Energy deposition in coolant of PWR under normal operation and accident conditions

In reactor core analysis, the energy deposition in the coolant is often regarded as a fixed amount. In this work we study the validity of this assumption at normal operation and under accident conditions in PWR. A fuel pin cell and other lattice elements from the VERA benchmark's 2-D 17 x 17 fuel assemblies are used to calculate photon and neutron energy deposition in the coolant by OpenMC. The adoption of a subcooled-boiling model for the coolant with non-uniform void fraction in the sub-channel is taken into account, without revealing however significant differences on the reaction rates used to score the coolant heating. Specifically, we study the effects of void fraction, pressure and coolant temperature on deposited energy. Chemical shim operated by diluted boric acid in water is also considered. Results show that coolant heating fraction can change significantly under accident conditions. Moreover, it can be expressed as a function of pure water density and boron concentration, where water is always considered as a homogeneous mixture also in presence of two-phase flow. Specifically, we provide a correlation fitting the numerical results from OpenMC pin cell calculations by least squares regression, in order to predict energy deposition in the coolant of the subchannels present in a fuel lattice. This correlation is explicitly intended for implementation in thermal-hydraulics computer codes used for coupled full-core calculations.