CFD Modeling of the Moderator Tank of a PHWR Nuclear Power Plant

Abstract

A steady state CFD simulation of the moderator tank of Atucha II Nuclear Power Plant (a Pressurized Heavy Water Reactor PHWR) was performed. Three-dimensional (3D) detailed modeling of the tank was achieved including inlet and outlet ring-shaped distributors, the coolant channel (CC) tubes and the control and safety rod guide tubes. Two heat sources were taken into account: the conduction/convection from the coolant channels’ fluid and the heat transfer by thermal neutron moderation. For the former, suitable boundary conditions (wall temperature) at the CC tube walls were arisen from 2D estimation of the conduction/convection heat through the coolant walls. The coolant temperature profile along each CC (obtained from a previous 1/3D model) along with correlations for the in-channel side convective coefficient were considered. The effective conduction coefficient was estimated by fitting the overall expected transfer power. For the latter, a homogeneoussource was implemented. Simulations allowed a thorough understanding of the complex flow and the heat transfer phenomena, while acquiring useful information about the temperature distribution in the moderator. The most relevant conclusion is that the power transferred from the CCs to the moderator does not show linear dependence on the fission power but on the coolant temperature, which is very similar for all CCs. These results become of prime importance when defining more accurate boundary conditions for modeling the in-channel flow with a previously developed, in-house, 1/3D multidimensional model of the reactor pressure vessel (RPV). The 3D model developed is the starting point to carry on unsteady simulations in the moderator tank, such as reactor Safety Control Rod Axe Man (SCRAM), heat removal during a primary pump shut down or boron distribution during a fast shut down injection.