Clifford J. Stanley
Radioactive Waste Management Complex (RWMC)
Idaho National Engineering and Environmental Laboratory (INEEL)
Lockheed Martin Idaho Technologies Company (LMITCO)
P.O. Box 1625, Idaho Falls, ID 83415-4201
526-5637, Internet: stancj@inel.gov

The Radioactive Waste Management Complex (RWMC) located within the INEEL contains facilities and equipment to manage low-level, mixed Department of Energy (DOE) nuclear weapons program legacy Transuranic (TRU) stored waste generated by the INEEL and other DOE laboratories and operations. This waste will be retrieved, nondestructively examined, assayed, and shipped to a centralized DOE disposal facility, such as the proposed Waste Isolation Pilot Plant (WIPP) facility in New Mexico.

The process includes waste container warm-up, container venting, radiological survey and weighing activity, radioscopic examination, fissile material assay, container integrity inspection, gamma-ray spectrometer assay, and container gas generation testing. This effort is currently concentrating on the TRU waste contained in 55-gallon drums stored at the RWMC. Each step in the process is designed to provide data associated with either drum physical content verification, fissile material quantity determination, thermal wattage generation, and structural integrity of the drum in accordance with the transportation requirements and the acceptance criteria requirements for disposal at the WIPP. The TRU waste characterization and certification is critical path for the start of shipment of TRU waste out of the State of Idaho in accordance with the DOE and State of Idaho Settlement Agreement by 1998. Approximately 15,000 drums of TRU waste must be shipped out of the State of Idaho by the end of 2002.

This work is supported by the U. S. Department of Energy Assistant Secretary for Environmental Management under DOE Idaho Operations Office Contract DE AC07 94ID13223.

The purpose of this paper is to provide an update on the status of the current and future nondestructive assay (NDA) and nondestructive examination (NDE) waste characterization activities associated with the Stored Waste Examination Pilot Plant (SWEPP) in the Radioactive Waste Management Complex (RWMC) located at the Idaho National Engineering Laboratory (INEEL). Included in this paper is a description of each of the SWEPP NDA/NDE systems and associated activities.

The RWMC located within the INEEL contains facilities and equipment to manage low-level, mixed, and transuranic (TRU) solid radioactive waste generated by the INEEL and other Department of Energy (DOE) laboratories and operations. The primary mission of the RWMC is safe disposal of INEEL-generated low-level waste (LLW) and temporary storage of mixed and TRU waste. Much of this waste will be retrieved, nondestructively examined, treated if necessary, and shipped to a centralized DOE disposal facility such as the proposed Waste Isolation Pilot Plant (WIPP) facility in New Mexico. Figure 1 shows an aerial view of the INEEL Radioactive Waste Management Complex.

The Transuranic Storage Area (TSA) within the RWMC is used for examination, segregation, certification, and interim storage of TRU waste. Within the TSA, a facility referred to as the Stored Waste Examination Pilot Plant (SWEPP) provides space for the nondestructive radioassay and radioscopic examination systems used to characterize and certify the various waste forms for shipping and disposal to WIPP. The process through SWEPP includes waste container warmup, radiological survey and weighing activity, radioscopic examination, fissile material assay, container integrity testing, container overpacking, and gamma-ray spectrometer evaluation.

Since 1970, the Radioactive Waste Management Complex (RWMC) at the Idaho National Engineering Laboratory (INEL) has served as the major U.S. Department of Energy storage site for defense-generated, contact handled transuranic (CH-TRU) waste. Approximately 65,000 cubic meters (2.3 million cubic feet) of CH-TRU waste is stored at the RWMC on above-ground concrete or asphalt pads, either covered with an earthen berm, or inside air support buildings, or in storage facilities that are approved by the Resource Conservation and Recovery Act (RCRA)⁽¹⁾. The waste is contained in approximately 132,000 drums and 11,400 boxes.

Before TRU waste shipments from the INEL to WIPP can begin, the TRU waste must be characterized and certified to meet the WIPP Waste Acceptance Criteria (WAC). The SWEPP facility was initially developed to evaluate the capability of various technologies to examine the various waste forms and to certify the contents of waste containers for shipment to WIPP. Over the years, the mission of the facility has changed from technology demonstration to production. The current mission of SWEPP is to perform the non-destructive examination (NDE) and non-destructive assay (NDA) of TRU waste contained in drums for shipment to WIPP.

