Nusselt correlation development in unsteady laminar gas flows for CABRI multiphysic simulations with CATHARE2

Abstract

CABRI is an experimental pulse reactor, which aims at studying and thus at better understanding RIA (Reactivity Initiated Accident) effects on nuclear fuels. The restart of the CABRI reactor in 2015 offers the opportunity to validate tools involving multiphysic calculation schemes on reactivity insertions (RI) transients at a system scale. The reactivity insertion in CABRI is mastered by the depressurization of a neutron absorber (3He), contained into transient rods. This enables the realization of various types of transients. As this depressurization highly influences the reactivity insertion kinetics, it is essential to accurately simulate the 3He density evolution which depends on the heat exchanges inside the circuit. The challenge is thus to manage the simulation of the various CABRI complex transients. Previous work allowed to model with CATHARE2 the whole reactor with its specific phenomena as well as phenomena generally involved in RIA, for instance strong neutronic feedback effects, effect of the pellet-clad interaction on the gap conductance and transient heat exchanges between fuel rods and water. These studies also revealed that some CABRI transients were still difficult to model with CATHARE2 and this paper shows the necessity to catch the transient heat exchanges in the transients rods for the simulation of the power pulse in CABRI reactor. In order to achieve this objective, a correlation for the Nusselt evolution in an unsteady compressible laminar 3He flow, inside a closed tube, has been realized from numerical resolution of an analytical model and implemented in the scientific calculation tool CATHARE2. This paper describes this analytical model and its numerical resolution. After having been validated on results of the literature in steady flow inside open tube, the correlation established for the evolution of the Nusselt in a transient rods’ tube is given. Finally, Best-Estimate simulation results of CATHARE2 are compared with experimental data obtained on CABRI commission tests: core power and 3He pressure evolution.