Preliminary shielding design of CSNS day-one neutron instruments

Abstract

The China Spallation Neutron Source (CSNS), intended to operate in 2018, is designed with 1.6 GeV proton energy and 25 Hz repetition rates, with an initial proton beam power of 100 kW that can be upgradable to 500 kW. Twenty neutron beam lines are provided by the CSNS target station for neutron scattering and neutron science research. The three neutron scattering instruments to be operated on day-one are the general purpose powder diffractometer, the multi-purpose reflectometer and the small angle neutron scattering instrument. These instruments present dissimilar shielding issues and their shielding calculation methods are presented in this paper. The thermal, fast and high-energy neutron source terms with angular distribution are individually calculated using target-moderator-reflector geometry and surface tally at a 1 m distance to the moderator. The thermal neutron losses in the guide system are calculated using a ray-tracing simulation. Possible worse case scenarios are also considered in the shielding analyses. Variance reduction methods, such as source biasing, are employed to speed up the convergence of calculations. As an example, multi-purpose reflectometer shielding analysis is introduced in this paper. All calculations are normalized to 500 kW proton beam power and a 2.5 µSv/h dose limitation are applied in accessible locations.