Modeling of the $^3$He Density Evolution Inside the CABRI Transient Rods During Power Transients

Abstract : CABRI is an experimental pulse reactor, funded by the French Nuclear Safety and Radioprotection Institute and operated by CEA at the Cadarache research center. It is designed to study fuel behavior under reactivity-initiated accident conditions. In order to produce the power transients, reactivity is injected by depressurization of a neutron absorber ($^3$He) situated in transient rods inside the reactor core. The shapes of power transients depend on the total amount of reactivity injected and on the injection speed. The injected reactivity can be calculated by conversion of the $^3$He gas density into units of reactivity. So, it is of upmost importance to properly model gas density evolution in transient rods during a power transient. The $^3$He depressurization was studied by computational fluid dynamics (CFD) calculations validated by measurements using pressure transducers. The CFD calculations show that the density evolution is slower than the pressure drop. Studies also show that it is harder to predict the depressurization during the power transients because of neutron/$^3$He capture reactions that induce a gas heating. Surrogate models were built based on CFD calculations and validated against preliminary tests in the CABRI transient system. Two methods were identified to evaluate the gas density evolution: CFD calculations and reverse point kinetics (PKs). The first one consists in adding a heat source in transient rods based on the experimental power conversion. The second one consists in using the measured power by boron ionization chambers to evaluate the net reactivity by a reverse PKs method and to subtract the reactivity feedbacks calculated with the DULCINEE multiphysics code.