Convective heat transfer in a semi-closed BWR-fuel assembly in absence of water

•Experiments were made about air cooling of BWR-FA with blocked bottom nozzle.•Stagnation region inside the experimental apparatus occurred.•Main heat transport mechanism is radial conduction.•Heat removal from FA will strongly depend on heat removal from canister.

In case of a partial drain-down or a specific boil-off scenario it can be presumed that fuel assemblies in a spent fuel pool are cooled by air. Their lower end can be blocked by residual water or debris (from containment etc.). The detailed heat and mass transfer mechanisms are yet unknown. Therefore, reliable cladding temperatures cannot be predicted. For a better understanding of the mechanisms experiments were conducted at the test facility ADELA-II at the TU Dresden. The facility simulates a quarter of a generic fuel assembly in original scale. Different experiments at low rod power levels (5 W and 10 W per rod) were conducted. The results show that the convective heat transfer in a fuel assembly under such boundary conditions is strongly inhibited. A stagnation region evolves where convective heat transfer inside the fuel assembly can be neglected. As a result the coolability of it depends on the heat removal from the fuel assembly box to the surrounding.