Vessel coolant mixing effects on a PWR Main Steam Line Break transient

A Main Steam Line Break transient analysis for a Swiss operated Pressurized Water Reactor has been performed using the code RETRAN-3D, which included a 3-D core model applying a one-to-one correspondence between the thermal-hydraulic and the neutron kinetics nodalizations (nodal/channel) and dynamically coupled to a thermal-hydraulic plant system model. The analysis was postulated at Hot Zero Power conditions and was based on a real UO2-MOX core to whom conservative modifications were made to bring about a power excursion despite the control rods insertion. The objective of this work was to assess the impact on the core power evolution, at both the global and local scales, of the level of detail adopted for the description of the thermal-hydraulics upstream and downstream of the core. To that aim, a parameterized coolant mixing model in the Reactor Pressure Vessel was developed to control the level and the radial details of the coolant mixing at the inlet and outlet of the core. For the core considered in this work and using different credible coolant mixing patterns, it was observed that the coolant mixing in the Reactor Pressure Vessel could have different, and sometimes even opposite, effects depending on the scale that was considered and on the details of the assumed coolant mixing pattern.