Electrogravimetric Studies of 20Cr-25Ni Austenitic Stainless Steel to Characterize Spent Nuclear Fuel Cladding Corrosion

Corrosion processes are inherently electrochemical in nature. The economic costs and risk to human health as a result of corrosion induced failure can be catastrophic [1]. Current policy calls for UK spent nuclear fuel (SNF) to be sent to a geological disposal facility (GDF) for its permanent disposal. However, it may be up to 100 years before a UK GDF becomes available [2]. Consequently, all spent fuel arisings for the foreseeable future will be sent to interim wet pond storage at Sellafield. SNF is comprised of cylindrical UO₂ pellets, encased in 20Cr-25Ni-Nb austenitic stainless-steel (SS) cladding. Consequently, cladding plays a major role in the containment of SNF. A loss in cladding integrity, due to in-situ corrosion processes, would result in pond or ground (in the cases of interim storage or ultimate disposal respectively) water becoming contaminated with water soluble radioactive fission products – such as, ¹³⁷Cs, ⁹⁰Sr, ⁹⁹Tc, ³H. A cladding failure scenario greatly increases the costs and severity of protection measures necessary for the safe handling and storage of SNF.<br/><br/>Using the electrochemical techniques of open-circuit potentiometry (OCP) and linear-sweep voltammetry (LSV), this paper presents studies on the corrosion susceptibility under wet storage conditions of as received cladding manufactured at Springfields. Corrosion mechanisms imply mass changes at the cladding surface and the quartz-crystal nanobalance (QCN) is a gravimetric analysis technique, that can be coupled with electroanalytical methods, to measure electrochemically induced material mass changes with nanogram sensitivity [3]. Thus, quartz-crystals coated in 20Cr-25Ni austenitic steel were specially manufactured to replicate SNF cladding composition. As a result, novel electrogravimetric analysis has also been performed, with 20Cr-25Ni coated QCN crystals (cladding simulant) as working electrode in the electrochemical cell.<br/><br/>LSV experiments were conducted on cladding as a function of temperature in pH 11.4 dosed storage water simulant with [Cl] = 20uM. Observed corrosion potentials, passive ranges and cathodic shifts in pitting potential with increasing temperature were in agreement with literature studies on 20Cr-25Ni-Nb and other austenitic stainless steels [4]. The effect of increasing chloride concentration was also investigated. This indicated that whilst corrosion inhibition is maintained up to 1mM chloride at ambient temperature 298K, a greatly increased corrosion susceptibility was observed for at the same 1mM chloride concentration at the predicted pond operating temperature of 318K. This suggests that either an alternative cladding corrosion inhibition strategy is necessary under these conditions, or that chloride content of the storage ponds needs to be carefully controlled.<br/>Finally, QCN experiments were performed on cladding simulant material in order to better determine the effect of pH 11.4 dosing on passive layer behaviour, and the nature of trans-passive corrosion under these conditions. Previous studies on 316 SS in 3.6M HNO₃ have revealed that no net growth in the passive layer occurs below an applied overpotential of ~400mV, above which the steel of the QCN crystal is catastrophically stripped. In contrast, analogous QCN studies of a 20Cr-25Ni-Nb SS simulant layer in simulant pond water at T = 298.15K°, pH =11.4, and [Cl] = 20uM, reveal that significant mass/passive layer growth occurs when held at the open-circuit potential, with no mass loss indicated in the trans-passive region at overpotential &gt; ~1000mV. This highlights the advantages of using the QCN - which essentially combines conventional coupon testing with electrochemical corrosion experiments - as a means to better characterise the effect of pond dosing strategies on cladding corrosion inhibition. Accordingly, studies are now ongoing at elevated temperatures and chloride concentration in order to determine the envelope of protection afforded by the current strategy.