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

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

ESS facility layout Neutron Science Systems Proton Linear Accelerator 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 Target

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

Key features of the ESS Target Station proton beam proton beam window proton beam instrumentation plug Moderator and reflector target wheel neutron beam extraction port monolith vessel target monitoring plug 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 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 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 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

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

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

Accident Analyses Bounding 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

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

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

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

TOAST Experiment Setup (Tungsten Oxidation AeroSol Transport) Flow control 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 Filter Impactor TC2 DMS2 Aerosol Measurements DMS1 – Differential Mobility Spectrometer 3 * 1 m, 1.5” More details in hidden IR thermometer Insulation Pipe Sample Stainless Sacrifice Heating coil Air Inlet Outlet Window cooling Thermocouple ~0.5 m/s TC1 – Thermocouple Flange Sample Pressurised air Inductive Heating T IR

Oxides from TOAST Test 3 ARF ~ 0.4 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 27 g / 74 g ~ 0.4 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

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 Material Mass Damaged Airborne Release Fraction Release time Helium coolant 30 kg 1 10 s Filter particles 10 g 0.01 Tungsten 50.3 kg 0.41 1000 s (17 min) Moderator coolant water 400 kg 0.44 20000 s (5.5 hrs) Beryllium 15.7 kg 1000s 29.6 kg 0.005 hydrogen explosion at t= 5000 seconds 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 Dose Consequences Public On the order of 100 mSv Dose Consequences Public On the order of 100 mSv gMitigation required Public limit 0.1, 1.0 mSv Unmitigated revised: More realistic analysis due to TOAST New amount released, new timing, new leak path rules g new doses

Significant Changes in AA3: 2017 g 2018 AA3 changes 2017 AA3 2018 AA3 Why? Amount of tungsten oxidized 1121 kg 50 kg Refined analysis ARF oxidized tungsten 0.005 g 5.6 kg 0.41 g 20.5 kg TOAST Timing 9 hour release H2 explode 10 min 17 min W, 5.5 hrs H2O H2 explode 83 min Emission height 10 m 30 m Worst case refined Event category H2, H3 Likely H3 Refined definition of accident Dose to public 75 mSv Order of 100 mSv New source term Updated AA3 report undergoing document review & approval process

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

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) Event Public Dose AA1 – wheel stop AA2 – beam aggressor AA3 – LOCA HEx AA3 – LOCA flow AA3 – LOCA leak Event Public Dose AA1 – wheel stop AA2 – beam aggressor AA3 – LOCA HEx AA3 – LOCA flow AA3 – LOCA leak Event Public Dose AA1 – wheel stop AA2 – beam aggressor AA3 – LOCA HEx AA3 – LOCA flow AA3 – LOCA leak Event Public Dose AA1 – wheel stop AA2 – beam aggressor AA3 – LOCA HEx AA3 – LOCA flow AA3 – LOCA leak Event Public Dose AA1 – wheel stop AA2 – beam aggressor AA3 – LOCA HEx AA3 – LOCA flow AA3 – LOCA leak 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

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

Thank you! europeanspallationsource.se

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)

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 ESS Safety Objectives SSM Operating conditions Initiative event likelihood Public limit (effective dose) Normal operation - H1 0.1 mSv/year (0.05 mSv/year GSO) Anticipated events - H2 F > 10-2 0.1 mSv/occurrence Unanticipated events - H3 10-4 < F < 10-2 1 mSv/occurrence Improbable events (DBA) – H4 10-6 < F < 10-4 20 mSv/occurrence Highly improbable events – H5 10-7 < F < 10-6 100 mSv/occurrence 22

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

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

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 Active cells Utilities block Beam expander hall