Since the reactor accident in Fukushima Daiichi, the vulnerability of spent fuel pools (SFP) is more focused in nuclear safety research. In case of a structural damage of the SFP through an external event with a loss of coolant, the coolability of the spent nuclear fuel is endangered. If the pool is completely drained, the fuel assemblies (FA) are fully uncovered and only cooled by air. A sufficient decay heat transfer depends on the arising air mass flow from the containment through the spent FA from bottom to top (chimney effect). Beside analytical approximations from the U.S. Nuclear Regulatory Commission (NRC) that are partially not publicly accessible only little experimental data about loss of coolant accidents in SFP exist.
This paper presents the experimental findings about the convective heat transfer of a boiling water reactor (BWR) spent FA under the absence of water. These studies are performed within the joint project SINABEL that is funded by the German Federal Ministry of Education and Research to investigate the thermal hydraulics of selected accident scenarios in SFP experimentally and numerically.
For the experimental investigation, the test facility ALADIN was build up in 2016. This mock-up is a full-scale electrically heated 10x10 rod bundle surrounded by additional heated rods to simulate the surrounding and to adjust nearly adiabatic boundary conditions. The rod bundle is equipped with a unique 3-D net of thermocouples that allows the observation of the cladding temperature distribution and temperature development combined with filling level measurement and video monitoring. Different experiments at single rod powers between 20-100 W were conducted.
The results show that the convective heat transfer of a BWR FA that is only cooled by air is strongly inhibited. Inside the FA channel where axial flow is not appreciable, the entire heat has to be transferred in radial direction requiring large temperature differences.