Multi-pin modelling of PWR fuel pin ballooning during post-LOCA reflood

Experiments investigating post-LOCA reflood generally indicate little coherence to fuel pin ballooning, and an absence of significant blockages and un-coolable regions of the core. Computational modelling is unable to predict this; the usual ‘representative pin’ models neither take into account the heterogeneity of the core, nor incorporate the dynamic coupling between the changing core geometry, and the flow paths taken by the coolant. In this paper we present a composite model, able to treat distinct pins mechanistically, and able to incorporate their (distinct) swelling behaviour into the thermal–hydraulic model of the reflood, allowing the cooling of a pin to be directly affected by the deformation of itself and its neighbours.