1. The Status of ESS Accelerator Shielding and Accident Scenarios Lali Tchelidze www.europeanspallationsource.se May 26, 2014

2. Outline Prompt radiation shielding design criteria. Accidental cases. Shielding results (with an attention to earth berm shielding on top of the linac). – Skyshine radiation. Conclusions. 2

3. November 2013 Review Recommendations related to beam loss and shielding : ”Basic assumption of 1 W/m used for shielding calculations needs to be confirmed by realistic end-to- end beam simulations, showing sufficient margin. Normal operations scenario should include accidental beam loss scenarios.” “Accidental beam loss: need to establish “worst case” scenarios based on risk analysis and to conduct simulations to determine appropriate shielding and system interlocks for personnel protection.” 3

4. Event classes and exposure limits at ESS Frequency (1/y)NameLimit to rad workersLimit to public H1> 1Normal operation10 mSv/y0.05 mSv/y H210 -2 - 1Anticipated events20 mSv/event0.5 mSv/event H310 -4 - 10 -2 Unanticipated events50 mSv/event5 mSv/event H410 -6 - 10 -4 Design basis accident (DBA) 50 * mSv/event20 mSv/event 4 * Used to be 100 mSv, until SSM (Swedish Radiation Safety Authority) asked to reduce the limit.

5. Normal operation H1 Limit is driven by requirements for activation/hands-on maintenance limits. Adopt 1 W/m (SNS specification above 100 MeV) – dose rates below ~1 mSv/h measured at 30 cm from component surface for 100 day/4 hour irradiation/cooling condition. 1.1 mSv/h At 30 cm 1 W/m 1 GeV protons on beam pipe 100 days/4 hours. FLUKA simulations from I. Strasik et al., Phys. Rev. ST AB 13, 2010. 5

6. Anticipated events H2 Adopt 30 MJ beam spill (LANSCE design criterion) – A burn through at LANSCE at 800 MeV proton coupled cavity linac occurred with an estimated 40 J of beam spill. – At PSI, at 600 MeV proton beam experience showed that leak at vacuum seal occurred at deposited energies of 30 kJ. – TJNAF (Thomas Jefferson National Accelerator Facility), 10 MJ energy spill. – No spill events greater than 10 MJ were identified! 6

7. Beam Spill Limits Summary Comparing (accidents) LANSCE 1.7 mSv/MJ TRIUMF 2.2 mSv/MJ SLAC 0.6 mSv/MJ ESS 0.08 mSv/MJ Frequency (1/y)NameDescriptionExposure limit > 1Normal operation1 W/m10 mSv/y 10 -2 – 1Anticipated events30 * MJ/event20 mSv/event 10 -4 – 10 -2 Unanticipated events 150 * MJ/event50 mSv/event 10 -6 – 10 -4 Design basis accident (DBA) 600 * MJ/event50 mSv/event 3 μSv/h for normal operation 7 * LANSCE design criterion

8. Accident duration as a function of lost beam power 8

9. Outcome of the recommendations 1.Beam spill limits for all event classes H1 – H4 (normal operations – incidents of various types) – ESS-0008309 “Beam Spill Limits for Various Event Classes”. – ESS-0008351 “Hands-on Maintenance Conditions at ESS Accelerator”. Both documents reviewed and approved at SAG. Final step – get an approval at EPG – Scheduled for 2/6/2014. – This will close this recommendation. 9

10. Tools used Verifying that 5 m earth berm shielding is sufficient for all types of events. – Analytical approach. – Monte-Carlo simulations code MARS. 10

11. Normal operation 1 W/m “Sullivan”-s analytic approach Lateral shielding for 1 W/m beam loss. – H=(H 0 /R) exp(-t/0.94λ) – H 0 is a source term, representing a dose equivalent per interacting proton, at 1 m from the interaction point and no attenuation, equals to 0.4 Sv/h. – λ is a hadron attenuation mean free path – t is a thickness of a shield – R is a distance from beam loss to outside of the shield. 5 m earth berm – 0.26 μSv/h (limit is 3 μSv/h). 11

12. Normal operations 1 W/m Monte-Carlo simulations Geometry: – Linac tunnel, tunnel walls and earth berm around it. – Klystron gallery building. – Waveguide penetrations. – Cable penetrations. – Personal emergency exits. – HEBT loading bay. – Smoke evacuations. – Alignment penetrations. – Cryogenic transfer line. 12

