Critical Experiments with Homogeneous PuO₂-Polystyrene at 50 H/Pu

The results and analyses are presented from the latest series of experiments for determining the critical parameters of plutonium mixtures over the undermoderated region of the neutron energy spectrum. This latest series of experiments was conducted with PuO₂-polystyrene fuel having anatomic H/Pu ratio of 49.6 and a ²⁴⁰Pu isotopic content of 18.53 wt%. In addition to providing basic criticality data, the experiments were designed to (a) establish the degree of correlation between the PuO₂-polystyrene and the plutonium nitrate solution fueled critical experiments performed at the Hanford Critical Mass Laboratory, (b) provide data for checking previously reported differences obtained by the neutron diffusion theory code HFN and the neutron transport theory code DTF-IV in calculating critical sizes of plutonium-water systems between about 500 and 1000 g Pu/liter, and (c) substantiate that high exposure, subcritical, fixed volume, plutonium-water systems can be made critical by dilution with water. In contrast to the data previously obtained on this same PuO₂ material unmoderated, the density effect of having the plutonium in the oxide form was found to have essentially no effect on the reactivity at 49.6 H/Pu. At 49.6 H/Pu, the minimum critical slab thickness was determined to be 5.22 ± 0.07 cm for ²³⁹Pu-H₂O as compared to 5.34 ± 0.07 cm for ²³⁹PuO₂-H₂O However, the neutron poisoning effect of the ²⁴⁰Pu at 49.6 H/Pu was at least five times that observed for the PuO₂ unmoderated. At this near-optimum concentration for maximum ²⁴⁰Pu effects, the percent change in spherical critical mass per percent change in ²⁴⁰Pu content was determined to be 12.1 for the reflected case and 10.2 for the bare case. Based on the data derived from these experiments at 49.6 H/Pu, the HFN code was found to accurately calculate the plutoniumwater systems at 521 g Pu/liter and should be used at this and lower concentrations in preference to the DTF-IV code for calculating critical parameters. As with the plutonium nitrate solutions at high concentrations, the DTF-IV neutron transport theory code calculations resulted in too small a critical size; however, the data obtained from the PuO₂-polystyrene experiments correlated well with those that have been obtained from plutonium nitrate solutions. The experiments also showed that, for high exposure plutonium-water systems, a given size vessel could be just critical (K_eff =1.0) at three different concentrations below about 1000 g Pu/liter. Consequently, a critically safe vessel could be made critical by dilution alone if the ²⁴⁰Pu content was high enough.