The Hanford Site near 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 have ever been used 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) corrosion monitoring system to improve Hanford?s corrosion monitoring strategy. Three systems are now installed and operating at Hanford. System design, performance history, data and the results of a recent analysis of tank vapor space data are presented.
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 and non-radioactive constituents resulting from plutonium separation processes. Leaks began to appear in the single-shell tanks (SSTs) shortly after the introduction of nitrate-based wastes in the 1950s. Leaks are now confirmed or suspected to be present in a number of SSTs.1 The probable modes of corrosion failures for the SSTs are nitrate induced stress corrosion cracking (SCC) and pitting.2 No leaks have ever been confirmed in the DSTs, but a study in 1996 identified SCC and pitting as the greatest potential threats to the long-term integrity of these tanks.3
Corrosion monitoring and control of the DSTs at Hanford has historically been provided through a waste chemistry sampling and analysis program. 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.4 This method has been effective, but is expensive, time consuming, and does not yield realtime data. In 1996, the Department of Energy Tanks Focus Area launched an effort to improve Hanford?s DST corrosion monitoring strategy and to help address questions concerning the remaining useful life of these tanks. Several new methods of on-line localized corrosion monitoring were evaluated. The EN technique was selected for further study based on numerous reports that showed this technique to be the most appropriate for monitoring and identifying the onset of localized corrosion.
