ABSTRACT
Millions of gallons of high-level radioactive waste are stored in underground carbon-steel storage tanks at the Hanford site. The waste chemistries are mixtures of organic and inorganic species and are predominantly alkaline. Tank corrosion conditions are partly monitored by measuring corrosion potentials of the tank carbon-steel. In addition, independent studies are conducted using chemical simulants representing the tank waste chemistries. It has been observed that several tanks’ corrosion potentials are invariant with time, however, laboratory experiment data with the simulants exhibit time-dependent evolutions of the corrosion potentials for several tank waste chemistries. Laboratory experimental data is used to assess the risk of pitting and stress corrosion cracking of the tank walls, and corrosion potential is one of the important parameters in the risk assessment matrix. Therefore, it is important to understand the source of corrosion potential evolutions and their implications on the risk assessment. To this end, laboratory experiments were conducted with the simulants of three tank chemistries. Corrosion potential evolutions were measured. The tank steel conditions were represented in each simulant using three coupons with the following surfaces: (i) polished, (ii) mill-scale with corrosion products, and (iii) partly polished surface of a mill-scale coupon. The electrochemical experiments were conducted for approximately 4 months, and corrosion potential evolutions were recorded. This paper presents the electrochemical test results and provides quantification of the evolutions as a function of surface characteristics and waste chemistries. The paper also explores sources of corrosion potential evolutions observed in the experiments.
