ESS Target Station Radiation Safety Accident Analyses

1. ESS Target Station Radiation Safety Accident Analyses Linda R. Coney European Spallation Source (ESS) ASW 2018 www.europeanspallationsource.se 22 August 2018

2. Outline • ESS Target Station Introduction • Hazard Analysis & Accident Analysis • Scope & Quantitative (AA) Procedure • Accident Analyses Results • Wheel & helium system • Safety SSC (structure, system, component) identification & impact • Conclusions

3. ESS facility layout 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

4. 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 • Cold Moderators • Liquid hydrogen with 2-5 year 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 100,000 inhabitant region

5. 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

6. Outline • ESS Target Station Introduction • Hazard Analysis & Accident Analysis • Scope & Quantitative (AA) Procedure • Accident Analyses Results • Wheel & helium system • Safety SSC identification & impact • 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 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 for group

8. 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 – includes AA4, AA7, AA8 PIEs • Leakage from target cooling circuit into monolith – beam OFF • Global bypass or local blockage of wheel 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) • 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

9. Accident Analyses:Evaluate unmitigated events • Describe system • Identify baseline assumptions – safety functions present in normal operations • Quantitative determination of event category (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 • 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 • Identify need for risk reduction measures to prevent or mitigate event

10. Outline • ESS Target Station Introduction • Hazard Analysis & Accident Analysis • Scope & Quantitative (AA) Procedure • Accident Analyses Results • Wheel & helium system • Safety SSC identification & impact • Conclusions

11. Accident Analyses: Focus on Loss of Cooling Accident (LOCA) Target & Helium Cooling Events • Target wheel rotation stop during beam on • Beam Event: Focused & non-rastered beam • Loss of target wheel cooling during beam ON – includes AA4, AA7, AA8 PIEs • Leakage from target cooling circuit into monolith – beam OFF • Global bypass or local blockage of wheel 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 Evaluated in 2017 Releases: Helium coolant, Tungsten, Moderator water, Beryllium Conservative event development analysis Tungsten mass damaged: 1121 kg Oxidized W ARF: 0.005 g 5.6 kg released g Unmitigated dose: 75 mSv

12. 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 – Thermocouple IR thermometer Flange Sample Sample Pressurised air Inductive Heating Stainless Sacrifice T IR Thermocouple

13. Oxides from TOAST Test 3 Remaining = 155 g(of 229 g) 74 g W removed Scales in vessel ~ 25 g W Deposits in pipe ~ 22 g W 34 g WO3 Captured in filter ~ 27 g W ARF = W mass in filter/W mass lost from block (27/74) W mass in filter calculated from the filter mass of oxide assuming it to be tungsten trioxide. ARF ~ 0.4

14. Unmitigated Monolith Events H2, H3 event Emission 30 m • Wheel • Stops rotating (AA1) • Beam as aggressor (AA2) • Loses cooling (AA3) • Leak in system • Fail HEX, int H20 system • Loss flow • Temperature increases, shroud melts, helium released g loss of monolith confinement • Moderator water released/evaporated • Part of spallation material spills into monolith, oxidizes & released • TOAST results required update to ARF and AA • Part of spallation material melts • Worst leak path – directly out of monolith via pressure relief system Public limit 0.1, 1.0 mSv gMitigation required • Unmitigated revised: • More realistic analysis due to TOAST • New amount released, new timing, new leak path rules g new doses

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

16. Outline • ESS Target Station Introduction • Hazard Analysis & Accident Analysis • Scope & Quantitative (AA) Procedure • Accident Analyses Results • Wheel & helium system, Monolith, ACF • Safety SSC identification & impact • Conclusions

17. Selection of Safety Functions:Monolith – Public Prevent release • TSS stops beam on low wheel speed, high temperature and low flow in He cooling (AA1, AA3) Limit event progression/limit release • TSS stops beam on high monolith pressure & low He cooling pressure (AA2, AA3) Control release • Monolith confinement (AA2, AA3) • Controlled relief path to height (AA3) • Determine required safety functions • Hierarchy applied – passive engineered over active, engineered over administrative, preventative over mitigative

18. Summary • ESS has established a well-defined, systematic process to identify, evaluate, and control potential radiation hazards and accidents related to the Target Station. • In alignment with overall ESS process & regulatory body (SSM) conditions • First iteration of 22 Target Station Accident Analyses for Operations completed in 2017 • Events evaluated, impact assessed, Safety SSCs & supporting safety-related SSCs defined, first iteration of DiD analysis completed • Refinement of analyses initiated and ongoing due to measurement of increased ARF (TOAST) & optimization of safety functions and system classifications

19. Thank you! europeanspallationsource.se

21. Accident Analyses:Dose calculations • Input from AA • Inner source term STinner= MAR * DR * ARF • Duration from release & flow rates • Public reference person • Located 300 m from emission point • Gaussian dispersion model for LPF • 95% worst weather for material transport • Dose calculation includes • External cloud dose while it passes • External ground dose from ground contamination – 8 hrs/day for 1 yr • Inhalation dose from cloud passage • Ingestion dose from contaminated food and drink over 1 yr 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)

22. Accident Analysis Input – Dose Limits • Protect workers and public from unsafe levels of radiation • Prevent the release of radioactive material beyond permissible levels • Defining consequence = dose to public 22

23. Accident Analyses – Select Safety SSCs: Evaluate mitigated events • Determine required safety functions • Hierarchy applied – passive engineered over active, engineered over administrative, 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 & dose consequences with safety measures implemented

24. Hazard Analysis Scope • Moderator Systems • Water Moderator • Reflector System • Cold Moderator • Target Moderator Cryoplant System • Fluid Systems • Active Liquid Purification system • Primary Water Cooling Systems • Intermediate Water Cooling Systems • Intermediate Water Cooling for Water Systems • Contaminated Tanks • Gas Delay Tanks • Target Systems • Target Wheel & He Cooling • He Purification Target HVAC System Remote Handling Systems Active Cells Operations • Monolith Systems • Monolith vessel incl. covers and penetrations, NBW, PBW • Shielding systems • Tuning Beam Dump

25. 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