[en] In order to protect a nuclear reactor efficiently also when the fuel elements begin to melt, it is proposed instead of the core catcher provided for this case, to arrange a larger number of smaller collecting vessels, each below a fuel element, intended to catch melted particles, to cool them well, and if necessary, to discharge them. The containers are made of high-temperature resistant material (stainless steel, ceramics). 9 drawings illustrate the invention. (UWI)

[en] In its fourth phase, a hypothetic core melt interacts with the concrete of the reactor foundation. This phase may last several days. Experimental laboratory investigations and theoretical models on the basis of model experiments aim at determining the time curve of the temperature of the core melt in order to quantify the processes up to the solidification of the melt and the end of concrete destroyal. Material interactions: 1) The two phases of the core melt, oxidic and metallic, remain separate for a long period of time. In dependence of the degree of oxidation of the system, the elemental distribution and, in particular, the fission products in the melt may be assessed. 2) The changes in the material values of the core melt in dependence of the temperature curve may be qualitatively assessed. 3) The solidification temperature of the oxidic phase of the core melt may be given in dependence of (UO₂ + ZrO₂) content. Thermal interactions: 1) The ratio vertical/radial erosion, which determines the cavity shape, is described in the correct order of magnitude by the extended film model. 2) The correct order of magnitude of the erosion rates is described by the concrete destruction model coupled with the film model. 3) The effects of the different concrete destruction enthalpies and concrete compositions (amount of gaseous decomposition products) may be estimated by the model calculations. (orig./HP)

[en] Melting ultimately requires the heat production rate to exceed the removal rate. In a nuclear reactor this situation is initiated by either overpower or undercooling conditions. The latter is related to reduced cooling through loss of coolant flow or loss of coolant itself and may lead to general meltdown of the reactor core. Molten material is accumulated in the lower plenum of the reactor pressure vessel. The separation of phases results from different components densities of molten material. High temperature molten UO₂ will slowly melt through the wall of RPV lower plenum. To avoid disruption of RPV, an instantaneous heat transfer is needed. To simulate the situation described above, general multiphase flow equations were developed. In order to describe the multiphase flow fully, the continuity, momentum and energy equations were derived using ensemble averaging rather then time or spatial averaging. Because of geometry of the lower plenum a spherical coordinate system was used, to enhance the accuracy of the calculation on the border. (author)

[en] This paper gives briefly the characteristics of the RBMK reactors and describes the possible history of the accident of the reactor Tchernobyl-4

No abstract available

[en] The severe core meltdown accident, which is not included as a design basis accident, has high consequence and low probability of occurrence and turns out to be a major risk factor in the overall risk assessment. The physical mechanisms of containment failure in core meltdown accidents are identified as steam explosion, debris bed coolability, hydrogen burning, steam spike and core-concrete interaction. The state of technology review is made for each subtopic about the previous and current researches for better understanding of the phenomenon. (Author)

[en] The TMI-2 accident has initiated a new phase of safety research. It is necessary to consider severe accidents with degraded or molten core. For NRC there was a need for an improved understanding of this reactor behaviour and the 'Severe Fuel Dage Program' was initiated. Planned are in-pile experiments in PBF, NRU and ESSOR and in addition separate effects tests and results from TMI-2. The analytical component of the program is the development of different versions of the code SCDAP for the detailed analysis during severe accident transients. (Author)

[en] A volumetrically-heated pool with gas injection at the boundaries is used to simulate the heat transfer processes taking place in molten core debris-concrete systems. Measurements of the upward, downward, and sideward heat transfer rates at the pool boundaries have been made over wide ranges of power density, superficial gas velocity, and pool aspect ratio. Pools with either a free upper surface or an isothermal solid upper boundary to simulate an overlaying solid crust of oxides have been examined. Detailed measurements of the temperature distribution within the pool have also been made. A total of nearly 400 experiments, generalized correlations for the downward and sideward Nusselt numbers have been developed. The results indicate that the Nusselt numbers are nearly independent of the internal Rayleigh number and depend only on the nondimensional quantity (pV³/μg) where V is the superficial gas velocity, p is the pool density, μ is the viscosity, and g is the gravitational acceleration. The results also indicate that the downward and sideward Nusselt numbers in pools with gas release at the boundaries are comparable. This result is in contrast to the natural convection case where the sideward Nusselt numbers are significantly higher than those in the downward direction. The correlations developed in this investigation cover a wide range of the relevant nondimensional quantity (5 x 10^-10 <= (pV³/μg) <= 5 x 10^-3). They can be used to determine the heat transfer rates at the pool boundaries, and hence, the pool growth rate into the concrete. Such information is necessary since it constitutes the last line of defence against violation of the containment. (orig.)

No abstract available

No abstract available