Zircaloy-4 and M5® oxidation by air between 600 and 1200°C
Congress title :15th International Symposium on Zirconium in the Nuclear Industry
Congress town :Sunriver
Congress date :25/06/2007
In the field of nuclear plant safety analysis, different kinds of accidental scenarios can result in cladding exposure to air containing atmospheres. Major examples are spent fuel storage pool loss of water inventory accident and reactor pressure vessel breaching following a thermal shock event. In such situations, the evolution of the fuel cladding up to its failure is of critical importance and will have direct implications on the source term (inventory of radio-nuclides released in the environment). The prediction of this evolution requires a comprehension of the oxidation behaviour of zirconium alloys when exposed to air at high temperature.
Zirconium alloys oxidation at high temperature has been widely studied in steam, but the experimental database for air is much more restricted. An experimental program dedicated to the study of air-oxidation of Zircaloy-4 and M5TM has been launched at the French Institut for Nuclear Safety and Radioprotection (IRSN) at the end of year 2004. Using a commercial thermo-gravimetric analyser (TGA), detailed kinetic data are obtained in isothermal conditions, in a 600-1200°C temperature range. The alloys are investigated first in a as received state, then pre-oxidised in steam at low temperature to simulate in-reactor corrosion. TGA experiments are completed with metallographic inspections and some EPMA investigations.
The main results can be summarised as follows:
At the beginning of air exposure, the oxidation rate obeys a parabolic, diffusion-limited, behaviour. The parabolic rate constants are comparable, for Zry-4 as well as for M5TM, to the correlations recommended for high temperature steam-oxidation of Zircaloy-4.
However, a transition to high oxidation rates rapidly occurs. Thanks to the TGA data, the transition has been clearly correlated to the radial cracking of the first-formed protective dense oxide scale. This breakaway process occurs for a scale thickness that strongly increases with the temperature. A corrosion scale favours the transition, which occurs earlier for the pre-oxidised samples. The M5TM alloy is breakaway-resistant, showing a late transition compared to Zircaloy-4.
The post transition regime is linear only for the lowest temperature investigated. From 800°C, it is rather observed a continuously accelerated regime up to very high degrees of oxidation. The mechanism of zirconium nitride particles formation and re-oxidation is shown to be responsible of this accelerated degradation, associated with the formation of a highly porous oxide.