Refined Radiation Safety Analysis and System Classification for the ESS Target Station

1. Refined Radiation Safety Analysis and System Classification for the ESS Target Station Linda R. Coney European Spallation Source, ERIC (ESS) ICANS XXIII www.europeanspallationsource.se October 15, 2019

2. Outline • Introduction • Hazard Analysis & Accident Analysis • Purpose and Scope • Quantitative (AA) Procedure Overview • Updates to AA analysis approach • Impact on AA results • Impact on Classification • Continuing Efforts • Conclusions

3. ESS Target Station Parameters • 5 MW long pulse spallation source • 2.86 ms proton bunch at 14 Hz rep rate • Raster pattern where protons incident upon Target wheel • Rotating tungsten target • 5.4 tons (4900 kg) wheel only, (11 tons wheel + shaft) • rotating at 23.3 rpm (sector receives beam pulse every 2.4 seconds) • 5 year lifetime, He gas cooled • Moderators • Thermal (water) and liquid hydrogen, 2-5 year moderator lifetime • Inventories • High power beam impacting the tungsten target will create an inventory of nuclides • Primary inventory is in wheel (monolith and storage in active cells) • Keeping in Mind: • 450 – 500 employees, • 2000 – 3000 users/year • facility is located in 2,000,000 inhabitant region

4. Key features of the ESS Target Station Moderator and reflector • Target Safety System • Monitors target coolant flow, pressure and temperature, monolith pressure, & target wheel rotation • Prohibit beam on target if parameters are outside specified limits • Helium cooling of target material • Mass flow 3 kg/s • Pressure 11 bar • Temperature inlet/outlet 40 °C/240 °C proton beam instrumentation plug monolith vessel • Rotating solid tungsten target • 36 sectors • Mass, total 11 tonnes, with 3 tonnes of W • Rotates 23.3 rpm, synchronized with pulsed proton beam 14 Hz neutron beam extraction port target monitoring plug protonbeam target wheel • Diagnostics and instrumentation • Controlled and integrated commissioning and operation of the accelerator and target • Fluorescent coating of PBW and target front face • Optical paths, grid profile monitor, aperture monitor • Wheel monitoring including position, temperature, vibration, as well as internal structure protonbeam window • Moderators • Provisional locations of moderators above and beneath the target wheel, i.e. monolith centre • 1st MR plug exploits the upper space, offering: • Cold, 30 mm high, liquid H2 moderators, 17 K • Thermal, 30 mm high, H2O moderator, 300 K

5. Key features of the ESS Target Station • Utilities and cooling plant • Helium cooling of target wheel • Water cooling of moderators, plugs and shielding • Intermediate water loops between primary circuits and conventional facility utilities • Helium cryoplant for refrigeration of cold moderator system • Nuclear grade HVAC system • Remote handling systems • Large active cells for safe storage and processing of spent radioactive target components • Shielded casks for transfer of spent components from monolith to active cells High bay 22 m 130 m 37 m Transport hall Target monolith Beam expander hall Active cells Utilities block

6. Outline • Introduction • Hazard Analysis & Accident Analysis • Purpose and Scope • Quantitative (AA) Procedure Overview • Updates to AA analysis approach • Impact on AA results • Impact on Classification • Continuing Efforts • Conclusions

7. Hazard Analysis Purpose • Protect ESS workers and the public from accidents with potential radiological consequences • Hazard Analysis is a systematic process to identify, evaluate, and control potential hazards and accidents related to the Target Station. • Identify & understand hazardous scenarios associated with the facility operations, work activities, and natural phenomena • Define necessary preventing & mitigating measures g Input to design requirements • Accident Analyses – set of formalized design basis accidents identified from hazard analysis • 350 Top Events from qualitative HAs • Selected 22 bounding scenarios for further study • If prevented or mitigated, no additional safety functions required • Perform check of all top events to ensure coverage

