Optimization of the neutron monochromator shielding for a cold triple-axis spectrometer at the FRM-II research reactor by Monte Carlo simulations

The former neutron monochromator shielding of the FLEXX-instrument at the research reactor BER-II (Berlin) will be moved to the research reactor in Munich (FRM-II) in order to reuse it for a new cold-neutron triple-axis spectrometer. The neutron flux at the monochromator position at FRM-II is roughly 20 times higher compared to its former application. In order to comply with the radiation protection requirements and to keep the radiation background low, the shielding has to be upgraded. The changes should be as small as possible to keep the effort and the expenses low. For planning and optimization of the upgrade, Monte Carlo simulations were carried out by help of the Monte Carlo codes MCNP6 and McStas. The results are presented in this paper. Different material combinations and arrangements and the influence of guide windows are investigated. It is shown that with minor modifications and without increasing the shielding thickness the desired dose rate level outside the shielding can be obtained despite the higher neutron flux at the FRM-II.