The Molten Salt Reactor (MSR) is one of the six Generation IV systems capable of breeding and transmutation of actinides and long-lived fission products, which uses the liquid molten salt as the fuel solvent, coolant and heat generation simultaneously. The MSR neutronics, such as the distribution of the delay neutron precursors (DNP), is significantly influenced by the fluid flow, which is quite different from the conventional reactors. Therefore, it is very important to do some research on MSR, especially in accident conditions. The present paper studies the natural convection through which the heat generated by the fuel is removed out of the core region (simply a square cavity in this paper). The neutronic theoretical model is founded based on the conservation law, which consists of two-group neutron diffusion equation for the fast and thermal neutron fluxes and that for one-group DNP, in which the convection terms are included to reflect the fuel salt flow. The SIMPLER numerical method was used to calculate the natural convection heat transfer to the molten salt inside a closed cavity for which the boundary temperature was spatially uniform. The equations were discretized by finite volume method based on collocated grids, in which QUICK defect correction was adopted for the convection terms and the central difference was for the diffusion terms. The discretization equations were calculated by ADI (Alternative Direction Implicit) with block-correction technique. The distributions of the dimensionless temperature, the dimensionless velocity, the fluxes and the DNP in the cavity were obtained. The calculated results showed that: a) the distribution of the DNP was correlated both with that of the fluxes and with the fuel salt flow and when Rayleigh number increased, the latter one was of much more importance; b) the distribution of the local Nusselt number varied with different Rayleigh numbers; c) the distribution of the dimensionless velocity and the dimensionless temperature were also closely related to Rayleigh number; d) the maximum dimensionless temperature decreased as Rayleigh number increases.

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