压水堆燃料包壳破损条件下裂变气体非稳态释放数值模拟研究

压水堆燃料包壳破损后，芯块-包壳间隙内积累的裂变气将释放到冷却剂中，其内部的微观机理还尚不清楚。为了揭示裂变气体释放过程中冷却剂与气体的相互作用规律，基于三维计算流体力学（CFD）方法对该物理过程展开数值模拟，所利用的模型为VOF模型以及k-ε模型。模拟结果表明，包壳破损后冷却剂首先进入芯块-包壳间隙，在芯块-包壳间隙内蒸发，引起芯块-包壳间隙内压强上升，而后裂变气体释放到子通道；裂变气体从芯块-包壳间隙释放到子通道可分为2个阶段。第一阶段:芯块-包壳间隙与子通道间压差较大，气体射流进入子通道，该阶段持续时间较短，裂变气体释放率较大，且变化也较大。第二阶段:芯块-包壳间隙与子通道间压差较小且相对平稳，裂变气体通过破口内涡的对流传质进入子通道，该阶段持续时间较短，裂变气体释放率较小，且相对稳定。 

Computational Simulation Research of Unstable Mechanics of Fission Gas Release under Condition of Fuel Cladding Breach

When the claddings of the PWR fuels breach, the fission gas accumulated in the gap of pellets and claddings releases into the coolant. Naturally, the process of fission gas release is a process of two-phase flow, while the microscopic mechanics is still undiscovered. To reveal the law of the interaction between the coolant and the gas during the process of fission gas release, a CFD method is built to simulate the transient process of fission gas release, in which VOF and k-ε model is used. The result shows that the coolant will flow into pellet-cladding gap, then it will be evaporated, which will lead to the increasing of the pressure in the gap, and the fission gas will release into the subchannel; the processing that the fission gas releases from the pellet-cladding gap can be divided into 2 steps. In step 1, the differential pressure between the pellet-cladding gap and the subchannel is relatively larger, which makes the gas jet into the subchannel, and the duration of this step is short, and the ratio of the release of the fission gas is higher, meanwhile, it changes violently.? In step 2, the difference of the pressure between the pellet-cladding gap and the subchannel is relatively smaller and stable, and the fission gas enters the subchannel by the convective mass transfer through the vortex in the breach. The duration of this step is short, and the ratio of the release of the fission gas is lower and stable.