Coolability of Blocked Regions in a Rod Bundle after Ballooning under Loca Conditions. Main Findings from a Review of past Experimental Programmes

IRSN has carried out a State-of-the-Art-Review of the main experimental programmes related to fuel behaviour under Loss-of-Coolant-Accident (LOCA) conditions conducted from the 70s until now, that has been split in three parts. The second part is devoted to the question of the coolability of blocked regions in a rod bundle after ballooning in a LOCA. The main findings from this part are presented here. The experimental characteristics and main results of the FEBA, SEFLEX, THETIS, ACHILLES, CEGB and FLECHT SEASET programmes, as well as several analytical developments performed in association with these experimental programmes, were examined in detail in this review. The comparison and combination of conclusions drawn from these results and studies were used to improve our understanding of the physical phenomena governing the behaviour 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 ratio and length) and thermohydraulic conditions. Thus, 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. However, a severe blockage with considerable axial extension (> 15 cm) and a high blockage ratio (> 80%) can lead – under low reflood conditions – to a significant increase in blockage surface temperatures, hindering the final coolability of this blockage. It is important to underline that these results were obtained in out-of-pile experiments performed with electrically heated fuel rod simulators with a large gap between simulator and cladding bulge, thus not allowing to simulate the possible fuel accumulation occurring in cladding balloons (fuel relocation), as was observed during all in-pile tests with irradiated fuel rods. The impact of fuel relocation upon blockage coolability therefore remains to be investigated.