DSMC analysis of a Loss-of-Coolant Accident from a He-cooled blanket in the EU DEMO Vacuum Vessel

Safety is a crucial aspect to deal with when designing a nuclear reactor. Among all of the possible accidental transients considered in the section Design-Basis Accidents (DBAs) of the Probabilistic Risk Assessments (PRAs), the in-vessel Loss of Coolant Accident (LOCA) is definitely one of the most critical, because of the significant damages caused to the components inside the Vacuum Vessel (VV). In this field proper system-level codes already exist. Due to their intrinsic lumped nature, these tools show good performances when studying the behavior of the entire system, but they are completely incapable of predicting localized quantities that are instead fundamental to assess the integrity of the VV in the most conservative way as possible. As a consequence, it is necessary to take into account 2D or 3D transient models. The main objective of this thesis is to critically analyze the helium flow inside the VV following a LOCA in the helium cooled blanket of the EU DEMO by means of a 2D Direct Simulation Monte Carlo (DSMC) transient analysis of the hypersonic flow (Ma >> 1) developed during the accidental scenario. In the initial phase of the transient, the expanding gas is highly rarefied, meaning that the continuum assumption is not applicable at the first instants of transitory after the accident occurrence. As a consequence, conventional Computational Fluid Dynamics (CFD) codes cannot be applied in the first phases of the transient. In order to correctly simulate the physics of the problem, a kinetic model represents the best option. The reference parameter in this field is the Knudsen number Kn, a dimensionless value defined as the ratio between the mean free path of a particle and a characteristic length of the system (in this case assumed as the distance between the inlet and the wall in front of it). Because of its good performances in the evaluation of the solution of the kinetic Boltzmann equation for mediums into a wide range of values of Kn number, a successful fluid simulation tool in this field is represented by OpenFOAM (Open-source Field Operation And Manipulation). In this thesis the transient evolution in the VV (initially at high vacuum conditions, 1 mPa) of helium entering at a pressure of 1 kPa and at a temperature of 683.15 K is carried out until the level of collisionalities is such that usual CFD models can be applied. An underexpanded jet grows inside the plasma chamber during the accident. Because of the significant computational effort into performing a DSMC analysis for the entire VV domain, the simulation was carried out until 160 μs. At that time the solution was interpolated into a smaller domain (whose height is approximately 1/3 of the original) and whose inlet was shifted right after the one of the first simulation, in order to neglect those regions where the Kn number had already reached values in the continuum range. The front evolution was monitored by means of progressive manual mesh refinements and mappings of the solution when switching from a mesh to a new one. The cell values were defined in accordance with the pressure and temperature magnitudes measured in each specific region of the domain. It was estimated into approximately 1 ms the contact between the helium entering in the VV and the wall facing the front of the jet. The transient is carried out up to 20 ms, time step required to approach steady-state conditions for the flow completely filling the domain of interest.