Calculation of radiation damage of key components of China Spallation Neutron Source II target station

Abstract

China Spallation Neutron Source (CSNS) I project passed the national acceptance in 2018, and current beam power has reached 140 kW. In order to further improve the output neutron strength of the target station moderator, a 500 kW power upgrade plan has been proposed for CSNS II. The target station is an important part of the spallation neutron source. In the target station, a large number of neutrons are produced by the spallation reaction between high energy protons and the target, these neutrons are moderated by the moderator and become neutrons for neutron scattering experiments. During operation, the target and other key components such as the target container, the moderator reflector container, and the proton beam window are irradiated by high-flux and high-energy particles for a long time, which will result in serious radiation damage. It is important to assess the accumulated radiation damage during operation to determine the service life of each component. At present, the physical quantities used to evaluate the radiation damage degree of materials include displacement per atom (DPA), H and He production. In this work, the displacement damage cross sections of protons and neutrons and the H, He production cross sections for W, SS316 and Al-6061 materials are obtained by using PHITS. The effects of the Norgett-Robinson-Torrens (NRT) model and athermal recombination corrected (ARC) model on the calculation of displacement damage are analyzed. The results show that the cross section calculated based on ARC model is lower than that based on NRT model, because the NRT model does not take into account the resetting of the atoms before reaching thermodynamic equilibrium. On this basis, DPA, H and He production of the key components of the target station operating for 5000 h at a power of 500 kW are calculated by combining the baseline model of the second phase target station of the spallation neutron source in China. The results show that the yields of NRT-dpa, ARC-dpa, H and He produced by irradiation are 8.01 dpa/y (in this paper, 1 y = 2500 MW·h), 2.39 dpa/y, 5110 appm/y and 884 appm/y, respectively. The radiation damage values of the target vessel are 5.34 dpa/y, 1.92 dpa/y, 2180 appm/y and 334 appm/y, respectively. The radiation damage values of the moderators and reflectors are 3.78 dpa/y, 1.77 dpa/y, 124 appm/y, and 36.7 appm/y. The radiation damage values of the proton beam window are 0.35 dpa/y, 0.19 dpa/y, 962 appm/y, and 216 appm/y. Subsequently, the life of each component is estimated by analyzing the radiation damage. These results are very important for analyzing the radiation damage of these parts, and constructing reasonable maintenance programs.