Anisotropic diffusion effect on criticality of plate lattice fast assembly

The neutron diffusion in plate lattice is generally somewhat anisotropic. In case of usual plate cells for the mockup of LMFBR composition, the diffusion coefficient for parallel direction to lattice plate, based on Benoist's theory, proves to be larger by 2 -- 4% than that for perpendicular direction, which is considered to affect the criticality of plate lattice fast assembly. A practical treatment of the anisotropic diffusion effect on criticality has been proposed, in which, like the transport correction, the anisotropic diffusion effect is treated as a correction term to be applied to the conventional isotropic diffusion calculation. The method is applied to actual plate lattice critical assemblies, already built in FCA, ZPR or ZEBRA. The anisotropy correction on criticality turns out to amount to the order of -0.2 -- -0.4% Δk/k for the normal plate lattice-core. The amount of anisotropy correction is further enhanced in case of an assembly consisting of plate lattice-blanket or sodium-voided core. The anisotropic diffusion effect on criticality is, therefore, important for the analysis of criticality of plate lattice assembly, and should be corrected in addition to the conventional heterogeneity effect. The present method, based on the perturbation theory, is practical and useful. (auth.)