Flow Effect on Core Burnup in a Molten Salt Reactor

Abstract

The molten salt reactor (MSR) adopts the fissile materials dissolved in fluoride salt as fuel. In the core, nuclear fission occurs in the fuel salt, while only nuclear decay happens when the fuel salt flows to the outer-loop . It would make the core burnup to perform differently from that in the reactor adopting solid fuel. To simulate the fuel salt circulates through the core and outer-loop, an auxiliary code based on three-dimensional (3-D) diffusion code CITATION is developed. In the core burnup calculation, accurately evaluating nuclide density of 135Xe is important because of the large neutron absorption cross section. 135I is the main source of 135Xe, which should also be precisely assessed. In this paper, the flow effect on nuclide density of 135I and 135Xe is investigated at various flow speeds, core to outer-loop volume ratios, as well as core power scales. The result shows that the nuclide density of 135I decreases significantly when the fuel salt flow is considered due to nuclide decay of 135I in the outer-loop. At low core power scale, the flow effect on nuclide density decrease of 135Xe is apparent, which results in hundreds of pcm increase of the core reactivity, because the nuclide decay of 135Xe in the outer-loop is considerable comparing with the 135Xe generation in the outer-loop resulting from 135I nuclide decay. It is mitigated as the core power scale is increased which increases the nuclide density of 135I and the nuclide generation of 135Xe in the outer-loop, which decreases the effect of 135Xe nuclide decay in the outer-loop on the 135Xe nuclide density.