Effect of steam and oxygen starvation on severe accident progression with air ingress

The CODEX-AIT-3 experiment simulated an in-vessel air ingress scenario after failure of bottom head of a nuclear reactor. 1600 °C maximum temperature was reached with the electrically heated bundle and slow cool-down was applied at the end of the test to demonstrate the bundle state before quench. Limited air and steam flow rates were intentionally selected in order to slow down the high temperature chemical interactions and provided unique data on nuclear fuel behaviour under oxygen and steam starvation conditions. The individual oxidation and nitriding phenomena were well separated from each other and it was indicated by the complex microstructures in the zirconium cladding with nitrides, oxides and transition zones. The competition between different chemical reactions could be excluded in some locations and some time periods. The post-test numerical analyses of the experiment could be used for the evaluation of the role of numerical models developed for individual chemical reactions in severe accident codes.