Neutronics design for the coupled para-hydrogen moderator for CSNS

We performed a neutronics design for the coupled para-hydrogen moderator for the Chinese Spallation Neutron Source (CSNS), which relied on the Monte-Carlo simulation code MCNPX. We chose a cylindrical shape moderator with a diameter of 150 mm after considering the peak and integrated cold neutrons. Under the cost limitations of the CSNS project, the target station may use light water rather than heavy water as a target coolant during phase one. Using light water as the coolant decreased the cold neutron intensity of various energies of the coupled moderator by about 15–20%. If the neutrons were extracted from one side of the moderator instead of being extracted from both sides, the fluxes increased by about 5–10%. The decay power and radiation damages of the moderator vessel were calculated for 100 kW and 5000 h.