Study on loss-of-cooling and loss-of-coolant accidents in spent fuel pool. (2) Fuel cladding oxidation

In spent fuel pool (SFP) loss-of-cooling and loss-of-coolant accidents, fuel rods will be exposed in atmosphere containing air and steam. For the detail investigation of SFP accident, severe accident (SA) codes such as MAAP and SAMPSON need to be improved, because the oxidation models utilized in these codes are which constructed for steam environment in the internal of pressure vessel during the accident. Therefore oxidation behavior of Zircaloy-2 (Zry2) cladding in dry air simulating SFP accident condition was evaluated by using a thermobalance, and the obtained data were applied to construct oxidation model for SFP accident condition. For the validation of the constructed oxidation model, oxidation tests using a long cladding tube 500mm in length were conducted in conditions simulating SFP accidents, such as flow rate of the atmosphere in spent fuel rack, temperature gradient along the axis of cladding, and heating-up history. Thickness of oxide layer formed on the surface of cladding samples was evaluated by cross sectional observation, and compared with calculated results obtained by computational fluid dynamics (CFD) analysis using the oxidation model. The calculated oxidation level of cladding in the SFP accident condition in dry air well reproduced the experiment. Oxygen starvation in the testing chamber was suggested both in experiment and in calculation. Influence of oxygen starvation for cladding oxidation behavior, as well as oxidation behavior of the cladding plug material were evaluated for further discussion considering the actual SFP accident. (author)