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

在轻水堆中，冷却剂损失事故通常是限制设计基础的事故。在这样的事故中，裂变链式反应会自动关闭，然而，在事故发生后的几个小时内，仍有“衰变热”产生，可能占运行功率的7%。消除这种衰变热需要足够的冷却剂能够进入堆芯，并且在此期间堆芯保持“可冷却的几何形状”。这是不能保证的——过热的、内部加压的燃料销会变形——也就是所谓的“包层膨胀”——并可能对气流形成阻塞。因此，反应堆安全案例的一个主要焦点是确保就地安置的后果是可控的。要做到这一点，我们必须理解和模拟燃料和外包层的复杂机械行为，以及燃料销上的冷却剂流动。事实上，这些影响是相互依赖的。目的是开发一种最先进的计算机代码系统，以预测在冷却剂丢失事故（LOCA）期间轻水反应堆（LWR）燃料束中棒的3-D包层气球行为。该代码系统将包括最先进的3-D燃料棒性能代码与最先进的3-D热工水力代码的动态耦合，将使用实验数据进行验证，并将在轻水堆燃料组件中进行演示。

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.