuated isolation valve (SAV100, see figure 9), the supply line from the supply manifold is connected to a gas handling system (GHS) located outside the experimental cave, see figure 4. The plumbing on the gas handling system consists of welded components and Conflat flanged and VCR-based joints constructed to typical (better than 10-9 atm cc/sec) helium leak tight


specifications. All the demountable joints that are exposed to hydrogen, are either in ventilated cabinets (gas manifold cabinet, #1, and relief/isolation cabinet, #2) or they are immersed in helium gas (this method is used inside the cave). In addition to the hydrogen fill line, the gas handling system also possesses a vacuum system for evacuation of the cryostat isolation vacuum, for leak testing of components, and monitoring of the main vacuum content with a residual gas analyzer. Two turbo pumps on the gas handling system will be used to evacuate the target vessel and lines. Pressure gauges on the gas handling system monitor pressures in the target and in the main vacuum chamber. The manifold of the GHS is enclosed inside a metal enclosure with its own ventilation line to the outside of the SNS experimental hall. Helium and nitrogen manifolds are used to provide a small flow of helium gas to all vacuum seals and weld joints inside the cave. Nitrogen gas is used to fill the vent stack. When the target is operating in the steady-state mode most of the GHS will be valved off at MV126, evacuated, and filled with helium gas except for the portions which contain the relief valves, rupture disks, and the residual gas analyzer. The vent stack is filled with nitrogen.

The hydrogen gas enters the hydrogen vessel in the cryostat through a combined fill/vent line (1.5” OD hydrogen line coaxially inside a 6” OD vacuum line). This fill/vent line possesses a slightly tilted ~6 m horizontal section whose exit is connected to vent isolation chamber that holds the relief valves and rupture disks and a vent stack that vents hydrogen safely outside of the Target Building. The fill/vent line bends 90 degrees to enter the target vessel from above. At the bend there is a section of the hydrogen line which is cooled by a mechanical refrigerator to cool the incoming hydrogen gas during the filling of the target and thereby reduce the fill time. The cooled hydrogen gas enters the extended cryostat top head. Inside the cryostat the gas travels vertically into the liquification chamber which is thermally connected to the cooling stages of a pulse-tube cryo-refrigerator (see figure 4). It then enters the hydrogen target vessel through the internal ortho-to-para converter chamber which is thermally connected to a second mechanical refrigerator (see figure 4). The experiment requires that the hydrogen is more than 99% in the para hydrogen molecular state. The refrigerators are mounted on the cryostat and their associated compressors are located outside the BL13 cave boundary, on the top of the roof shielding. The cooling powers of the refrigerators suffice to keep the liquefied hydrogen at 17 K and completely perform the ortho-para conversion for the given flow rate. Gas produced by the heat of conversion during filling is recondensed in the liquefying and ortho-to-para converter chambers and gas produced by boil-off in the chamber re-circulates until essentially all (99.8% at 20 K) of the liquid in the target is converted to the para-state. The thermodynamic state of the target is determined using pressure and temperature measurements on the target (no sensors are inside the hydrogen volume), cryo-refrigerators, ortho-to-para converter, and exhaust line. Pressures, feedback power to the refrigerators, the status of remotely controlled valves on the GHS, and the information from the RGA (residual gas analyzer) is recorded by the DAQ and interlock system to

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monitor the status of the target, to record and display the history of these parameters, to communicate the status of the target to the facility.

Fig. 5. Schematic of the LH2 target showing cryostat, vacuum/fill/vent line, and relief/isolation chamber with relief devices and three cryocoolers


Fig. 6. The LH2target piping and flow diagram.

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2.2 Overview of safety aspects of the target

The basic safety goal of the target design has been to keep the hydrogen gas from coming into contact with air. First of all, the target vessel itself is classified as a pressure vessel according to the ASME code. The pressure vessel consists of the shell, the entrance and exit heads, and the inlet and outlet nozzles and therefore the pressure vessel boundary is at the aluminum nozzles. The target consists of three layers of containment (“triple containment”). The LH2

target vessel (first containment) connected to the condenser unit by a fill/vent line contained inside the isolation vacuum chamber (IVC) and vacuum line, (second containment) which provides thermal insulation together with the 80 K radiation shield. Helium channels (third containment) surround the demountable joints and o-ring seals of the IVC and the hydrogen piping system inside the experimental cave.

The target safety is designed first to protect the personnel in the experiment and Target Building and second to prevent property damage. The target design is aimed to handle the worst possible target failures which are either a loss of isolation vacuum or the rupture of the hydrogen vessel. The target relief system must handle boil-off hydrogen in either accident scenario. This requirement along with the BL13 shielding structure sets constraints on the length and diameter of the fill/vent line and the nature and properties of the gas conduction devices (relief valves and rupture disks) described below. Each of the components of the LH2 target system – hydrogen volume and isolation vacuum – has separate pressure relief systems which are sufficiently robust to respond safely to any maximum credible accident. The conductance of each safety relief system has been designed to be large enough that a pressure rise will not lead to a rupture of the weakest component in the system.

