Abstract The baffle jetting phenomenon, intensified by pressure difference across baffle plates, may cause excessive fuel rod vibrations and possible damage. The downward flow direction through the baffle-barrel region opposite to the upward flow through the core in pressurized water reactors affects increase of pressure gradient. Gaps between the baffle plates widen with reactor operating time and water jets become more pronounced. During the 2015 outage the NPP Krsko has undergone the upflow conversion (UFC) modification in order to minimize baffle jetting and prevent fuel rod failure. The modification consisted in altering reactor vessel internals in such a way that the coolant downflow path in the baffle-barrel region was converted to an upflow path. The impact of the UFC modification on the nuclear power plant behaviour was analyzed in the paper. The double ended guillotine break of one direct vessel injection (DVI) line was selected as a representative scenario because the DVI line break is exhibiting the highest peak cladding temperature among the small break LOCA cases. The thermal-hydraulic analysis was carried out with the RELAP5/Mod3.3 code for core average channel conditions, while additional FRAPTRAN runs were performed to evaluate peak cladding temperatures for high power and high burnup fuel assemblies. Distribution of pressure and velocity fields in the reactor pressure vessel baffle-barrel region and change in bypass flow rates due to modification were determined by the ANSYS-Fluent calculation. The accident analysis has been performed for both the downflow configuration (pre-UFC) and the upflow configuration (post-UFC) to assess the influence of the modification. Additional cases with different reactor coolant pump trip times, higher core decay heat and reduced capacity of safety systems were also analyzed to evaluate the impact on reactor safety.

中文翻译：

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某压水堆厂升流转换改造及其对选定LOCA事故的影响分析

摘要 挡板喷射现象由于挡板之间的压差而加剧，可能导致燃料棒过度振动并可能造成损坏。在压水反应堆中，通过隔板筒区的向下流动方向与通过堆芯的向上流动相反，影响压力梯度的增加。挡板之间的间隙随着反应器运行时间而变宽，并且水射流变得更加明显。在 2015 年停运期间，NPP Krsko 进行了上流式转换 (UFC) 改造，以最大程度地减少挡板喷射并防止燃料棒故障。修改包括以这样的方式改变反应堆容器内部构件，即挡板-筒体区域中的冷却剂向下流动路径被转换为向上流动路径。论文分析了UFC改造对核电站行为的影响。选择一条直接血管注入 (DVI) 线的双端断头台断裂作为代表性场景，因为 DVI 线断裂在小断裂 LOCA 案例中表现出最高的峰值包层温度。使用 RELAP5/Mod3.3 代码对核心平均通道条件进行热工水力分析，同时执行额外的 FRAPTRAN 运行以评估高功率和高燃耗燃料组件的峰值包壳温度。通过 ANSYS-Fluent 计算确定反应堆压力容器挡板-筒体区域中压力和速度场的分布以及由于修改引起的旁通流速变化。对下流配置（UFC 前）和上流配置（UFC 后）都进行了事故分析，以评估修改的影响。还分析了具有不同反应堆冷却剂泵跳闸时间、更高的堆芯衰变热和安全系统容量降低的其他情况，以评估对反应堆安全的影响。