STUDY ON THE IMPACTS OF BURN-UP MEASUREMENT ERRORS ON THE DISCHARGE BURN-UP DISTRIBUTION OF PB-HTR

摘要

The burn-up measurement is an essential procedure for the multi-pass refueling strategy of pebble bed high temperature reactors (PB-HTRs), which presents the systematic and random measurement errors. The random feature of entrance position of loaded pebbles at the top of core gives rise to a statistical distribution of discharge burn-up values. In this work, the impact of burn-up measurement errors on the actual distribution of discharge burn-up and further on the neutronics features of equilibrium core is investigated. A Monte Carlo model based on random sampling the entrance position of loaded pebbles and summing the pre-calculated burn-up increment values is presented to analyze the discharge burn-up distribution. Furthermore, an additional random sampling and a definite offset are performed before summing the burn-up increment after one passage through the core, to simulate the random and systematic errors of burn-up measurement. If the burn-up increment values at different radial position are close, the ideal discharge burn-up distribution without burn-up measurement errors behaves like a Gaussian with just one peak. However, the multi-peak mode with several damping Gaussian peaks emerges if the random errors of burn-up measurement is considered. The larger the random uncertainty is, the stronger the side peaks grow. On the other hand, the systematic errors almost only make the burn-up distribution deviate from the original non-systematic-error one. It is revealed that the burn-up measurement errors could give rise to significant impact to the neutronics and safety features of PB-HTRs.