Severe accidents arising from the fusion of a nuclear reactor core must be anticipated to enhance the efficiency of their mitigation. Such accidents have occurred at TMI-2 (1979) and Fukushima (2011). Following a loss of coolant accident, core heating and oxidation of the fuel cladding followed by reflooding (injection of water) may lead to the collapse of fuel rods and formation of porous debris bed in the core. Steam produced upon reflooding may activate the exothermic oxidation of Zircaloy leading to partial melting of materials. Such evolution generates zones with reduced porosity limiting coolant penetration and/or impermeable blocked zones. In this situation, the efficiency of injecting water into the core to stop the progress of degradation and prevent the reactor core melting may be significantly reduced.
In this scope, IRSN launched PEARL program to investigate the thermal hydraulics of reflooding of hot debris beds surrounded by a more permeable zone simulating the presence of intact or less damaged zones in the core. The PEARL experiments were modeled and simulated using ICARE/CATHARE code to assess the evolution of a bottom reflooding of a superheated debris bed surrounded by a bypass of larger permeability. The thermal hydraulics of the quenching process has been analyzed and the effect of each of the initial conditions on the reflooding behavior was assessed. The effect of pressure was investigated and related to the entrainment of injected water at quench front level into the bypass.
An analytical model was then developed to investigate thoroughly the reflooding of a superheated heterogeneous porous medium, composed of two layers of contrasting permeability and porosity, and to describe the water entrainment in the bypass. This model computes the main variables characterizing the reflooding process suchas quench front velocity, water-to-steam conversion ratio, and the flow rate of water entrained in the bypass. It provides good qualitative and quantitative results for the two-phase flow redistribution as compared to experimental results. This model has several advantages. It is written in a rather general form including the Forchheimer correction terms and non-zero cross-terms in the generalized Darcy-Forchheimer momentum equation. Variations of proposed momentum equations including changes in correlations and interfacial friction laws can be tested easily and efficiently. Comparison of the calculations against experimental results indicated that it is necessary to include an interfacial friction law to obtain good predictions. This model allows performing fast evaluations of the efficiency of cooling by computing the fraction of the injected flow rate that participates in cooling. Upscaling to the reactor scale is straightforward and calculations were performed to assess the impact of geometric parameters of the debris bed (particle size, porosity, dimensions) as well as thermal hydraulic conditions (temperature, pressure, injection flow rate) on the reflooding process. Thus the model is very useful to estimate the total quenching time and the maximum temperature that could be reached by the hot debris bed at large scales. This allows assessing the probability of a successful quenching of a hot debris bed formed during a hypothetical accidental scenario.