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The vapour explosion in the post-disassembly phase needs careful analysis because of the potential to convert thermal into mechanical energy. Whereas vapour explosions in water are relatively easily obtained, kilogram scale interaction tests between molten UO-2 and sodium executed in the laboratories of UKAEA at Winfrith, of CEA at Grenoble and of JRC at Ispra show that, to this day, no such explosions could be demonstrated in sodium. On the contrary, several sodium tests generated bumps with low efficiency. The difference in explosive behaviour is indicated by being category 1 and 2 events. Category 1 events were analysed as thermal detonations. The key physical phenomenon which allows an explosion is film boiling upon fuel-coolant contact, which favours a coarse premixture. When exploding, fragmentation propagates through the mixture with shock velocity. The contact temperature necessary to guarantee film boiling in water is 300circC, but in sodium it is 1788circC. Category 2 events in sodium were analysed as heat exchange transients. The speed of heat exchange is analysed by mere parametric fragmentation, by thermal stress governed peeling of UO-2 particles, or by a superheat transient. These three explanations were not held to be fully satisfactory. A new physical interpretation for category 2 events is proposed which allows one to retain most of the modelling work done in the past. It consists in the concept that transition boiling is maintained during the heat exchange transient, and is therefore called the sustained transition boiling concept. This means that the speed in heat exchange is dominated by fragmentation governed by sodium vapour bubble and collapse forces. Typical fragmentation time is 50 ms. Ongoing research at Winfrith with the MFTF facility concerning category 1 events, and at Ispra with the FARO- TERMOS facility concerning category 2 events, is expected to confirm present understanding of fuel/sodium interactions and their consequences, if any, for FBRs in the post-disassembly phase.

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Bibliographic Reference: RES MECHANICA, VOL. 23, PP 65-88, 1988
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