The S₄ and Few-Group Diffusion Calculations of Fast Reactors

Abstract

To economize a large amount of numerical work in the calculation of fast breeders, the present work has been done in the hope that few-group diffusion theory might give acceptable results in some cases. A hypothetic 233TJ-232Th system with large core size (～670/) as well as a hypothetic 239Pu-238U fast reactor with small core size (～50l) are adopted. These systems are assumed to be of spherically symmetric geometry. One dimensional calculations are applied to obtain the static characteristics of the systems. The results from few-group S4 and diffusion method are investigated. These results seem to indicate that fouror three-group diffusion calculation might at least be used in place of three-group S4 computation for both large and small fast reactors. A new convergence criterion imposed upon the static parameters is proposed. The leakage rate of neutrons from the blanket is selected as the sensitive measure of convergence. This rate is estimated in two ways, i.e. with the aid of neutron current and by neutron inventory. The sufficiently converged state can be reached when these two values coincide with each other. One is also able to infer the necessary number of spatial mesh points by comparing these two values.