ABSTRACT
The Hanford Nuclear Reservation uses large underground, carbon steel tanks for interim storage of liquid radioactive waste. The Hanford Waste Treatment and Immobilization Plant (WTP) is being constructed to treat the high-level waste (HLW) and low-activity waste (LAW) in the tank. The WTP LAW recycle stream will be generated by condensation and scrubbing of the LAW melter off-gas stream. A portion of this stream, which will contain substantial amounts of chloride, fluoride, ammonia, and sulfate ions, may be returned to the tank farms for storage and evaporation. Presently, there are no restrictions on the halide or sulfate concentrations of this return stream prior to transferring to the carbon steel waste tanks. Prior testing has shown the waste tanks would be susceptible to pitting corrosion due to the halide contents of the return stream compositions investigated without adequate inhibitor concentrations. Cyclic potentiodynamic polarization tests were performed with statistically designed compositional test matrices to determine and optimize the nitrite inhibitor requirements needed to safely store and process the return stream in the waste tanks.
INTRODUCTION
Weapons, space, and medical research programs led by the U. S. Department of Energy have created a legacy of nuclear waste over the past 70 years. At the Hanford site in Richland, WA, the liquid waste is being stored on an interim basis in 149 single shell tanks (SST) and 28 double shell tanks (DST). The final disposition of the liquid waste into a solid vitrified form will likely take several more decades. Understanding potential degradation mechanisms is a key to preserving the structural and leak integrity of the waste tanks, particularly the DSTs, until final disposition of the waste. Presently, the site implements a corrosion control program to maintain the waste chemistry within requirements that mitigate corrosion mechanisms such as stress corrosion cracking, pitting and general corrosion.
With the eventual start-up of the Hanford Waste Treatment and Immobilization Plant (WTP), secondary wastes generated during the Direct Feed Low Activity Waste (DFLAW) portion of the mission may be returned to the DSTs. The tank farm facility is reviewing and evaluating material balance projections to determine the impact of these processes on the waste chemistry. Initial projections suggest that the waste chemistry of the receiving DSTs may shift to a broader range of pH and higher aggressive anion concentrations (e.g., chloride, sulfate, etc.) than the waste compositions that are presently stored. Therefore, in conjunction with this evaluation, a corrosion control program is being developed to ensure that it is robust and can adjust to these anticipated changes.
