Multiphysics analysis of the MSFR helium bubbling system: A comparison between neutron diffusion, SP3 neutron transport and Monte Carlo approaches

The Molten Salt Fast Reactor is a fast-spectrum molten salt reactor under development in the framework

of the European H2020 SAMOFAR Project (http://samofar.eu/). Among the design peculiarities, this circulating

fuel reactor features a helium bubbling system aimed at removing on-line gaseous fission products,

and metallic particles as well. From a modelling point of view, the presence of helium bubbles in the core

needs to be assessed both from a neutronics and thermal-hydraulics point of view. In this paper, the

attention is paid to the first aspect, analysing the void reactivity effect induced by the bubbles by means

of a Monte Carlo, an SP3 neutron transport and a neutron diffusion approach. Since the distribution of

helium bubbles is not uniform in the core but strongly depends on the fuel salt flow conditions, as well

as on the location of their injection and extraction, a coupling scheme between Serpent 2 and OpenFOAM

codes is adopted for the Monte Carlo analysis. In this way, the Monte Carlo code Serpent 2 is provided

with a real bubble distribution calculated by means of a two-phase Euler-Euler solver implemented in

OpenFOAM. The outcome puts in evidence the difference arising when a uniform and a real distribution

of bubbles is considered in a Monte Carlo calculation. These results are also compared with an SP3 neutron

transport and a neutron diffusion solver implemented in OpenFOAM, highlighting the difference

among the three neutronics approaches herein adopted.