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.