Criticality Parameters for Mixtures of Plutonium Oxide, Uranium Oxide and Water

A number of articles have appeared in the literature in 1964-5, concerned with various aspects of reactor fuels containing mixtures of plutonium and uranium. So far as the author is aware only one of these, a report by Marchuk, is concerned with criticality. The present paper considers homogeneous mixtures of plutonium and uranium oxides with water in various proportions and presents estimates of the critical parameters of these mixtures based upon diffusion theory calculations using four energy groups. No experimental results are available to directly check the accuracy of these calculations. Using the same method of calculation, however, the experimental results quoted for 0,372 in. diam. rods in hexagonal lattice arrangement in water were reproduced to within ±1.1% in keff. These rods contained 1.5 wt.% PuO2 in UO2 depleted to 0.22 wt.%235U. The results of some Monte Carlo check calculations on homogeneous mixtures are quoted ; these predict lower critical masses. Comparison is also made with Marchuk's results. The mixtures considered in this paper are assumed to be mechanical mixtures of water with PuO2/UO2 having a crystal density of 11.2 g/cm3. Marchuk's calculations are based on a PuO2/UO2 density of 6.0 g/cm3. The comparisons with his results assume the same density. (author)