In the last 10 years large-scale experiments and separate effect tests have shown the potential of cladding destruction during oxidant starvation by formation of zirconium nitride (ZrN) in presence of air or nitrogen which is used as "inert" gas in boiling water reactor containments. This reaction plays an important role during loss of cooland accidents in spent fuel pools. PSI and KIT have started a project to develop a standalone computer model to calculate the behavior of the cladding material under different concentrations of oxygen and nitrogen. More than 70 separate effect tests were used to define the temperature dependent reaction rates for the pre-oxidation phase, the annealing phase, the nitriding phase and the re-oxidation phase. The later one has to be separated into oxidation of
ZrN and oxidation of the remaining metal. The model distinguishes also between two regimes, the breakaway regime at temperatures below 1050°C and the non-break-away regime above 1050°C. In the high temperature region, the acceleration of the oxidation results from the morphology change due to the nitriding reaction (porous structure of re-oxidized ZrN).
Embedded in the former PSI air oxidation and break-away model, which is implemented in the MELCOR and SCDAP/RELAP5 codes, the standalone code is able to calculate the production of several components like oxygen stabilized alpha zirconium (α-Zr(O)), zirconium nitride (ZrN) and zirconium dioxide (ZrO2). In addition, the consumption of oxygen, steam and nitrogen as well as the release of nitrogen during ZrN oxidation is calculated. The results of the standalone code will be compared with MELCOR calculations (without nitriding) in the different phases of integral experiments and separate effect tests. In the present work, the model concept and the first calculations of separate effect tests and integral experiments will be presented for all phases of pre-oxidation, nitriding and re-oxidation. The implementation of the new model in severe accident codes is planned to support the calculation especially for spent fuel pool accidents and for cladding behavior during mid loop accidents.