Evaluation of fast neutron population in aged CANDU pressure tube: lattice calculations

In CANDU (Canada Deuterium Uranium) reactor design, the fuel bundles are placed inside fuel channels, each of which is comprised of concentric tubes called the pressure tube (PT) and the calandria tube (CT). The pressure tube undergoes physical deformations both in the axial and radial directions, as the reactor ages. Such deformations are caused by continuous exposure to extreme neutronics and thermal hydraulic conditions. One of the contributing factors to the PT deformation, namely the fast neutron (i.e., neutron with energy greater than 1 MeV) in the pressure tube, is evaluated in this paper. The knowledge of the fast flux in the PT can be used for estimating the amount of deformation, in particular the axial elongation that will be experienced by the PT throughout its lifetime. An understanding of how the fast flux distribution changes in the PT as it undergoes a diametral creep due to aging is sought. The results from the study indicate that the use of a proper crept PT dimension in determining the fast flux value used for estimating the axial growth of the PT is important in order to get more accurate predictions of the PT axial elongation. (author)