Thermohydraulic evaluation of a MOX (U, Th)O2 fuel application in an AP1000 reactor typical fuel assembly

In the present work, we propose a three-dimensional model of the typical fuel assembly of the AP1000 reactor with the change from conventional fuel to (U, Th)O2. Owing to the complexity of the real model, we made some simplifications to reduce the computational cost and optimise the calculation time. The absence of spacer grids, burnable poisons, and the simulation of only 1/8 of the fuel assembly, culminated in a simplified model, and at the same time, capable of representing the reactor's operating situation under normal conditions. We obtained the power density distribution in the fuel assembly through a neutronic-thermohydraulic coupling, using MCNP6 and ANSYS CFX-19, respectively. One of the main differentials of this model is the consideration of the temperature dependence of thermophysical properties, which shows a direct influence on the results. We assessed the correlations published in some reports about thermal conductivity, density, and specific heat to obtain expressions for these parameters. We determined the temperature profiles, the axial distribution of the coolant density, and the pressure drop in the fuel assembly to assess the thermohydraulic limits. The proposed model presented results according to the project limits for the normal operating conditions of the reactor. The implementation of a robust model provided consistent results, in addition to a methodology capable of carrying out more complex analyses.