DOE's TRU waste characterization criteria is specified in the Waste Acceptance Criteria for the Waste Isolation Pilot Plant (WIPP-WAC)⁽²⁾ and the Quality Assurance Program Plan (QAPP)⁽³⁾. The QAPP identifies the quality of data and techniques required to ensure that the specific objectives associated with the TRU Waste Characterization Program are met. It describes the activities that will characterize TRU waste for disposal at the WIPP facility, including both the managerial and technical aspects of program management and the data quality requirements that facilities must meet. The WIPP-WAC identifies criteria and requirements that regulate the safe handling, preparation, transportation, and emplacement of TRU waste packages in the WIPP repository.

The SWEPP facility currently performs NDA/E activities for waste contained in 55-gallon drums. The primary techniques being used in SWEPP to examine waste packages non-destructive radioscopic and radioassay examinations for container contents. These examinations are performed using the real-time radiography (RTR) system, the passive/active neutron (PAN) assay, and gamma-ray spectrometer (GRS) system. An interior view of the SWEPP facility is shown in Figure 2. Surveys for contamination, radiation levels, and drum weight are also made at a container weigh station within the facility. Supporting and associated systems include the Drum Vent Facility and the Transuranic Packaging Transporter (TRUPACT) Facility.

The Stored Waste Examination Pilot Plant (SWEPP) facility contains a high sensitivity, non-destructive assay (NDA) unit based on the combined passive-active neutron (PAN) measurement principle and analysis techniques to evaluate transuranic (TRU) waste packages. The unit was originally designed and constructed by Los Alamos National Laboratory (LANL) and was installed at SWEPP in 1984 and accommodates standard 55-gallon (208-liter) drums.⁽⁴⁾ A second unit installed in 1987 to accommodate metal or wood crates as large as 84 x 54 x 54 inches which did not perform up to expectations has been removed from the facility. The PAN drum assay unit is shown in Figure 3.

In the PAN system, the passive neutron unit detects the number of time-correlated neutrons being emitted spontaneously from the waste package. Passive neutron measurements are then used to quantify the spontaneous fission neutron yield of the source material. The passive portion of the measurement records neutrons being emitted spontaneously, using electronic processing that separates neutrons detected in clusters of two, three, or more at a time from neutrons that are detected singly. The cluster events can be related to spontaneous fission of Pu-240. Since most of the TRU waste processed at SWEPP has a constant Pu-240 isotopic fraction, the Pu-240 measurement can be used to estimate the total Pu mass in a waste package. The single events are related to alpha-neutron reactions in the waste matrix, which for most of the SWEPP wastes are produced by Americium (Am)-241. After processing, this data can be used to estimate the total Am-241 mass present.

Active neutrons are measured by inducing fission in fissile material via an interrogation neutron source and then quantifying the resulting induced fission neutron signal. The interrogation neutrons are provided by a Zetatron neutron generator which produces bursts of 14 MeV neutrons. These neutrons are thermalized and used to interrogate the waste packages and to produce fissions in Pu-239 and other fissile TRU isotopes. The prompt fast fission neutrons that result are counted and can be related to the mass of Pu-239 present. The PAN system is currently configured for a sensitivity sufficient to measure 100 nCi/g quantities of Plutonium (PU) for most waste matrix categories. The presence of lumps of fissile material with dimensions comparable to the mean free path of thermal neutrons in the material can result in underestimation of the fissile content by the PAN system due to self-shielding considerations. As a result, self-shielding can be a problem at the Pu quantities that need to be measured in order to segregate the waste at 100 nCi/g if lumps of fissile material exist within the waste matrix.

Wastes originating from the Rocky Flats Plant (RFP) constitute most of the SWEPP inventory. These wastes consist of primarily weapons grade material have a constant ratio of Pu-239 and Pu-240, and thus both the active and passive assay measurements can be used to estimate total Pu mass. This allows a considerable amount of cross-checking to be done. Since the active measurement is more sensitive, it is used generally to determine Pu contents below 20 g, and especially to determine if a package is below the 100 nCi/g "non-TRU" limit.

