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[en] At the request of the CSNI, a state-of-the-art report (SOAR) has been prepared on melt coolability and core concrete interactions that captures the last thirty years of international research in this area. These efforts revealed that significant progress has been made in terms of understanding phenomenological behaviour, but also led to the conclusion that a few issues that may warrant further investigation to reduce residual uncertainties. These issues include specific realistic reactor conditions from short to long term, and proposition to improve the efficiency of melt coolability under top flooding conditions.
[en] Objectves: More realistic estimation of consequence of melt release into Cavity/ Pedestal space in CV; • Evaluation of probability of MCCI/ CV failure under various water depth conditions.
[en] Highlights: • The different ablations regimes for LCS and siliceous concretes are explained. • Ablation instabilities are linked to transitions between these regimes. - Abstract: This paper deals with the extension of the Transient Interface Model (TIM), initially developed for in-vessel and ceramic dissolution situations, to molten core concrete interaction (MCCI) conditions. The application of the TIM model to MCCI leads to evidence of two interaction regimes, depending on the nature of the concrete. For siliceous concrete, there is no crust nor mushy zone at the ablated corium/concrete interface and the interface temperature is close to the liquidus temperature of concrete (Low Interface Temperature – LIT regime). For limestone common sand concrete, a mushy zone or crust can form at the interface between the corium and the concrete and the interface temperature (between mushy zone and melt) is close to the pool liquidus temperature (High Interface Temperature – HIT regime). No intermediate regime exists, but rather regime transitions (HIT to LIT, or LIT to HIT), which may occur on a limited number to all interfaces during MCCI. The work described herein demonstrates that these regime transitions are abrupt and happen within a short time period. These regime transitions are shown to cause ablation instabilities. Many combinations of regime transitions on various interfaces at various pool temperatures (in comparison to pool liquidus) are possible. Only a limited number of these transitions have been calculated; the results indicate that different trends in temperature evolutions observed during MCCI tests can be reproduced. New calculations of the CCI2, CCI3 and M3B tests are presented.
[en] This paper presents the results of the modelling of the loss of cooling accident in at-reactor spent fuel pool of VVER-1200. The calculations of the accident were performed with 3 Russian codes: best-estimate severe accident code SOCRAT/V1 (processes in the spent fuel pool), containment lumped-parameters code ANGAR (parameters of the containment atmosphere during the accident) and calculation code GEFEST-ULR (molten corium-concrete interaction). Such combination of codes allowed to perform complex evaluation of the severe accident: from the initial event (loss of cooling) to the melt-through of the spent fuel pool concrete bottom, taking into account change of the containment atmosphere during the accident. (author)
[en] Highlights: • The explicit pressure model was incorporated into MPS code to enhance computational speed. • 3-D simulation of a MCCI test was presented with improved MPS method. • The phenomenology of MCCI progression was revealed by improved MPS method. - Abstract: The molten corium-concrete interaction (MCCI) is an important phenomenon after the failure of pressure vessel in a severe accident of nuclear reactor. It has been verified that the original Moving Particle Semi-implicit (MPS) method has the capacity to simulate some MCCI experiments. In this study, the original MPS method has been improved by including the explicit pressure calculation model to reduce the computational cost and enhance the computational speed. Then, the improved MPS method was validated by simulating the classical dam break problem, and the results agreed well with that of the original MPS method. Afterwards, the HECLA-4 transient MCCI test performed by VTT was simulated by the improved MPS method with a three dimensional particle configuration of about one million particles. The basemat and sidewall ablation fronts, melt pool temperature and concrete temperature at different positions predicted by MPS were in good agreement with the experimental results. All the above-mentioned simulations proved that MPS method using explicit pressure model is capable of simulating MCCI and related heat and mass transfer in multicomponent phase flow.
[en] Future work: • Krško NPP will calculate source term for severe accident including MCCI -filtered containment venting release; • Krško NPP will model release and calculate doses to environment/population for the same SA scenario; • Emergency protection areas will be re-evaluated; • New ERDS tool with extended data set will be developed; • SNSA is following international activities with new tools for quick determination of source term; •SNSA is following development of generic assessment methods for source term determination (revision of the IAEA TECDOC 955).
[en] In the case of hypothetical scenarios of severe nuclear reactor accidents, the core could melt and spill out of the vessel. In this case, the bath, called corium, resulting from the fusion of the core, would interact with the concrete of the reactor pit. In this case, there may be simultaneous partial melting of the concrete and the formation of a corium crust at the interface. In the event that the crust is not stable, the molten concrete, less dense and more viscous than the corium, will be subjected, on horizontal or inclined walls, to Rayleigh-Taylor instabilities, generating a solutal convection of which a semi-analytical model is proposed
[fr]Dans le cas de scenarios hypothetiques d'accidents graves de reacteurs nucleaires, le coeur pourrait fondre et se repandre hors de la cuve. Dans ce cas, le bain, appele corium, issu de la fusion du coeur, interagirait avec le beton du puits de cuve. Dans ce cas, on peut avoir a l'interface simultanement fusion partielle du beton et formation d'une croute de corium. Dans le cas ou la croute ne serait pas stable, le beton fondu, moins dense et plus visqueux que le corium, sera soumis, sur des parois horizontales ou inclinees, a des instabilites de Rayleigh-Taylor, generant une convection solutale dont un modele semi-analytique sera propose. (auteurs)
[en] Model of Erosion Due to Interaction of CorIum with basement Substance: MEDICIS contains: - A model of the structure of the corium concrete interface; - Models of corium coolability in case of water injection upon the corium pool surface; - Models of evolution of corium pool configuration; - Models to evaluate the release from the ex-vessel corium pool of concrete aerosols and the release of fission products during MCCI; - interface with the physico-chemistry MDB package (Material Data Bank), to evaluate the corium layers properties.
[en] In a hypothetical severe accident in a light water reactor (LWR) the failure of decay heat removal might lead to the destruction of the reactor core. After the reactor pressure vessel has melted through, the molten core of the reactor reaches the concrete basement. This leads to a Molten Core - Concrete Interaction, in which the melting of the concrete starts. In order to assess the safety and the development of the so-called fourth phase of a core decomposition accident, the consequences of such an interaction are of great interest.
[en] With the ALISA Co-operation between the European Union and China on SA (Severe Accident) facilities, CGNPC (China General Nuclear Power Corporation) has proposed to CEA (Commissariat à l'Energie Atomique et aux Energies Alternatives) a series of experiments to determine physical properties of two corium compositions representative from Molten Core Concrete Interaction (MCCI): LOCA and LOOP. This article describes those tests performed on the two compositions in the VITI facility, by means of the Maximum Bubble Pressure (MBP) method. The measurement procedure using the Maximum Bubble Pressure method is described. Both density and surface tension measurements are successfully performed for both LOCA and LOOP corium compositions, which constitutes to our best knowledge the first measurements ever performed for ex-vessel (MCCI) liquid corium compositions. (author)