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Separate-effect tests on zirconium cladding degradation in air ingress situations

Funded under: FP7-EURATOM


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.
A state-of-the-art review performed before SARNET started showed that the existing data on 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 behaviour in air + steam or nitrogen-enriched atmospheres (encountered in oxygen-starved conditions), almost no data were available.
New experimental programmes comprising small-scale tests have therefore been launched at FZK, IRSN (MOZART programme in the frame of the International Source Term Program-ISTP) and INR. Zircaloy-4 cladding in PWR (FZK, IRSN) and in CANDU (INR) geometry are investigated. On-line kinetic data are obtained on centimetre size tube segments, by thermogravimetry (FZK, IRSN and INR) or by mass spectrometry (FZK). Plugged tubes 15 cm long (FZK) are also investigated. The samples are air-oxidised 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 degrees Celsius up to 1500 degrees Celsius. Systematic post-test metallographic inspections 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 (Q-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.

Additional information

Authors: DURIEZ C, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR);STEINBRÜCK M, Forschungszentrum Karlsruhe, FZK, Institut für Materialforschung, Karlsruhe (DE);OHAI D, Institute for Nuclear Research, Mioveni Arges (RO);MELEG T, Institute for Nuclear Research, Mioveni Arges (RO);BIRCHLEY J, Paul Scherrer Institute, Villigen (CH);HASTE T, Paul Scherrer Institute, Villigen (CH)
Bibliographic Reference: An article published in: Nuclear Engineering and Design, Article in Press, Corrected Proof
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