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Description
ISIS neutron and muon source, STFC, Rutherford Appleton laboratory Didcot, UK
The ISIS spallation and muon facility uses a proton beam accelerated to 800 MeV in a linac and synchrotron to generate neutrons and, via an intermediate carbon target, muons. The muon intermediate target is located on the extracted proton beam before the neutron spallation target on ISIS target station one. After the muon target there are a series of collimators followed by quadrupole magnets to shape the proton beam before the neutron target.
In 2020 the ISIS muon collimator 1 had a small leak in an area which had previously been repaired, the decision was made to replace the muon collimator. This paper details the radiation transport simulations performed both for the calculation of the induced activity in the collimator as a result of the proton beam interactions and the shielding design of the flasks to enable the safe removal of the highly active muon collimator.
The induced activity calculations were performed using FLUKA [1,2], this was chosen due to the ability to perform proton and neutron transport, activation and decay dose rates calculation in a single simulation.
The shielding flask design calculations were made using MCNP [3] based on the decay photon source term calculated using FLUKA. There were several flasks designed for the different high dose rate components including a steel inner layer into which the most active component was winched before it could be removed from the area and placed within its final shielded storage cask.
The MCNP simulations utilised global variance reduction using the FWCADIS method built into Advantg [4] to generate weight windows in order to achieve low variance around all sides of the casks.
MCNP simulations were also performed to investigate the shielding design of the collimator and its surrounding shielding, to determine if there were any improvements and if the dose rates during proton beam operations would be acceptable within the ISIS target station one hall.
The results of the simulations are compared against the health physics measurements taken as the collimator was removed. The agreement is very good given the differences in cooling times between the planned and actual removal times, the uncertainties the irradiation history of the collimator due to varying beam conditions over its lifetime and the material composition uncertainties.
[1] "The FLUKA Code: Developments and Challenges for High Energy and Medical Applications" T.T. Böhlen, F. Cerutti, M.P.W. Chin, A. Fassò, A. Ferrari, P.G. Ortega, A. Mairani, P.R. Sala, G. Smirnov and V. Vlachoudis,, Nuclear Data Sheets 120, 211-214 (2014)
[2] "FLUKA: a multi-particle transport code" A. Ferrari, P.R. Sala, A. Fasso`, and J. Ranft,
CERN-2005-10 (2005), INFN/TC_05/11, SLAC-R-773
[3] MCNP6 User's Manual, Version 1.0, LA-CP-13-00614, Los Alamos National Laboratory, 2013.
[4] ADVANTG―An Automated Variance Reduction Parameter Generator, ORNL/TM-2013/416 S. Mosher et al. 2013