A statistically-engineered approach for assessing ageing effects on thermal-hydraulic elements for CANDU reactors

摘要

The purpose of this study was to develop a valid method to assess ageing effects on thermal-hydraulic elements for CANDU reactors. This method consisted of six steps. The ageing elements used in this method allowed for the immediate consideration of the code input without adjusting preexisting NPP simulation codes, and it also comprehensively considered the change in NPP's thermal-hydraulic elements due to ageing effects. Each ageing element was selected from among the many thermal-hydraulic factors in which each element would have the greatest effect on the thermal-hydraulic conditions determined by the analysis of the ageing effect on the reactor. In this process, sensitivity analysis for each ageing element was done to understand the effects of each ageing element on the thermal-hydraulic conditions and peak cladding temperature. In addition, a degradation model capable of anticipating the values of the ageing elements over time was developed based on statistical interpretation methods and measured data, and the results' conservativeness was guaranteed by conservatively selecting optimized combinations of ageing elements and their effects. The inherent uncertainty found in the complex nature of ageing for thermal-hydraulic elements can be reduced by being very conservative. Thus, the concept of the safety margin was introduced to propose a criterion for the assessment of ageing effects on thermal-hydraulic elements in NPPs. In addition, a preliminary analysis of Wolsong Unit 2 has been done. The results show that the 3rd highest value of PCT during LBLOCA was higher than that of the baseline with a value of 21.4 K. Thus, the ageing effect which is not taken into consideration in existing accident analysis evaluation methods was evaluated in this study. Moreover, it was found that meaningful differences may occur from the consideration of the safety analysis of NPP accidents. Accordingly, this method could synthetically assess the ageing effects on thermal-hydraulic elements in CANDU reactors, and this is expected to make considerable contributions to secure reliable safety margins for NPPs.