A state-of-the-art review of past programs devoted to fuel behaviour under LOCA conditions - part2 Impact of clad swelling upon assembly cooling

Rapport technique SEMCA-2006-183 - Juin 2006

This technical report provides a state of the art review of the main LOCA programs carried out in the 80s and focusing on the coolability of an assembly containing a partial blockage in a group of ballooned fuel rods under accidental LOCA conditions.

The experimental characteristics and main results of the FEBA, SEFLEX, THETIS, CEGB and FLECHT SEASET programs, as well as several analytical developments performed in association with these experimental programs, are reviewed in detail in this document.

The comparison and combination of conclusions drawn from these results and studies were used to improve our understanding of the physical phenomena governing the behavior of a partially blocked rod array during a LOCA reflood scenario. It has also been possible to determine the limits of blockage coolability under the most severe geometric (blockage length) and thermohydraulic conditions. Therefore, even a severe blockage ratio (90%) of a moderate length (<10 cm) does not cause any particular problems in terms of coolability during two-phase reflood. A severe blockage with considerable axial extension (> 15 cm) and a high blockage ratio (> 80%) can lead –with low reflood rate conditions – to a significant increase in blockage surface temperatures, hindering the final coolability of this blockage.

It is important to stress the fact that these results were obtained in out-of-pile tests performed on simulator assemblies containing electrically heated rods, making it impossible to simulate the possible fuel accumulation occurring in cladding balloons (fuel relocation), as was observed during in-pile tests with irradiated fuel rods. The impact of fuel relocation upon blockage coolability remains to be investigated. It must also be underlined that reflood conditions – representative of a large break LOCA scenario – were the main thermohydraulic conditions studied in these experimental programs. Blockage coolability during an intermediate break scenario has not been investigated in tests performed under representative thermohydraulic conditions and therefore will not be discussed in this paper.