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

冷却剂损失事故（LOCA）通常是轻水堆中的极限设计基准事故。在这种事故发生时，裂变链式反应会自动关闭，但在事故发生后的几个小时内，仍会产生“衰变热”，大约占运行功率的7%。要去除衰变热，需要有足够的冷却剂进入堆芯，并且在此期间，堆芯保持“可冷却的几何形状”。这一点无法保证--过热、内部受压的燃料棒可能变形--所谓的“包壳膨胀”--并可能形成流动堵塞。因此，反应堆安全案例的主要重点是确保LOCA的后果是可控的。要做到这一点，我们必须了解和模拟燃料和外覆层的复杂机械行为，以及燃料细棒上的冷却剂流动。事实上，这些影响是强烈的相互依赖的，其目的是开发一个国家的最先进的计算机代码系统，以预测在轻水反应堆（LWR）燃料棒束在冷却剂损失事故（LOCA）的3-D包壳气球行为。该代码系统将涉及一个国家的最先进的3-D燃料棒的性能代码与国家的最先进的3-D热工水力学代码的动态耦合，将使用实验数据进行验证，并将被证明为轻水堆燃料组件。

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.