EFFECT OF NEUTRON-SPECTRUM AN EFFECTIVE CROSS-SECTION IN EVALUATION OF RESIDUAL RADIOACTIVE INVENTORY IN NUCLEAR-REACTORS

The method of shielding calculations, and production and decay of nucl ides being employed for estimate of radioactive inventory frequently a ssumes the Maxwellian distribution (MD) for thermal neutron spectrum a nd 1/E for epithermal neutron spectrum. Actually each spectrum deviate s largely from the ND and 1/E spectrum respectively in reactor pressur e vessel (RPV) and biological shielding concrete (BSC). These assumpti on then leads activation calculation for residual radioactivity to not iceable error. In this paper it is studied how the deviation of the ne utron spectrum affects activation, effective thermal neutron cross sec tion and effective epithermal neutron cross section. It has been prove d that they change according to position in the RPV, BSC and other rea ctor components. Therefore, neutron energy group structure is required to be so fine as to define neutron spectrum accurately.