Decontamination & decommissioning
Non-invasive pipe characterisation1 November 2012
Radiological characterization using tracers (RCUT) is a minimally-invasive method for detection and location of residual radiological contamination in pipes and ducts. It involves injecting a gaseous tracer into the pipe, waiting for it to react with any contaminants, injecting an inert gas to push the tracer out, and monitoring the outflowing gas. The time of arrival of the tracer can be used to locate the contaminant in the pipe. By Wesley L. Bratton, Joseph W. Maresca and Deborah A. Beck
The United States Department of Energy (DOE) continually seeks safer and more cost-effective remediation technologies for use in the decontamination and decommissioning (D&D) of nuclear facilities. One of the major efforts within the DOE complex is the closure of tank farms and their associated ancillary systems. Ancillary systems consist of those components used to both transfer waste (for example, transfer lines, pump tanks and pits, diversion boxes and valve boxes) and reduce waste volume though evaporation (for example, the evaporator systems) . Ancillary systems and tanks must have the residual radiological inventories accounted for as part of facility closure.
Safely closing ancillary systems, as with waste tanks, involves an intricate set of steps that includes removing as much of the residual waste as possible. After completing ancillary equipment cleaning operations, a small amount of residual radioactive waste may still remain. As with the tanks, these residuals will need characterization to confirm that radionuclide and hazardous constituent concentrations meet performance objectives to ensure protection of the public and the environment.
For all of the sites, the piping that was used to transport process materials represents one of the larger challenges. As the demolition of these systems occur, disposal of this piping has become a costly issue. Currently, all process piping is cut into sections measuring three metres or less, and the ends of the piping are wrapped and taped to prevent the release of any potential contaminants into the air. The piping is then placed in roll-off boxes for eventual repackaging and final disposal. Alternatives that allow for the onsite disposal of process piping are greatly desired due to the potential for dramatic savings in current offsite disposal costs.
To address this need for an in situ determination of radiological contamination, a minimally-invasive method for detection, location, and quantification of residual beta and gamma radiological contamination in pipes and ducts using gaseous tracers has been developed. The method is called RCUT, which is an acronym for radiological characterization using tracers [2,3]. RCUT is very similar to an existing method called PCUT (pipeline characterization using tracers), which was developed for the purpose of detecting, locating, and quantifying non-radiological contaminants in pipes and ducts. PCUT was demonstrated under a Phase I and II SBIR for the US Department of Energy (DOE) [4-6]. PCUT has been demonstrated for piping of various lengths ranging from 3m to 45m that have been contaminated with chlorinated solvents including TCE, PCE and CCl4, various petroleum products, and heavy metals such as mercury. The location and quantification measurements have been demonstrated to within 5% and 10%, respectively, during previous PCUT studies [4-6].
There are a variety of ways to implement RCUT and PCUT. The method that is used depends on the tracers being used and how they interact with the contaminant. For some of the PCUT methods, a conservative tracer, that is, a tracer that does not react with the contaminant, is utilized as a reference or control. For RCUT, only the reactive tracer is required. RCUT (and PCUT) are best implemented by injecting enough reactive tracer to completely fill the entire pipe being inspected and allowing sufficient time (minutes to tens of minutes) for the tracer to interact with any contaminant that may be present. Then an inert gas is injected into the pipe at one end to push the tracer gas and reaction products at a known or constant flow velocity along the pipe and out the exit port/sampling location. The concentration of the tracer gases can be measured online with a gas chromatograph (GC) as they flow out the pipeline; alternatively, the concentration of the tracer gases can be sampled at set time intervals and measured with gas chromatography/mass spectroscopy (GC/MS), or some other analytical system.
