The Hanford radioactive waste storage site outside of Richland, Washington has 177 underground waste tanks that store approximately 253 million liters of radioactive waste from 50 years of plutonium production. No online corrosion monitoring systems are in place at Hanford to facilitate the early detection of the onset of localized corrosion should it occur in a waste tank. Because of this, a program was started in 1995 to develop an electrochemical noise (EN) system to improve Hanford?s corrosion monitoring strategy. The design of the latest systems and the results of operation are described herein.
The Hanford Site has 177 underground waste tanks that store approximately 253 million liters of radioactive waste from 50 years of plutonium production. Twenty-eight tanks have a double shell and are constructed of welded ASTM A537-Class 1 (UNS K02400), ASTM A515-Grade 60 (UNS K02401), or ASTM A516-Grade 60 (UNS K02100) material. The inner tanks of the double-shell tanks (DSTs) were stress relieved following fabrication. One hundred and forty-nine tanks have a single shell, also constructed of welded mild steel, but not stress relieved following fabrication. Tank waste is in liquid, solid, and sludge forms. Tanks also contain a vapor space above the solid and liquid waste regions. The composition of the waste varies from tank to tank but generally has a high pH (>12) and contains sodium nitrate, sodium hydroxide, sodium nitrite, and other minor radioactive constituents resulting from plutonium separation processes. Leaks began to appear in the single-shell tanks shortly after the introduction of nitrate-based wastes in the 1950s. Leaks are now confirmed or suspected to be present in a number of single-shell tanks.1 The probable modes of corrosion failures are reported as nitrate stress corrosion cracking (SCC) and pitting.2 No leaks have appeared in the double shell tanks. The effort to condense and transfer all single shell tank waste to DSTs is near completion.
Corrosion monitoring of DSTs is currently provided at the Hanford Site through a waste chemistry sampling and analysis program.3 In this program, waste tank corrosion is inferred by comparing waste chemistry samples taken periodically from the DSTs with the results from a series of laboratory tests done on tank steels immersed in a wide range of normal and off-normal waste chemistries. This method is expensive, time consuming, and does not yield real-time data. Previous efforts to directly monitor nuclear waste corrosivity at other sites have included linear polarization resistance (LPR) and electrical resistance techniques.4 These techniques are most effective for monitoring uniform corrosion, but are not well suited for detection of localized corrosion (pitting and SCC). Numerous laboratory studies and recent reports on field applications have reported that EN analysis is well suited for monitoring and identifying the onset of localized corrosion, and for measuring uniform corrosion rates.5-11 In late 1995, the Department of Energy Tanks Focus Area helped to launch an effort to develop EN based systems to improve Hanford?s corrosion monitoring strategy. Proof of principle lab testing was conducted at Oak Ridge National Lab and Pacific Northwest National Lab in 1995 and 1996 to provide a technical basis for using EN in Hanford nuclear waste tanks.12 Based on this study, a prototype system was constructed and deployed for a year in August, 1996. Since that time five additional systems have been deployed in the 241-AN tank farm (a DST farm). Each system has improved upon the design of the previous system. The three systems most recently deployed are of the same design and are currently operating at the site.