8. Objectives for dose levels for different event classes (GSO)

9. Accident AnalysesBounding Events for Operations Monolith/Utility Block Events Active Cells Facility Events • Target wheel rotation stop during beam ON • Beam Event: Focused & non-rastered beam • Loss of target wheel cooling during beam ON • Leakage from target cooling circuit into monolith • Global bypass or local blockage of cooling • Increased radiation level in target helium cooling system • Leakage between target He cooling system & intermediate water system • Loss of confinement in target He system – release into utility rooms • Moderator hydrogen combustion • Water leakage in monolith • Water leakage in connection cell and utility rooms • NBG/Chopper failure – projectile effect on monolith system (part of AA9) • High power beam on beam dump • Earthquake scenario – monolith • Fire in He purification getters • Active Cells: Worker inside maintenance cell when intrabay door opens • Active Cells: Worker exposed to worst case inventory in process cell • Active Cells: Worker exposed to worst case inventory in maintenance cell • Active Cells: Loss of HVAC (loss of dynamic confinement) • Active Cells: Loss of confinement – open doors/hatches • Active Cells: Fire • Active Cells: Earthquake scenario Target & He Cooling

10. Accident Analyses:Evaluate unmitigated events • Describe system • Identify baseline assumptions – radiation safety functions (RSFs) present in normal operations • Quantitative determination of event class (H2,H3,H4,H5) all PIEs • Describe evolution of event • Include simulations, calculation of effect on local & neighboring systems • Calculate inventory • Determine amount of radioactive material released during accident • Source term = MAR(material at risk)* DR (damage ratio)* ARF(airborne release fraction) • Identify leak paths & points of emission for released material • Identify flow rates along leak path • Emission points – 10 m, 20 m, 30 m, 45 m above ground level • Calculate dose consequence to public reference person • Apply point of emission – use worst case weather conditions • Duration based on release & flow rates • Risk ranking for unmitigated event as compared to GSO • Identify need for risk reduction measures to prevent or mitigate event

11. Outline • Introduction • Hazard Analysis & Accident Analysis • Purpose and Scope • Quantitative (AA) Procedure Overview • Updates to AA analysis approach • Impact on AA results • Impact on Classification • Continuing Efforts • Conclusions

12. Deterministic Analysis Approach • Assess normal operations • Identify RSFs needed g Operational Group functions • Assign Safety-related SSCs (Structures, Systems, Components) to execute the RSFs • Assess hazardous events • No RSFs designed to mitigate the event • No SSCs designed to behave in specific way during the event • If Operational Group RSF exacerbates the scenario g it does • Identify necessary Safety Group functions, assign Safety SSCs • Manual action to intervene shall not be considered for the unmitigated scenario • H1 and H2 events should largely be handled by the Operational Group RSFs and the safety-related SSCs that execute those functions. • The facility shall be designed so that events and circumstances that are anticipated to occur during the facility’s lifetime can…be managed through the measures and the SSCs that maintain the fundamental safety functions in defence in depth levels 1 and 2. Ch 4, C5

13. Risk Assessment Criteria – Dose to Public • Events with frequency < 10-7 are at an acceptable residual risk level • Event category – the only probabilistic part of the analysis • Operational Group functions can lower the event category • Event category the same for unmitigated g mitigated • Reduce consequences, not likelihood

14. Recent refinements in the analyses:Event Category • Determine based on SSC failure rates • Different initiating events may have different event classes • If Operational Group RSFs exist that would prevent or mitigate the event, failure rates of the SSCs performing those functions can be included in the assessment of the event probability. • Impact: DiD L2 functions become very important for the radiation safety analyses • Ex. MP failure rates set the event category for LOCA

15. Recent refinements in the analyses:Event Category – AA3 LOCA • Radiological consequence requires two simultaneous sub-events • Disturbance (ex. failure of target helium cooling function) • PFs need to fail • Event frequency – only probability of failure in the PFs • PIE (helium compressor stop) • Failure of both PF4 and PF3 with added conservative factor • Large uncertainty in PF5 reliability g not used g H3 event

