Neutronic and thermal-hydraulic simulations for Molten Salt Fast Reactor safety assessment

Molten Salt Fast Reactor (MSFR) is an innovative and challenging reactor concept in the frame of Generation IV International Forum. The unique and distinctive feature of MSFRs is the liquid state of their fuels, which call for new safety systems that should be studied, as the Emergency Draining System. Molten salt fuel draining is a research activity in the frame of SAMOFAR project. Thesis objective is to provide an accurate description of core dynamics during a fuel draining. In case of emergency, the salt could be drained out from the core, actively or passively triggered by melting of salt plugs, and stored into a draining tank underneath the core. During the draining transient, the fuel should evacuate the thermal power due to decay heat to avoid mechanical damages to core internal surfaces and to EDS structure. In addition, subcriticality of the fuel should be granted during all the draining transient. The aim of this thesis is the development of modelling tools for investigating and assessing the temperature and reactivity variation during draining transients in MSFR. A simplified multiphysics analysis of the molten salt draining is firstly proposed, consisting in a 0-D semi-analytical model, able to capture the coupling among salt fluid-dynamics, system energy and neutronics. Temperature and system reactivity time evolutions are described and the general dynamics of the draining phenomenon is figured out. Particular emphasis is given to develop reactivity coefficients related to temperature (Doppler effects) and to volume change (increase in neutron leakages). From the 0-D model results, it is deduced that the problem is intrinsically related to spatial features of the system, i.e. the location of the draining shaft that affects the outflow salt enthalpy. Therefore, a multi-dimensional model is required to fully characterized the salt draining. The second part of the thesis focuses hence on developing a preliminary 2-D axial-symmetric CFD-based numerical model. Salt fluid-dynamics are modelled with RANS equations along with realizable k-e turbulence model, while energy balance is described with a local temperature partial differential equation. As far as neutronics is concerned, monoenergetic neutron diffusion and precursors' balances are included in the model. Finally, the computational model executable is implemented in the OpenFOAM software. CFD simulation results show that the highest temperature local hot spot never overcomes the critical value to bring damages to internal wall structures and, furthermore, confirm the intrinsically stability of molten salt reactors, since the supercritical phase ends within 1 second from the transient onset and consequently subcritical conditions are established. Ultimately, during the drainage, thermal damages could occur because of fuel salt flowing along the draining shaft. Fluids flowing in channels and featuring an internal production of heat have significantly different heat transfer mechanisms with respect to conventional thermal-hydraulic streams. Therefore the third part of the thesis provides a CFD-based model in order to obtain a correlation for the Heat Transfer Coefficient to predict the real wall-bulk temperature difference and indeed the thermal load on tube surfaces (Fiorina et al., 2013). CFD-based correlation is finally used to study the thermal-hydraulics of the draining shaft in the frame of a fuel draining scenario, demonstrating no worrying thermal loads on shaft walls