Separate-Effect Tests on Zirconium Cladding Degradation in Air Ingress Situations
Congress title :ERMSAR 2007 2nd European Review Meeting on Severe Accidents
Congress Location :Karlsruhe
Congress Date :12/06/2007
Journal title : Nuclear Engineering and Design Volume : 239 Issue : 2 pages : 244-253 Publication date : 2009/02/01
In the event of air ingress during a reactor or spent fuel pond low probability accident, the fuel rods will be exposed to air-containing atmospheres at high temperatures. In comparison with steam, the presence of air is expected to result in a more rapid escalation of the accident, mainly because (i) the Zr+O2 reaction enthalpy release is about 85% higher than the Zr+H2O reaction, (ii) the zirconium alloy oxidation in air is much faster because reactions with nitrogen degrade the protective oxide scale.
A state of the art review done before the SARNET project showed that the existing data on the zirconium alloy oxidation in air were scarce. Moreover, the exact role of zirconium nitride on the cladding degradation process was poorly understood. Regarding the cladding behavior in air + steam or nitrogen enriched atmospheres (encountered in oxygen starved conditions), almost no data were available.
New experimental programs comprising small scale tests have therefore been launched at FZK, IRSN (Mozart program in the frame of ISTP) and INR. Zircaloy-4 and M5TM cladding tubes in the PWR (FZK, IRSN) and in the CANDU (INR) geometry are investigated. On-line kinetic data are obtained on centimeter size tube segments, by thermogravimetry (FZK, IRSN, INR) or by mass spectrometry (FZK). 15 cm plugged tubes (FZK) are also investigated. The samples are air oxidized either in the as received state, or after pre-oxidation in steam. Analytical tests at constant temperature and gas composition provide basic kinetic data, while more prototypical temperature transients and sequential gas compositions are also investigated. The temperature domains extend from 600°C up to 1500°C. Systematic post-test metallographic inspections of the samples are performed.
The paper gives a synthesis of the results obtained, comparing them in terms of kinetics and oxide scale structure and composition. A comparative analysis is performed with results of the QUENCH-10 bundle test, which included an air ingress phase. It is shown how the data contribute to a better understanding of the cladding degradation process, especially regarding the role of nitrogen. For modelling of the oxide scale degradation under air exposure, important features that have to be taken into account are highlighted.