Nuclear Instrumentation Tools For Lower Cost And Higher Reliability Decommissioning Of Buildings And Grounds
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Prepared for: TOPSEAL '96,
European Nuclear Society,
June 10-12, 1996,
Stockholm, Sweden
Introduction
The two most expensive things in the decommissioning of buildings and grounds with radioactive contamination are the labor for the contamination assessment and for the decontamination, and the cost of radioactive waste disposal. There have been many studies and papers presented on the decontamination techniques themselves, but these have been primarily about the contamination removal operation. In this paper, four new tools are presented to guide the implementation of the decontamination effort, to better characterize the radioactivity, and to reduce the volume and therefore the cost of radioactive waste disposal. A very important side benefit in this day of intense regulatory and public scrutiny is the resulting improved documentation of the radioactivity levels in the waste or other items leaving the site, and in the buildings and grounds left behind at the site.
While these tools are new, the principles behind them are not. They have been in use in quality laboratories for years. What is new is modern detectors and electronics that have made practical the transition of these laboratory quality measurements to field instrumentation.
Advances in Nuclear Instrumentation
The past 10 years have seen a very rapid growth in the capabilities and the practical implementation of nuclear instrumentation.
- Spectroscopy provides much more information about the sample and
proof of the quality of the results, and is the preferred radiological
assessment technique where possible. Germanium detectors are now large
enough and reliable enough to replace NaI detectors for most of these
applications. Ge detectors have more definitive nuclide identification,
more accurate nuclide quantification, lower detection limits, a wider
dynamic range, better stability, and much higher overall confidence
in the results.
- Although Ge detectors still must be cooled to operate, warm-ups
cause no harm, and the cooling systems are much more practical. Efficient
Dewars allow small light-weight detector packages that hold enough
LN for several days of operation. Multiple-attitude cryostats allow
the same detector to be used in all orientations. For long term autonomous
operations, reliable LN autofill systems, and also cooling systems
using gaseous refrigerants [freon-like, and Helium] are available
for non-portable applications.
- A dramatic change has happened in the electronics industry. Integrated
and now micro electronics have allowed the creation of full laboratory
quality Multi Channel Analyzers in small totally computer controlled
packages. Two such examples are Canberra's ICB NIM series for fixed
instrumentation on an Ethernet network, and Canberra's InSpector which
provides the same full computer control capabilities in a small lightweight
battery powered package for portable applications.
- And the computers that can control these input electronics are incredibly
powerful for large scale instrument operations, or very compact for
portable applications. This allows extremely sophisticated software
to automatically and reliably operate the instrument, analyze the
data, record the results in a database, interpret the results, and
then implementation an appropriate action.
- The most recent addition to this impressive evolution has been the ability to predict the performance of these instruments before they are built, and to calibrate them for quantitative analysis of radioactivity without building numerous and expensive calibration sources. Canberra has developed the use of MCNP for this, and is currently completing the development of more practical software for customer use.
Using these new tools, Canberra has developed a variety of task-specific instrumentation specifically for the contaminated sites remediation marketplace. These instruments are supplied as integrated turnkey solutions. The integration, setup, testing, calibration, and validation are already done. The customer can begin counting immediately.
Proposed Solution to Contaminated Grounds Environmental Remediation
Current practice today involves taking a large number of samples in an attempt to assess the location and the volume of the contaminated soil. For the most part, this has not been very accurate, even when large numbers of expensive samples are taken. In almost all cases, the final contaminated soil volumes are higher than the initial estimate. Current practice uses the sample results to create excavation patterns. To assure removal of all radioactive material, this excavation plan always removes more material than is really contaminated, which greatly increases the radioactive disposal costs. Current practice is to send these samples to an off-site laboratory for analysis. This is expensive, and also quite time consuming to wait for the results. Commonly, after the review of the initial sample results, more samples must be taken, and analyzed, which delays the project even more. After the excavation is complete, then more samples must be taken [and perhaps retaken] to prove that the site is clean.
Figure 1 presents two new complementary tools to provide a more cost effective and more timely solution to this problem.
Figure 1.
Instrumentation for environmental restoration.
