Dimensional Effects in the Modelling of MSR Dynamics: Moving on from Simplified Schemes of Analysis to a Multi-Physics Modelling Approach

The MSR (Molten Salt Reactor) is one of the six innovative concepts of nuclear reactors envisaged by the GIF-IV (Generation IV International Forum) initiative for the long term evolution of the nuclear technology, in the direction of a more sustainable, safe, proliferation resistant, and economic power generation. The MSR is characterised by a complex and highly non-linear behaviour, which requires a careful investigation, as a consequence of some unusual features like the presence of a fluid fuel and the drift of delayed neutron precursors (DNP) along the primary circuit. In this paper, the MSR primary circuit dynamics is analysed with reference to the MSRE (Molten Salt Reactor Experiment), due to the availability of both a detailed design and experimental data. Numerical models featured by increasing complexity are presented. In particular, a zero-dimensional model is developed, and two other models introducing a one-dimensional discretization for the DNP drift and/or the heat convection are also elaborated. These simplified models are then compared with a more complex model, obtained according to an innovative Multi-Physics Modelling (MPM) approach to the dynamic analysis, where the partial differential equations governing the different phenomena in a core channel are solved in a two-dimensional domain, within the same computational environment. A one-dimensional closure of the primary circuit is also provided. The MPM approach gives a unique insight into the influence of local effects on the overall dynamic behaviour of the reactor, while the variety of developed models allows a systematic investigation about the dimensional effects in the modelling of MSRs. This work represents a starting point in the set-up of a Multi-Physics (MP) simulation tool, suitable for calculations with different degrees of accuracy and physical complexity, and paves the way towards the development of MP models capable of a point-by-point coupling of all the phenomena characterising the MSRs, and the nuclear reactors in general.