AGR Cladding Corrosion: Investigation of the Effect of Temperature on Unsensitized Stainless Steel
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AGR Cladding Corrosion: Investigation of the Effect of Temperature on Unsensitized Stainless Steel Elizabeth Howett1, Colin Boxall1 and David Hambley2 1 Engineering Department, Lancaster University, Lancaster, LA1 4YW, United Kingdom. 2 National Nuclear Laboratory, Sellafield, Cumbria, CA20 1PG, United Kingdom. ABSTRACT The corrosion of unirradiated and unsensitized Advanced Gas-cooled Reactor cladding material, 20/25/Nb stainless steel, was studied as a function of temperature and [Cl-] typical of those found in interim spent fuel storage pond waters. With respect to preventing corrosion, it found to be advantageous to dose the ponds to pH≈11.4. At pH lower than 7, the initiation of pitting is observed at ~0.4V vs Ag/AgCl, an undesirable condition as pits are considered to be initiators of stress corrosion cracking (SCC) which may contribute to loss of cladding integrity during storage. Such pits are not seen at a pH 11.4 for the expected and projected pond operating temperatures of 24-60°C. There generally appears to be no localised corrosion threat to cladding as the temperature is increased in this range, although substantial pit formation is observed at the extreme maloperation temperature of 90°C at pH 11.4 indicating a loss of protection. INTRODUCTION The bulk of the UK’s currently operating fleet of nuclear reactors are of the Advanced Gas-cooled Reactor type (AGR). These were constructed during the 1970s and 1980s, and utilize a graphite moderator and CO2 gas coolant. The fuel used in AGR reactors consists of low enriched UO2 annular ceramic pellets, encased in a 20/25/Nb stainless steel cladding. Currently, spent nuclear fuel (SNF) from these reactors undergoes reprocessing at the Thermal Oxide Reprocessing Plant (THORP) in Sellafield, Cumbria, where the spent fuel components are separated. Unused uranium and plutonium can be sent to form new fuel, and the fission products present in the fuel can be separated for disposal as high-level waste. However THORP is due to close within the next 5 years. The plan for current holdings of un-reprocessed AGR SNF and SNF that will be discharged from AGRs in the future is to send it to a national GDF (geological disposal facility). The intention is for the GDF to start taking intermediate level waste (ILW) in the late 2030s, with fuel to follow after the disposal of the bulk of the accumulated ILW. It is therefore likely that the GDF may not be open for receipt of spent fuel until around 2075. Until then AGR SNF will be kept in interim wet storage in ponds at Sellafield, ultimately being consolidated into a single, long term storage pond. The fuel will be stored in demineralized water with a chloride concentration typically < 0.1 ppm. This will be dosed with NaOH to pH≈11.4 at 24°C to act as a corrosion inhibitor, a treatment that has been shown to be effective for current storage requirements [1]. In the absence of the inhibitor, cladding corrosion is known to be possible, leading to perforation within 1- 2 years, if the pond chloride concentration is increased to 0.2
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