Since the RFP wastes are segregated by matrix type, experience obtained in assaying thousands of packages of the same type matrix has led to specialized waste matrix specific analysis algorithms.⁽⁵⁾ Less common waste types, such as those from another DOE site, are processed using a generic algorithm. Both specialized and generic algorithms reside in the operating system software.

The drum assay PAN unit data generation system consists of the assay chamber and its associated polyethylene and graphite moderating materials, structural aluminum, and steel; drum rotator; vertical door opening and closing mechanism. A Zetatron-type pulsed neutron source; active, passive, and interrogation flux neutron detector packages (both shielded and unshielded); mechanical, electrical, and nuclear safety interlock hardware; and various assay chamber signal acquisition and processing electronics. Also associated with the PAN system is a calibration verification drum (designated the "Pink Drum") containing a californium (Cf-252) source and depleted uranium (DU).

The SWEPP gamma-ray spectrometer (GRS) system uses passive gamma-rays to analyze radioactive waste packaged in 55-gallon drums. The GRS is a modified Canberra-supplied, quantitative and qualitative (Q²) low-level waste assay system. The system includes a shielding enclosure, drum turntable, turntable motor, motor controls, high-resolution high-purity germanium (HPGe) detectors, associated amplifiers, high-voltage (HV) power supplies, analog-to-digital converters, acquisition and interface modules, the calibration pulsers, the ethernet communication link, and a VAXstation computer. Also included is the liquid nitrogen (LN) support system required for operation of the HPGe detectors and a drum containing four ¹⁵²Eu₅₂ sources for energy calibration of the system. The GRS system is shown in Figure 4.

During radioactive decay, a given radionuclide emits a number of gamma-rays with specific energies. This gamma ray distribution is known as the gamma-ray energy spectrum for the isotope of concern. Measuring the gamma-ray energy emitted by an unknown sample (analyzing the spectrum) identifies the radionuclides in the sample. The GRS is capable of identifying gamma-ray emitters at very low activity levels packaged in a wide variety of matrix materials. Each of the four HPGe detectors in the system detects and records gamma-rays emitted from the waste, generates a gamma-ray spectrum. The individual detector spectra are normalized and summed prior to final processing and analysis by the VAXGAP analysis code.^(6,7) The software processes the resulting information in a specific energy region to arrive at calculated ratios of the mass of selected isotopes to the mass of ²³⁹Pu and/or to the mass of ²³⁵U. The reported isotopic mass ratios assume that the ratios are constant throughout the entire contents of the drum. The isotopic mass ratio data verify or adjust the quantitative results from the passive-active neutron (PAN) waste assay system. Without these measured isotopic mass ratios from the GRS, the PAN assay system assumes the radioactive material to be weapons-grade plutonium which consists of ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, and ²⁴¹Am (a decay product of ²⁴¹Pu) in nominal weapons-grade proportions. Because of the variety of chemical separation processes used by the waste generator, the material in a drum may not be weapons-grade plutonium. If the contents were found to be enriched in ²⁴¹Am and/or ²³⁵U, then without this data isotopic mass values obtained from the PAN assay system would be in error.

The (Q²) shield chamber was modified to accept four detectors. The chamber is constructed of low radiation background steel. Four circular penetrations have been cut into the back wall of the chamber for insertion of the snouts of the four HPGe detectors. The detectors are positioned at different elevations in the shield to provide overlapping coverage of the waste drum. The detectors are nominally 10% efficient and have energy resolution of better than 800 electron volts (eV) at 122 keV.