One new tool is in situ gamma spectroscopy. This consists of a portable Ge detector and detector holder, portable MCA, laptop PC, data analysis software, and calibrations for the likely sample geometries. Other accessories are also available. A typical In Situ System can perform measurements at the 0.002 Bq/g level for most common contaminants from reactor facilities [Cs-137, Co-60] and from ore processing facilities [Ra-226, Th-232], and measurements at the levels of 0.02-0.1 Bq/g for fuel processing and reprocessing facilities [Am-241, U-235, U-238]. These are easily possible with common technology and with short [15 minute] counting times. The results are available immediately for use to determine the need and strategy for further measurements, or to guide the excavation effort. Global Positioning Systems [GPS] can be easily integrated to provide automatic sample location [latitude, longitude, and elevation] to within a meter, and to record this with the sample information. Geographic Information Systems [GIS] are commercially available to rapidly present this information to the user in a variety of cartographic schemes. The field gamma spectroscopy results can even be transmitted back to a base station via telemetry link and directly into the GIS for an immediate view of the survey effort, and to guide the selection of the next measurement point. In situ gamma spectroscopy can be effectively used to provide the initial site assessment, to locate the extent of surface activity to plan the next day's excavation, and to verify that the site is clean.
The second new tool is an Automatic Conveyor Monitor and sorting system [ACM] for radioactive dirt. In consists of conveyors to deposit the dirt onto the counting system, a shielded counting chamber housing two Ge detectors for sensitive accurate assay of the dirt, and a diversion system to place the effluent into one of three output streams based upon the nuclide-specific criteria for that site. Sensitivity is of the order of 0.05 Bq/g for Cs-137, Co-60, Ra-226 and Th-232. With the use of the ACM, all dirt excavated is measured, and any that is less than the site deminimus limits can be left at the site. Soil with an intermediate activity level can be disposed at an inexpensive waste disposal site, or perhaps processed to remove the radioactivity and then be returned to the site as clean. Soil above this level is disposed as radioactive waste. Processing limits are up to 50 tons/hr at these sensitivities. Because the operation of the system is automatic, the labor requirements are minimal; therefore there is little penalty for over-excavation, as the clean dirt will be reliably separated and will not be classified as radioactive waste. Because full laboratory quality gamma spectroscopy is performed, there is no need for additional measurements of the clean effluent stream, nor of the radioactive effluent stream. Finally, the system is mobile, allowing it to be placed near the excavation site.
For a typical site, the In Situ System would first be used to provide a preliminary estimate of the volume of material on the site. This would be from a combination of above surface and subsurface measurements. The subsurface measurements can be most economically performed using holes drilled with well-drilling equipment and lined with plastic, and with a Ge detector placed at various locations down the hole. This volume assessment would then determine the economic applicability of the ACM. If the analysis/transportation/disposal costs are $5000/ton, and with a decontamination factor of 50% [conservative estimate] then sites with approximately 500 tons of radioactive dirt will completely pay for the equipment through cost savings on this one week job! [note: a ton, metric ton, cubic yard, and cubic meter of loosely compacted dirt are all approximately the same] For larger sites or when an on-site engineered radioactive waste disposal cell is possible, the disposal costs are much less [commonly $70/ton]; but even there, when the radioactivity exceeds 25,000 tons the equipment payback is complete on that one three-month job alone!
Figure 2 shows how these two instruments would be combined in a typical field operation. The in situ detector would be used to determine where to excavate. But since it is a surface weighted measurement, the excavations should be carried out in thin layers of perhaps 10-15 cm. The dirt would then be placed onto the ACM without mixing, which would likely increase the radioactive disposal volume. After removal of that layer, the In Situ System would be used again to see if the newly exposed layer of dirt is below acceptable levels. If not, dig again, and measure again until the site is clean. If other volume reduction methods like soil washing are used to process the ACM's intermediate output stream, then the ACM would also be used to verify that the soil washing output is clean.
Figure 2.
Flow plan for dirt environmental restoration.
These two tools are designed to greatly reduce the labor for the typical operation. One person can survey approximately 1000 m2/day (1/4 acre). One person can excavate approximately the same area per day to a depth of 15 cm using a simple front end loader. And the ACM can process approximately this volume of material in a day. Because the data acquisition and analysis is automatic, then the labor to convert field samples into lab samples into analysis results into reports can also be greatly reduced.
