Improvement of a calculation procedure of neutron-flux distribution for radioactivity inventory estimation for decommissioning of nuclear power plants

We improved a calculation procedure of neutron-flux distribution in the Primary Containment Vessel (PCV) to get reliable estimation of radioactivity inventory, which is crucial for specification of a decommissioning plan of nuclear power plants. We calculated two-dimensional (2D) distribution of neutron-flux in cylindrical coordinate (r-z) with a 2D discrete ordinate (Sn) code DORT by utilizing knowledge of the characteristics of neutron-flux distribution in the PCV of Tsuruga Nuclear Power Plant unit 1 (TS-1). The knowledge was obtained from measurements and three-dimensional (3D) calculation of neutron-flux distribution in cylindrical coordinate (r-θ-z) with a 3D Sn code TORT. The measurements were performed by using activation foils at 30 locations where the distribution characteristics could be observed in the PCV. The 3D calculation provided us better qualitative understandings about the characteristics of neutron-flux distribution in the PCV. An improved model of 2D calculation of neutron-flux distribution was generated by utilizing the knowledge from the measurements and the 3D calculation in the PCV. The reliability of 2D calculation was confirmed by comparing calculated fluxes with measured ones at locations where measurements were performed. The comparison between them showed that neutron-flux distribution calculated by the improved 2D model has met our criteria of the reliability. Therefore, the improved procedure provided a reliable 2D distribution of neutron-flux.