Browsing by Author "Schröder, Maxim"

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Three-dimensional modeling and simulation of vapor explosions in Light Water Reactors
(2012) Schröder, Maxim; Lohnert, Günter (Prof. Ph.D.)
Steam explosions can occur during a severe accident in light water nuclear reactors with the core melting as the consequence of interaction of molten core materials with water inside the reactor pressure vessel (in-vessel steam explosions), or after a failure of the reactor vessel due to the release of molten materials into the reactor cavity likely filled with water (ex-vessel steam explosions). Such steam explosions may significantly increase risks of severe accidents threatening the integrity of the reactor pressure vessel, of the primary containment and possibly even of the reactor building. The loss of integrity of the primary containment and reactor building would cause a release of large amounts of fission products into the atmosphere and a contamination over a large area. Eliminating the risk of steam explosions in reactor accident scenarios would contribute to enhancing the effectiveness of accident management procedures, e.g. concept for the external cooling of the reactor vessel or the cooling of the molten core in the flooded reactor cavity. The main parameters influencing the outcome of a strong steam explosion are a limitation of the fragmented melt mass mixing with water, the melt jet fragmentation, the void buildup during premixing, the solidification at the surface of melt drops during the premixing and pressure escalations during detonation. Asymmetries caused by geometrical constraints (e.g. wall proximity, distributed melt pouring) are likely during an accident with core melting and can have a strong impact on the explosion strength. Previously, asymmetric configurations have been investigated with two-dimensional models using 2D approximations. Until now, open questions concerning the fragmentation of the melt, the mixing phase with water, the extent of the mixing region and pressure increases under asymmetric conditions remained due to the uncertainties existing with the approximated approach. This led to unsatisfactory answers as to the role of geometrical restrictions. In order to give a more adequate solution to the problem and be able to predict the explosion strength in a conservative manner, the two-dimensional premixing and explosion codes IKEJET/IKEMIX and IDEMO were extended to 3D in the present work. Additional modeling improvements have been made with regard to applicability to real reactor conditions. The enhancements focus in particular on the breakup of thick melt jets in deep water pools and on the solidification of melt fragments during the mixing phase with water. Asymmetries and their impact on the formation of explosive mixtures were investigated using the extended program codes. Variations to the melt delivery, pool depth and melt pouring configuration are considered. The melt fragmentation, void production, extent of the mixing zone due to geometrical constraints and the loads on adjoining structures are discussed in detail. The focus is on the assessment of three-dimensional effects in the mixing and detonation phases. In this regard, two- and three-dimensional calculations were performed for each configuration. An investigation of the influence of the 3D geometry, i.e. geometrical restrictions on mixing, extent and distribution of melt and coolant, is discussed. Pressure loads and impulses on adjoining structures are obtained and the results are critically discussed. The calculations performed show the capability of the codes to correctly represent the main aspects of premixing and explosion stages of steam explosion in non-symmetrical scenarios and to adequately predict the pressure loads on the adjoining structures.