Estimates of noble metal particle growth in a molten salt reactor

Within the H2020-Euratom project SAMOSAFER, research is currently ongoing into the Molten Salt Fast Reactor (MSFR) concept in which nuclear fuel is dissolved in a liquid salt. A major challenge in the MSFR is the online and offline treatment of salt in order to remove a plethora of fission products. Noble metals are a type of such fission products, the atoms of which are thought to coagulate into particles as a result of diffusion. Due to their low solubility, noble metal particles will deposit onto interfaces such as solid walls but also on bubbles, as was observed in the Molten Salt Reactor Experiment (MSRE) at Oak Ridge national laboratory. The relatively large interfacial area generated by bubbles makes online or offline bubbling, therefore, an interesting option to extract noble metals from the MSFR and to limit wall deposition. However, to understand the performance of the bubbling process, particle sizes must be known. Particle size distributions arise from a complex interplay of atomic formation by fission and decay, growth by coagulation and removal by interfacial deposition. In this paper, theory is developed to understand these mechanisms on a semi-analytical level, in order to provide estimates of noble metal particle growth in the MSFR. The governing partial differential equation is reduced to a set of ordinary differential equations using the method of moments, revealing the dynamics of the underlying physics. We show that large noble metal particles, up to the micrometer scale, can arise in the reactor, but only at very long operational times. Moreover, we show that the previously assumed 'cycle time' of noble metal particle removal of 30 s is only feasible at very high bubbling void fractions. The theory developed in this work contributes significantly to a qualitative understanding of the behavior of noble metals in an MSR. The theory can potentially help to explain certain phenomena observed in the MSRE and can assist in the future design of safe and reliable MSRs.