3.0 DESIGN OF THE LH2 TARGET SYSTEM

The physics goals of the experiment coupled with the known properties of cold neutron and MeV gamma interactions with materials, the properties of hydrogen, and the need for the target system to be consistent with the other subsystems of the experiment implicitly define the following design criteria for the target:

(1) The target must absorb as much of the polarized cold neutron beam flux as possible without depolarizing the neutron beam before capture. The need to prevent neutron depolarization requires the target to consist of > 99.9% parahydrogen at a temperature no higher than 17 K. Given the 10 cm x 12 cm beam size on the polarizer exit, the phase space of the beam from the cold neutron guide using m=3 supermirror neutron guides, and the scattering cross section of cold neutrons in parahydrogen, the target size of 27 cm diameter and 30 cm length has been chosen on the basis of Monte Carlo simulations


using MCNP and the LANL hydrogen neutron scattering kernel. This target will absorb 60% of the incident cold neutron flux. The target system therefore requires a cryostat to liquefy gaseous hydrogen and an ortho-to-para converter to catalyze the formation of parahydrogen. The details of the target sizing can be found in [2].

(2) The target must possess negligible attenuation for the incident neutrons and for the 2.2-MeV gammas from neutron capture. This requires the use of low Z materials in the target vessel and associated radiation shields as well as the vacuum vessel.

(3) To ensure that the statistical accuracy of the measurement is not compromised by extra noise due to density fluctuations in the target, we require a liquid target in which bubbles are suppressed to acceptable levels and in which fluctuations in the pressure and temperature of the target are held to acceptable levels. The suppression of bubbles will be insured by the following design features: (a) using two cryorefrigerators which will be capable of cooling the radiation shield surrounding the target vessel to a temperature below 80 K, thereby reducing the heat load on the 17 K target vessel, and (b) the use of a heater on the exhaust line of the target which can maintain the pressure in the (re-circulating) target chamber at a value above that of the equilibrium vapor pressure in the 17 K target.

(4) To ensure that no false effects are introduced by gammas produced by polarized slow neutron capture on target materials other than parahydrogen, we must select the target vessel material carefully. The window materials seen by the incoming neutron beam will consist of Al alloy (type 6061) as is the target vessel itself. The remainder of the components around the target vessel, although made of Al and Cu, is protected from polarized neutron capture by a 6Li-rich plastic neutron shield outside of the target vessel. This shield will possess a small exit hole which will be small enough for polarized neutron capture in the Al exit window to produce a negligible contribution to the detector signal but large enough to permit efficient monitoring of the neutron beam exiting the target. Materials like Al, Cu, B, and In are used in the structures of the cryostat and in some seals (In) and they have to be studied as needed to make sure that they do not produce a false gamma-ray asymmetry in the NPDGamma experiment.

(5) To ensure that the interaction of circularly polarized gammas from neutron capture on pH2 produces negligible systematic effects and that the magnetic field in the target can be maintained with sufficient uniformity, the target materials in the vicinity of the neutron beam must be nonmagnetic (relative magnetic permeability has to be less than r<1.02). Any magnetic components in the target system must result in negligible magnetic field gradients.

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3.1 Hydrogen gas handling

3.1.1 H2 supply manifold

Figure 6 gives the overall of the flow diagram of the target system, figure 7 shows the diagram of the hydrogen supply manifold. Three 2000 psi compressed gas cylinders size A are connected to the manifold, three cylinders are required for one fill of the target vessel, with a room temperature ortho-to-para ratio (3:1) are connected to pressure regulators PR101, PR102, and PR103. In addition the supply manifold has a He cylinder for flushing the lines before use of hydrogen and then filling the lines with helium gas when the loading of the target is completed

Fig. 7. Diagram of the hydrogen supply manifold located outside of the Target Building in the gas cylinder shed. MV108 is located in the gas handling manifold in the cabinet #1 near the experimental cave.


Between the cylinders and the manifold are filters to remove particles. In addition the manifold has two flow restrictors in series with relief valve PR104 between to keep maximum flow out from the manifold at about 20 standard liters per minute (SLPM) at 100 psig defined by PR101. In a case of a relief the hydrogen gas is conducted outside the storage building. After the regulator PR104 the gas flows through a remotely controlled valve, SAV100 (normally closed) which is controlled by the interlock circuit and will be closed if the interlock circuit is activated. This action will stop hydrogen flow to the gas handling manifold in the cabinet #1. The function of the MV132 is to allow effective pumping and flushing of the manifold. MV131 allows isolation of the manifold from the supply piping. From the supply manifold the gas is conducted to the gas handling manifold (GHM) located in the cabinet #1 by 1/4” stainless steel piping. This line is welded line with 1/4” VCR connections in the both ends.

3.1.2 Hydrogen supply shed

Three H2 cylinders, one He cylinder, and the manifold are located in the prefabricated chemical storage shed that has approved fire barrier. The shed is located on the Northeastern part of the slab it has to independent rooms, one of the rooms is holding our gas cylinders. Both rooms have their own forced ventilation located in the floor level, therefore for the hydrogen storage and operation, a small diameter penetration has been done to the side wall in the ceiling level to allow the escaped of the very buoyant hydrogen gas from the manifold to bleed outside the shed.The relief valve RV101 has a tube that conducts the gas outside the shed in a case that the gas pressure is higher than the set pressure of 100 psig.

3.2 Gas handling system

The gas handling system (GHS) regulates the flow rate of the gas, cleans the gas, allows pumping of hydrogen out from the piping, allows cleaning of the piping by pumping and flushing. The gas handling manifold (GHM) is located in the cabinet #1 as also a hydrogen proof mechanical pump MP101. Figure 8 shows the diagram of the GHM. Components in the main hydrogen line in the GHM are mainly welded. A few have VCR connections like the relief valves (to allow set point verification) and the cold trap (to allow cleaning of the trap).

