Methodical investigations for calculating modified few-group diffusion parameters of pressurized water reactor core edges

The description of neutron transport in a PWR reflector by the few-group diffusion approximation gives rise to problems due to anisotropic neutron scattering by hydrogen nuclei and heterogeneity of materials. The few-group diffusion parameters usually used are defined for the isotropic case and calculated for the asymptotic case and do not sufficiently reflect neutron physics conditions in the reflector and in the near-reflector region of the fuel. For that reason diffusion parameters modified with respect to anisotropic transport and spectrum have been calculated by use of the SN method and tested in one-dimensional and two-dimensional diffusion calculations. In the numerical case considered, modified few-group diffusion parameters have been calculated for the WWER-440 core edge, which differ by maximally 30% from ten conventional diffusion parameters. The effect of parameter modification on the results of two-dimensional fine-mesh diffusion calculations is estimated. It amounts to maximally 5% with respect to power density microdistribution at the core edge and to more than 20% with respect to few-group neutron fluxes far from the edge. (author)