Preliminary analysis of randomized configuration patterns in modified STACY core

In collaboration with the Japan Atomic Energy Agency (JAEA), the Nuclear Regulation Authority (NRA) has taken an experimental approach to determining the criticality of pseudo-fuel debris since 2014. In addition, JAEA continues to modify the STAtic experiment Critical facility (STACY) and prepares for critical experiments that simulate pseudo-fuel debris. In a critical experiment plan using the modified STACY, randomized core configuration patterns composed of fuel and structure rods, etc. were proposed Various rod patterns were considered in the critical experiments; however, the experimental patterns should be narrowed down as reasonable. For this reason, to evaluate the impact of rod patterns on the criticality characteristic etc., hundreds of randomized configuration patterns with two types of spacer grid plates (1.27 and 1.50 cm pitches) assembled in the modified STACY core were generated using the Fisher-Yates shuffling algorithm, the criticality characteristics of which were then evaluated using Tools for Sensitivity and Uncertainty Analysis Methodology Implementation in three dimensional systems (TSUNAMI-3D). In addition, pseudo-fuel debris models that consider the volume ratio of pseudo-fuel debris to moderation (V_m/V_f) were built and the sensitivity coefficient-based similarity index (G) between the patterns and pseudo-fuel debris models were then calculated using TSUNAMI-Indices and Parameters (TSUNAMI-IP). As a result, the impact of the rod patterns on reactivities was within 500 pcm. Especially G values in V_m/V_f > 2 exceeded 0.80 for all the patterns with the 1.50 cm-spacer grid plate. The impact of rod patterns to G values in the pseudo-fuel debris models was extremely small. (author)