Numerical Analysis of Dryout Phenomena in Light Water-Cooled Nuclear Reactors using Particle Method

在沸水反应堆(BWR)中，堆芯上部出现环状雾流。干涸是热工水力设计的极限条件。准确的干化分析对反应堆的设计和安全具有重要意义。运动粒子半隐式(MPS)方法是目前研究人员发展起来的一种方法。MPS方法不需要网格，因此液滴的分离和合并很容易分析。在本研究中，采用MPS方法对干燥过程进行了直接模拟，干燥现象包括液滴被燃料棒上流动的液膜吸收的沉积过程和液滴从液膜中释放出来的携带过程。首先，用MPS方法分析了沉积过程中单个液滴的行为。当液滴直径较小或撞击速度较慢时，液滴完全被液膜吸收。当液滴直径较大或撞击速度较高时，液滴撞击对液膜产生扰动，产生新的液滴。在沸水反应堆(BWR)条件下，这种新的液滴(再夹带)是不可忽略的。基于有限体积法和粒子方法(MPS方法)，发展了一种粒子-网格混合方法。该混合方法可以有效地分析气液两相流动。该混合方法引入了低雷诺数k-ε模型。采用混合方法模拟沸腾堆堆芯燃料棒上的液膜流动和不稳定性。在这项研究中，发展了数值方法来分析沸腾堆条件下干涸现象的基本过程。对二次夹带过程也有了新的认识。

In Boiling Water Reactors (BWRs), annular mist flow appears in the upper part of the core. Dryout is a limiting condition of thermalhydraulic design. It is important to analyze dry out accurately for the reactor design and safety. Moving Particle Semi-implicit (MPS) method has been developed by the present researchers. The MPS method does not need meshes, so that droplet separation and merging are easily analyzed. In the present study, direct simulation of dry out was carried out using the MPS method.Dryout phenomenon consists of the deposition process where droplets are absorbed by the liquid film flowing on the fuel reds, and the entrainment process where droples are released from the liquid film. First, single droplet behavior in the deposition process was analyzed by the MPS method. When the droplet diameter is small or the impact velocity is slow, the droplet is totally absorbed by the liquid film. When the droplet diameter is large or the impact velocity is high, the droplet impact disturbs the liquid film and new droplets are generated. This new droplets (re-entrainment) cannot be neglected in the Boiling Water Reactor (BWR) conditions.A particle-mesh hybrid method was developed based on the finite volume method and the particle method (MPS method). Gas-liquid two-phase flow can be analyzed efficiently using the hybrid method. A low-Reynolds number k-ε model was incorporated into this hybrid method. Simulation of liquid film flow and instability on the fuel rods in BWR cores using the hybrid method.In this study, numerical methods have been developed to analyze the basic processes in the dryout phenomenon m the BWR conditions. Anew knowledge about the re-entrainment process was also obtained.