Analysis of the Fast Neutron Spectrum Inside the Materials Test Bundles in the NRU Loops

Abstract

The 30-element Materials Test Bundles (MTB) in the NRU reactor provide a facility in which the effects of irradiation on CANDU reactor materials may be studied. Each bundle contains 1.71% enriched uranium in its 12-element inner fuel ring and 1.25% enriched uranium in its 18-element outer fuel ring. The MTB is installed in the high-flux position of a fuel string in the NRU loops, and supplies fast-neutron fluxes (E > 1 MeV) up to 5.3 x 10^17 n.m^-2s^-1. This paper describes how the MTB with its experimental insert was modeled to calculate the fast-neutron spectrum in the test bundle. Calculations were performed using the WIMS-AECL code, which is a multi-group transport code with two-dimensional capabilities using the collision probability method. Results of the fast-neutron spectrum above 1 MeV are presented in nine groups. The analysis confirms that the spectrum in the MTB is representative of the actual irradiation spectrum for fast-neutron damage in a CANDU reactor. The calculated fast-neutron fluxes (E > 1 MeV) at the specimen irradiation locations of the MTB were compared with integral measurements that were obtained from iron-wire flux monitors by determining the ~n% activities arising from the Fe54(n,p)Mn54 reaction. The measured and calculated values for the fast- neutron fluxes agree to within 12%.