Reaction Rates in a Uranium Pile Surrounding a 14-MeV Neutron Source: Calculations of the Weale Experiment

To validate the neutronics analysis of hybrid fusion-fission reactor blankets, calculations were made of an experiment by Weale et al., where a 14-MeV neutron source was surrounded by a natural uranium metal pile. The evaluated nuclear data libraries, ENDL and ENDF/B, were used. The calculated parameters were found to be in closer agreement for present versions of these libraries than for preceding versions; however, there were still 15% differences in the ²³⁵U(n,f) and ²³⁸U(n,f) reaction rates. The present version of ENDL gives the results that are the closest to the experimental values for these reactions. For the ²³⁸U(n, γ) reaction, the calculations with the ENDF/B libraries are closer to the measured values. Both the ENDL and ENDF/B evaluations, however, fail to calculate correctly the neutron leakage or derived values for the ²³⁸U(n,2n) and ²³⁸U(n,3n) reaction rates. The spatial variations of the ²³⁵U(n,f), ²³⁸U(n,y), ²³⁸U(n,f), and ²³⁹Pu(n,f) reaction rates show that the penetration of high-energy neutrons in the pile is better described by the calculation with ENDL, which gives a greater penetration. The effects of resonance self-shielding were investigated and found to require a much smaller correction than the differences between calculations with different data libraries.