Before leaving the GHM the gas goes through a liquid nitrogen cooled trap to remove water and reduce other contaminants. The cold trap is equipped with necessary relief valves RV102 and RV103. The trap is operated under the GHM operating procedure given by the LH2TgtOperatingProcedures.pdf

A hydrogen proof mechanical pump MP101 is connected to the GHM allowing pumping of the hydrogen out from the target vessel and associated piping. The

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exhaust of the pump is connected through a check valve CV102 to the vent stack. The maximum gas flow rate (and therefore the liquefying rate) is determined by the cooling power of the refrigerators, and the properties of hydrogen, and will be approximately 10-20 SLPM. The rate is regulated by the regulators on the cylinders, flow restrictors and PR104 in the supply manifold, and the main flow control valve FCV100 on the GHM. From the GHM the gas is conducted into the vent/isolation box and then to the fill/vent line.


Fig. 8. Gas handling manifold

3.3 LH2 vessel

Aluminum is the only practical material for the LH2 vessel which can meet the experimental goals (nonmagnetic, low Z, no LH2 embrittlement (see Hembrittlement.pdf). The question then becomes the shape and wall thickness of the vessel. The target vessel has gone through several design phases, a

25


summary of these designs prior to the ORNL design can be found in the NPDGamma Liquid Hydrogen Target Engineering Document (LH2EngDoc2.00.pdf). For the SNS experiment, the neutron entry window had to be thinner than that used at LANSCE. The mechanical strength of the thin window vessel was FEA’ed by Luttrell, LH2_FEA_Luttrell.pdf. Her analysis indicated that the LH2 vessel can be a stamped pressure vessel.

The LH2 vessel is a code-stamped pressure vessel with two weld seams, one at the convex entrance dome and the other at the concave exit dome fabricated by an ASME authorized vendor (Ability Engineering). The concept of the vessel design was arrived at through FEA calculations performed at ORNL,LH2_FEA_Luttrell.pdf.. The thickness for the entrance window of the target vessel is 0.0625” and for the exit window is 0.125”. The target vessel has a Finite Element Analysis (FEA) calculated MAWP of 71 psia and was successfully tested at 72 psia by the vendor. AbilityVesselReport.pdf contains the FEA analyses of the vendor as required by ASME. ThermalCyclingBimetalJoint.pdf describes our thermal cycling and helium leak checking performed at Indiana University to confirm the performance of the aluminum to stainless steel bimetallic joints in the hydrogen target design. The tests are documented in AbilityVesselReport.pdf is an overall vendor report on the design, QA, and testing/certification of the target vessel. QAvesselwindows.pdf provides the documentation of the procedures and instruments used in the tests of the vessel and vacuum windows.

3.3.1 Bi-metallic joints

Commercial friction welded bimetallic joints are used to join the aluminum target vessel to the stainless steel fill and vent lines: stainless steel is required to minimize cryogenic heat loads. The bimetallic joints were designed and procured by Indiana and welded to the target vessel by certified welders at Ability Engineering. The document QAvesselwindows.pdf also provides information on the pressure and helium leak testing of these components since this was performed after the bimetallic joints has been welded to the vessel. Testing was performed on these joints to confirm that they can withstand the required internal pressure during accident scenarios and thermal cycling. The results of these tests are contained in ThermalCyclingBimetalJoint.pdf .

3.4 Fill/vent line

Design and fabrication of the fill/vent line is one of the largest modification to the target system. In addition to the ability to respond to the most serious accident scenarios mentioned above, the required functions of this fill/vent line system also include:

(1) safe conduct of the hydrogen gas to the target vessel during the target loading process

(2) cooling of the incoming hydrogen gas during filling to help minimize the overall time of target filling


(3) evacuation of the target and cryostat isolation vacuum (4) consistency with the vacuum monitoring requirements for hydrogen safety

involving the use of a RGA on the main vacuum to monitor for leaks into the main vacuum system.

(5) Proper design to withstand an earthquake according to SNS PC-2 requirement.

(6) Proper design to handle the thermal contraction and expansion of the hydrogen line experienced during the target venting.

(7) proper design of the support structure to handle about 5000 lb trust load in the 6” line caused by use of bellows

Figure 9 shows an assembly drawing of the hydrogen fill/vent line. It consists of a welded hydrogen fill/vent tube inside of a vacuum tube with a long horizontal section and a short vertical section with a portion of the hydrogen line cooled by a mechanical refrigerator. On the target/cryostat end, the fill/vent line connects to the target vacuum system and the liquid hydrogen target vessel. On the end past the cave boundary the fill/vent line connects to the relief/isolation chamber and the hydrogen target gas handling system. The distance from the target to the outside of the cave is about 30 ft.

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Fig. 9. Assembly drawing of the coaxial fill/vent line which allows pre-cooling of incoming gas and then conducts the gas into the target vessel from the GHS and serves as the relief path for the hydrogen from the vessel and the vacuum line that connects the isolation vacuum to the vacuum pumps and relief system outside the cave and also serves as a buffer between hydrogen volume and air. The bellow is required near the cryostat to allow for fine tuning of the orientation and angle with respect to the vent/isolation chamber.

