SELF-SHIELDING AND ENERGY-DEPENDENCE OF DILUTION CROSS-SECTION IN THERESOLVED RESONANCE REGION

In the calculation of flux weighted multigroup neutron cross-sections of a nuclide present in a homogeneous mixture of several nuclides, by the well known Bondarenko approach, the contributions from the other n uclides to the flux weighting function, are contained in a parameter c alled 'dilution'. Under the narrow resonance approximation and with as sumed non-overlap of resonances, the energy fluctuations of the diluti on are usually ignored within an energy group. The 'self-shielding fac tor' (SSF), which is the ratio of the group cross section for a given dilution to that at infinite dilution, is a smooth function of dilutio n. A conventional multigroup cross section set, apart from the infinit e dilution cross-sections, gives SSF over a dilution grid, from which the effective SSF for any dilution could be obtained by interpolation. The effective SSF multiplied by the infinite dilution cross-section t hen gives the effective cross-section to be used in the neutronic anal yis. Though this conventional procedure works well, the accuracy of th is procedure depends on the interpolation scheme used and on the finen ess of the dilution grid. The effect of fineness of the dilution grid and the effect of energy dependence of the dilution on the effective S SF are observed, for a specific case, and the results presented in thi s paper. The JENDL-2 basic nuclear data was used and the benchmark fas t critical assembly ZPR-6-7 anlaysed for a temperature of 300K. The st udy was restricted to the resolved resonance regions of the nuclides i nvolved. The results appear significant with respect to the target acc uracies demanded for the SSF. (C) 1998 Elsevier Science Ltd. All right s reserved.