Thermal-hydraulic impact of failure of highly irradiated fuel pins on LMR passive safety

The EBR-2 SHRT-45 loss-of-primary-flow-without scram test and SHRT 302 unprotected loss-of-heat-sink test have demonstrated that liquid metal reactors can be built with a high degree of passive safety. No fuel pins were observed to fail in these tests; but if the burnup of the fuel were pushed further, one would face the question of what could be the impact of pin failures and rapid release of plenum gas in such a transient. Also, there are questions about the impact of an unprotected rapid release of plenum gas following the failure of highly irradiated fuel during normal operation. One area of concern, and the one addressed in this document, is whether the release of gas from failed pins could interfere with the coolant flow enough to cause melting of the fuel, possibly leading to blockage of the coolant channels. To address this question, the coolant voiding model in the SASSYS-1 code was modified to account for voiding due to gas release from failed pins. This model was used to analyze both unprotected pin failures during normal operation and pin failures during a SHRT-45 type of unprotected loss-of-flow transient. 3 refs., 5 figs.