ABSTRACT
This paper presents the results of a laboratory investigation performed to evaluate the impact of key Hanford tank waste chemical constituents on corrosion of Type 304L stainless steel (UNS S30403), which is the material of construction for the site’s evaporator. Simulated chemistries of the effluent were used to evaluate the susceptibility of the stainless steel to localized corrosion. The work is intended to improve the technical basis for establishing waste chemistry limits and controls that protects the integrity of the site evaporator system while processing tank waste and potentially effluent from the Hanford Waste Treatment and Immobilization Plant (WTP). The results of the testing indicate that the environmental conditions expected to be present in the evaporator during the processing of retrieved waste and WTP effluent are not likely to cause localized corrosion attack of UNS S30403.
INTRODUCTION
The Hanford site contains approximately 2.1 × 108 liters (56 million gallons) of radioactive and chemically hazardous wastes arising from plutonium production for the nuclear weapons program beginning with World War II and continuing through the Cold War. The wastes are stored in 177 carbon steel underground storage tanks, of which 149 are single-shell tanks (SSTs) and the remaining are doubleshell tanks (DSTs). The mission of the River Protection Project is to retrieve the waste from the underground storage tanks and then treat and immobilize it for disposal. The Hanford Waste Treatment and Immobilization Plant (WTP) is being designed to separate the waste into high level waste (HLW) and low activity waste (LAW) portions and then treat and immobilize those portions by vitrification.
The site evaporator will be used to reduce the volume of waste recovered from the underground storage tanks for continued storage and subsequent treatment and immobilization. While the evaporator has successfully processed waste retrieved to date, future retrieval operations may give rise to evaporator feeds containing key constituents at concentrations higher than previously experienced. Additionally, the WTP will generate a dilute effluent stream from condensation and scrubbing of the LAW melter off-gas. During the early part of the WTP mission, a portion of this effluent may be returned to the tank farms for management and/or evaporative concentration before recycling to the WTP. This dilute effluent will be rich in chloride, fluoride, and sulfate ions, but poor in traditional corrosion inhibitors, such as nitrite, that are present in the tank waste. The main circulation loop of the evaporator, consisting of a reboiler, vapor- liquid separator vessel, recirculation pump, and inter-connecting piping, is fabricated from AISI Type 304L stainless steel (UNS S30403).1,2 The evaporator heats liquid waste under a vacuum of about 60 torr (0.08 atm) so that it boils at a temperature of about 50°C. Water vapor from the boiling waste is captured, condensed, filtered and sent to an effluent treatment facility for treatment and disposal. The concentrated waste is then returned to the double-shell tanks for storage.
