The paper presents some results of the ISTC (International Science and Technology Center)-financed project 'Investigation of Corium Melt Interaction with NPP Reactor Vessel Steel' (METCOR). In the METCOR experiments the metallic phase of a two-liquid system was produced by the interaction between hot suboxidized corium and cooled VVER vessel steel, with the steel being corroded. Models of corrosion mechanisms in the considered conditions are used to systematize data on the limiting temperature of corrosion/(dissolution) of the vessel steel. A considerable influence of thermal gradient conditions is shown, which has to be taken into account in the analysis of molten pool behaviour.

Experimental, theoretical and numerical studies of oxidation kinetics of an open surface corium pool have been reported. The experiments have been carried out within OECD MASCA program and ISTC METCOR, METCOR-P and EVAN projects. It has been shown that the melt oxidation is controlled by an oxidant supply to the melt free surface from the atmosphere, not by the reducer supply from the melt. The project experiments have not detected any input of the zirconium oxidation kinetics into the process chemistry. The completed analysis puts forward a simple analytical model, which gives an explanation of the main features of melt oxidation process. The numerical modeling results are in good agreement with experimental data and theoretical considerations. (C) 2013 Elsevier B.V. All rights reserved.

The systems FeO-UO2-ZrO2 (in inert atmosphere) and Fe2O3-U3O8-ZrO2 (in air) were studied. For the FeO-UO2-ZrO2 system, the eutectic temperature was found to be 1310°C, with the following component concentrations (mol %): 91.8 FeO, 3.8 UO2, and 4.4 ZrO2. For the Fe2O3-U3O8-ZrO2 system, the eutectic temperature was found to be 1323°C, with the following component concentrations (mol %): 67.4 FeO1.5, 30.5 UO2.67, and 2.1 ZrO2. The solubility limits of iron oxides in the phases based on UO2(ZrO2,FeO) and UO2.67(ZrO2,FeO1.5) were determined.

Experimental results of the investigation of the FeO1+x–UO2–ZrO2 system in neutral atmosphere are presented. The ternary eutectic position and the composition of the phases crystallized at this point have been determined. The phase diagram is constructed for the FeO1+x-enriched region and the onset melting temperature of 1310°C probably represents a local minimum and so will be a determining factor in this system and its application to safety studies in nuclear reactors.

Results of the investigation of the FeO1.5–UO2+x–ZrO2 system in air are presented. The eutectic position and the content of the phases crystallized at this point have been determined. The temperature and the composition of the ternary eutectic are 1323±7°С and 67.4±1.0 FeO1.5, 30.5±1.0 UO2+x, 2.1±0.2 ZrO2mol.%, respectively. The solubilities of FeO1.5 and ZrO2 in the UO2+x(FeO1.5, ZrO2) solid solution correspond to respectively 3.2 and 1.1mol.%. The solubilities of UO2 and ZrO2 in FeO1.5 are not significant. The existence of a solid solution on the basis of U(Zr)FeO4 compound is found. The ZrO2 solubility in this solid solution is 7.0mol.%.

Within SARNET, the corium topic covers all the behaviors of corium from early phase of core degradation to in or ex-vessel corium recovery with the exception of corium interaction with water, direct containment heating and fission product release. The corium topic regroups in three work packages the critical mass of competence required to improve significantly the corium behavior knowledge. The spirit of the SARNET networking is to share the knowledge, the facilities and the simulation tools for severe accidents, so to reach a better efficiency and to rationalize the R&D effort at European level. Extensive benchmarking has been launched in most of the areas of research. These benchmarks were mainly dedicated to the recalculation of experiments, while, in the next periods, a larger focus will be given to integral experiments or reactor applications. Eventually, all the knowledge will be accumulated in the ASTEC severe accident simulation code through physical model improvements and extension of validation database. This paper summarizes the progress that has been achieved in the frame of the networking activities. A special focus is placed on the melt pool and debris coolability and corium-concrete interaction, in which, the effects due to multidimensional geometries and heterogeneities has been shown, during SARNET, to play a crucial role and for which further research is still needed.

Phase-relation studies of the UO2–FeO1+x system in an inert atmosphere are presented. The eutectic point has been determined, which corresponds to a temperature of (1335±5)°C and a UO2 concentration of (4.0±0.1)mol.%. The maximum solubility of FeO in UO2 at the eutectic temperature has been estimated as (17.0±1.0)mol.%. Liquidus temperatures for a wide concentration range have been determined and a phase diagram of the system has been constructed.

Phase transformations in the oxide binary section of the UO2-FeO-Fe ternary system were studied. The melting onset point of the UO2-FeO heterogeneous system (1335±5°C) was determined and the fusion curve of this system was constructed. The limiting solubility of FeO in the UO2 solid solution was measured. The changes in crystal parameters in formation of the solid solution were determined. Uranium dioxide was found to be insoluble in the wüstite phase (FeO).

We present the results of the investigation of the ZrO 2 -FeO system under an inert atmosphere. We have refined the position of the eutectic point, which lies at 1332 ± 5°C and 10.3 ± 0.6 mol % ZrO 2 . The iron oxide solubility boundaries in zirconium dioxide have been determined over a wide temperature range taking into account the polymorphism in ZrO 2 . A phase diagram for the system has been designed.

The results on the ZrO2–FeO system studies in a neutral atmosphere are presented. The refined eutectic point has been found to correspond to a ZrO2 concentration of 10.3±0.6mol% at 1332±5°C. The ultimate solubility of iron oxide in zirconia has been determined in a broad temperature range, taking into account the ZrO2 polymorphism. A phase diagram of the pseudobinary system in question has been constructed.

In a postulated core meltdown accident in a light water reactor there are bound to be interactions, in the ex-vessel phase, among the core melt and the structural materials within and below the reactor cavity. In existing plants, these structural materials normally are structural concrete, while future, evolutionary reactor lines are to have sacrificial and protective materials specially designed for this hypothetical case. To add to the state of knowledge about the phenomena occurring, experiments need to be conducted under conditions as realistic as possible. Within the research programs funded by the European Union, the German Federal Ministry for Economics, and the German nuclear power plant operators, experiments on a laboratory as well as an industrial scale on these problems are being carried out in the two projects called CORESA (COrium on REfractory and SAcrificial materials) and ECOSTAR (Ex-vessel COre melt STAbilization Research). The experiments are accompanied by an extensive analytical theoretical program also serving to advance and validate computer codes on the problems under investigation. The projects, which are carried out with international European participation, are expected to allow a concept to be developed for managing postulated accident scenarios involving core meltdown for innovative nuclear power plants, and to provide findings on risk evaluation of plants now in operation so as to further develop accident management measures.