High-level nuclear wastes at the Henford Site are stored underground in carbon steel double-shell end single-shell tanks. The installation of a three-channel prototype corrosion monitoring system into double-shell tank 241-AZ-l 01 was completed in August 19%. A fully operational eight-channel system baaed on the design and operation of the prototype was installed into double- shell tank 241 -AN- 107 in September 1997. The 241-AN-107 system monitors fluctuations in corrosion current and potential (electrochemical noise) occurring on eight electrode arrays immersed in the waste liquid and in the vapor space above the waste. The electrodes are designed to detect the onset of pitting and stress corrosion cracking should tank conditions change to allow these phenomena to occur. This paper focuses on the design of the 241-AN-107 corrosion probe.
INTRODUCTION AND BACKGROUND
The Hanford Site has 177 underground waste tanks that store approximately 253 million liters of radioactive waste from 50 years of plutonium production [1]. Twenty-eight (28) tanks have a double shell and are constructed of welded ASTM A537-Class 1 (UNS K02400), ASTM AS 15-Grade 60 (UNS K02401), or ASTM A516-Grade 60 (UNS K02 100) material. The inner tanks of the double-shell tanks (DSTS) were stress relieved following fabrication. One hundred and forty-nine (149) 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 [1-4]. 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 significant number of single-shell tanks [1]. The probable modes of corrosion failures are reported as nitrate stress corrosion cracking (SCC) and pitting [2].
Previous efforts to monitor internal corrosion of waste tank systems have included linear polarization resistance (LPR) and electrical resistance techniques [5-6]. These techniques are most effective for monitoring uniform corrosion, but are not well suited for detection of localized corrosion (pitting and SCC). The Savannah River Site (SRS) investigated the characterization of electrochemical noise (EN) for monitoring waste tank corrosion in 1993, but the teats were not conclusive [7]. The SRS effort has recently been revived and additional testing is underway,
For many years, EN has been observed during corrosion and other electrochemical reactions, and the phenomenon is well established [8- 19], Typically, EN consists of low frequency (<1 Hz) and small amplitude signals that are spontaneously generated by electrochemical reactions occurring at corroding or other surfaces [20], Laboratory studies and recent reports on field applications have reported that EN analysis is well suited for monitoring and identifying the onset of Iodized corrosion, and for measuring uniform corrosion rates [14-26], A two year laboratory study was started at Hanford in 1995 to provide a technical basis for using EN in Hanford nuclear waste tanks [27], Based on this study, a prototype system was constructed and deployedinDST241 -AZ-1 01 in August, 1996 [28]. Based on the successful demonstration of this prototype for more than a year, a more complex system was designed and was installed into DST 241-AN-107 in September 1997, This paper summarizes the design and operational req
