Modeling cladding oxidation with coupled thermal-mechanics and thermal-hydraulics solvers

The zirconium cladding in typical light-water reactor nuclear fuels oxidizes during normal operation forming a protective oxide layer. The oxide layer has a lower thermal conductivity than that of the metal, and therefore affects heat transfer from within the fuel rod. The temperature of the cladding is most important in determining the extent of cladding oxidation. However, the cladding temperature is not solely determined by the thermal behavior of the fuel rod itself, but in large part by the heat transfer conditions from the cladding to the coolant. This coupled phenomenon provides a suitable case for validating the predictions of a coupled simulation including fuel rod thermal mechanics and coolant thermal hydraulics.

The fuel behavior module FINIX has been developed at VTT for providing a reasonably accurate description of fuel thermal-mechanical behavior in coupled applications. In addition, quite recently a new thermal-hydraulics solver Kharon has been developed. The closed channel two-phase steady state solver is based on the porous medium approach and in this work it provides the description of cladding-coolant heat transfer. From thermal hydraulics the steady-state axial temperature distribution in the cladding is obtained, which then determines the axial variation in the cladding oxide thickness in the fuel behavior description.

A pressurized water reactor assembly from the literature is modeled with both codes and the resulting cladding oxidation predictions are compared to experimental data. A somewhat idealized linear heat generation rate history is used for the rods in the assembly, as Kharon models the system at the assembly level and FINIX at the rod level. The predictions of this approach are compared to the oxidation predictions of standalone FINIX, which contains basic thermal-hydraulic correlations for the calculation of the axial temperature distribution in the cladding.