In October of 2000 work was started on a project to develop a temporary electrochemical noise (EN) based corrosion monitoring system for Bethel Valley Evaporator Storage Tank (BVEST) W-23 at Oak Ridge National Laboratory (ORNL). The project was undertaken to assess the corrosivity of the waste in the tank and to assist in the overall development program for nuclear waste tank corrosion monitoring at the Hanford Site and other DOE facilities. Prior to the fabrication and deployment of the Tank W-23 system, laboratory testing was performed at ORNL to characterize the behavior of 304L (UNS $30403) stainless steel in simulated Tank W-23 waste solutions. Following the laboratory work a system was deployed in Tank W-23 in June of 2001. Data collection was terminated in June of 2002. Both laboratory data from the development effort and field data from system operation in Tank W-23 are presented herein.
Underground storage tanks made of mild steels or stainless steels are used to contain radioactive wastes generated by facility operations at numerous Department of Energy (DOE) sites. Most of these sites are now involved to varying degrees in environmental restoration and site cleanup. Because corrosion-related failures of waste tank walls could lead to the leakage of radioactive contaminants to the soil and groundwater, it has become essential to monitor corrosion conditions of the tank walls as tanks approach their design life.
In October of 2000 work was started on a project to develop an EN based corrosion monitoring system for BVEST W-23 at ORNL. The project was undertaken to assess the corrosivity of the waste on welds in the tank and to assist in the overall development program for nuclear waste tank corrosion monitoring at the Hanford Site and other DOE facilities. Prior to the fabrication and deployment of the Tank W-23 system, laboratory testing was performed at ORNL to characterize the behavior of 304L (UNS $30403) stainless steel in simulated Tank W-23 waste solutions. Following the laboratory work a system was deployed in Tank W-23 in June of 2001. Data collection was terminated in June of 2002. This paper describes the laboratory development effort, final system design, and operational history of the Tank W-23 system.
BACKGROUND
Tank W-23 is a 50,000 gallon (189,270 L) waste tank fabricated from ½ inch thick (1.27 cm) welded 304L (UNS $30403) stainless steel plates. The tank measures approximately 12 feet (3.6 m) in diameter and 61 feet (18.6 m) in length. It is housed in a concrete vault lined with stainless steel in the BVEST facility near the center of the ORNL campus. 1 The BVEST facility is used to collect liquid low- level waste (LLLW) from waste stream producers at ORNL. The LLLW is reduced in volume through processing at an evaporator prior to being moved to the BVEST facility. Tank W-23 is primarily used as a storage tank but can also serve as a feed tank for the evaporator. Because of its role as a storage tank that could see waste streams of varying composition, Tank W-23 was selected as an ideal tank for the application of on-line corrosion monitoring.
Prior to 1995, only a small number of traditional electrochemical techniques had been tried within the DOE complex to determine the corrosivity of nuclear waste stored in underground tanks. Coupon exposure programs, linear polarization resistance (LPR), and electrical resistance (ER) techniques have all been tried with limited degrees of success. 25 These techniques are most effective for monitoring uniform corrosion, but are not well suited for early detection of localized forms of corrosion such as pitting and stress corrosion cracking (SCC). Over the last ~20 years, EN b
