Materials aspects of SRP waste storage: corrosion and mechanical failure

Studies were made on the causes of observed leaks in mild steel tanks used to store liquid waste from nuclear fuel reprocessing at the Savannah River Plant. Leaks were observed in the walls of some primary vessels and in some cooling coils. The studies revealed that: wall leaks resulted from nitrate stress corrosion cracking caused by alkaline nitrate solution; and coil leaks resulted from pitting corrosion caused by diluted waste solutions during sludge removal. Stresses that caused wall cracking were residual stresses, primarily those produced during tank fabrication. Pitting of coils was caused by the concurrent dilution of nitrite in the waste supernate, and leaching of sulfate from the sludge. Results of these studies provided the bases for modifications in fabrication and operation of waste tanks. The modifications included: selection of more stress corrosion resistant steels for tank fabrication, heat treatment of newly fabricated tanks to relieve stresses, operation of tanks at temperatures above the nil ductility transition temperature of the steel to preclude brittle fast fracture, operation of tanks at temperatures below specified upper limits to reduce incidence of nitrate stress corrosion cracking, specification of limits on concentrations of nitrate, nitrite, and hydroxide ions in waste supernate to prevent stress corrosion cracking of walls, and specification of limits on nitrite ion concentration in sludge slurry to prevent pitting corrosion of cooling coils