There is also a great improvement in the quality of the job. Samples are notoriously unrepresentative if there is a non-uniform distribution of radioactivity. Unfortunately, this is typically the case for these decommissioning activities. Both in situ gamma spectroscopy and the ACM greatly reduce these errors by looking at a very large sample. And since the ACM looks at the entire volume of excavated dirt, the resulting assays of dirt returned to the site, and of dirt sent to the waste disposal site are also more accurate, and completely documented.
Proposed Solution to Contaminated Building Decontamination and Decommissioning
This solution is a parallel one to that for environmental remediation. There is an in situ gamma spectroscopy tool to quickly and economically characterize the structure. Traditional decontamination tools are used to remove materials that are above site-specific nuclide-specific levels. These are placed in approved radioactive waste shipping containers [generally of a few cubic meters of size]. Those building components that are likely not radioactive, and are scheduled for removal are placed into larger containers typically used for building rubble [generally of 20-30 cubic meters]. These containers are then measured in a box counter using gamma spectroscopy to provide a final confirmatory measurement and record of the proper classification and the radioactivity level.
Current practice today involves taking a large number of samples of the building surfaces in an attempt to assess the volume and the location of the contamination. Again, this sampling process is generally not very accurate for non-homogeneous distributions. And these samples are even more labor intensive [expensive] than those of soil. Current practice uses these sample results to define areas for decontamination or for removal. Current practice is to send these samples to an off site laboratory for analysis. This is also expensive, and adds to the project time while waiting for the results. If after the review of the initial sample results, more samples are needed, then the cost and time are further increased. If decontamination is performed, then more samples must be taken [and perhaps retaken] to prove that the object is clean.
Figure 3 presents two additional new complementary tools to provide a more cost effective and more timely solution to this problem. One tool is the In Situ Object Counter System [ISOCS] and the other tool is the Box Counter. The ISOCS consists of a Ge detector, a lead collimator and portable stand, portable MCA, laptop PC, and data analysis software. Software to compute the efficiency for the various sample geometries likely to be encountered is also included. ISOCS has adequate sensitivity to detect and quantify surface contamination on wall/floor/ceilings at small fractions of commonly used free release levels. Detection limits are in the 0.01-0.1 Bq/cm2 range for Am-241, U-235, Cs-137, Co-60, Ra-226 and Th-232, with a 15 minute measurement time, for a 3 m x 3 m area. It can also be used to quantify radioactivity [or confirm the absence of it] inside objects [pumps, valves, ductwork, piping, tanks, etc.] without inherent problems associated with sampling. For these tasks, the sensitivity is generally adequate to free-release items from reactors and uranium processing facilities, and to classify as not-TRU [not alpha waste] those items with suspected Pu.
Figure 3.
Instrumentation for decontamination and decommissioning.
The Box Counter also uses Ge gamma spectroscopy to quantify the radioactivity in the large containers. Typically four Ge detectors would view the entire surface of both sides of each box. Multiple measurements are made vertically and horizontally to provide best accuracy and homogeneity information. The detectors automatically move to do this task. Manual systems with fewer detectors and manual placement of the detector into the appropriate position are also available for less cost, but longer assay time. With a processing time of 0.5 hour for small boxes [2 cubic meters] or 2 hours for larger boxes [30-40 cubic meters] detection limits of the order of 0.05 Bq/g for Cs-137, Co-60, Ra-226, and Th-232 are achievable.
Figure 4 shows how the ISOCS and the Box Counter would be combined in a typical field operation. The ISOCS detector would be used to determine which part of the structure was likely to be clean, and which part is contaminated. These determinations are done quickly and on a large area basis. If reuse of the building is economically feasible, then traditional techniques are used to remove the radioactivity. But commonly, demolition is the more economical choice, as then the extensive measurements necessary to prove the absence of potential contamination in pipes, sewers, and below floors is not necessary. The common use of small 208 liter drums for radioactive waste requires much labor to reduce items to small size. Cost savings will be realized if larger containers are used. Generally 2-30 cubic meter containers [B-25 boxes to ISO shipping containers] are suitable for radioactive waste transportation and disposal in many countries. The Box Counter is designed to provide direct quantitative radioassay of the contents. For heterogeneous wastes as expected here, sampling is quite error prone, and the large "effective sample size" measured by the Box Counter reduces the error bounds. This lowers the upper error bound of the radioactivity content, which generally reduces the radioactive disposal fees, and also uses up less of the disposal site's total radioactivity limit. For those portions of the building expected to be clean, very simple demolition tools can be used. The material removed is placed in large containers [roll off boxes, or ISO containers] as typical in the building demolition industry for transportation to the local sanitary landfill. But since the in situ characterization process was just preliminary, a final measurement of the container in the Box Counter is necessary for the definitive and official proof that the radioactivity levels are acceptable.
