CFD analysis of the effects of a fuel assembly blockage in a liquid metal cooled fission reactor

The Belgian nuclear research centre SCK•CEN endeavours to develop the new generation reactor MYRRHA. The characteristics that differentiate this technology are the LBE coolant and spallation target, which sustains the fission chain reaction, being it a subcritical reactor. To ensure the safety of this complicated project, some postulated accidental conditions must be considered, to guarantee its safe operation and reduce the probability of such accidents with the appropriate design. In the present work the fluid behaviour in presence of a local internal blockage in a 19-pin wire-wrapped rod bundle representing the MYRRHA fuel assembly is studied. Indeed, through numerical CFD simulations, the hydraulic as well as the thermal fields in presence of both solid and porous blockage are analysed, to tackle fuel assembly safety aspects. Firstly, a single solid blockage occupying one single subchannel is modelled, to approach the validation of the code through the comparison with the experimental data performed in the KIT-KALLA tests, within the European project MAXIMA. Particular attention has been given to the process to define the most appropriate mesh for the domain, both on the meshing algorithm itself and on the level of refinement desired to obtain meaningful results. Similar treatment has been applied to the solving modelling method decision. In spite of this, the results of these numerical assessments are exhibiting big differences in the prediction of temperatures at the thermocouple positions present in the test facility, as it has been described in the numerical simulations done by the partner institution NRG. For this reason, a detailed sensitivity analysis on the influence of different parameters on the temperature distribution within the fuel bundle has been done, to understand the difference between the numerical results and the experimental data. Specifically, the influence of buoyancy, thermocouple position, turbulence model, mesh refinement has been evaluated and all these studies show little sensitivity. Only the inclusion of conduction in the electrical heater has showed a higher impact by decreasing the blockage temperatures. In the second place, both the case scenarios with non- and heat-emitting porous blockage has been investigated as well. As far as the first aspect is concerned, results obtained show that compared to a solid blockage, the implemented degree of porosity leads to lower temperatures at the cladding surface where the TCs are embedded, since even if limited, the flow of coolant is allowed to a certain extent. Porous blockages with self-heating present an opposite trend: the effect of the internally generated power is clearly the substantial increase in the temperatures in the blocked region. In conclusion, due to the heterogeneous formation origin of such blockages, a parametric study on the impact of blockage properties, i.e. porosity and self-heating, on the thermohydraulic flow flied has been performed. Given the simulations performed, it has been possible to conclude that, although the overall behaviour observed is common for all cases simulated with the different porosities, the maximum temperatures are affected only marginally by the change in this parameter. Contrarily, the impact of the change in the generated power level shows that overall behaviour of the temperature profiles is enhanced with increasing power, and the maximum temperature steadily increase while going to full power conditions.