Fission density in plutonium-uranium mixed oxide fuel irradiated by pulsed power in NSRR experiments

Fuel behavior during a reactivity initiated accident condition is recognized to be predominantly related to energy deposition in the fuel. The first stage of NSRR in-pile experiments addressing the behavior of PuO2-UO2 mixed oxide fuels evaluated the energy deposition per unit integrated reactor power by γ-ray spectrometry. Solid samples were used to measure the γ-rays because the facility is permitted to handle solid plutonium only. Determination of the penetration ratios of γ-rays from the actinides contained in the fuels allowed correction for the self-attenuation of γ-rays in the solid samples. Evaluation of the effect of epithermal neutron fissions was also necessary since the fissile nuclides of 241Pu and 239Pu have high resonance cross sections in the epithermal energy region. For this evaluation, the fission density was first calculated for the fission products as a function of the contribution ratio of the fissions of epithermal neutrons. Accurate fission density was then determined using the contribution ratio which minimized the deviation of the calculated values for the fission density. The fission densities determined by this simplified method agreed well with the values calculated using the computer codes CITATION and GGC-4. (author)