Abstract During the lifetime of fuel assemblies, irradiation and fluid mechanical forces can cause a permanent deformation in the lateral direction that leads to larger interassembly water gaps in the reactor core. The standard reload safety analysis for the reactor core is developed for a uniform distribution of corewise interassembly water gaps. A nonuniform distribution of water gaps with locally larger or smaller water gaps could lead to a significant change in the positions of the hot-spot factors. Thus, such modifications could also impact boundary conditions for safety analysis or boundary conditions of the reactor core surveillance systems. To analyze the impact of a nonuniform water-gap distribution on the safety analysis and the reactor core surveillance systems, TÜV Nord EnSys is developing a new methodology that allows the incorporation of assembly bow effects in core analysis. For this methodology, functions linking the maximal relative power increase in the vicinity of the modified water gap to the fuel properties had to be derived. This was accomplished by simulating for gaps between different fuel types at selected positions in a full-core model of a generic four-loop Siemens/Kraftwerk Union pressurized water reactor using the bow model of the two-group diffusion code SIMULATE-3. The data of the maximal relative power increase were linearly correlated with the spectral indices and the coolant densities of the two gap-adjacent assemblies. Then a function was derived that provides a firsthand approximation of the maximal relative power increase using only the physical properties of the unbowed core configuration. The maximal absolute positive deviation of the function from the simulation results was 2.4%.

中文翻译：

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西门子/KWU-PWR燃料组件弯曲引起功率增加的经验计算

摘要 在燃料组件的寿命期间，辐照和流体机械力会导致横向永久变形，从而导致反应堆堆芯中的组件间水隙变大。反应堆堆芯的标准重装安全分析是针对堆芯间组件间水隙的均匀分布而开发的。局部较大或较小水隙的水隙分布不均匀可能导致热点因子位置的显着变化。因此，这种修改也可能影响安全分析的边界条件或反应堆堆芯监视系统的边界条件。分析非均匀水隙分布对安全分析和反应堆堆芯监视系统的影响，TÜV Nord EnSys 正在开发一种新方法，允许在岩心分析中结合装配弓效应。对于这种方法，必须导出将修改后的水隙附近的最大相对功率增加与燃料特性联系起来的函数。这是通过使用两组扩散代码 SIMULATE-3 的弓形模型模拟通用四回路 Siemens/Kraftwerk Union 压水反应堆全堆芯模型中选定位置的不同燃料类型之间的间隙来实现的。最大相对功率增加的数据与两个间隙相邻组件的光谱指数和冷却剂密度线性相关。然后推导出一个函数，该函数提供最大相对功率增加的第一手近似值，该函数仅使用未弯曲核心配置的物理特性。该函数与模拟结果的最大绝对正偏差为 2.4%。