Hydrogen line

Vertical section connected to cryostat

Mechanical refrigerator

Horizontal section connected to rlief/ isolation chamber

Bellow

Vacuum line


Fig. 10. Assembly of the gas precooling cryo-refrigerator, radiation shields at the 90 degree bend, and bellow of the fill/vent line.

The hydrogen line is a 1.5” outer diameter stainless steel tube (inner diameter: 1.37”) interspersed with bellows. All joints in the hydrogen line will be welded except for the end joint in the vertical section connected to the liquid hydrogen vessel with a 2.75” Conflat flange. The 280” long horizontal section will have an upward tilt angle of 0.5 degrees from the cryostat to the hole in the cave wall for venting hydrogen gas. Several internal support structures mechanically support the hydrogen line, avoid thermal contact and stabilize the line against possible uncontrolled radial motion relative to the vacuum line. The outside surface of the hydrogen line vertical section will be wrapped in cryogenic superinsulation to reduce the radiative heat load.

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The hydrogen line will be surrounded by a 6” outer diameter (inner diameter: 5.75”) stainless steel vacuum tube. One end will be mounted to the top of the cryostat isolation vacuum chamber and the other end will extend through the hole in the cave wall and be welded to a 4-way Conflat cross which in turn connects to the relief/isolation chamber and gas handling system.

The assembly procedure for the hydrogen line inside the vacuum line, both of which consist primarily of welded components, is nontrivial and requires a coordinated set of welds, weld inspections, flux dye penetrant and radiography tests, and subsequent helium and subsequent helium leak checking/testing with weld joints strategically located to ensure that assembly in this mode is possible. A plan for the proposed assembly procedure is described in the document Ventlineassemblyplan.pdf.

The use of vacuum-compatible components inside the vacuum with minimal outgassing is consistent with use of a sensitive partial pressure monitoring by RGA to verify the absence of leaks and therefore the maintenance of the triple containment condition.

All the Conflat flange joints in the vacuum line inside the experimental cave will be surrounded by helium gas contained in plastic covers which seal on the outside of the vacuum tubing. An example of such a cover in the horizontal/vertical transition region is shown in figure 11. The vacuum line will connect to the cryostat vacuum with a flange possessing a double o-ring joint (indium on the inside and viton on the outside) with helium gas between the rings. FailureHazardAnalysis.pdf discusses some hazards and mitigations specific to the fill/vent line.


Fig. 11. Design of a polyethylene box assembly to immerse CF flange joints on the main vacuum line inside the cave in helium gas to maintain the triple containment requirement.

3.4.1 Bellows

Stainless steel bellows in both the hydrogen line and the vacuum line are used for reducing stresses from thermal contraction during the filling and operation of the target system and the venting of the cold hydrogen gas and also to help meet mechanical tolerances in alignment/assembly. The specifications for the bellows are included in document Hlinebellowscertificationinfo.pdf.

In the horizontal section the central 1.5” tube is radialy constrained in the 6” line by centering spiders. At each end there is a 1.5” bellows with the outer ends fully constrained by welded brackets. This allows for thermal contraction due to cooling of the central line. Since the bellows are axially constrained at each end, deformations short of full squirm (at 178psi) should not occur. Since this section is axially aligned and there are fixed points just outside the bellows, loads due to pressure differential are not taken up on the bellows (other than on the convolutions themselves).

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The bellows between the cold head and 6” bellows is also fully axially constrained between the welded bracket and an axial spider. Here the loads due to thermal and pressure differentials are necessarily taken up axially by the cold head and differential contraction is taken up by the bellows, estimated to be on the order of 0.06” at 20 K.

For the bellows just before the connection to the LH2 vessel, a bellows frame is required to prevent large cantilever loads on the cold head due to thermal contraction and pressure differentials. We will pre-position the LH2 vessel so that after cooling it will end up centered in the beam.

3.5 Cryocooler and electrical feedthroughs

Two electrical feedthroughs on the cryostat will provide signal paths for all thermometers in the target and control of heaters. All target signal wiring is located in the main vacuum chamber; there is no penetration into the hydrogen volume.

A 6-way 6” CF cross in the fill/vent line is used to allow the connection of the horizontal and vertical parts of the line and coupling of the precoling cryocooler. One of the 6” access flanges will possess helium leak tight ceramic electrical feedthroughs for instrumentation connections (electrical heaters and thermometers) from the target cryostat. Another access flange will connect to the mechanical refrigerator. The MAWP for the electrical feedthoughs is 165 psig. 3.6 Precooling of H2

In order to reduce the time to liquefy the hydrogen gas and fill the target, and thereby increase the target safety by reducing the time when the hydrogen gas handling system is fully utilized, a mechanical refrigerator will be used to accelerate the filling by pre-cooling the incoming gas. The pre-cooler will consist of a mechanical refrigerator with compressor located outside the cave. This refrigerator will be mounted to the 6-way cross which connects the main vacuum line vertical and horizontal sections, see figure 8. A copper sleeve with direct thermal contact to the second stage of the refrigerator is brazed onto the outside of the hydrogen line to cool down the stainless steel tubing and the incoming gas flow, and a radiation shield will be connected to the first stage of the refrigerator and surround a portion of the cooled fill/vent tube. The available cooling power is 35 W at 77 K and 12 W at 20 K. The temperature of the refrigerator will be monitored using diode thermometry. None of these elements change the integrity/safety of the hydrogen line. There will not be any new welded or other type of joints. The refrigerator will cool the hydrogen gas under the maximum flow rate of 10 SLPM to a temperature of about 90K with a copper sleeve of about 20K in surrounding the hydrogen line. The estimated target filling time with the pre-cooler is about 24 hours, this should be compared to three day filling at LANSCE. The cooling rate calculations are shown in the document


LH2filltimeestimate.PDF. Refrigerator3coolingtest.PDF shows that the refrigerator thermal contact can be chosen so that it never gets cold enough to liquefy hydrogen.

