Hafnium Oxidation in steam at high Temperature

Due to its high thermal neutron absorption cross section and good corrosion resistance at low temperature, hafnium has been used in the nuclear reactors as control rods or shut-off rods since the late 1950’s. Its use in commercial reactors declined in the 1990’s, due, in part, to its ability to pick-up hydrogen and swell. However, hafnium received a renewed interest, theses last years. For instance, in France, it is planned to generalize the introduction of absorber rods of hafnium not sheathed in fuel assemblies positioned in the core periphery in order to reduce the neutronic flux received by the vessel in the 900 MWe power plants. This paper presents IRSN and KIT results on the oxidation behaviour of hafnium up to 1400°C, i.e. in temperature conditions relevant to severe accidents. Samples, with different surface conditions (rods/tubes) were oxidized in steam/argon mixtures, either in a furnace or in a thermogravimetric analyser (TGA). Metallographic examinations, hydrogen measurements and Electron Probe Micro Analysis (EPMA) oxygen profiles were then performed. All the tests confirm that hafnium exhibits better resistance to oxidation at high temperature than zirconium alloys. For hafnium rods, metallographic examinations show the presence of a dense and protective oxide film, whereas, on hafnium tubes, a white and porous layer is observed. No or little hydrogen is measured in the metallic part of the rod specimens while significant hydrogen amount is picked up by hafnium tubes. Regarding hafnium rods, the reaction rate may be described by a parabolic law in the investigated temperature range, and the value determined from the experimental data in steam is in good agreement with the ones published in the literature for oxygen. The oxygen diffusion coefficient was estimated at each temperature by fitting the experimental profile obtained on hafnium rods. A linear regression of the data allows an estimate of the temperature dependence of the diffusion coefficient of oxygen in hafnium in the temperature range 700-1400°C. Hafnium tubes led to weight gain twice as high as the ones measured on hafnium rods. However, it remains well below the oxidation rate of zirconium alloys in the same temperature range. Above 800°C, the reaction rate for hafnium tubes cannot be modelled by a parabolic law, which confirms that the oxide layer formed on this material, is not protective. These tests highlighted the influence of the surface conditions on the oxidation rate of hafnium in steam.