Development of CANDU 6 moderator models for simulating the flow circulation and predicting temperature distribution inside the Calandria vessel

This paper describes the work of developing two 3D CFD models using ANSYS CFX for simulating the flow circulation of the moderator in a CANDU 6 nuclear reactor and predicting temperature distribution inside the Calandria vessel under normal operation. This work is important as the estimation of the local subcooling of the moderator inside Calandria vessel is an input parameter in assessing the reactor response to an accident in which the moderator acts as a heat sink. For assessing the flow in the calandria, two models were developed and compared. The first one uses the porous media approach, in which the flow around individual calandria tubes is not modelled in the analysis, and a pressure loss coefficient is applied. The second model implies simulating the fuel channels, in order to consider the effect of the calandria tube matrix on flow distribution. The results obtained with the first model are compared and validated against the results obtained with MODTURC_CLAS using similar input data, the results being almost similar. This work is the first step in the effort to improve the thermal hydraulics characteristics in the calandria vessel and obtain a more uniform temperature distribution in the core region.