Steam Generator Tube Rupture accident scenario in Lead Fast Reactor: assessment of numerical modelling for shock wave generation and propagation.

In Lead-cooled Fast Reactors (LFRs), a type of Gen-IV nuclear reactor, the limited chemical reactivity between water and lead allows for innovative designs in which the primary circuit, comprising of high-temperature molten lead pool, and the secondary circuit, containing the steam generator with high-pressure subcooled water, are placed in direct contact. Although this configuration offers several technological and thermodynamic advantages, it also introduces a new potential accident scenario, known as Steam Generator Tube Rupture (SGTR). An SGTR occurs when a tube in the secondary circuit breaks within the molten lead pool. The consequences of this accident have not yet been fully characterized in the scientific literature. However, it is already known that the initial effect of such an event is the propagation of a shock wave through the molten lead pool, caused by the significant pressure difference between the two fluids in contact. Since the management of all possible accident scenarios is required for the safety evaluations of nuclear reactor designs, the SGTR event must be thoroughly assessed in the coming years. In the absence of experimental data, numerical simulations are currently the most effective tool for supporting safety analysis. This study implements and applies two models available in the literature to reconstruct and analyse the evolution of the flow field in an SGTR scenario. The first model is a 3D shock propagation model involving two distinct compressible fluids, that is, a water-vapour mixture at saturated conditions and molten lead. Spherical symmetry is assumed. The governing Euler-type equations are the same for both fluids; the equations of state, however, are specific to each fluid. These equations are discretised using the finite volume method, and an Osher solver is adopted to calculate interface fluxes and perform time integration. The level-set method is employed to track the motion of the water-lead interface over time, and a ghost fluid method is implemented to address multi-material interactions during time integration. The second model is presented in the literature as a simplified version of the first. Since the involved velocities in the lead are largely subsonic, the lead is modelled as an incompressible fluid. Additionally, the physical properties of the water are described as volume-averaged. These assumptions result in a simplified model that is useful for describing the evolution of the water–lead interface over time. The entire model is derived as a system of ordinary differential equations (ODEs), which describes the evolution of the water-lead interface in terms of its radial coordinate, velocity, and pressure. Then, by exploiting the continuity of pressure and velocity across the interface, the physical quantities of the lead (radial velocity and pressure) can also be obtained. A comparison is provided between the results from the literature and both model outcomes, in order to verify the correctness of the calculations. This is also necessary because the mathematical methods adopted differ from those in the reference papers. Thorough analyses are then performed and described.