Corrosion of Agr Fuel Pin Steel Under Conditions Relevant to Permanent Disposal

The majority of the UK nuclear reactor fleet is made up of the Advanced Gas-cooled Reactor type (AGR). These utilise fuel rods made from ceramic UO₂ fuel encased in a 20/25/Nb stainless steel cladding. Currently, spent fuel rods undergo reprocessing after removal from the reactor, but the option of direct disposal of spent fuel in a Geological Disposal Facility (GDF) is now under examination. Direct disposal is the current policy in several mature nuclear states (USA, Sweden, Spain, and Finland are examples). The vast majority of spent oxide fuel in these states is from Light Water Reactors (LWR), and the disposal of the UK’s gas-cooled reactor fuel may behave differently due to differences in materials and operating temperature.

The GDF proposal would involve spent fuel rods being placed in copper or steel canisters and buried several hundred meters underground in tunnels backfilled with clay. It is assumed that over several thousand years groundwater from the environment will penetrate these barriers and come into contact with the fuel surface and steel cladding.

The corrosion behaviour of the fuel ceramic and steel cladding is of interest as part of the safety case for the construction of a GDF. The GDF is an anoxic environment, but alpha-radiolysis of water from the activity of the fuel will create a source of near-field peroxide oxidants.

Anaerobic corrosion of the cladding is expected to yield Fe²⁺ and H₂ at the surface of the spent AGR fuel, along with species such as Ni(II) and CrO4^2-. These species are expected to inhibit the dissolution of the UO₂ into the groundwater, but the situation may be complicated by the interaction of Fe(II) with radiolytically generated H₂O₂ via the Fenton cycle.

The ceramic UO2 fuel can be oxidised by the species generated by radiolysis, converting the uranium from U(IV), which has very low solubility in water,  to U(VI), which is much more soluble. This oxidation will lead to dissolution of the uranium fuel, allowing uranium and fission products to escape the ceramic fuel matrix and enter the groundwater. This contaminated groundwater could pose a danger to life as it is transported through the biosphere.

The effect of hydrogen peroxide on the corrosion behaviour of AGR fuel pin steel has been investigated under conditions relevant to permanent disposal in a geological disposal facility. Electrochemical experimental methods including open circuit potentials and linear sweep voltamograms were used to understand the corrosion susceptibility over a range of potentials. Comparisons to other grades of steel and the effect of the high proportion of nickel present examined.

Linear polarisation of the steel shows pitting beginning to occur on the steel surface, localised around the NbC precipitates present, at a potential of around -150mV vs. SCE before undergoing severe trans-passive corrosion at around 400mV vs. SCE. The potentials where the onset of pitting is seen are similar to the expected potentials in repository conditions.

An increase in concentration of hydrogen peroxide in the electrolyte was found to reduce the region of passivity in the steel, leading to an increase in corrosion potential. The range of potentials expected in a repository was identified. The corrosion products released in this region (Fe, Ni, and Cr species) and their expected concentrations were investigated by ICP OES analysis.

The effect of corrosion on the surface of the steel was examined using Electrochemical Impedance Spectroscopy and Scanning Electron Microscopy. Changes to the UO₂ corrosion behaviour under the influence of the cladding corrosion products are discussed.