Our Portfolio: Monitoring Projects

CMST Technology Index | Monitoring and Sensors | TFA

Integrated Raman Sensor for In-Tank Corrosion Chemistry Monitoring

Information
Resources

Technology
Summary Sheet

Technology
Summary Book

Images

 

Tech ID: 2015
Project Overview
 The DOE has over 300 underground tanks used to process and store over 100 million gallons of radioactive and mixed chemical waste. These tanks contain up to one million gallons; many are of single-shell steel construction. These tanks present a considerable risk to the public and the environment should a wall be compromised due to corrosion or otherwise. Corrosion is of particular concern because many of the tanks contain high concentrations of nitrate, which attacks steel. To minimize the effects of nitrate, tank contents are maintained at an elevated pH and at an optimum nitrite level. To ensure that tank waste chemistry is being maintained to minimize corrosion, it is important to monitor concentrations of NO3, NO2, and OH. If significant changes in the concentrations are observed, then appropriate measures can be exercised to restore optimum anti-corrosive conditions. Current analytical protocols for high-level waste tanks involve liquid sampling, preservation, transport, storage, preparation, and analysis with a pH meter and an ion chromatograph in a hot cell. This process is slow, expensive, and dangerous. High costs serve to limit the number of samples collected from any one tank. There is also considerable opportunity for the sample composition to change from the time it is collected to when it is prepared for analysis. An attractive alternative to existing protocols is to use one or more sensors to monitor in situ the three species of interest. Small, inexpensive devices that can be sacrificed, if necessary, are most appealing. Techniques employing fiber optic probes are promising for this application because they are affected less by radiation than electrical devices. The crucial challenge for in-situ fiber optic sensors is that of providing reliable analytical data at relevant concentrations under harsh conditions ranging up to 1000 rad/hr, 90°C, and 10 M (molar) OH-. Furthermore, the measurements of the specified analytes must be performed in turbid solutions without interference from other sample components.

Technology Description
 A fiber-optic Raman spectroscopy sensor and deployment platform for the in situ analysis of chemical species in waste tanks was developed. Specifically, the instrument can detect nitrate, nitrite, and hydroxide (NO3- , NO2- , and OH- ) over the full range of concentrations of significance for controlling corrosion in U.S. Department of Energy (DOE) high-level waste (HLW) tanks. A 670-nm diode laser is used for analyte excitation and an echelle spectrometer with a charge-coupled device (CCD) detector is used for spectral acquisition. The instrument operates remotely with only a small, lightweight probe and fiber-optic cable deployed in the tank. The miniature fiber-optic Raman sensor is housed in a miniature sampling chamber with a filtering mechanism to allow only liquid waste to enter the sampling area. A remotely operated deployment platform that interfaces with the riser opening in the waste tank will be used to deploy the Raman probe into the waste. This is done via a reel mechanism inside the platform. The deployment platform is completely sealed to protect workers from being exposed to the radiation plume emanating from the riser opening.

Using a Raman sensor for in-tank monitoring has several advantages that make it the technique of choice for monitoring anionic species such as nitrate, nitrite, and hydroxide. First, the Raman spectrum is unique for every molecule and can therefore be used as a "chemical fingerprint" to identify unknown species. The Raman technique can also differentiate between solid and dissolved species, which is important in waste tanks where the physical composition can range from liquid to sludge to hard saltcake. Another advantage of the Raman technique is ease of sampling. Furthermore, use of the Raman technique requires no sample preparation, so samples can be easily analyzed in situ if a fiber-optic probe is used to deliver and collect the scattered light from the sample.

Electrochemical noise (EN) sensors developed at Hanford and at the Savannah River Site (SRS) are being co-deployed with the Raman sensor. The two sensors are being integrated into a single probe head. The EN technique has the capability of monitoring both localized and general corrosion. Localized corrosion, including pitting and stress corrosion cracking, is of concern in waste tanks. EN measures the potential and current fluctuations of metal in solution. These fluctuations are perturbations resulting from the electrochemical reactions occurring at the metal surface and are low-frequency (1 Hz), small amplitude signals.

Current sampling and analysis techniques require removal of samples from tanks and subsequent laboratory analysis using ion chromatography and titration. The advantage of the Raman probe technology is that it can readily be deployed into HLW tanks and provide in situ analysis of ions associated with corrosion.

Return to Top