Debris bed formation in degraded cores of light water reactors

Abstract

In the aftermath of the Fukushima Dai-Ichi nuclear accident, the issue of corium coolability has received considerable attention in the severe accident research. One of the accident mitigation strategies for the ex-vessel debris cooling is the employment of deep pool water in the cavity below the Reactor Pressure Vessel (RPV). During a hypothetical Severe Accident (SA) in Light Water Reactors (LWR), degraded core materials released from the RPV after its failure will be fragmented and quenched by contact with water. The solidified particles will settle on the bottom forming a porous bed. However, this strategy succeeds only if the residual decay heat is sufficiently removed and the bed is thermally stabilized and will not re-melt again damaging the containment integrity. One of the main factors determining the ability of decay heat removal and long-term coolability of debris bed is its geometrical configuration. A flatter and broader bed can be easier cooled than a higher bed with the same mass of debris. For this purpose, the present work focuses on the development of a two-dimensional continuum model describing the formation process of the debris bed resulting from the deposition of the settling particles and their relocation along the surface of the heap. The mathematical model is based on a hyperbolic system of partial differential equations determining the overall bed height, the distribution of the flowing particles layer depth and the depth-averaged velocity component tangential to the sliding layer. Because of the hyperbolicity of the system, a successful implementation of a solver is challenging, notably when large gradients of the physical variables appear, e.g., for a moving front in the flowing layer or possibly formed shock waves during the deposition. In this thesis, several numerical methods are applied to solve the system and compared. The implemented Roe-solver has provided promising results, which are verified with analytical solutions in the steady state. The spatial convergence is also reported and quantified with the use of the Grid Convergence Index (GCI). A sensitivity analysis is subsequently performed to study the influence of the uncertainties in the input parameters on the bed geometry. A dedicated test facility, named BeForE, is designed and built in the framework of this study, with the aim of providing the necessary experimental data for the model validation. A series of tests were conducted using different shaped and multi-size mixtures of particles. It could be evinced, that in addition to the modeled particles sliding the smaller particles (< 3 mm) are subject to the influence of a suspension and convection flows, which are affecting the final bed shape. The comparison between the numerical and the experimental results has shown a very good agreement, notably for the cases where the two last-mentioned phenomena are less present. Moreover, the test facility could also be used to gain an insight into the influence of steam production on the particulate bed spreading. The decay-heat-induced coolant boiling and the resulting two-phase flow serve as a source of mechanical disturbance, which might lead ultimately to leveling of the debris bed (a.k.a. self-leveling). A series of experiments were then conducted by discharging solid particles in the two-dimensional viewing vessel of the facility, while air bubbles simulating the steam production are injected simultaneously from the bottom. Depending on the quantity of the settled particles on the top of each section of the vessel, air flow rate is so monitored and adjusted in time to simulate the corresponding amount of steam produced by the similar quantity of hot debris. This study shows that, in most of the cases, the two-phase flow inside the vessel alters the sedimentation process resulting in a broader and flatter bed than under quiescent conditions. However, it was observed that in the case of customarily formed concave beds in the quiescent conditions, the presence of the gas flow can change the mound shape to a convex type with a higher bed height, at least in the beginning. It was also shown that, for high gas flow rates, the convective flows induced by the bubble plumes inside the bed would contribute to the diminishment of self-leveling effect and a slower particles redistribution. Lastly, it was mathematically described how the steam production could reduce the characteristic angles of repose of a debris bed, putting forth a physical explanation of the self-leveling phenomenon. With the coupling of the developed continuum model with a model simulating the two-phase flow within the bed, a full numerical simulation of the avalanche-like particles motion during the self-leveling process could also be successfully provided. This allows a more accurate simulation of the bed formation process under the influence of steam production, which is of particular importance for the bed coolability and a decisive requirement for the nuclear accident progression and termination.