1. ESS Target Station Radiation Safety Accident Analyses
Linda R. Coney
European Spallation Source (ESS)
ASW 2018
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 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

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 – users/year
facility is located in 100,000 inhabitant region

5. 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

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 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

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: g 5.6 kg released
g Unmitigated dose: 75 mSv

12. 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

13. 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

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
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

15. 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

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)
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

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
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

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
Active cells
Utilities block
Beam expander hall