Degraded core reflood: Present understanding and impact on LWRs

The severe fuel damage (SFD) research at Forschungszentrum Karlsruhe started with the single rod experiments [Hagen, S., Malauschek, H., Peck, S.O., Wallenfels, K.P., 1983. Temperature Escalation in PWR Fuel Rod Simulator Bundles due to the Zircaloy/steam Reaction Test ESBU-1. Test Results Report, FZK-3508] already before the TMI-2 accident revealed that the knowledge on accident initiation and progression was scarce. The general focus on primary circuit research was the understanding of the relevant processes and their interactions, and of possibilities for countermeasures. However, even today, the most prominent countermeasure, the flooding of a degraded core, is not yet completely understood, namely the influence of adverse effects such as enhanced core degradation and hydrogen spikes under the most probable scenarios.Our work shows that the design basis accident procedures of core reflood can be extended to peak core temperatures of approx. 2200 K, if sufficient reflood mass flow rate can be supplied. Assuming an initial core heat-up rate of 0.5 K/s, this gives only approx. 12 min additional time with respect to design basis accidents (DBA) cases, but moreover a strategy for a successful core reflood is seen to be feasible before a large in-core pool is formed, that might be uncoolable.