Development Of A Computer Code Model For Nitriding And Re-Oxidation Of Cladding Materials Under Severe Accident Conditions

Full scale experiments conducted in 2011 and 2012 at the Sandia National Laboratories in the Sandia Fuel Project (SFP), three QUENCH tests at KIT as well as many separate-effect tests in various labs have shown that nitrogen plays an active role in the degradation process of spent fuel cladding materials and may lead to the loss of the only barrier for the fission product release in case of a spent fuel storage accident. To account for the effect of nitrogen in the cladding degradation, PSI has conducted together with KIT an experimental programme during the years 2014 and 2015.More than 70 separate-effect tests were conducted at KIT. The results of the tests provide a data base sufficient for the development of a nitriding and re-oxidation model to describe the accelerated degradation of cladding materials under air ingress conditions during a severe nuclear accident. The samples were analysed not only for the mass gain during exposure to subsequently oxygen, nitrogen, and again oxygen, but also for the composition after the tests using a combined metallographic and image analysis. In addition, detailed examination of the samples was carried out to determine possible existence of Zr-oxy-nitrides as a result of the nitriding reaction. The conceptual model for nitriding includes formation of Zr-oxide and oxygen stabilized alpha-Zirconium (alpha-Zr(O)) during pre-oxidation in oxygen, fast nitriding and slow nitriding during the exposure to nitrogen, and the effect of breakaway oxidation.A standalone computer code to describe the nitriding reactions was developed based on the separate effect tests conducted at KIT. In the present paper, the model concept and the first calculations of separate-effect tests are presented for all phases of pre-oxidation, nitriding and re-oxidation. The model includes the nitriding process under oxygen and steam starvation conditions and the strongly accelerated process of re-oxidation when oxygen or steam is recovered as observed in several separate effect tests and also integral experiments such as QUENCH-16, QUENCH-18, and the Sandia Fuel Project tests (SFP). The model distinguishes between two regimes, the breakaway regime at temperatures below 1050 degrees C and the non-breakaway regime above 1050 degrees C. In the high temperature regime the acceleration of the oxidation is based on the morphology change due to the nitriding reaction (porous structure of ZrO2 formed by re-oxidation of ZrN).This model is an extension of the PSI air oxidation and breakaway model available in the severe accident codes SCDAP/Relap5 and MELCOR.