A Study on cell heterogeneity effects in the MONJU core; Evaluation using the continuous energy Monte Carlo code MVP

Morohashi, Yuko ; Ishibashi, Junichi; not registered

The criticality analysis of the MONJU initial critical core was conducted based on conventional methods developed by the JUPITER program. Effective cross sections were created, considering self-shielding effects, from the JAERI Fast Set (JFS-3-J3.2); group constants in 70 energy groups, which were processed from the Japanese Evaluated Nuclear Data Library (JENDL-3.2). These were used in the standard calculation method: a 3-Dimensional Hexagonal-Z whole core calculation by diffusion theory. This standard calculation, however, involves several approximations. The continuous neutron energy spectrum is divided into 70 discrete energy groups and continuous spatial coordinates are represented by assembly-wise spatial meshes. Original transport equations are solved by diffusion theory (isotropic scattering) approximation and fine structures in fuel assemblies, such as fuel pins or wrapper tubes, are processed into cell-wise homogeneous mixture. To improve the accuracy of the results, these approximations are compensated for by applying corresponding correction factors. Cell heterogeneity effects, among them, were evaluated to be 0.3-0.4% k/kk' by diffusion calculations based on the group constants, obtained by heterogeneous cell model calculations. This method, however, has the drawback that it assumes that there is no interdependency of the related approximations; energy grouping, diffusion approximation, etc. A study on cell heterogeneity effects has been conducted using the continuous energy Monte Carlo method to validate the adequacy of this non-interdependency assumption. As a result, cell heterogeneity effects slightly larger than those from conventional methods have been obtained; 0.54% k/kk' for the initial critical core, and 0.50% k/kk' for the initial full power core. Dependency on plutonium enrichment and fuel temperature has also been identified, which implies the dependency of the cell heterogeneity effects on the specific ...