Utvärdering av bränsledesignens prestanda : Med avseende på bränslestavens effektbegränsning

Sammanfattning

This study investigates the possibility of optimizing and improving fuel design by identifying the most limiting fuel rod at an early stage of the design process. This is of particular importance since the limiting rod, which is the first to approach the thermal power limit (specifically 380 W/cm inthis study), can have a decisive impact on the entire fuel assembly's lifetime and performance. When a single rod approaches or risks exceeding these safety limits, it means that the fuel assembly cannot be operated at such high power, despite the fact that the majority of the rods still operate well within their design parameters and have significant remaining capacity. This scenario clearly illustrates how a single fuel rod can function as a limiting factor or bottleneck in the system, which in turn directly affects the total operating time and thus the overall efficiency of fuel utilization. By identifying and addressing these potential limitations early in the design phase, opportunities are created to significantly improve the fuel assembly's total performance and lifetime. By using the simulation program SIMULATE5 and the training reactor Misterhult, an analysis tool was developed in MATLAB to systematically identify and map the limiting rods in the fuel assemblies. To evaluate the tool's effectiveness and validate its potential, an analysis was conducted between two case studies: the first based on the original baseline design, and the second on a modified configuration where the enrichment level in the identified most limiting rod was reduced from 4.8% to 4.4%. The purpose of this specific enrichment adjustment was to investigate whether such a relatively limited modification could result in a significant reduction of the local power peak, thereby ensuring that the entire fuel assembly consistently operates within the established safety margin (FLPD, Fraction of Limiting Power Density, kept below the critical value of 0.95). The results of the study showed clear and significant differences between the two cases: in the original configuration, the safety margin was exceeded in several fuel assemblies, which posed potential risks for both operational safety and fuel economy in the long term. In contrast, the modified design with reduced enrichment level showed significantly better performance, where all elements consistently remained within the established safety margins. The observed impact on reactivity was remarkably small, only about 2 pcm, which in practice would result in a marginal shortening of the operational cycle by barely two full-power hours. A particularly interesting observation was that the specific rod position that caused the limiting effect displayed a consistent pattern and was found in the same location in all analyzed elements. This recurring pattern represents a valuable insight that opens up significant opportunities to develop more systematic and generally applicable guidelines for optimal rod placement and enrichment distribution alreadyin the initial design phase. The study clearly demonstrates that the fuel's thermal performance can be significantly improved through strategic and well-planned adjustments of the enrichment level, without requiring extensive modifications to the existing core layout. This insight is particularly valuable as it enables cost-effective and time saving optimizations in future design work. The tool developed within the framework of this study has proven to be a valuable aid that can be used for systematic optimization of both fuel and core design in various educational and development environments. The tool also demonstrates significant potential for development and improvement, where a particularly promising development opportunity would be to integrate comprehensive dryout analyses to further refine and ensure the fuel's thermal performance under various operating conditions.