Some numerical achievements on Na boiling dynamics and next technical route

Abstract

The paper proposes an overview of an R&D program for Gen4 Sodium Fast Reactors that was launched in 2012 at CEA to investigate Na boiling dynamics under an Unprotected Loss of Flow scenario. Part of the program motivation was an incident periodic two-phase flow pattern (limit cycle) along the transient that was calculated at first. If proven, such a stable phenomenology could indeed attractively improve reactor safety by providing a time-window for recovering pumping power. The paper is restricted to the thermal–hydraulic side of the R&D initiative that targeted to credit prediction of the boiling flow dynamics. First, the 1-D numerical achievements obtained by simulating with CATHARE3 thermal hydraulic system code, some out-of-pile programs from the 70–80 s, are detailed. The latter were indeed already very informative on two-phase flow stability scenarios, by so offering a first sound qualification basis. Related benefits allowed by a recent air–water experimental program that supported some modifications of wall and interfacial closure laws, are pointed out. The second part of the paper outlooks the next technical route and its early advances. Part of it, is 3-D simulation by subchannel code TrioMC which first application shows its added-value to perform a detailed analysis of a scaled experiment. Predictive capabilities of 3-D approach will be also central to credit a full-scale transposition. Progress on flow stability analysis, including subcooled condensation as well as connected channel reflooding and pressure wave aspects, could be as such expected from recent developments on CFD. Indeed, advances for an all flow regimes representation opportunely suit Na boiling physics. Bifurcation and stability analysis with the developed BACCARAT model is finally identified as providing a tailored mathematical approach to connect the periodic flow pattern with a Hopf bifurcation. Its ability to provide a mechanistic view of the scenarios shift on a natural convection test-case, is reported. On the experimental side, the scope of a new program planed with IPPE on a wire-spaced 19 pins bundle, is briefly introduced. HARIBO program suits some Gen4 hydraulic design specifics and targets CATHARE3 and TrioMC qualification.