Abstract For economic reasons, the US nuclear industry is renewing efforts to build a technical basis to extend rod average burnup limits above the current regulatory burnup limit of 62 GWd/MTU. The primary driver is to increase pressurized water reactor cycle lengths to 24 months, reducing the number of fresh fuel assemblies and core design constraints, thereby making core energy utilization more efficient. However, fuel pellet fragmentation and pulverization, termed high burnup fuel fragmentation (HBFF), has been observed in the high burnup (>90 GWd/MTU) Halden loss-of-coolant-accident (LOCA) integral test series. The issue gained attention when fuel fragmentation and pulverization were also observed closer to the current US regulatory limit during the US Nuclear Regulatory Commission (NRC) sponsored out-of-core integral test at Studsvik Nuclear in early 2011. This led to NRC concerns with potential changes to fuel and core designs relative to fuel pellet pulverization. In a letter to the NRC Commissioners, the staff specifically identified a need to “…define the boundary of safe operation for key fuel design and operating parameters,” stating that “the staff is challenged to evaluate the acceptability of future fuel design advancements and fuel utilization changes.” As such, it can be concluded that HBFF and potential dispersal into the reactor coolant system introduces additional complications in light-water reactor (LWR) fuel safety evaluations. However, it is not clear how much fuel will be susceptible to HBFF; nor has there been a methodology developed to evaluate fuel susceptibility to HBFF. To that end, this paper proposes an analysis methodology to assess fuel susceptibility to HBFF during LOCA scenarios. The work presented here uses the BISON fuel performance code to evaluate a representative pressurized water reactor fuel rod exposed to a rod average burnup of 75 GWd/MTU. Sensitivity studies investigated the impact of the peak cladding temperature, transient fission gas released, and pre-transient fission gas release on cladding ballooning and burst timing. Subsequently, a methodology to assess fuel susceptibility to HBFF will be developed based on experimental data published in the open literature. The methodology will then be demonstrated by calculating the mass of fuel susceptibility to HBFF. The BISON results conclude that increasing peak cladding temperature drastically decreased time to failure, and decreased balloon size both of which have been confirmed experimentally. Additionally, the effect of pre-transient and transient fission gas release affected cladding balloon size and burst timing. Lastly, fuel susceptibility to HBFF significantly decreased as a function of peak cladding temperature.

中文翻译：

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在冷却剂瞬态损失下评估高燃耗燃料对粉化的敏感性的方法的开发和演示

摘要 出于经济原因，美国核工业正在重新努力建立技术基础，以将棒平均燃耗限制扩展到目前 62 GWd/MTU 的监管燃耗限制之上。主要驱动因素是将压水反应堆循环长度增加到 24 个月，减少新燃料组件的数量和堆芯设计限制，从而提高堆芯能源的利用效率。然而，在高燃耗 (>90 GWd/MTU) 哈尔登冷却剂损失事故 (LOCA) 整体测试系列中观察到燃料芯块破碎和粉化，称为高燃耗燃料破碎 (HBFF)。当美国核管理委员会 (NRC) 于 2011 年初在 Studsvik 核电站赞助的堆芯外整体测试期间观察到燃料碎片和粉化更接近当前的美国监管限制时，该问题引起了人们的关注。 这导致 NRC 担心潜在的与燃料芯块粉碎相关的燃料和堆芯设计的变化。在给 NRC 专员的一封信中，工作人员明确指出需要“……定义关键燃料设计和运行参数的安全运行边界”，并指出“工作人员面临着评估未来燃料设计进步和燃料的可接受性的挑战。利用率发生变化。” 因此，可以得出结论，HBFF 和可能扩散到反应堆冷却剂系统中会给轻水反应堆 (LWR) 燃料安全评估带来额外的复杂性。然而，目前尚不清楚有多少燃料会受到 HBFF 的影响；也没有开发出一种方法来评估燃料对 HBFF 的敏感性。为此，本文提出了一种分析方法来评估 LOCA 情景中燃料对 HBFF 的敏感性。此处介绍的工作使用 BISON 燃料性能代码来评估暴露于 75 GWd/MTU 的棒平均燃耗的代表性压水反应堆燃料棒。敏感性研究调查了峰值包壳温度、释放的瞬态裂变气体和瞬态裂变气体释放对包壳膨胀和爆发时间的影响。随后，将根据公开文献中公布的实验数据开发一种评估燃料对 HBFF 敏感性的方法。然后将通过计算燃料对 HBFF 的敏感性来证明该方法。BISON 结果得出结论，提高峰值包层温度会显着减少失效时间，并减小球囊尺寸，这两者都已通过实验得到证实。此外，瞬态前和瞬态裂变气体释放的影响影响包层气球尺寸和爆发时间。最后，燃料对 HBFF 的敏感性作为峰值包层温度的函数显着降低。