Radioactive and chemically hazardous wastes are stored at the Hanford site in underground double-shell tanks (DSTs) constructed of carbon steel. The corrosion management of these tanks has largely focused on the complex waste chemistries that reside within the interiors of the primary tanks. More recently, attention has been given to the corrosion that has been found on the exterior of the secondary liners of the DSTs. The cause of the external corrosion seems to be related to the intrusion and accumulation of groundwater in the tertiary leak detection systems, which exist under the concrete foundations of the DSTs. The external corrosion has led to significant wall-loss (up to 70% in one of the tanks), which has occurred in a localized manner (broad pitting). In addition, site measurements have indicated that the most significant areas of wall-loss are aligned with the slots (drain channels) of the concrete foundation.
Experiments were conducted to assess the penetration rates that could be associated with the external corrosion of the secondary liners. Carbon steel specimens were affixed to slotted concrete blocks and exposed to a synthetic groundwater solution that was periodically raised and lowered in the test vessel to create alternate immersion conditions. Pitting penetration rates were determined by measuring the maximum depth of localized attack as a function of exposure time in the groundwater environment. The penetration rates decreased with time and appeared to follow a power-law behavior. Also, it was found that the penetration rates of the concrete-contacted regions of the specimens were generally lower than the rates of the boldly exposed or slotted regions.
The Hanford site contains approximately 55 million gallons (2.08 × 108 liters) of radioactive and chemically hazardous wastes arising from weapons production, beginning with World War II and continuing through the Cold War era. The wastes are stored in 177 carbon steel underground storage tanks, of which 149 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). Historically, tank failures have been associated with the SSTs. However, in 2012, a failure was encountered for the first time in one of the DSTs: Tank AY-102. The failure of AY-102 occurred from internal corrosion of the primary liner,1 which allowed the waste to leak into the annular space between the primary liner and the secondary liner. The waste never reached the surrounding soils due to the containment provided by the secondary liner.