3.7 Cooling of the LH2 vessel

The two cryorefrigerators on the cryostat will both be two stage closed-cycle refrigerators. The liquid hydrogen flows down a fill line into an ortho-to-para converter and the bottom of a 16-liter cylindrical target chamber. The chamber is wrapped with Li-loaded flexible plastic neutron shielding (~2mm thick) and superinsulation (Mylar coated with aluminum on both sides with adjacent layers separated by polyethylene netting) and is supported and separated from the 80 K copper radiation shield by a thermally-insulating support structure made of a G-10 ring. Thermal connection of both refrigerators to the target chamber, ortho-to-para converter, and radiation shields is effected by both mechanical connection to the cold stage flanges, a thick aluminum and, where necessary, by flexible copper braid. A similar G-10 support structure separates the 80K radiation shield from the inside of the main vacuum chamber. This support structure allows the liquid target chamber to slide horizontally upon thermal contraction. Stresses on the target from differential thermal contraction in the vertical direction are accommodated with the use of a stainless bellows on the target line. Stress on the 80K radiation shield due to differential thermal contraction in the vertical direction is accommodated by the flexibility of the thin walls of the radiation shields introduced by cutting radial slots into the soft copper sheet near the thermal contact to the lower refrigerator. The inlet/outlet line of the target has bends to avoid excessive radiative heat loads from a line-of-sight view of room temperature surfaces. Thermometry, heaters, and associated electrical wiring are described in LH2targetinstrumentation.pdf.

3.8 Liquefaction chamber and ortho-para converter

The gas from the fill/vent line enters a copper liquefaction chamber thermally connected to the upper cryorefrigerator with internal grooves to increase the surface/volume ratio for hydrogen condensation. Hydrogen gas is liquefied in this chamber and then drips down into an ortho-para convertor. Stainless steel lines guiding the hydrogen gas into and out of the liquefaction chamber are either welded (to stainless parts) or silver soldered (to copper parts). A 1.33” Conflat flange with strengthening flanges to withstand accident scenarios is present between the fill/vent line and the liquifaction chamber.

An ortho-to-para converter (OPC) in the hydrogen loop on the cold stage of the cryorefrigerator at 17 K is required to convert in a short time period the liquid to the parahydrogen state required by the experiment. The converter body must possesses high thermal conductivity to be capable of removing the heat from ortho-to-para conversion process in the converter material. The converter volumes must be capable of being heated for reactivation. This operation requires opening of the isolation vacuum and pumping of the hydrogen line when

33


introducing heat to OPC. The body of the converter is made of copper to allow the converter material to be heated for regeneration if necessary and also to have a good thermal contact with the cryocooler head to remove the conversion heat. We will use hydrous ferric oxide Fe(OH)3 converter material. The Fe(HO)3 is prevented from leaving the annular region with fine wire mesh on the inlet and outlet tubes.

The inlet to the converter possesses a 2.75” Conflat flange to allow for filling of the converter powder. This inlet is located below the hydrogen liquification chamber. The outlet is connected to a small diameter stainless steel tube welded to the target vessel. Joints on the OPC are vacuum brazed. Details of the design and testing are described in TargetPlumbingOPC.pdf.

3.9 Cryostat – vacuum chamber

The purpose of the cryostat isolation vacuum is to thermally isolate the target vessel so that it can be cooled. In addition it provides the “triple containment” boundaries between the liquid hydrogen and air and be able to withstand the accident scenarios without uncontrolled release of hydrogen.

3.9.1 Vacuum chamber

The outer jacket of the isolation vacuum is constructed entirely of 6061-T6 aluminum by a contractor (Ability Engineering). The outer jacket beside of the new neutron windows, was used in the experiment at LANSCE in 2006. The isolation vacuum chamber (IVC) possesses a horizontal cylindrical region which inserts into the CsI gamma detector array and a downstream rectangular cross-section box whose downstream wall is removable with a viton o-ring seal and indium seal and nonmagnetic Helicoil inserts to avoid galling of the aluminum threads. The rectangular portion is machined out of a solid block of aluminum and the cylindrical portion is cut from extruded pipe to minimize the required amount of weld joints. The top of the chamber possesses threaded holes for lifting eye-bolts. The inside surface of the chamber and the inside surfaces of the windows are polished to a mirror finish and also covered with aluminum tape to reduce emissivity of heat radiation. Vacuumvesselreport.pdf contains the design/QA/testing report on the IVC. MAWP of the IVC is 31 psid.

3.9.2 Beam windows

The neutron entrance and exit windows on each end of the IVC are formed into a trapezoidal shape for increased strength and are made of 6061-T6 aluminum alloy. BeamWindowDesignSafety.doc contains a detailed description of the design calculations, FEA analyses, and safety mitigation involved in the


design of the vacuum windows. ThickAldomesreport.pdf contains the vendor report on the design/QA/fabrication/testing of the target exit windows. The entry window is 0.062” thick and has MAWP of 31 psid. The exit window is 0.125” thick. SNSBL13LH2MVVWINDOW.doc contains a comparison of the vacuum window properties with Code requirements.

