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Core Reflood

In the past, much effort has been dedicated to analyses of experiments on severe accidents in Light Water Reactors, largely in international co-operations. To define severe accident management guidelines, all possible countermeasures have to be evaluated for a successful mitigation or termination. For this purpose, the reliability and the safety margins of nuclear reactors are investigated at various levels from normal conditions and operational transients up to the sequence and consequences of anticipated severe accidents. Several large experimental series were performed at KIT and its predecessors. However, late phase data are still lacking because of the related experimental difficulties.

This work is the basis of our effort to give some general insight about the success of reflood under various conditions. The freely available literature on reflood experiments and reflood in a reactor were used to derive a first assessment of the success to reflood a nuclear reactor (see Figure). Such work is needed for PSA (Probabilistic Safety Assessment) level 2 analyses.

For a successful accident termination, the reflood mass flow rate should be beyond a minimum value, which was assessed based on the all free available reflood experiments to be about 1 g/s*rod. When design basis conditions regime are exceeded, i.e. above 1200 °C, the time to reach transition from localized debris to molten pool can range between 20 and 30 minutes depending on the scenario. Though this time is limited, it may help to bring additional systems on-line to reach a sufficient reflood mass flow rate and so to terminate the accident [1].

Reflood map for LWR accident termination

Reference

[1]

W. Hering, Ch. Homann, J. Stuckert: Update of the Reflood Map, Proceedings of ICAPP 2015, May 03-06, 2015 - Nice (France), Paper 15465.