Before it can be disposed of, nuclear waste must be accurately characterized to identify and quantify its radioactive content. Once it is characterized, appropriate measures can be taken to reduce the radioactive hazard it poses to the public. One of the most cost-effective approaches for characterizing radioactive waste is through the use of nondestructive assay (NDA) instrumentation, which provides quantitative determinations on the isotopes of interest using the characteristic radiation they emit. Such radiation includes but is not limited to gamma rays, neutrons, and heat. Validating the performance of waste assay methods such as NDA is critical to establishing the credibility of the assay results that will be used in choosing storage and disposal methods.
Figure 2: Two commercial mobile assay systems are being evaluated at LANL. These two NDA instruments are in the Canberra Industries trailer.
The DOE Carlsbad Area Office (DOE/CAO) is responsible for qualifying waste characterization equipment associated with the WIPP repository. Qualified equipment is then allowed to characterize waste destined for WIPP. Tests outlined in this article are part of the efforts to qualify such equipment. Under the Performance Demonstration Program (PDP), part of the overall qualification program, both radioactive standards and 55-gallon drums of various matrices to hold the standards have been provided to each site in the DOE Nuclear Weapons Complex. Nondestructive assay systems are then tested semiannually to determine if they meet the quality assurance objectives for precision and bias under the program.
Because the NDA capability at various radioactive waste-generator and waste-storage sites is limited, a number of NDA equipment suppliers have installed their equipment in mobile trailers that could travel around to such sites. To date, no commercial mobile assay systems have participated in the PDP tests because DOE/CAO is still in the early stages of determining how such systems should participate and because it currently lacks a site to test mobile NDA trailers. The manufacturers of these trailers (who have made substantial investments of resources) must quickly demonstrate their capabilities, validate their equipment, and urge DOE/CAO to allow mobile NDA trailers to participate in the program.
The LANL Plutonium Facility is uniquely qualified to test this equipment. We have a broad spectrum of plutonium standards that are traceable to the National Institute of Standards. This includes over fifty 239Pu standards in a wide range of geometries, ranging from 0.2 mg to 480 g. The TA-55 site also has locations where manufacturers of mobile NDA systems can power their trailers and take advantage of realistic test conditions. Having manufacturers come here also eliminates the need to transport radioactive samples for their use.
Two mobile systems, one from Canberra Industries, Inc. (Figure 2), and one from Pajarito Scientific Corporation (Figures 3 and 4), were installed at the LANL Plutonium Facility in August and November of 1996, respectively. The Canberra trailer has two passive NDA systems. One is a segmented gamma scanner waste assay system, and the other is a neutron coincidence counter, waste drum assay system with add-a-source correction (a neutron interrogation technique that corrects for the effects of the waste matrix on neutrons). Both the scanner and counter were pioneered at LANL by the Nonproliferation and International Security Division (NIS). The Pajarito Scientific Corporation trailer has both an active differential die-away technique and a passive multiplicity counter (both developed by NIS). The die-away technique actively interrogates items using thermal neutrons. For both trailers, plutonium isotope compositions are determined from measurements using the MGA (multigroup analysis) code pioneered by Lawrence Livermore National Laboratory.
Los Alamos provides peer review of the performance of the systems for plutonium, acts as a beta-test site to evaluate the waste assay software, and provides data for potential ³pre-certification² for future installations‹which would minimize the measurements required to qualify the systems for use at other sites. After instruments are calibrated by the manufacturers, assays determine bias, precision, and minimum detectable activity. A performance demonstration test has been conducted to measure the systems against the DOE/CAO quality assurance objectives.
Finally, the performance is evaluated for representative waste types at TA-55 (salts, metal, combustibles, leaded rubber, and HEPA filters). Although analysis of test results is still in progress, data from these tests have pointed out areas requiring modifications by the manufacturers. This effort has clearly demonstrated how essential it is to use plutonium, rather than other radioactaive sources, for testing.
Figure 3: An imaging passive/active neutron counter in the Pajarito
Scientific Corp. trailer.
The LANL Plutonium Facility benefits from these tests on a number of levels. First, they help fulfill the mission of the DOE Defense Programs Office's designated "User Facility/Technology Development Center" at TA-55 for the testing of NDA assay instrumentation. The LANL NDA laboratory equipment in the Plutonium Facility is currently in building areas with high gamma and neutron background levels. With the new focus on measurements of low-level waste, the facility is interested in quantifying any benefits from assay systems located outside of the plutonium facility's high background radiation levels. The trailers will help with this assessment.
Second, one of the most difficult aspects of NDA measurements is the measurement uncertainties associated with waste. This is particularly true when calibration and measurement control standards for NDA instrumentation do not match the matrix of the waste being generated. By bringing in independent instruments, such as the Canberra and Pajarito trailer instruments, and measuring a wide range of actual facility waste, LANL can make consistency checks between the trailer instruments and those of the Plutonium Facility's NDA Laboratory.
Figure 4: Keith Lash operates the Pajarito Scientific Corp.
console.
A third benefit is increasingly experienced and knowledgeable Los Alamos technicians, considered a key element in the Laboratory's ability to provide cost-effective and high-quality research. With the trailer manufacturers' request to provide beta-testing of the software interface, the LANL Plutonium Facility chose to have Canberra and Pajarito train LANL technicians to operate the equipment (see Figure 4). Technicians thus became intimately involved, and the process resulted in a fair and practical evaluation performed by those who normally operate this equipment on a routine basis. Finally, the current effort has demonstrated the Plutonium Facility's capability to provide a test site for mobile assay systems.
Contributors to this project include: M. Schanfein, C. Bonner, R. Maez, J. Martinez, M. Padilla, D. Vigil, NMT-4; and L. Tichnor, TSA-1.
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