Estimating the effects of homogenized fuel temperature in group constant generation using Serpent 2

We extend the multi-physics capabilities of the Serpent 2 Monte Carlo code to coupled burnup calculations by implementing the Stochastic Implicit Euler depletion scheme with thermal feedback. We use these new multi-physics capabilities for the verification of the traditional way of generating group constants using an effective flat fuel temperature profile during the burnup calculation. We investigate the effects of this approximation on the generated nuclide compositions, group constants as well as the results on the simulation of the initial cycle of the EPR reactor using the ARES core simulator. The main findings state that while the use of an effective temperature model leads to significant differences in the radial nuclide distributions, the assembly wide homogenized group constants are reproduced fairly well and the effects on the simulation of the EPR initial cycle are modest, although interesting axial and radial power redistribution can be observed due to the slower speed of gadolinium burnout when effective fuel temperatures were used. The results indicate that better results for the full core calculations could be obtained by using a separate effective temperature for the burnable absorber rods in the burnup calculation and by considering the fuel temperature history effect in the group constant parametrization.