Figure 4.
Flow plan of building decontamination and decommissioning.
Like the soil remediation, there is also a great improvement in the quality of the job. Sample taking is even more error prone because of the very non-uniform nature of structure contamination. Both the ISOCS and the Box Counter greatly reduce these errors by looking at a very large sample. And since the Box Counter looks at the entire volume of building debris, shipment results are also more accurate, and completely documented.
Regulatory Roadblocks in the Way of Full Realization of the Cost and Time and Quality Improvements
In most countries today, regulations relating to radioactivity are based instrumental capabilities that existed at the beginning of the nuclear age. They are only slowly starting to be restated in terms compatible with instrumentation available today.
Rarely is there a suitable definition of an radioactivity sufficiently low that the item can be free released. Most definitions are based upon surface radioactivity limits [X Bq/100 cm2], and generally further subdivided into fixed and removable limits. It is difficult to discover the dose basis for such limits; they are likely based upon the instrument capabilities in existence at that time. And also, commonly, there are additional area averaging limits [e.g. the surface activity averaged over an area of 1 square meter cannot exceed X units/100cm2, provided that no single 100 cm2 portion exceed Y units/100cm2]. This definition makes these cost saving total activity measurement techniques described here quite impractical unless the object has a well defined surface area, and even makes conventional smear and survey meter techniques impossible unless all portions of the object's surface area are accessible. It is much easier to determine an object's weight, and it's total radioactivity. And it is more likely the total radioactivity or concentration that generally determines the object's potential for harm [dose], than surface activity.
Another example of an expensive regulatory release criteria is a "hot spot" limit. Often, volumetric release levels of X Bq/g are negotiated for soil, but with the caveat that no small volume can be greater than Y, where Y>X. There is very little dose related basis for this restriction, but instead the motivation seems to be making released radioactivity difficult to detect, because if you can detect it, it is bad.
Another problem stems from the lack of nuclide-specific volumetric release regulations. Current practice is to assume that things from the site are potentially radioactive, and good practice warrants screening to prove that it is not. But if something is detected, then commonly, the entire sample must be disposed as radioactive because there are no defined acceptable levels. This is a significant disincentive for the conscientious licensee to use high quality [sensitive, nuclide-specific] equipment, as it will sometimes find things that instrumentation of lesser quality will miss.
In the case where free release limits are defined using surface activity, it is acceptable for items with clearly measurable radioactivity [but less than the limits] to be free released. But, if most of the surfaces of the items are indeed contaminated to just below the limit, the radioactivity is quite large and very easily detected. In fact, this amount of radioactivity would likely be considered unacceptable for dose based release limits.
The fate of the USNRC's Below Regulatory Concern [BRC] initiative was very disappointing. It was a true dose-based limit. Its failure was a political one, not a technical one. But, perhaps there is hope for the future. It is comforting to see the IAEA and ICRP propositions of deminimus radioactivity concentrations, and the US NRC/DOE/EPA proposed risk based criteria [0.15 mSv/yr incremental dose, which can then be used to compute radioactive concentration]. But these or similar regulations are not enacted in law yet in any of the major nuclear countries.
Conclusion
Modern technology allows the creation of a new class of tools to perform reliable and practical quantitative gamma spectroscopy in the field. These devices can greatly reduce the need for sample taking and laboratory analysis. These samples and laboratory analyses can now be reduced to those necessary to establish total nuclide inventory and non-gamma nuclide correlation to gamma emitters, and for independent analyses for quality assurance purposes. This reduction in reliance on the sampling process can greatly increase accuracy for heterogeneous items. These new devices can provide more information in the official record to document the decisions made about the fate of materials released to or from the site. And, these tools can accomplish this task in a manner that saves time and money over current methodology.
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