Abstract The moderator system in a CANDU reactor provides the unique ability to provide emergency cooling to the fuel in postulated severe accidents during the early phases of the transient. A key criteria which dictates the effectiveness of heat removal during these events is the integrity of the fuel channel assembly. This in turn relies on the prevention of dryout on the calandria tube. The main tool used to demonstrate adequate margins to dryout on the calandria tubes is Computational Fluid Dynamics (CFD). While several experimental datasets are available for validation of these tools, the historical facilities lack the specific geometrical features present in some of the older CANDU designs. The historic tests were typically scaled based on the so-called Archimedes number and non-dimensional heat flux. To extend the validation database two unique 1/16th scale (based on vessel diameter) experiments are performed which include features found in the moderator systems of two operating CANDU designs. The primary goal of these new experiments was to generate thermalhydraulic data for validation of CFD codes, but several additional tests were performed to examine the potential impact of scaling on the results. This paper demonstrates a consistent conservative bias in the CFD predictions with respect to maximum temperature in the experiments over the range of conditions tested. The combination of CFD and experimental observations concludes that the likely cause of the bias is due to CFD’s over prediction of inlet jet dissipation leading to less cold fluid penetration in the heated core of the test section. In terms of scaling, a combination of experimental evidence and supporting CFD simulations indicate that the historical scaling parameters do provide some level of qualitative similarity in the mixed-convection regimes in the vessel, but some revision to account for geometrical and flow pattern deviations may be necessary.

中文翻译：

---

慢化剂循环实验尺度考虑的实验和数值研究

摘要 坎杜反应堆中的慢化系统提供了独特的能力，可以在瞬变早期阶段为假定的严重事故中的燃料提供紧急冷却。决定这些事件期间散热有效性的关键标准是燃料通道组件的完整性。这反过来又依赖于防止排管管变干。用于证明排管管有足够余量干燥的主要工具是计算流体动力学 (CFD)。虽然有几个实验数据集可用于验证这些工具，但历史设施缺乏一些较旧的 CANDU 设计中存在的特定几何特征。历史测试通常基于所谓的阿基米德数和无量纲热通量进行缩放。为了扩展验证数据库，进行了两个独特的 1/16 比例（基于血管直径）实验，其中包括在两个运行的 CANDU 设计的减速器系统中发现的特征。这些新实验的主要目标是生成用于验证 CFD 代码的热工水力数据，但还进行了一些额外的测试，以检查缩放对结果的潜在影响。本文展示了 CFD 预测中关于测试条件范围内实验中最高温度的一致保守偏差。CFD 和实验观察相结合得出的结论是，偏差的可能原因是由于 CFD 对入口射流耗散的过度预测导致测试部分的加热核心中的冷流体渗透较少。在缩放方面，