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

Fuel pins from the UK's Advanced Gas-cooled Reactors (AGR) consist of ceramic UO2 fuel encased in a 20/25/Nb stainless steel cladding. Spent AGR fuel is currently reprocessed, but the option of direct disposal of spent fuel in a sealed, underground Geological Disposal Facility (GDF) is now under examination. 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.Electrochemical studies on unsensitised samples of 20/25/Nb steel in simulant groundwater electrolytes have been performed, and show low corrosion currents, typically of the order μA/cm2, at the oxidative potential stresses found in such repositories. Whilst the cladding may therefore be considered to be passive short time periods, the very long timescales involved in a GDF project mean that these currents are sufficient to cause corrosion of the full thickness of the clad in a matter of decades.Furthermore, an increase in electrochemical potential from that expected of less than 50 mV is sufficient to initiate severe pitting corrosion in a matter of hours. It can therefore be assumed that there is a risk of cladding corrosion in a repository environment, and the corrosion products created may have implications for the chemistry of the spent fuel ceramic.