3.9.3 Radiation shielding and superinsulation

Copper radiation shielding surrounds the LH2 vessel and the piping and is connected to the cryo-refrigerators to reduce radiative heat loads on the LH2 vessel. In the IVC we have – from outside to inside – 0.5 cm thick multilayer superinsulation to reduce radiative heating, a radiation shielding made from 0.04” thick copper foil connected to the second stages of the cryorefrigerators and operated at about 80-100K, a G-10 ring to support the target vessel, more superinsulation, a copper cylinder, and the 6Li plastic neutron shielding. None of these layers are vacuum tight. Superinsulation is used in areas outside the neutron beam and Al foils in the beam areas. The function of the superinsulation, Al foil, or Cu is to minimize radiative heating of the LH2, they also reduce the heat flow to the LH2 in a case of the failure of the vacuum.

3.9.4 He-channels

Inside the cave every demountable seal or welded joint in the cryostat or in the fill/vent line has a He channel. Helium gas is introduced into the space between the neutron entry and exit windows in the IVC, into gas channels machined into the IVC between the inner indium seal and outer viton o-ring seals in such a way that every seal and weld joint is exposed to helium gas to catch any leaks into the isolation vacuum. To prevent helium diffusion through the viton o-ring seals the inner seals are made with indium wire. If a leak occurs in a demountable joint or seal in the vacuum chamber, the leak can be detected immediately by a RGA monitoring helium content in the vacuum and the interlock system will be activated. Second, the helium channels prevent air or other gases from penetrating into the vacuum through such leaks. If gases other than helium (and hydrogen) get in contact with the LH2 vessel or the hydrogen piping from the refrigerators to the target, they will immediately freeze. Solidified gases are difficult to detect, as they will not produce a pressure increase.

3.11 Vacuum system and pumps

3.11.1 Vacuum system and pumps

Figure abc shows the cabinet #2 which holds the relief/isolation chamber as well as most of the vacuum system piping. Two turbo pumps TP1 and TP2 backed by roughing pumps MP302 and MP202 which establish the isolation vacuum are located outside the cabinet #2. The pumps are connected to the

35


target isolation vacuum by two air-actuated in-line gate valves PV204 (normally closed) and PV207 (normally closed) interlocked to the control/alarm system which can isolate the pumps from the main vacuum. In a case of the power failure these valves will close and isolate the vacuum from the pumps and in a case of a failure of the isolation vacuum, the valves will be closed by the interlock system or target specialist. The pumping system can also be used to evacuate the GHM through MV124. The vacuum system is shown in drawing XYz. The vacuum system components hat operate with electricity are located outside of the relief/isolation cabinet. The vacuum system is operated under procedure of “LH2TgtOperatingProcedure.pdf”. 3.11.2 RGA

A residual gas analyzer (RGA) on the gas handling system will be used to monitor water, N2, and He content in the vacuum during target testing prior to cooling, and to sample the main vacuum residual gas composition for helium and other gases during operation. The helium signal level is interlocked to the target safety system. See section control/interlock.

3.12 Relief/isolation chamber

3.13 Vent stack

A 6” OD pipe made from stainless steel conducts relieved gas from the RIC to outside the Target Building. Because of the short length of the line, xx ft, and large diameter, the vent stack contributes only little to the flow impedance as is shown in design document. During an operation the vent stack is filled with N2 gas to create a buffer between hydrogen and air. During a sensitive vacuum testing of the RIC, a sealed plate is installed to the vent stack to form sealed volume. The vent stack is designed to meet CGA G-5.5 – 2004 - Hydrogen Vent Systems. The vent stack has to meet also seismic PC-2 requirement. Figure 12 from model shows the vent stack.

3.14 Relief of overpressures from LH2 vessel and from isolation vacuum

The hydrogen fill/vent line and relief system have been designed with the two incident scenarios in mind. In the case of overpressure in the hydrogen vessel, a pressure relief system, consisting of a relief valve RV104 and a rupture disc RD101 in parallel in the relief/isolation chamber (RIC) will release the


hydrogen gas through the 30ft long, 1.5” diameter hydrogen line into the RIC directly connected to the vent stack which leads to the outside of the Target Building. During this event, the pressure in the LH2 vessel can reach aaa psig which is significantly less than MAWP of the vessel, bbb psig, see section relief/isolation chamber.In the case of a rupture of the target vessel or piping inside the vacuum chamber releases the LH2 into the vacuum, a pressure relief system with a small throughput relief valve RV201 and two parallel rupture disks RD201 and RD202 will also safely release the hydrogen gas through a 30ft long, 6” diameter vacuum line into the RIC and thus to the vent stack while maintaining the pressure within the isolation vacuum chamber at a safe level, less than xxx psig which is significantly less than MAWP of yyy psig of the IVC, see section relief/isolation chamber. Figure 13 shows a cut view of the RIC and figure 14 the RIC in the relief/isolation cabinet.

Fig. 12. Relief/isolation cabinet and vent stack on platform.

