Effect of Neutron Spectrum on Effective Cross Section in Evaluation of Residual Radioactive Inventory in Nuclear Reactors

Abstract

The method of shielding calculations, and production and decay of nuclides being employed for estimate of radioactive inventory frequently assumes the Maxwellian distribution (MD) for thermal neutron spectrum and 1/E for epithermal neutron spectrum. Actually each spectrum deviates largely from the MD and 1/E spectrum respectively in reactor pressure vessel (RPV) and biological shielding concrete (BSC). These assumption then leads activation calculation for residual radioactivity to noticeable error. In this paper it is studied how the deviation of the neutron spectrum affects activation, effective thermal neutron cross section and effective epithermal neutron cross section. It has been proved that they change according to position in the RPV, BSC and other reactor components. Therefore, neutron energy group structure is required to be so fine as to define neutron spectrum accurately.

KEYWORDS:

• radioactive inventories
• Maxwellian distribution
• thermal neutron spectra
• 1/E
• pressure vessels
• biological shielding
• activation
• neutron energy group structure
• neutron transport theory