Definition and Analysis of an Experimental Benchmark on Shutdown Rod Worths in LEU-HTR Configurations

Abstract Although high-temperature reactors (HTRs) are endowed with a number of inherent safety features, there are still aspects of the design that need particular attention. For concepts in which shutdown rods are situated outside the core region, as is the case in contemporary modular pebble bed designs, accurate calculations are needed for the worth of these shutdown rods not only in normal operation but also under accident conditions in which significant changes occur, for instance, due to inadvertant moderation increase in the core (ingress of water or other hydrogeneous compound). Corresponding validation experiments, employing a variety of reactivity measurement techniques, were conducted in the framework of the HTR-PROTEUS program employing low-enriched uranium pebble-type fuel. Details of the experimental configurations, along with the measurement results obtained, are given for two different HTR-PROTEUS cores, in each of which four different shutdown rod combinations were investigated. Comparisons made with calculations, based on both approximative deterministic models and geometrically “near-to-exact” Monte Carlo analyses, have clearly brought out the sensitivity of the experimental results to calculational correction factors when conventional (thermal) techniques are used for reactivity measurements in such systems. Considerably greater systematic accuracies are reflected in the experimental shutdown rod values obtained using specially developed epithermal techniques, and it is these results that are recommended for benchmarking purposes.