Improvements in the calculational methods of nuclear reactor assemblies

The standard procedure for the calculation of neutron leakage rate of an assembly is based on calculating this rate for a homogeneous medium representing approximately the assembly, but not the leakage rate of the assembly itself The method presented here allows to take into account, with some approximations, the influence of heterogeneous structure of the assembly on the leakage The anisotropic buckling-dependent migration area of the assembly in the direction k, which relates buckling in the same direction to the effective and the infinite multiplication factor, can be calculated using the anisotropic buckling-independent migration area corrected in an approximately way by the buckling-dependent and the buckling-independent migration areas of the homogenized medium The mentioned migration areas are calculated by means of the first flight collision probability and the first flight directional collision probability The numerical results show that for ordinary PWR fuel assembly the influence of heterogeneity on the leakage is relatively small and that the effect increases when the concentration of water decreases This effect could be much more important in more heterogeneous types of reactors (author) 10 refs, 1 tabSpecialists meeting on advanced calculational methods for power reactors; Technical committee meeting and workshop on LWR core design parameters; Cadarache (France); Rez (Czechoslovakia); 10-14 Sep 1990