Impact of reflected neutrons on accuracy of tritium production rate prediction in blanket mock-ups for fusion reactors

A series of breeding blanket neutronics experiments were carried out with DT neutrons in the Japanese ITER test blanket module at JAEA Fusion Neutronics Source (FNS), and tritium production rates (TPRs) inside different blanket mock-ups were measured in detail. Most of calculations made using FENDL-2.1 and MCNP agreed well with the measured TPRs. On the other hand, there were overestimations of the calculation for the TPR in the experiment with a reflector and the TPR around the boundary between the rear part of the breeder layer and the beryllium in the pebble bed mock-up experiment. In this study, we investigated the possibility of improvement of the calculation by modifying the part of angular distributions in backward directions in FENDL-2.1. By uniformly decreasing the backward elastic scattering cross-sections of 56Fe by 20% below the original data for incoming neutrons in the energy range from 0.5234 MeV up to 15.0 MeV, the overestimation of the calculations for the TPR in the enriched breeding blanket experiment with a reflector was reduced. Also, by uniformly decreasing the backward elastic scattering cross-sections of 9Be by 20% and 30% below the original data for the incoming neutrons in the energy range from 0.93 MeV up to 14.94 MeV, and by 20% for those from 0.62 MeV up to 14.94 MeV, the overestimation of the calculations for the TPR around the boundary between the rear part of the breeder layer and the beryllium was lessened.