Coupled neutronics and computational fluid dynamics for the molten salt fast reactor

The foremost fears in society concerning nuclear energy are reactor safety and long lived nuclear waste. A much safer reactor is needed to regain trust, boost the field o f nuclear physics and to supply the world’s ever increasing demand for clean electric energy. The molten-sa lt re ctor (MSR) has this promise. It is even likely that they can consume more nuclear waste than they produce. The nuclear fuel of MSRs is dissolved in a liquid salt, which h as several advantages. When temperatures exceed predetermined values, a passive safety system allow s for the liquid fuel salt to be drained in to several passively cooled sub-critical storage tanks, so that the MS Rs can be brought to a passive shutdown during which there is no need for the decay heat to be removed by active mean s. The molten salt also allows the gaseous neutron absorber xenon to be quickly removed from the reacto r core, which makes the breeding of uranium from thorium feasible, so that only thorium, and no uranium, has to be added when refuelling MSRs. The thorium fuel cycle has the advantage that the production of l ong iving transuranic waste is reduced by 5 orders of magnitude as compared to when natural or enriched uranium would have been used. One promising MSR is the molten salt fast reactor (MSFR). The MSFR does not have a complex graphite core, and only the salt slightly moderates the neutrons, whi ch results in an epithermal neutron spectrum. Due to the absence of any structural material in the core the flow r eaches very high Reynolds numbers there. For a correct physical description of the neutron flux in the M SFR, the transport of the precursors, within the flowing fuel, must be accounted for. Turbulent mixing aff ects the transport of the precursors, given that the fuel flow through the MSFR’s core is turbulent. In this work a model is presented that physically describes t h mutually dependent neutron flux, turbulent flow, and turbulent transport of heat and the precursors. For the flow the Reynolds averaged Navier-Stokes (RANS) equation, with Boussinesq’s closure hypothesis and the standardk − ǫ model are adopted. The multigroup diffusion equation is used for the neutron flux. The con vection diffusion equation for scalar transport in turbulent flows has been selected for describing the transpo rt of precursors. The resulting system of partial differential equations is s olved by making use of three different programs. The in-house CFD program HEAT was used to solve the RANS equat ion nd thek − ǫ model. The in-house program DALTON-MSR solved for the neutron flux and the precur sor concentration. A third program was used for generating the temperature-dependent neutron group cr oss-sections. The three mutually linked programs form the software package named IMP. Results obtained with IMP show that, when the heat extractio n and flow rate remain constant, the MSFR passively reaches a steady state operation. During steady s t te operation a large recirculation zone is found within the reactor core. As a consequence, unnecessarily hi gh temperatures are reached there, which will corrode the structural material at an unnecessarily high ra te. From results obtained with IMP it is concluded that different precursor concentrations would have been fo u d, had turbulent transport of the precursors been unaccounted for. Transient calculations performed with IM P show that there is a strong negative temperature feedback on the reactivity. For a reactivity introduction o f ∆ρ = 533pcm, the MSFR becomes super prompt critical. At first, the neutron flux quickly increases by 3 ord e s of magnitude, but within 50 milliseconds, the flux decreases to levels lower than it was before the transien t.