If a radiological contaminant (beta or gamma) is present, the presence of the tracer gas reaction product will be detected; there may also be a slight change in the concentration of the original tracer gas. The time of arrival of the tracer gas reaction products can be used to locate the contaminant, because the advection velocity of the inert gas is known. If the pipe is free of radiological contamination, no tracer gas reaction products will be detected. Figure 1 illustrates the basic components of the system. Figure 2, shows the expected results from a region of radiological contamination within the pipeline. Note that multi-peak concentration curves will occur if there is more than one region of contamination. If the entire pipe, or large sections of it, are contaminated, the concentration curve will be broad. The conservative tracer (the red line in Fig. 2) can be used to monitor for leaks and the presence of other additives within the pipeline.
Two methods were developed for locating contamination along the pipeline. Both methods used the time of arrival of the peak of the tracer concentrations. The first method uses only the average velocity within the pipe and the time of arrival of the tracer peak after flushing. The average velocity is computed by dividing the measured volumetric flow rate by the diameter of the pipe. The second method, which does not require information about the diameter or geometry of the pipe, utilizes the ratio of the time of arrival of the peak of the first tracer pulse, which travelled over the full length of the pipe, and the time of arrival of the second tracer pulse, which travelled only the distance from the contamination to the end of the pipe. After weighting the arrival times by the mean of the measured flow rates, the distance from the sampling location to the contamination can be determined.
RCUT and PCUT can be used in support of D&D of piping and ducts that may be contaminated with radioactive or hazardous materials. These methods can be used to:
- Demonstrate that the pipe or duct is not contaminated and can be removed without special equipment or special safety precautions
- Determine the location of any contamination that might be detected
- Determine the amount of the detected contaminant.
The potential amount of cost savings and schedule reduction related to the first bullet is immense. Knowing that a pipe or duct is free of contamination, especially radioactive contamination, is key. Since many of the DOE pipelines are buried, the ability to characterize the pipe in place and possibly avoid excavation represents a significant cost savings.
RCUT and PCUT methods of detecting radiological and non-radiological contamination of pipes offer significant advantages over conventional pipe inspection techniques, including:
- Tracer movement is not impacted by pipe diameter or configuration, and therefore can be used on small-diameter piping
- No information about the pipe diameter or configuration is required beforehand for the method to work (although configuration information is needed if a contaminant is detected and its location is desired)
- There are no moving parts or equipment that must be introduced into the pipe
- There is no sparking potential or ignition source with gaseous tracers
- There are no decontamination requirements.
In addition, the RCUT technology has immediate application for the D&D of piping exposed to radiological contamination at the DOE’s Hanford Site and Savannah River Site. Many of these piping systems are either buried underground or are otherwise inaccessible, where external inspection techniques requiring direct or safe access to the outside wall of the pipe cannot be used.
The tracers identified for use with RCUT dissociate (that is, split into simpler groups of atoms, single atoms, or ions) when exposed to gamma and/or beta radiation emitting from a radiological contaminant in a pipe or duct.
For RCUT, the tracer gas is carefully selected to insure that (1) the tracer will react with a radiological contaminant, (2) the tracer does not break down if radioactive sources are not present, (3) the decomposition products are not rapidly recombined to form the original compound after exposure to a radioactive contaminant or when no longer in the presence of the radioactive contaminant, and (4) the tracers are safe (that is, nonflammable and nontoxic). All of these criteria were validated for the chosen RCUT tracer SF6.
Our laboratory tests indicate that the tracer pair of sulphur hexafluoride (SF6) and O2 will form SO2F2 when exposed to a gamma or beta radioactive field of sufficient strength. Thus, we know that the pipe contains radioactive contaminants if SO2F2 is detected at the measurement point. If only SF6 is detected, then the pipe is free of radioactive contamination. To date our testing has focused on rather high exposure levels as noted below to develop a tracer strategy. Additional efforts will evaluate the time/exposure relationship to determine the detection limits and the strategies to reach regulatory levels for disposal in place.
The feasibility of using SF6 as an RCUT tracer was tested by subjecting samples of SF6 combined with O2 and H2O (liquid and vapour forms) to various levels of gamma irradiation at the PNNL High Exposure Laboratory located in the Radiological Calibration and Standards Facility.
