Abstract During a loss of coolant accident (LOCA), blocked sub-channels may appear due to the swelling of the fuel rods’ cladding, which results in flow redistribution during the reflooding phase. For this reason, special attention has been paid to the effect of fuel rods ballooning on the thermal-hydraulics in LOCA conditions. Due to the practically impossible physical or optical access to blocked sub-channels, no experiment so far has performed precise three-component velocity field measurements in the presence of ballooned regions. In this study, we used magnetic resonance velocimetry (MRV) to obtain three-component velocity fields of water flow within two 7 × 7 fuel rods bundles built mainly in plastic, one regular and one containing siafforganizationafforganizationxteen ballooned fuel rods with 90% blockage ratio and 240 mm blockage length. We present herein results with 50 lpm water flow rate, which corresponds to a Reynolds number of 1936. With the regular bundle, the performance of spacer grids’ mixing vanes to homogenize the flow was notable. With the ballooned bundle, we observed transverse velocities upstream of the ballooned zone that are as intense as the bulk mean velocity. Furthermore, there are substantial decreases in the axial velocity within blocked sub-channels up- and downstream of the ballooned zone, reaching near-zero and even negative values downstream, indicating flow recirculation. Although the flow is highly affected by the ballooned zone, the mixing spacer grid placed downstream remarkably homogenized the flow and effects of the flow redistribution disappeared. Finally, with the present ballooned bundle configuration, about 90% of the flow that should pass through blocked sub-channel deviates towards less resistant regions, which suggests a predominant geometric effect on the flow redistribution.

中文翻译：

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用磁共振测速仪测量的气球状 7×7 燃料束中的速度场和流量重新分布

摘要 在失冷事故（LOCA）中，燃料棒包壳膨胀可能会导致子通道堵塞，从而导致再注水阶段的流量重新分布。出于这个原因，在 LOCA 条件下，燃料棒膨胀对热工水力的影响得到了特别的关注。由于实际上不可能通过物理或光学方式访问阻塞的子信道，因此迄今为止还没有任何实验在存在膨胀区域的情况下进行过精确的三分量速度场测量。在这项研究中，我们使用磁共振测速法 (MRV) 获得了主要由塑料制成的两个 7 × 7 燃料棒束内水流的三分量速度场，一个是规则的，一个包含 siafforganizationafforganizationx1 个具有 90% 阻塞率的气球燃料棒和240 毫米阻塞长度。我们在本文中给出了 50 lpm 水流速的结果，这对应于 1936 年的雷诺数。对于规则束，间隔网格的混合叶片使流动均匀的性能是显着的。对于膨胀束，我们观察到膨胀区上游的横向速度与整体平均速度一样强。此外，膨胀区上游和下游阻塞的子通道内的轴向速度显着降低，下游达到接近零甚至负值，表明流动再循环。虽然流动受到气球区的高度影响，但放置在下游的混合间隔网格显着地使流动均匀并且流动重新分布的影响消失了。最后，使用当前的气球束配置，