Abstract

In the high-temperature reactor design, it is common practice to leave gaps between the graphite blocks of the reflectors to accommodate thermal dilatation and material swelling, as well as to provide an additional cooling source during operation. These gaps give rise to bypass flows entering the reactor core. The bypass flows can change friction factors and heat exchange coefficients obtained in the bulk of the pebble bed. In this paper, a coupled computational fluid dynamics–discrete element method (CFD-DEM) model is proposed. In this model, the pebbles are resolved by the CFD grid and the turbulent field is partially captured using a detached eddy simulation method. The DEM model is first validated against empirical correlations for the packing of the pebbles, and the coupled model is then tested against thermal measurements in the SANA experiment. Then the model is used to perform three-dimensional studies of the effects of the bypass flows in a representative pebble bed configuration. It is determined that the effect of cross flows can be approximately bounded to the first two layers of pebbles next to the reflector wall. Additionally, an increase of ~12% in the Nusselt number in the pebbles next to the reflector is predicted, with a maximum local increase in the pebble of ~100%.

摘要

在高温反应堆设计中，通常的做法是在反射器的石墨块之间留出间隙以适应热膨胀和材料膨胀，并在运行期间提供额外的冷却源。这些间隙引起进入反应堆堆芯的旁通流。旁通流可以改变在大部分球床中获得的摩擦系数和热交换系数。在本文中，提出了耦合计算流体动力学-离散元法（CFD-DEM）模型。在该模型中，鹅卵石由 CFD 网格解析，湍流场使用分离涡流模拟方法部分捕获。DEM 模型首先根据卵石堆积的经验相关性进行验证，然后根据 SANA 实验中的热测量值对耦合模型进行测试。然后使用该模型对具有代表性的球床配置中的旁通流的影响进行三维研究。确定横流的影响可以近似地限制在靠近反射器壁的前两层鹅卵石上。此外，预计反射器旁边鹅卵石的努塞尔数增加约 12%，鹅卵石的最大局部增加约 100%。