Monte Carlo simulations for the ODIN shielding at ESS Florian Grünauer 24.8.2018 preliminary results Monte Carlo code: MCNP6 1

Monte Carlo model of the ODIN shielding version 1 horizontal cut FOC1 FOC2 T0 FOC3 shutter FOC4 FOC5 WFMC1+2 bunker wall NBPI 60cm ordinary concrete 33m vertical cut 3.1m 14.1m 48m The shielding in version 1 consists of 60cm ordinary concrete all shuters and all choppers are opened no specimen no beam catcher 2

Monte Carlo model of the ODIN guide horizontal cut guide wall (borkronglas NBK7) vacuum tube (Al6061) (Cu) neutron beam port insert (steel 1.0570) y x 4900 bunker wall Up to a distance of 20.49m from the focal point the guide wall consists of Cu. For bigger distances borkronglas NBK7 is applied. Guide wall thickness: 1cm Coating: Ni/Ti 3

Monte Carlo model of the ODIN guide vertical cut vacuum tube (Al6061) guide wall (borkronglas NBK7) (Cu) neutron beam port insert (steel 1.0570) z x 4900 Al window (0,5mm) The guide is surrounded by a vacuum tube made from Al; wall thickness: 1cm bunker wall There are no Al windows outside the bunker wall except at the end of the guide (inside ODIN cave) no Al windows 4

Fictive neutron source for the ODIN Monte Carlo model Neutron flux tally for verification of the fictive source yield (5m from focal point) fictive neutron source at the guide entrance (2m from focal point) Generation and implementation of the fictive source data: Step 1: Simulation of the neutron flux (energy resolved) in the ODIN channel 5m from the focal point with the ESS Target MCNP model (with ODIN collimator included, entrance window cross section 4.4cm x 3.4cm) Step 2: Placement of a fictive source in the ODIN MCNP model at the guide entrance 2m from the focal point which causes the same flux at 5m as in step 1. Si-Wavers at the guide entrance for enhancement of cold neutron flux are not considered! Total neutron flux: 6109 cm-2 sec-1 5

Neutron total reflection Neutron total reflection is not considered in the MCNP simulations. Cold and thermal neutron flux is underestimated therefore . The corresponding gamma production due to cold/thermal neutron capture is underestimated at certain locations. To overcome this problem additional fictive cold/thermal neutron sources are included along the guide in further simulations. 6

Total horizontal neutron dose rate distribution opitcal chamber The neutron dose rate on the outer surface of the guide shielding is ca. 10µSv/h big background dose rate [µSv/h] The red line is the 1µSv/h border 7

Horizontal neutron dose rate distributions in energy groups 1/2 E<0.41eV 0.41eV<E<0.1MeV dose rate [µSv/h] dose rate [µSv/h] 0.1MeV<E<10MeV 10MeV<E<100MeV dose rate [µSv/h] dose rate [µSv/h] Biggest contribution to the dose rate from the energy region 0.1MeV<E<10MeV 8

Horizontal neutron dose rate distributions in energy groups 2/2 100MeV<E<500MeV 500MeV<E<1GeV dose rate [µSv/h] dose rate [µSv/h] 1GeV<E<2GeV dose rate [µSv/h] Above 100MeV forward scattering dominates. The contribution to the total neutron dose rate outside the guide is small. Contributions above 500MeV to the neutron dose rate outside the guide shielding are negligible 9

Horizontal generated g dose rate distribution The generated g dose rate on the outer surface of the guide shielding is between 1µSv/h and 10µSv/h dose rate [µSv/h] The red line is the 1µSv/h border 10

Horizontal generated g dose rate distributions in energy groups E<0.5MeV 0.5MeV<E<3MeV dose rate [µSv/h] dose rate [µSv/h] 3MeV<E<30MeV 30MeV<E<100MeV dose rate [µSv/h] dose rate [µSv/h] Biggest contribution to the generated g dose rate from the energy region 0.5MeV<E<30MeV 11

fictive cold/thermal neutron source in front of closed chopper g-production in the closed FOC5 by thermal neutron capture fictive cold/thermal neutron source in front of closed chopper R=34.6cm A fictive cold/thermal neutron source is placed in front of FOC5 (total reflection in source data considered) fictive cold/thermal neutron source 0.15mm Al 2mm Graphite 0.27mm B4C (95% B10 enrichment) 12

g-production in the closed FOC5 On the outer surface of the FOC5 shielding 1µSv/h is slightly exceeded dose rate [µSv/h] 13

g-production in the closed FOC5 E<0.5MeV 0.5MeV<E<3MeV dose rate [µSv/h] dose rate [µSv/h] 3MeV<E<30MeV dose rate [µSv/h] The chopper disk becomes an intensive source of gamma radiation with an energy of 0.478MeV 14

Conclusions for shielding version 1 Neutron dose rate outside the guide shielding is too big (10µSv/h) Increase shielding thickness The gamma dose rate for the closed FOC5 slightly exceeds 1µSv/h on the outer surface of the chopper shielding The background in the optical chamber is quite high. Possible reasons are: scattering in the last part of the guide radiation transport in the gap between vacuum tube and shielding. Reduce the empty space between vacuum tube and radiation shielding Add a shielding around the vacuum tube in the entrance window to the cave 15

Monte Carlo model of ODIN shielding version 2 Changes in version 2: shielding thickness around neutron guide and FOC5 increased to 75cm empty space around guide reduced: inner width of shielding reduced from 37cm to 27cm Upper inner surface shifted 5cm to beam axis empty space around guide in the entrance window of the cave filled with borated PE 16

Comparison of horizontal neutron dose rate distributions version 1 dose rate [µSv/h] 75cm shielding thickness is still not sufficient Filling the empty space in the entrance window to the cave around the vacuum tube and reduction of the inner width of the shielding did not reduce background in the cave significantly version 2 dose rate [µSv/h] The dominant reason for the high background in the optical chamber is scattering in the last part of the guide but not radiation transport outside the vacuum tube 17

Comparison of horizontal generated g dose rate distributions version 1 dose rate [µSv/h] The generated g dose rate on the outer surface of the guide shielding is now 1µSv/h or below version 2 18