1962 Volume 4 Issue 11 Pages 746-753
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 VPu/VFuel=0.05 and 0.2% for VPu/VFuel=0.1, where VPu and VFuel are the volume fractions of 239Pu and fuel (239Pu+235U) respectively. For the reflected Tworegion 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.