Coupling Serpent and OpenFOAM for neutronics - CFD multi-physics calculations

The main goal of this work was to couple the Monte Carlo neutronics code Serpent~2 with a CFD solver from the OpenFOAM toolbox. The coupling was implemented with the already available multi-physics interface of Serpent. The interface allows the passing of high fidelity density and temperature distributions from an external solver to Serpent and also the passing of fission power distribution from Serpent to the external solver. The coupled CFD-neutronics problem was solved by iteration. The coupling was tested by modelling a mock-up 5x5 fuel assembly cooled with water in a steady state condition at full power. The effects of boiling were neglected and the flow was modelled as single phase. The main results of the coupled calculation were high fidelity temperature and density distributions. The effect of distribution fidelity on neutronics was studied by running Serpent simulations with varying level of axial refinement. Most obvious differences were observed in the axial power density and in the collision density in the coolant, with minor differences in e.g. the effective multiplication factor. In addition the convergence of the coupled calculation was briefly studied. The default Serpent relaxation scheme based on the stochastic approximation performed well in the coupled calculations. This work can be continued in the future for example by including the effects of boiling in the CFD solution.