16. Recent refinements in the analyses:Impact of existing systems • Role of Operational Group systems • Describe systems involved, implicitly or explicitly, in the accident scenario with respect to public • Which systems, how applied in analysis? • Sensitivity studies expanded • Evaluate the impact of assumptions applied • Impact on dose consequences due to changes in the role of Operational Group systems • Understand which systems and operating parameters have an impact on the accident analysis and dose consequences • Information feeds into ESS Operational Limits and Conditions • Impact: Leads to identification of many systems important to the AAs g more Operations Group RSFs/Safety-related SSCs

17. Recent refinements in the analyses:Sensitivity Analyses • AA19: Active Cells Facility – loss of confinement due to unplanned openings • Emission height • Change in inventory • Twice the considered normal operation inventory • All tritium released from heated zone • HVAC – with and without

18. Recent refinements in the analyses:ALARA • Include ALARA philosophy consistently…

19. How have the analyses changed? • Event class dominated by H3 in Monolith/Utility area • Evaluation ongoing for ACF analyses • Operational Group impact g evaluate many emission heights • Changes in the analyses lead to results in the Tolerable range

20. Recent refinements in the analyses:Public Workers • Analyses with respect to the consequences for the public have been separated from that with respect to the workers • System classification determined solely by radiological impact with respect to the public • Classification of many target systems and the corresponding requirements have changed

21. Safety Function Selection:Prevent or Control events • Determine required safety functions • Hierarchy applied with preference to passive engineered safety features over active, engineered over active or administrative controls, and preventative over mitigative • Consider several options and optimize • Technical feasibility, reliability, efficiency, impact on operations, impact on other systems, cost • Specify safety SSCs (structures, systems and components) for each event • Reassess evolution and dose consequences with safety measures implemented, check compliance with GSO

22. Classification:Recap of previous situation • Safety SSCs executing Safety Group RSFs • Target Safety System (TSS) • Primary water cooling systems • Helium cooling system • Monolith vessel • Rupture disk in monolith vessel • Relief path to stack • Windows on perimeter of monolith • Safety interlock on ACF doors/hatches/floor valves • Isolation device in proton beam pipe • Impact: MQC3 classification for mechanical systems • Impact: EICPA classification for TSS safety group RSFs

23. How have the RSFs changed? Operational Group

24. How have the RSFs changed?Safety Group • Safety SSCs executing Safety Group RSFs • Target Safety System (TSS) • Primary water cooling systems • Helium cooling system • Monolith vessel • Rupture disk in monolith vessel • Relief path to stack • Windows on perimeter of monolith • Safety interlock on ACF doors/hatches/floor valves • Isolation device in proton beam pipe • Impact: MQC3 classification for mechanical systems • Impact: EICPA classification for TSS safety group RSFs • Impact: MQC4 classification for mechanical systems • Impact: EICPA, EICPB, MQC4 classification for TSS

25. Outline • Introduction • Hazard Analysis & Accident Analysis • Purpose and Scope • Quantitative (AA) Procedure Overview • Updates to AA analysis approach • Impact on AA results • Impact on Classification • Continuing Effort • Conclusions

26. Continuing Effort • New events under analysis

27. Continuing Effort:Events for Operations Monolith/Utility Block Events Active Cells Facility Events • Target wheel rotation stop during beam ON • Beam Events • Loss of target wheel cooling during beam ON – includes AA4, AA7, AA8 PIEs • Leakage from target cooling circuit into monolith – beam OFF • Global bypass/ local blockage of cooling • Increased radiation level in target helium cooling system • Leakage between target He cooling system & intermediate water system – beam OFF • Loss of confinement in target He system – release into utility rooms – beam OFF • Moderator hydrogen combustion • Water leakage in monolith • Water leakage in connection cell and utility rooms • NBG/Chopper failure – projectile effect on monolith system (part of AA9) • Beam Dump: High power beam, LOCA • Fire in He purification getters • Active Cells: Worker inside maintenance cell when intrabay door opens • Active Cells: Worker exposed to worst case inventory in process cell • Active Cells: Worker exposed to worst case inventory in maintenance cell • Active Cells: Loss of HVAC (loss of dynamic confinement) • Active Cells: Loss of confinement – open doors/hatches Target & He Cooling Global Events 14. Earthquake – monolith 21. Earthquake – ACF 20. Fire – ACF 23. Fire – D02 Utility areas 24. Lightning 25. Flooding

