Hanford stores millions of gallons of high-level waste in 27 carbon-steel double shell, underground tanks. A secondary shell surrounds the primary shell, where the bottom plate of the secondary shell rests on a channeled concrete pad. There have been instances of metal loss on the secondary shell bottom plates in contact with the concrete basemat where groundwater accumulation in the channels may have caused corrosion. In addition, uneven contact between the basemat and shell could create occluded areas where localized corrosion is possible. In previous studies, vapor corrosion inhibitors (VCIs) were tested for their ability to mitigate concrete-basemat side corrosion of the secondary shell bottom. The previous study indicated that VCIs are effective in mitigating corrosion in both immersed and vapor space conditions. However, the tanks being large with approximately 70-ft bottom, it is important to understand VCI distribution rate after VCIs are injected. Experiments were conducted with VCI injection in the groundwater along with coupons positioned at several locations with respect to the ground water. Coupons' potentials were monitored, and corrosion rate data were analyzed. It was determined that corrosion potential is a good indicator of VCI concentration in the simulated groundwater solution. The paper presents result and analysis of the experimental data.
High-level radioactive waste generated during reprocessing of spent nuclear fuel at Hanford has been stored in several single- and 27 double shell tanks (DSTs). Each DST consists of a primary shell (inner) surrounded by secondary (outer) liner. The secondary liner rests on a concrete foundation. Rainwater may seep in and accumulate in the drain slots and may corrode the exterior of the secondary liner. Evidence of wall thinning has been detected via ultrasonic inspections of the annulus floor between the primary and secondary tank shells. Since the inspection is confined to this region, there is a concern that corrosion is widespread on the underside of the bottom plate. Since the rainwater level can vary in the drain slots based on accumulation, corrosion could be caused by direct contact with the accumulated water; when the leak detection pit (LDP) water level is below the structural limit, vapor space corrosion (VSC) could also occur. Accumulated water is drained through the sumps in the LDP. The LDP water was analyzed for its constituents, and two simulants were developed considering the chemical composition range of the accumulated water. The simulants are identified as leak detection pit and ground water (GW); compositions are listed in Table 1. A previous study established that GW simulant is more corrosive than the leak detection pit, therefore GW was used in the VCI effectiveness study.1
