3-D Simulation of Fuel Assembly Blockage in MYRRHA

In the framework of the KIT and SCK•CEN R&D cooperation and as continuation of earlier studies performed for the EURATOM FP7 MAXSIMA project, accidental transients caused by a single fuel assembly (FA) block-age were simulated with the SIMMER-IV (3-D) code for the MYRRHA core, while assuming no power variation during the accident. A 7-FA model that includes mesh cells for inter-wrap-per gaps between FAs was applied, with the blockage of the central FA. Sensitivity analyses on the gap flow rate, fuel chunk jamming fraction, insulator pellet material were performed in order to identify a conservative case that maximises the chance of damage propagation from the blocked FA to the neighbouring ones. All calculations with different options and parameters did show the same sequence in the blocked FA, including melting of pin cladding, fuel pellet failure, small can-wall break-up, steel particle and fuel chunk accumulation leading to additional blockages, and large can-wall break-up. Finally, fuel chunks are swept out from this FA through the inter-wrapper gaps. In the calculations performed for several tens of seconds and longer, no canwall break-up in the neighbouring FAs has been observed. ... mehrNevertheless, different options for simulation of the insulator break-up lead to signifi-cantly different results in the later phases of calculations. If the insulator pellet breaks up when the cladding is lost, a fuel/steel blockage is formed, which results in a large canwall break-up, but this blockage is dissolved as soon as the upper steel structure melts. If no insulator pellet breaks-up, the fuel/steel block-age is kept in place by the ceramic insulator, which has a very high melting temperature. This observationsupports the use of an insu-lating material with low melting temperature; this option may prevent or reduce blockage of fuel/steel particles coming from failed pins that eventually may prevent or reduce the risk of damage propagation to the intact core.