High-fidelity modelling of clad ballooning during a loss-of-coolant accident

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The loss-of-coolant accident (LOCA) is generally the limiting design-basis accident in a LWR. In the event of such an accident, the fission chain reaction is automatically shutdown, however there remains 'decay heat' generation, perhaps 7% of operating power, for some hours following the accident. Removal of this decay heat requires that sufficient coolant can be brought into the core, and that the core, during this time, retains a "coolable geometry". This is not guaranteed - excessively hot, internally pressurised fuel pins can deform - so called 'clad ballooning' - and possibly form blockages to the flow.A major focus of the reactor safety case is therefore to ensure that the consequences of a LOCA are manageable. To do so, we must understand and model both the complex mechanical behaviour of the fuel and outer cladding, and the coolant flow over the fuel pins. Indeed, these effects are strongly interdependent.The aim is to develop a state-of-the-art computer code system to predict the 3-D clad ballooning behaviour of rods in a light water reactor (LWR) fuel bundle during a loss-of-coolant accident (LOCA). The code system will involve the dynamic coupling of a state-of-the-art 3-D fuel rod performance code with a state-of-the-art 3-D thermal-hydraulics code, will be validated using experimental data, and will be demonstrated for an LWR fuel assembly.