For operational implementation, the preferred gaseous tracers are comprised of SF6 and O2, although the method works equally well for SF6 and water vapour. When SF6 is irradiated, chromatographs of the exposed samples indicate the presence of SO2F2 as determined by a peak at a retention time of 8.5 minutes. The mass spec results  showed that the peak seen at 8.5 minutes has mass to charge (m/z) ratios of 102 and 83 and are the results of the presence SO2F2. The peak at 102 m/z represents the intact SO2F2 molecule (molecular weight 102), and the peak at 83 m/z represents the primary ionic breakdown compound of SO2F2. This was confirmed with an analysis of pure SO2F2.
While further optimization is needed for RCUT, the key first step of identification of a tracer compound appropriate for the application of detecting radioactive pipeline contamination through the detection of decomposition products of SF6 has been demonstrated. Other tracer gases that will also undergo radiolysis will be considered in the future.
The next step for the RCUT development process is conducting laboratory-scale tests using short pipelines to define analytical requirements, establish performance boundaries, and develop strategies for lower exposure levels. Studies to identify additional analytical techniques using equipment that is more field rugged than a GC/MS would also be beneficial.
Wesley L. Bratton, Ph. D., Joseph W. Maresca, Jr., Ph.D., and Deborah A. Beck, Vista Engineering Technologies, L.L.C., 355 Columbia Park Trail, Richland, WA, 99352.
This article is based on paper no. 12514 presented at the WM 2012 conference, February 26-March 1, 2012, Phoenix, Arizona. This article was first published in the October 2012 issue of Nuclear Engineering International
1. H. H. Burns, S. L. Marra, A. P. Fellinger, and C. A. Langton, "Technology Needs and Status on Closure of DOE Radioactive Waste Tank Ancillary Systems 9312" Waste Management 2009 Conference, March 1-5, 2009, Phoenix, AZ, SRNL-STI-2009-00064, Rev. 1, Savannah River National Laboratory Savannah River Site, Aiken, SC 29808
2. W.L. Bratton, G.R. Golcar, D.A. Beck, T. Crotwell, and R.R. Baker, "Radiological Characterization Using Tracers Technology Development," Final Report, VET-1625-07-RPT-002, Vista Engineering Technologies, 1355 Columbia Park Trail, Richland, WA 99352, Prepared for M. Skorska, Washington River Protection Solutions, P.O. Box 850, Richland, WA 99352 (December 1, 2010).
3. W. L. Bratton, "Remote Characterization of Radiologically Contaminated Pipelines Using Gaseous Tracers," Final Report to Phase II STTR/SBIR Program, Grant No. DE-FG02-08ER86366, Vista Engineering Technologies, L.L.C., Kennewick, WA 99336 (2008).
4. W. L. Bratton, and J. W. Maresca, Jr., "PCUT, a Non-Invasive Method for Detection, Location, and Quantification of Contaminants in Pipes and Ducts Using Interactive Tracers," Technical Note, Vista Engineering Technologies, 1355 Columbia Park Trail, Richland, WA 99352 (February 2009)
5. W. L. Bratton, J. W. Maresca, Jr., and C. M. Haas, "Using Tracer Technology to Characterize Contaminated Pipelines,"Final Report to Phase II STTR/SBIR Program, Grant No. DE-FG02-03ER86173, Vista Engineering Technologies, L.L.C., Kennewick, WA 99336 (December 2005).
6. W. L. Bratton, and J. W. Maresca, Jr., "A New Method for Detecting, Locating, and Quantifying Residual Contamination in Pipes and Ducts in Support of D&D Activities," Technical Paper, Proceedings of the Waste Management 2004 Conference, Tucson, Arizona, Vista Engineering Technologies, L.L.C., Kennewick, WA 99336 (February 29 - March 4, 2004).