We consider a hypothetical scenario of reactivity initiated accident in a nuclear power plant. The violence of the so-called fuel-coolant interaction phenomena, that we arbitrarily assume to occur during the accident, depends strongly on the flow rate of particles out of the gas pressurized rod in which they were initially confined.
The aim of this thesis was to study how this discharge rate is driven by the internal geometry and the pressurized gas. Fuel particles were experimentally simulated by a dense granular material discharging out of a confined and pressurized silo. The controlled parameters were the particle size, density and shape, the outlet size and the gas pressure, whereas the granular and gas flow rates and the pressure along the silo are measured. Discrete and continuous numerical simulations were performed on similar configurations. We focused firstly on the discharge of a rectangular silo of thickness W, with a lateral outlet of size D and an inclined bottom with a parametric angle. For a small inclination angle, the granular flow orientation depends on the aspect ratio D/W due to the wall friction, whereas a large inclination angle fully determines this orientation. These results were successfully reproduced by numerical simulations. Secondly, we focused on two configurations with pressurized gas: a case with constant gas overpressure at the top of the silo and a more transient case for which an initial larger overpressure initiates the rupture of an orifice. The granular flow rate increases significantly with the gas flow, especially for the finer particles and the large overpressures. In both cases, the flow rate scales with a modified Beverloo law where the gas pressure gradient near the outlet acts as an additional driving force. The pressure gradient is well described by a Forchheimer resistance law through the granular medium.
We therefore propose a quasi-steady model for the transient description of the granular flow rate. The two configurations were successfully reproduced by numerical simulations based on a continuum multiphase model. For the larger flow rates, instabilities of the granular jet were found to be initiated by pressure oscillations in the outlet region. The presence of water surrounding the silo only acts through an additional hydrostatic pressure effect.