Hafnium oxidation at high temperature in steam

To assess the potential impact of using hafnium as absorber material in LWRs in high temperature accidental situations, the oxidation behavior of hafnium was studied up to 1400 °C, i.e. at temperature conditions relevant to severe accidents. Different sample geometries were tested and oxidized in steam/argon mixtures, either in a furnace or in a thermogravimetric analyzer. Metallographic examinations, hydrogen measurements and EPMA oxygen profiles were then performed. For hafnium rods/discs, metallographic examinations showed the presence of a dense and protective oxide film after steam oxidation. No or little hydrogen was detected in the metallic part of the rod/disc specimens. The reaction rate can be described by a parabolic law in the tested temperature range in the mid-to-long term, and the value of the effective activation energy determined from the experimental data in steam is in good agreement with the ones published in the literature. The diffusion coefficient of oxygen in hafnium was estimated at each temperature by fitting the experimental oxygen profile obtained on hafnium rods and its temperature dependence is derived in the temperature range 700-1400 °C. The hafnium claddings produced for the application in integral bundle tests exhibited a lower resistance to steam oxidation than hafnium rods/discs. Metallographic examinations showed a non-protective layer and a significant hydrogen amount was picked up by hafnium claddings. Above 800 °C, the oxidation rate for hafnium claddings follows a cubic to quartic law and the effective activation energy was determined in the temperature range 800-1100 °C. These tests highlighted the influence of the surface conditions on the oxidation rate of hafnium in steam. However, hafnium oxidation rate remains well below the oxidation rate of zirconium alloys in the same temperature range.