Session 4A, Working Group 2
Chairman P Tanguy
Discussion of Phenomena Associated with the Long Term Accident Sequence, Plant Recovery Measures and Radioactive Releases from the Plant.
Graphite fire, core damage, use of robotics, long term reliability of safety systems, recovery actions, radioactive release characteristics.
Non-Soviet delegates expressed interest in learning more about preliminary stages of fuel degradation, hydrogen production, possible water interactions, the fragmentation effects of fuel in the core, the extent of the ejection of graphite from the core, why one side of the reactor building (the north side) had been much more damaged than the other and the number of fuel channels ruptured.
The Russians said that they found delegates' comments helpful in indicating some of the many areas in which further research was necessary. The south side of the building backed on to the machine room and was structurally stronger. That was why the explosion particularly affected the weaker north side.
It was confirmed that the estimated energy stored in the oxide fuel at the time of the accident was 300 calories per gram a figure of this order tended to be confirmed by research carried out in the USA (presumably an experiment in PBF) and in Japan. The fact that the top protective plate blew off, amongst other indications, confirmed that all 2000 fuel channels were destroyed the zirconium was subjected to temperatures of 700 to 750°C and was then easily ruptured. When the upper plate lifted, all the coolant-exit pipes would have been destroyed.
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Visual examination confirmed that only a small amount of graphite (10%) had been ejected from the reactor building. Fragmented fuel was not found in the graphite analysed, as most of the graphite came from the reflector regions.
The Russians had not yet formed any views on the extent of possible hydrogen formation through interaction of fragmented fuel with water and would welcome any contributions in this field.
A French delegate indicated the results of some of the French calculations on the accident. It was agreed that a reactivity excursion was the likely explanation of the first explosion, the energy involved calculated to be 200 megajoules. The second explosion was either due to an excursion, a steam explosion or a hydrogen explosion.
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