Failure analysis of high-level radioactive waste tank purge ventilation vent pipe

High-level radioactive wastes generated during the processing of nuclear materials are stored in large underground storage tanks made of low-carbon steel. The wastes consist primarily of concentrated solutions of sodium nitrate (NaNO3) and sodium hydroxide (NaOH). Each of the tanks is equipped with a purge ventilation system designed to continuously remove hydrogen gas and vapors from the waste tanks while preventing the release of radionuclides to the atmosphere. Several intergranular cracks were discovered in the vent pipe of the purge ventilation system of a particular high-level waste tank. The vent pipe, fabricated from galvanized steel sheet, was connected to the final exhaust fan downstream of high-efficiency particulate air (HEPA) filters. The failure analysis investigation concluded that nitrate-induced stress-corrosion cracking (SCC) was the cause of the failure. © ASM International.