Estimation of the buckling of ²³⁹Pu fueled system from the data of composite ²³⁹Pu-²³⁵U fueled fast critical assembly

²³⁹Pu-²³⁵U fueled fast critical assemblies are of interest to estimate properties of ²³⁹Pu fueled assemblies because of safety considerations and or limited plutonium availability. Numerical analysis for bare and reflected homogeneous cores and reflected Two-region cores are previously reported. It is concluded that the errors in the calculated critical size for a 5.0 v/o ²³⁹Pu fueled system reduce more than a factor of about 10 to 20, if an accurate experimental value of critical size of a critical assembly fueled by 0.5 v/o ²³⁹Pu and 4.5 v/o ²³⁵U is known. In this report, a general mathematical formula, which is useful to explain the above conclusions more clearly, is obtained for bare homogeneous mixtures using multigroup diffusion approximation. In order to predict the critical radius for the bare homogeneous ²³⁹PU system in an accuracy, 1%, the errors‧for the experimental value of critical radius must be less than 0.1% for V^Pu/V^Fuel=0.05 and 0.2% for V^Pu/V^Fuel=0.1, where V^Pu and V^Fuel are the volume fractions of ²³⁹Pu and fuel (²³⁹Pu+²³⁵U) respectively. For the reflected Two-region cores, the experimental errors required for the above accuracy must be less than 0.5% for the ratio of central ²³⁹Pu core region volume to total core volume of 0.1. (author)