Mozart program to determine the air-oxidation kinetics of zirconium alloys at high temperature.
The Mozart analytical test program, conducted from 2005 to 2007, was dedicated to the study of oxidation in air of nuclear fuel cladding. This study was part of the International Source Term Program initiated by the IRSN, whose aim is to reduce uncertainties concerning the evaluation of radioactive product emissions into the environment in the event of a core meltdown accident in a pressurized water reactor or a spent fuel storage pool accident.
Certain accidents can lead to the exposure of irradiated fuel claddings to atmospheres containing air. These situations can include the uncovery of spent fuel assemblies in the event of an accident related to loss of water in a storage pool or the contact of a pressurized water reactor core with air penetrating the vessel via a breach during the final phase of a core meltdown accident. In such cases, zirconium alloy cladding becomes oxidized. Because this reaction is strongly exothermic, it can result in reaction runaway and lead to rapid destruction of the cladding, which constitutes the first containment barrier. Loss of integrity of the cladding leads to the release of radionuclides, which may be released into the environment, according to the circumstances. In such situation, to predict the behavior of cladding requires, among other things, an understanding of the mechanisms and a determination of the kinetics of the oxidation of zirconium alloys when exposed to air or a mixture of air and steamvapor at temperatures from 600° to 1200°C.
Bibliographic reviews reveal wide scattering of the existing kinetic data concerning the oxidation of Zircaloy-4 by air in the temperature range concerned. For recent alloys, such as M5TM and Zirlo, there is virtually no data published in the open literature.
Moreover, the mechanism by which zirconium alloys oxidize in air is not perfectly well understood: a study conducted in the USA in the 1970s demonstrated the decisive role played by the simultaneous presence of oxygen and nitrogen on the oxidation process of pure zirconium above 1050°C. More recently, the work carried out at the Forschungszentrum Karlsruhe (FZK) on samples of Zircaloy-4 between 750 and 1250°C showed that, for zirconium alloy, the effect of nitrogen also seems to predominate above 800°C. These studies do not, however, clearly specify to what extent nitrogen is responsible for the acceleration of the oxidation process observed by the authors; an acceleration that could have serious consequences on the progress of the accident.
Finally, most of the existing data were obtained on virgin samples. Only a study carried out at the Argonne National Laboratory (ANL) has investigated the effect of pre-existing corrosion and the presence of dissolved hydrogen in the cladding material on its subsequent oxidation in air, studied at the Argonne National Laboratory (ANL).
The Mozart programme
Because the current state of knowledge concerning the oxidation in air of zirconium alloy revealed unacceptably large gaps and large uncertainties, IRSN decided to launch the Mozart experimental program to supplement the databases and help to better understand and better model the mechanisms involved.
The temperature range studied was restricted to 600-1200°C, because beyond this temperature range, oxidation becomes catastrophic and precise knowledge of the phenomenon is not required. Different types of alloy (Zircaloy-4, M5TM and Zirlo) in different initial states (virgin, pre-oxidized, pre-hydrided to simulate the initial state of corroded cladding in the reactor) were studied. In these tests, synthetic air was used as the oxidizing medium, and the increased weight gain of the cladding due to oxidation was continuously recorded using a thermobalance.
For initially virgin cladding in the temperature range 800-1000°C, the experimental data revealed two kinetic regimes during oxidation at a given temperature. During the first phase, corresponding to the formation of a dense protective oxide on the initially bare metal, the oxidation rate falls, approximately following a parabolic function. After the cracking of this dense layer ('breakaway'), the second phase is characterized by a faster oxidation rate, which either remains constant or increases over time. The presence of nitrogen plays an important role in the degradation of the cladding in this accelerated regime, because a self-sustaining nitriding/oxidation mechanism generates the formation of a porous, non-protective oxide and causes creep in the cladding. Above 1000°C, the kinetic regime remains parabolic, and therefore rather slow, provided that there is sufficient oxygen in the oxidizing medium. Otherwise, nitrogen becomes preponderant in the oxidizing medium, and can then diffuse in the cladding and cause nitriding. The pre-oxidation layer can either have a protective effect or quite the opposite: an accelerating effect, according to the pre-oxide thickness and the temperature domain and the alloy concerned.
Based on these results, a new model for the oxidation of Zircaloy by air was incorporated in the Astec computational software package developed by IRSN to evaluate the consequences of a core meltdown accident.