Studies of liquid metal boiling in fuel assemblies of fast reactors in accident conditions

Abstract

The paper presents the results of experiments carried out at IPPE JSC on heat transfer and circulation stability at sodium–potassium alloy boiling on models of single fuel assemblies and in a system of parallel fuel assemblies, taking into account the influence of various factors on the boiling process at coolant natural convection.The obtained experimental data have shown that the stable nucleate boiling in the simulated fuel assemblies is observed only in a restricted region of heat fluxes, its transition to unstable pulsating slug boiling is determined by various factors, the transition boundary from nucleate boiling to slug, annular-dispersed and dispersed regimes of two-phase liquid metal flow in fuel rod assemblies are approximated by simple dependences.The onset of an oscillation process at coolant boiling in one of the parallel fuel assemblies leads to an antiphase oscillation process in another fuel assembly, the hydrodynamic interaction of the loops in various fuel assemblies may cause a significant increase in the amplitude of coolant flow rate oscillations (“resonance” of flow rate oscillations) and “choking” or inversion of the coolant flow rate in the loops, the temperature rise in the coolant and fuel rod cladding (the effect of inter-channel instability) and, eventually, a critical heat flux (CHF).In the assembly with low surface roughness of the rod simulators, evolution of an unstable (slug) regime with sharp coolant flow rate oscillations and overheating of the simulator wall results in a CHF, . For the fuel rod simulators with industrially-manufactured surface roughness, due to the appearance of a liquid film on the surface of the simulators, a transition from the unstable slug regime to the stable annular-dispersed one is observed.For sodium boiling in the fuel assembly model with a sodium “cavity“ located above the reactor core, which is designed for compensation of the positive sodium void reactivity effect in case of boiling onset, the feasibility of long-term sodium cooling of fuel rod simulators in the fuel assemblies has been demonstrated for these conditions.The data on liquid metal boiling heat transfer in the assemblies were generalized a cartogram of the flow regimes for a liquid metal two-phase flow in the assemblies was constructed, which significantly differs from a cartogram of the flow regimes for water. The paper presents the results of the calculated and experimental data comparison.