The charge-sensitive pre-amplifiers are an integral part of the detector, i.e. they are located within the detector housing. Each of the detectors has been fitted with an INEEL-designed calibration pulser. Information from the pulser pulses is used for real-time validation of the data collected by each detector. The pulser data are stored as a permanent part of the spectral record and are used in analyzing the spectral data for energy calibration, dead time correction, pile-up loss correction, and data quality verification. Each spectrum is individually validated using the stored pulser information. The individually acquired spectra are summed together to increase the resolution of the spectral lines and reduce the total count time under low count rate conditions. This spectral summing upgrade reduced the required count time for each drum significantly. The VAXGAP code contains the subroutines that analyze the pulser buffers of the four individual spectra prior to summing and analyze the gamma-ray spectrum and pulser buffer regions of the composite spectrum following spectra summing. For spectra summing, the energy scales of three of the four spectra collected with the SGRS detectors are normalized to the energy scale of one of the four spectra and then the three spectra are added to the fourth spectrum channel by channel to produce the sum spectrum. The energy scale for each spectrum is described by a quadratic energy function. A routine in VAXGAP calculates the coefficients of the energy function for each spectrum from the centroids of the two pulser peaks stored with the spectrum. The summing technique uses the energy function coefficients determined from one detector as the reference values. The channel contents of the remaining spectra are shifted so that the energy scales of the shifted spectra are identical to the energy scale of the reference spectrum.

The VAXstation computer controls the GRS and the drum turntable, and analyses the spectral data. The spectral data are analyzed using VAXGAP, a spectral analysis code developed at the INEEL, along with the method for calculating isotopic mass ratios. Reduced data is then sent to the SWEPP Data Management System which is linked to the PAN assay system measurement data.

The Real Time Radioscopy (RTR) waste NDE System uses an X-ray source and image intensifier (radioscope) to allow the image of the object under examination to be displayed at the same time the examination is taking place (i.e., in "real" time). The RTR system is shown in Figure 2. This is especially desirable in the identification and characterization of radioactive or hazardous waste because container contents can be examined without breaching the container. The system also provides digital data and image recording capabilities.

The real-time X-ray radioscopy system is used to determines or verify the proper waste Item Description Code (IDC) and visually examines the contents of the waste containers to determine if the contents meet certain waste form requirements. Information on the presence of liquids, gas cylinders (e.g., aerosol cans or other potentially pressurized containers), small particulates (fines), and drum liners is provided using the RTR system.

A cart mounted on rails transports drums or boxes into a lead-shielded enclosure. The cart is has an attachment designed to carry up to three drums. The attachment has three pedestals on which individual drums may be placed. All pedestals turn at the same time from a common drive and will rotate the drums at a speed selected by setting a console potentiometer. Direction of rotation can be switched at the console. Boxes cannot be rotated during RTR inspection but are first inspected from one side removed from the enclosure rotated 180 degrees reinserted into the RTR enclosure and inspected from the opposite side.

The X-ray tube head is a variable potential, oil-cooled, hooded anode Philips Model MCN 421 X-ray head. The maximum operating voltage of the tube is 420 kV dc. The output of the X-ray head is 11,000 R/min at 20 cm when operating at 420 kVp. A beam focus selector for the tube head may be set on wide or narrow focus. The head is a vacuum tube with an anode and a cathode supplied by high voltage DC (+/-210 kV). When high voltage is applied, the electrons produced at the tungsten filament are accelerated into the face of the anode target. The reaction between electrons and the tungsten target produces X-rays.

The imaging system consists of an image intensifier, high resolution CCD camera and a digital X-ray video processor. The digital video processor captures images in real time and can enhance critical or low contrast features of an image through a variety of image enhancements, filters, and real time frame averaging hardware and software to improve signal-to-noise resolution efforts. The output of the imaging system is supplied to a monitor at the operator's station. The operator makes a real time digital video of each drum examined. The video is recorded at 30 frames per second and includes the operator's voice comments with respect to observations during the examination. The video is stored on optical media which provides the following benefits; video duplicating with no loss of image quality, no degradation of video quality over time, random access to all examinations, and a highly stable media for archival storage.

The SWEPP facility is in full production operation processing in excess of 100 drums per week of TRU waste and collecting and storing the required data for characterization and eventual shipment to WIPP for disposal. The SWEPP radioassay systems have successfully passed the testing associated with the WIPP radioassay Performance Demonstration Program Plan (PDP) for the TRU Waste Characterization Program⁽⁸⁾ evaluation cycles 1, 2, 3, and 4. Research, development and demonstration efforts into waste characterization technologies continues, and the integration of software and hardware upgrades and new technology are continuing to be evaluated for implementation in SWEPP. Continuing waste characterization software development and implementation of identified upgrades is expected to increase both process efficiency and the percentage of certified waste containers.

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