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Fig. 13. A view of the relief/isolation chamber inside the ventilated relief/isolation cabinet with the cabinet cover removed. Picture is created by model

The pressure relief valve for the target (RV104) has an ASME rated mass flow capacity of 0.306 lb/s and a 20 psid set pressure. Therefore a minimum requirement for the items in the hydrogen line is that they can withstand an internal pressure of 38 psia. In addition, a rupture disk (RD101) with a set point of 30 psid is used in parallel for the case RV104 does not function properly. The LH2 target pressure in steady-state mode will not exceed ¼ above the atmospheric pressure: 1.25 14.7 psia = 18.4 psia. See table a.

Vent isolation chamber

Coaxial fill/vent line


Fig. 13. Cross section view of the relief/isolation chamber with assembly of the relief valve, RV104, and rupture disks, RD101, RD201, and RD202. Gas enters from the fill/vent line port on the left side and exits on the right side to vent stack. See detailed drawings in drawing series 315337.

Transient pressures in the hydrogen line must be stay below the MAWP during the relief process, and the hydrogen flow must remain subsonic. For a given rate of vaporization of the liquid hydrogen in the target, this requirement sets a minimum gas conductance for the path between the target vessel and the vent stack. We have experimental data on the rate of vaporization of the liquid hydrogen in our system under these conditions from operation at LANSCE, and we have calculated the gas conductance for our design. We estimate that the pressure in the target vessel will rise to no more than 35 psia with the pressure relief devices open, see table 2 Similar considerations apply to the second accident scenario. We estimate that the pressure in the isolation vacuum chamber will rise no more than 19 psia. Again, this is far below the MAWP, see table b. Detailed calculations for both of these scenarios are presented in the document Designreliefvent.doc. Accidentscenariotesting.doc describes testing performed in support of these calculations. Ventisolationchamberreport.doc contains the calculations and specifications for the relief/isolation chamber. Tdistributioninreliefventline.doc calculates the temperature distribution during venting. Detailed calculations can be found in xxxx.pdf.

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Table a. Internal pressures for components in the hydrogen lines and pressure set points for relief valve RV104 and rupture disk RD101.

ComponentOperating

internal pressure

(psia)

Calculated maximum internal

pressure(psig)

Tested internal

maximum pressure

(psig)

Set pressure of

RV104 (psid/psia)

Set pressure of

RD101(psid/psia)

LH2 vessel 18.4 72 72 20/35 30/45Bellows 18.4 ? 75 - -

1.3” Conflat joint 18.4 - ? - -

Pressuregauge

18.4 - ? - -

Table 2: Internal pressures for devices connected to the isolation vacuum volume and pressure set points for rupture disks RD201 and RD202 and relief valve RV201

ComponentOperating internal pressure

Calculated maximum internal

pressure(psia)

Tested internal

maximum pressure

(psig)

Set pressure of relief valve

RV201 (psid/psia)

Set pressure of rupture

disks(psid/psia)

Isolation vacuum chamber

vacuum I32 28 3/18 7/22

Bellows vacuum 50 75 - -8” Conflat

jointsvacuum ? 200 - -

Ceramic electrical

feedthroughvacuum na 165 - -

3.15 Deflagration and explosion in fill/vent line; 150-psig one-time-event boundary

The triple containment of the liquid hydrogen, helium filled unused piping, helium gas surrounding all welds and demountable joints, combined with the RGA monitoring of partial pressures in the isolation vacuum, the thorough helium


leak checks prior to target fillings, and the absence of electrical feedthroughs into the hydrogen volume, all reduce the probability that hydrogen will come into contact with air in the presence of an ignition source to negligible levels. Nevertheless one can ask whether or not the hydrogen remains contained in such an event: namely, a deflagration or explosion in the fill/vent line.

According to CGA G5.5 - 2004, if the appropriate “one-time event boundary” of the volume defined as the venting volume (in this case the vacuum volume formed by the cryostat isolation vacuum, vacuum of the fill/vent line up to the relief devices) is capable of withstanding an internal pressure of 150 psi, then even in this extreme event the hydrogen can be contained. We have, therefore, conducted a series of tests and analyses to evaluate whether or not our system meets this requirement. The answer is yes. The analysis consists of a combination of test measurements on components, FEA analyses, and use of standard Code estimation procedures. These measurements and calculations are summarized in the document deflagration.pdf. AnalysisConflatsUnderPressure.pdf contains relevant FEA calculations. Ventlinestress.doc calculates the stress in the tubing. SNSBL13CROSSEVALUATION.pdf contains a FEA analysis of the 6-way cross on the fill/vent line. DraftExemptioncomponentlistV1.2.pdf lists the relevant properties of components in the fill/vent line.

3.16 Cabinet ventilation line

Outside the cave the demountable VCR and conflat joints are in the two cabinets, #1 and #2. The cabinet are equipped with x OD ventilation line. In a case of leak in one of the joints the buoyancy drives leaking hydrogen outside the Target Building. The designed ventilation rate is 20 SLPM. The cabinet #2 is equipped with hydrogen sensors. SNS drawing xxx gives details of the design of the ventilation line.

4.0 ISTRUMENTATION

The instrumentation required for the operation of the hydrogen target can be divided into systems internal to and external to the isolation vacuum system. Inside the vacuum system, the instrumentation consists of thermometry in the target vacuum and pressure gauges in the GHS and RGA. See figure 5 for the locations of the diode temperature sensors. Outside the system, the instrumentation consists of flow meters, pressure and vacuum sensors, and hydrogen sensors, readouts, computer.

4.1 Pressure/flows/thermometry/heaters

TargetThermometerConnections.pdf shows a wiring diagram for the target thermometers and a map of their locations.

