Modeling of Heterogeneous Nucleation from Surface Nuclei and Application to the Evaluation of the Void Ratio in a Molten Salt Reactor

Abstract

In molten salt reactors (MSRs), the nuclear fuel is dissolved in the liquid salt coolant. This design allows prolonged exposure of the fuel to the neutron flux, facilitating the transmutation of minor actinides. Thus, MSR research is being conducted with the goal of reducing the radioactive toxicity of nuclear waste. In this design, the gaseous fission products pose a challenge because they are poorly soluble and tend to precipitate as bubbles. These bubbles affect neutronic reactivity and hence reactor stability.

In particular, the heterogeneous nucleation of gas bubbles on the vessel walls is studied. When liquid salt is poured into the vessel, small amounts of filler gas, called nuclei, can be trapped in the cracks in the walls. The dissolved gaseous fission products can diffuse to such nuclei, leading to their growth and eventually to the detachment of bubbles. These detached bubbles migrate in the salt where they can remain for tens of seconds, modifying the void ratio and thus the reactivity in the core. Our aim is to evaluate the effect of this heterogeneous nucleation on the void ratio.

A theory of the nucleation process is developed, including mass transfer in a turbulent flow, the kinetics of gas exchange with the bubbles, and the forces triggering detachment, in order to determine the frequency of bubble production. The transport of the detached bubbles in the salt and their behavior in the reactor are modeled to finally give an estimate of the void ratio due to these bubbles.