Model of terminal debris bed formation after a CANDU core collapse

Abstract A CANDU reactor core comprises several hundred horizontal fuel channels spanning a calandria vessel. Loss of sufficient cooling during a severe accident could result in collapse of the core to the bottom of the calandria. A simple computational tool for simulating, in two dimensions, the resulting build-up of a terminal debris bed is described. The tool is used to model a variety of core collapse scenarios. Computed debris beds are generally lower in the middle, ∼10 fuel channels deep, and have higher decay power in their interiors. The initial debris bed porosity is estimated to be 0.65 ± 0.15. High porosity could augment in-vessel hydrogen generation and fission product release during subsequent debris bed heat-up.