A new brief diffusion scheme for three-dimensional calculation of a BWR core

A modified one-group diffusion scheme and a coarse-mesh diffusion method were newly developed independently, and are combined together to make an effective brief scheme for the solution of few-group diffusion equation in a 3-dimensional BWR core. In the above two methods, the local spatial behavior of the neutron spectrum-the ratio of thermal neutron flux to the fast neutron flux-and of the fast neutron flux within a volume element are approximately taken into account by using analytical procedures, therefore the resultant numerical equation, which is slightly more complicated than those of the conventional brief schemes, gives sufficiently accurate results. The special behavior of the spectrum in the neighborhood of the core-reflector interface is also treated in the appendix of this paper. An analytical expression for the spectrum on the core-reflector boundary of various shapes is derived and some numerical examples are presented. The results obtained from a series of test calculations indicate that the present method assures an accuracy comparable to the standard few-group fine-mesh finite-difference calculation without requiring much more computing time than those of the conventional brief calculation methods. Thus the present method is particularly suited for improving the computational accuracy of the BWR simulation codes. (auth.)