13. Earth berm thickness Normal Operation 5 m earth berm - max 8 μSv/h (limit is 3 μSv/h) Berm will be fenced! -Linac length – 450 m. -Proton beam energy 5 MeV – 2 GeV. -Loss distribution: 1 W/m upwards at 3 mrad angle on a 2 mm stainless steel beam pipe. -Tunnel concrete roof thickness – 70 cm. 13

14. Accidents – Analytical solution -Point beam loss is considered: -H=H 0 /R 2 e(-t/λ) -H 0 is a source term, representing a dose equivalent per interacting proton, at 1 m from the interaction point and no attenuation, equals to 1.2 Sv/h. -λ is a hadron attenuation mean free path -t is a thickness of a shield -R is a distance from beam loss to outside of the shield. -5 m earth berm - 17.2 μSv/MJ max dose rate outside of earth berm. -Equivalent to 0.31 Sv/h for full point beam loss. 14

15. Accidents – Monte-Carlo results -Point beam loss upwards with a shallow angle at 2 GeV proton beam energy. -127.8 μSv/MJ max dose rate outside of earth berm. -Equivalent to 2.3 Sv/h for full point beam loss. DET (mSv/h) Side view 15

16. Shielding results - summary NameDescriptionWith 5 m berm (Anal.) With 5 m berm (MC) Exposure limit Normal operation 1 W/mNo limit1250 h limit10 mSv/y Anticipated events 30 MJ/event0.5 mSv/event3.8 mSv/event 20 mSv/event Unanticipated events 150 MJ/event2.6 mSv/event19.2 mSv/event 50 mSv/event Design basis accident (DBA) 600 MJ/event10.3 mSv/event76.8 mSv/event 50 mSv/event No Access on top of berm during operation! 16

17. Earth berm thickness Current observations. -From the accidental prompt dose rate consideration point of view: -The top of the berm has to have no access during operations! -Or the amount of earth berm thickness has to be increased. 17

18. Skyshine Close to public limit (5.7 nSv/h or 50 µSv/y) at the site boundary. Skyshine radiation around ESS site (contribution from the linac, excluding the A2T, 2013) 18

19. Earth berm thickness Current observations. -From the accidental prompt dose rate consideration point of view: -The top of the berm has to have no access during operations! -Or the amount of earth berm thickness has to be increased. -From the skyshine prompt dose rate consideration point of view: -The amount of earth berm thickness might need to be increased. -We are building an accelerator tunnel allowing placement of up to 7 meters of earth berm on top of it if necessary! 19

20. Preliminary to detailed shield design -Things that affect the final shielding configuration: -Level of details in the geometry description. -Accelerator component and shield material - material’s composition. -Beam loss assumptions. -We expect the total berm thickness to decrease in the detailed design stage, but as a back-up strategy we have a possibility of increasing the thickness to 7 meters. 20

21. Sensitivity to linac components (Example) DET (mSv/h) Side view across the tunnel. 2 mm stainless steel 20 cm copper A factor of 5-10 less total dose equivalent outside of berm shielding when a bulk copper is considered! 21

22. Shielding towards the klystron gallery DET (mSv/h) Top view at 2 m < 0.1 µSv/h with no penetrations. 54 µSv/h max – with penetrations and no extra shield. 3 µSv/h max – with penetrations and extra shield block. Equivalent to 28.8 mSv/event for DBA (limit – 50 mSv). 22

23. Outcome of the recommendations 1.Beam spill limits for all event classes H1 – H5 (normal operations – incidents of various types) – ESS-0008309 “Beam Spill Limits for Various Event Classes”. – ESS-0008351 “Hands-on Maintenance Conditions at ESS Accelerator”. Both documents reviewed and approved at SAG. Next/final step – get an approval at EPG. 2.Shielding reports. Normal operations: Technical note ESS-0007143 “ESS Linac Shielding Strategy and Calculations” – reviewed and approved internally and externally (March, 2013). Accidental cases: Technical note ESS-0011759 ” Earth berm shielding for accidental beam losses” – reviewed internally. 23

24. Summary Recommendation about establishing accident scenarios and assigning beam spill limits to them. (ESS-0008309 & ESS-0008351) Recommendation about verifying that the earth berm shielding is enough for accident scenarios. (ESS-0007143 & ESS-0011759 ) 24

25. Thank you!