Thermal-hydraulic conditions in a partially uncovered nuclear fuel assembly mock-up are studied with particular focus on the influence of the horizontal air flow above the rod bundle. The investigations are performed at the ALADIN test facility, which models a boiling water reactor fuel assembly at a 1:1 scale both axially and radially. In the scenario studied, the main heat transfer mechanisms – conduction, convection and radiation – are strongly coupled and all are of similar importance. A combination of measurements and CFD simulations serves to analyze the heat transfer processes in detail. Contrary to previous studies in this field, all heat transfer mechanisms were considered in the simulation with sophisticated models. The numerical results show a good agreement with the measurements, given the inevitable differences between the approaches. Although the successive evaporation of cooling water in a fuel assembly is a transient, multiphase process, the steady, single-phase simulation yields acceptable results. While single effects are overestimated in the simulation, the important dependencies are predicted similarly. A general result is that the maximum cladding temperature rises with decreasing water level. Further results indicate an impact of the horizontal air flow on the residual heat removal for moderate rod powers. Higher horizontal velocities above the fuel assembly lead to slightly higher temperatures inside. A characteristic flow field forms in the test facility that prevails for all studied water levels and horizontal velocities. However, it has only a minor effect on the temperature distribution in the central rod bundle. By combining experiments and numerical simulations, the study provides important information about the decisive parameters for the heat exchange in a spent fuel pool in case of an accident with loss of cooling. The exposed length of the fuel rods is of much more importance than the magnitude of the horizontal velocity above the fuel assembly.

研究了部分未覆盖的核燃料组件模型中的热工条件，特别关注了棒束上方水平气流的影响。研究是在ALADIN测试设备上进行的，该设备以轴向和径向1：1比例对沸水反应堆燃料组件进行建模。在所研究的场景中，主要的传热机制（传导，对流和辐射）是紧密耦合的，并且都具有相似的重要性。测量和CFD模拟的结合可用于详细分析传热过程。与该领域的先前研究相反，在模拟中使用复杂模型考虑了所有传热机理。考虑到两种方法之间不可避免的差异，数值结果与测量结果吻合良好。尽管燃料组件中冷却水的连续蒸发是一个短暂的多相过程，但稳定的单相模拟仍可得出可接受的结果。尽管在模拟中高估了单个效果，但重要的依存关系也被类似地预测。一般结果是，最高包层温度随着水位的降低而升高。进一步的结果表明，水平气流对中等杆功率的余热去除有影响。燃料组件上方较高的水平速度会导致内部温度稍高。测试设备中会形成一个特征流场，该流场适用于所有研究的水位和水平速度。但是，它对中心棒束中的温度分布影响很小。通过结合实验和数值模拟，该研究提供了有关发生事故而损失冷却的情况下，乏燃料池中热交换的决定性参数的重要信息。燃料棒的暴露长度比燃料组件上方水平速度的大小重要得多。