Fuel rod analysis and rationalization of fuel rod integrity criteria for Super LWR

超级轻水堆是东京大学正在开发的一种热谱SCWR，被认为是第四代反应堆的概念。分析了超轻水堆的燃料棒性能，提出了燃料棒完整性准则的合理化原则，并将日本原子能机构开发的FEMAXI-VI应用于燃料棒的分析。由于包层表面温度是燃料棒分析的关键边界条件之一，因此首先使用子通道分析和统计热设计方法来评估正常工作条件下的最高包层表面温度。由于轴向功率分布向包层温度较高的燃料棒上部移动，每个循环的最大芯块中心线温度向EOC方向增加。由于FP气体的释放，燃料棒的内部压力朝着EoC方向线性增加。即使可能发生PCMI，包层外径的变化也是由包层的热膨胀决定的。对熔覆层的机械强度要求进行了评估，并提出了降低强度要求的原则。减少FP气体释放和PCMI的设计努力处于权衡关系中，因为增加初始颗粒尺寸会减少FP气体释放，但会增加颗粒膨胀率。将气体真空室放置在燃料棒的下部，有效地降低了包层上的应力。消除了燃料棒内压低于冷却液压力的约束也是有效的。提出了合理的异常瞬变判据的原则。燃料棒分析表明，为了保证燃料的完整性，可以确定燃料棒过功率瞬变的最大功率和冷却损失瞬变的最大包壳温度的允许限值。

The Super LWR is a thermal spectrum SCWR, recognized as a Generation-IV reactor concept, being developed by the University of Tokyo. Fuel rod behaviors of the Super LWR are analyzed and the principles for rationalizing the fuel rod integrity criteria are developed.The FEMAXI-VI, developed by Japan Atomic Energy Agency, is applied to the fuel rod analyses. The maximum cladding surface temperature for normal operating conditions is evaluated first with subchannel analyses and a statistical thermal design procedure since the cladding surface temperature is one of the key boundary conditions for the fuel rod analyses.The fuel rod behavior is analyzed for normal operating condition. The maximum pellet centerline temperature increases towards EOC of each cycle since the axial power distribution shifts to the upper part of the fuel rod where the cladding temperature is high. The fuel rod internal pressure linearly increases towards EOC due to FP gas release. Even though PCMI can occur, change of the cladding outer diameter is governed by thermal expansion of the cladding. The mechanical strength requirement for the cladding is evaluated and the principles for reducing it are developed. Designing efforts in reducing FP gas release and PCMI are in the tradeoff relationship since increasing the initial pellet grain size reduces FP gas release but increases pellet swelling rate. Placing the gas plenum in the lower part of the fuel rod is effective in reducing the stress on the cladding. Eliminating the constraint that the fuel rod internal pressure is kept below the coolant pressure is also effective.The principle of rationalizing the criteria for abnormal transients is developed. Fuel rod analyses show that allowable limits to the maximum fuel rod power, for overpower transients, and maximum cladding temperature, for loss-of-cooling transients can be determined to assure the fuel integrities.