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4.2 Target monitoring system

ComputerDAQ.pdf describes the computer, software, and safety features of the PC which monitors data from the LH2 target.

5. TARGET OPERATION

The target system will be operated under procedures LH2TgtOperatingProcedure.pdf and AuxilaryTgtProcedures.pdf , which presently are in draft states and will be finalized when the construction of the target system is completed. The Procedures have to be authorized as described by MOU. These procedures describe how to operate target in normal situation and then how to respond to abnormal situations. In addition, they describe how to perform target auxiliary operations like, calibration of hydrogen detectors.

The target operation is responsibility of Target Specialists, we propose to have two levels of the specialists, Junior and Senior. Rough difference between these two specialist is that Junior knows how to monitor the target system but is not allowed to change running conditions of the target without permission of the Senior Target Specialist. When target is filled or emptied a Senior Specialist who is in charge has to be on experiment with a Junior Specialist. During a normal operation, the target is running and we are taking data, there has to be at least a Junior Specialist on experiment and a Senior Specialist on call. The Junior and Senior Specialists have to have typical SNS/P-Division NPDGamma Users training and additional target specialist on-job-training described by drafts NPDGammaOJTPlanSenior.pdf and NPDGammaOJTPlanJunior.pdf. Target Specialists are authorized as described in the MOU.

5.1 Target Signal Processing

Target performance is monitored as described above, by the monitoring system which does not have a feedback to the target system. In addition of the monitoring there are signals that are used to warning or alarm Target Specialist in shifts, that the set point has been passed. The signals and the action and optional actions are given in table aa.

Table aa. Signal processing, warnings and alarms


Location of sensor

Set point Action/ Optional action

Cabinet #2: H2 sensor 1 10% of LEL Warning H2 sensor 2 25% of LEL Alarm / shut off

PV100Cave: H2 sensor 3 10% of LEL Warning H2 sensor 4 25% of LEL Alarm / shut off

PV100Outside cabinet#2; Vacuum pressure

Higher than set point

Warning / close PV204 and PV303

RGA: He concentration in vacuum

Higher than low set point

Warning

RGA: He level; He concentration in vacuum

Higher than high set point

Alarm / close PV100

H2 pressure Pressure >16psig

Warning

On floor:He source pressure in He channels

Pressure <3psig

Warning

He flow in He channels

Flow less than set point

Warning

N2 source pressure for vent stack

Pressure < 3psig

Warning

N2 flow to vent stack

Flow less than set

Warning

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point

LEL=lower explosive limit for hydrogen, in our case it is LFL=lower flammable limit in air = 4.1% (deflagration). Lower detonation limit is 18.3%.

Currents of the two heaters are interlocked to temperature of the target vessel. A target specialist can change the temperature limit. If the pressure in the hydrogen line passes the set point, warning will be activated. A change of this set point has to go through the target parameter change process If the total pressure in the vacuum passes the set point, PVxxx will be closed and warning circuit activated. A change of this set point has to go through the target parameter change process. If the He partial pressure in the isolation vacuum passes the set point, the warning will be activated. A change of this set point has to go through the target parameter change process. If the one of the hydrogen sensor pairs reaches the lower set point, warning is activated, If one of the pairs reaches the upper set point, alarm is activated and the target emptying process will be started. A change of these set points has to go through the target parameter change process.

The target parameter change process: The senior target specialists agree and sign off the change. When the interlock circuit is activated, target specialist can interrupt the activated process only when the parameter caused the process has returned below the set point.

During the target filling process and normal operation, target specialist can operate PV204 to stop pumping of the isolation vacuum, and PV100 to stop the gas flow from the supply to GHS. A target specialist can also operate the manual vales according to the Procedures.

6. Operation of the LH2 target

The LH2 will be operated under the RSS 8305.0 “Installation, Commissioning, and Operation of the NPDGamma Experiment at the FNPB and by following the authorized Target Operating Procedures (TOP) which includes several sub procedures for different target tasks. A draft TOP can be found in the website. It covers for instance, cooldown, steady-stae running, normal target em\ptying process, and emergency emptying as well as hydrogen gas cylinder change. The authorization of the procedure is given by MOU. The LH2 target can only be operated by authorized LH2 Target Specialist. Authorization is possible after the on-job training. The target specialist has to have a normal NPDGamma user’s training and then in addition the LH2 target on-job training. There will be two seniority level of the target specialist; Junior specialist and senior specialist, The allowed target operations are defined but in general junior can monitor the


target performance and do a simple operations when supervised by the senior operator. The planed OJT for the target specialists in given in web site by “,,,” and “ ..” Authorization is comin from/..Training approval….

7. QUALITY MANAGEMENT

7.1 Configuration management plan

…., the technical drawings, and other technical documents related to the target system such as review reports, test reports, and operating procedures, training procedures, design calculations.

8 FAILURE AND HAZARD ANALYSIS

The RSS 8305.0 “Installation, Commissioning, and Operation of the NPDGamma Experiment at the FNPB” includes general hazard analysis of the NPDGamma experiment at the BL13. Next we perform failure and hazard analysis of the target system, this analysis is a part of the RSS. The analysis is given by FailureHazardAnalysis.pdf.

REFERENCES

[1] M.T. Gericke et al., Parity Violation in Neutron-Proton Capture: The NPDGamma Experiment, Nucl. Inst. Meth. A611, 239 (2009).

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