Analysis of the CABRI power transients -Prediction improvements using a combination of measurements and calculation

Abstract

International audienceCABRI is an experimental pulse reactor, fundedby the French Nuclear Safety and Radioprotection Institute (IRSN) and operated by CEA at the Cadarache research center. It is designed to study fuel behavior under RIA (Reactivity Initiated Accident) conditions. In order to produce the power transients, reactivity is injected by depressurization of a neutron absorber (3He) situated in the so-called transient rods Inside the reactor core. The CABRI reactivity injection system allows us to generate structured transients based on specific sequences of depressurization. For such transients, the time difference between the openings of two valves of the reactivity injection system has an important impact on the shape of the power pulses. A kinetic point code SPARTE was created in order to replace the DULCINEE code dedicated to the modeling and prediction of CABRI power transients. The new code includes a new model of 3He depressurization based on CFD calculations, a model of variable Doppler coefficient based on Monte Carlo calculations and variable axial neutron flux profile. The density model and Doppler model have a big impact on power transients prediction. However uncertainties remain in calculations. For low initial pressure transients, the major uncertainty comes from the reactivity injected by the 3He depressurization. For high initial pressure transients, the 3He heating during the power pulse (TOP effect) is responsible of an additional injection of reactivity that needs to be modeled precisely