28. Continuing Effort • New events under analysis • Re-analysis of worker cases ongoing • Permitted to apply probabilistic assessments with respect to workers • Can apply existing RSFs to the worker scenario • Residual risk = 10-6 • No H5 • Document normal operations • Evaluate Maintenance mode/Maintenance Accidents

29. Remaining challenges • Clearly define rules for worker analyses • Areas where DiD is not clear remain difficult ex. ACF • Ongoing design changes that impact radiation safety analyses • Often assumptions in accident analysis imply requirements on SSCs • Managing those requirements not yet optimized

30. Conclusions • ESS has established a systematic process to identify, evaluate, and control potential radiation hazards & accidents related to the Target Station. • Aligned with ESS process & regulatory body (SSM) conditions • Events evaluated, impact assessed, Safety-related and Safety SSCs defined • Refinement of Target Station Accident Analyses – significant changes: • AA event categories largely g H3 • Importance of Operational Group systems much easier to see • System classification based on impact to public • TSS is only remaining Target Station Safety SSC • Challenges remain • Communicate requirements efficiently to safety-related SSC owners • Clarify worker analysis rules • Perform maintenance analyses

32. The ESS facility layoutMain parts Target Neutron Science Systems • 22 planned and budgeted neutron scattering instruments • Neutron beam line lengths ranging from 10 m to 150 m from source Proton Linear Accelerator • Long pulse proton beam 2,86 ms • Pulse frequency 14 Hz • Proton energy 2 GeV • Beam power, time averaged 5 MW, within pulse 125 MW • Beam current 62,5 mA

33. Defence in Depth and Grouping of Radiation Safety functions

34. How have the RSFs changed?Operational Group • Controls (BPCS) • MP • Water cooling system pipes, • Helium cooling system pipes • Helium purify • H2 moderator system • Off-gas system • Gas purging system • Chopper design • Rad monitoring • …

35. Accident Analyses:Dose calculations • Input from AA • Inner source term STinner= MAR * DR * ARF • Duration from release & flow rates • Public reference person • Located 330 m from site boundary • Gaussian dispersion model for LPF • Dose calculation includes • Dose for 1 year due to: Inhalation, external dose due to cloud, external dose due to ground contamination • Food surface contamination • 95% worst weather for material transport Source Term for Release ST = MAR * DR * ARF * RF * LPF MAR: Material at Risk DR: Damage Ratio ARF: Airborne Release Fraction RF: Respirable Fraction (1) LPF: Leakpath Factor (Calculation)

36. TOAST Experiment Setup(Tungsten Oxidation AeroSol Transport) Flowcontrol Outlet Filter • How much tungsten becomes airborne by tungsten oxidation at high temperatures (> 1400 C)? • Measure Airborne Release Fraction, ARF= mass fraction of the oxidised amount that is airborne after passage through the system Windowcooling Air Inlet Insulation Pipe Impactor TC2 ~0.5 m/s Heating coil DMS2 AerosolMeasurements DMS1 – Differential Mobility Spectrometer 3 * 1 m, 1.5” TC1 IR thermometer Flange Sample Sample Pressurised air Inductive Heating Stainless Sacrifice T IR Thermocouple

37. Oxides from TOAST Test 3 Remaining(of 229 g) 74 g W removed Scales in vessel ~ 25 g W Pipe depositsrest ~ 22 g W Captured after34 g WO3 ~ 27 g W ARF ~ 0.4

38. Significant Changes in AA3: 2017 g 2018 • Updated AA3 report undergoing document review & approval process