ANALYSIS OF EPITHERMAL NEUTRON BEAM EXPERIMENTS AT THE HIFAR REACTOR

A calculational model of the entire core of the DIDO class reactor HIF AR has been used to analyze epithermal neutron beam experiments. In th e experiments, an off-center fuel element was replaced by a dummy fuel element voided by a dry liner in which an aluminium spectrum shifter was suspended at core center to extract the beam. Various combinations of the filter materials aluminum, iron, sulfur, titanium, and cadmium were inserted near the top of the dry liner, and liquid argon was pla ced in a cryostat above the dummy element. Reaction rates were measure d in a fission chamber, sandwiched between various thicknesses of poly ethylene, in order to assess the accuracy of the calculational model f or different regions of the neutron energy spectrum of the beam. The n eutron source distribution of the HIFAR core was obtained from a three -dimensional diffusion calculation, with burnup-dependent fuel composi tions and fission products included, using the AUS modular code scheme . Argon cross sections were generated from ENDL-84 data and resonance parameters taken from Neutron Cross Sections (1984). A whole-core MCNP source calculation was used to analyze the experiments giving good ag reement between measured and calculated reaction rates. This whole-cor e model of HIFAR may be applied with confidence to predict the perform ance of filtered beams for boron neutron capture therapy and also to o ther HIFAR calculations.