Severe accident conditions in a LBE-cooled reactor after core degradation: current considerations in the framework of MYRRHA

In heavy-metal-cooled reactors lead or lead-bismuth eutectic (LBE) are used as a coolant. They offer a good conductivity and a high boiling temperature. Both properties can contribute to effective cooling in post-severe-accident conditions. The density of these heavy liquid metals is close to the density of oxide nuclear fuels. In the hypothetical event of a core degradation, fuel debris might either float towards the surface of the coolant or sink towards the bottom. Furthermore, the expected amount of fuel that will potentially float
or sink depends on parameters such as fuel burn up and temperature distribution of the coolant.
Here, we report our considerations about what can be expected, in an LBE-cooled fast reactor, during the post-accidental phase of a hypothetical event that has led to a core degradation, when a given amount of fuel reaches the bottom of the vessel. We discuss the phenomena involved with the formation of the debris bed and its cooling. Then, we assume the presence of a stable deep bed and proceed with the adaption of a one-dimensional Post-Accidental-Heat-Removal model, developed for light coolant such as Sodium or Water, to the LBE case. With this model, we can investigate how well the bed is cooled, including phenomena such as downward boiling.