Non Destructive Assay Box Counter

Many long established US DOE facilities are in possession of poorly documented wastes in a variety of forms spanning a broad range of waste containers, waste matrices and isotopic mixtures. Existing assay technologies such as the SuperHENC can address many of the expected waste configurations. However, there are also expected waste items that fall outside the normal operating range of these exceptional but “standard” systems. Given the inherent difficulties and high costs associated with repackaging even a small number of these containers, an assay solution combining multiple techniques has been studied with the objective of maximizing the likelihood of successfully assaying waste for eventual shipment to WIPP or low level waste disposal. The Integrated Crate Interrogation System (ICIS) is comprised of a Box Segmented Gamma Scanner (BSGS) and a passive/active neutron counting system called the Super IWAS. These are two physically independent assay systems mounted within separate ISO Containers but connected via Ethernet to allow automated integration of the assay results from the two systems. In operation, the waste containers will first be assayed within the BSGS system to obtain both quantitative gamma-ray assay results and relative isotopic data using well-known algorithms such as the MGA or FRAM. In addition to scanning in front of an array of HRGSs the item is also stepped past a Co transmission station on the same line. The operator will then move the container to the neutron assay system. The Super-IWAS concept considered, based on the successful IWAS installations at the AMWTP, provides both highefficiency passive neutron coincidence analysis and active neutron interrogation using the Differential Die-Away technique (DDA). The prospect of combining all three complementary assay modes to provide a reliable assay result is discussed for realistic waste forms along with the extensive modeling results. Our objective was to devise an NDA solution to this pressing problem that stretched the current state of the practice but which could be implemented with low technical risk using a reasonably sized resource allocation in a predictable and timely fashion. At the conclusion of the study, a design without the DDA capability was selected for construction. INTRODUCTION A feasibility study for an integrated waste assay system for the characterization of suspect transuranic (TRU) waste was undertaken by Canberra Industries as part of the U.S. D.O.E. Program Research and Development Announcement (PRDA) No. DE-RA09-03SR22278. The ICIS represents one of several system types examined in that study. The following sections of this paper discuss the performance of the Super-IWAS system for the characterization of wastes within large containers such as the Solid Waste Liner Box (SLB-2) and the Ten Drum Over Pack (TDOP). The assay of large waste containers is not a new application. The currently deployed box counter technologies have been used with varying degrees of success for a variety of matrix types and measurement conditions. However, they all share important limitations. First, all existing box counter measurements, whether gamma or neutron based, are dependent on waste matrix composition and source distribution. Matrix composition parameters that affect measurements include elemental form, bulk density, presence and distribution of multiple matrix materials, and the concentration of interfering materials (such as neutron moderators and absorbers) in the matrix. Although matrix effects are generally smaller for passive neutron measurements than for gamma-ray or active neutron instruments, all of the existing techniques are susceptible to some extent. Self shielding is another effect that has (provided the moderator content is not excessive) the potential for increasing the bias in active neutron and gamma-ray measurements. Although passive neutron analysis is also less vulnerable to this effect, it suffers from poorer sensitivity and significantly higher background effects. All existing systems are affected by non-uniform distributions of radioactive sources. Non-uniform source distribution, including non-uniform plutonium composition, can bias NDA measurements either high or low. Several attempts have been made in the recent past to correct biases due to matrix composition uncertainty, self shielding, source distribution effects, and background irregularities in nondestructive measurements. Some of these attempts include random triggering of the accidentals gate to reduce backgrounds in passive neutron measurements; matrix non-uniformity correction algorithms for gamma-ray analysis; neutron imaging and tomography for neutron and gamma-ray analysis, respectively; the ‘add-a-source’ technique to compensate for matrix effects, etc. In addition, there have been attempts to provide matrix-specific calibrations for each different matrix form that the system is intended to measure. To some extent, all of these approaches have led to improvement in measurement accuracy. However, the improvements have generally been over a limited range of matrix types, under a limited set of conditions, or have required great effort to implement. Most of the correction approaches have also been applied to drum-sized, or smaller, containers. Their usefulness for analysis of boxed waste has not yet been validated. The ICIS concept attempts to improve the likelihood of successfully characterizing radioactive wastes by integrating the assay results from three distinct assay modes, high efficiency passive neutron counting, active neutron interrogation, and box segmented gamma scanning. The ICIS would consist of two physically separate counters, the Super-IWAS passive active neutron system and the Box Gamma Segmented Gamma Scanner (BSGS). These systems would be based on modifications to existing counter designs and techniques with an automated data integration technique tailored to these large systems. The Multi-Modal Approach The ICIS provides three distinct assay modes. The different analysis methods are complementary in that the weaknesses of one assay mode are often times the strengths of another. The ICIS approach provides all the data that can be gathered on the waste container to minimize misclassification of wastes. Passive neutron counting provides the following benefits: • The passive neutron assay does not suffer from self shielding or self absorption effects, providing generally more accurate assay results than the HRGS or active neutron assays for higher plutonium and uranium loadings. • More accurate than active neutron interrogation because it is less sensitive to matrix effects including thermal absorbers and less sensitive to source distributions. Matrix effects in passive neutron are smaller and more predictable, therefore, the errors from matrix effects can be bounded. This feature is necessary for approval of the Total Measurement Uncertainty (TMU) method for WIPP certification. • Significantly less sensitive to the presence of high-Z material in the waste matrix than HRGS. • Interference from uranium is small. For passive assay U is a weak interference but its effects can be corrected for in the neutron analysis when combined with the gamma-ray assay. • The total neutron rate is proportional to the total alpha activity that must be reported as part of the characterization requirements for WIPP. While this measurement is subject to the chemical form of the alpha emitter it can be used to bound the sample’s alpha activity. • The Pu-effective from the passive measurement can be combined with a gamma isotopic measurement to provide reliable reporting of plutonium and other isotopes. High resolution gamma spectroscopy provides: • Measurement of the plutonium, americium and/or uranium isotopic ratios. Standard isotopic analysis codes such as Multi-Group Analysis (MGA) and FRAM code are available and have been approved for isotopic measurements by DOE-CAO during multiple site audits. • Quantitative measurement of plutonium and uranium for low density matrices. By summing the spectra from all eight detectors, the proposed system can achieve detection levels well below the 60 nCi/g TRU detection level for plutonium wastes. • Detection levels for U-235 < 0.5 grams for uniform source/matrix distributions for matrix density <1.2 g/cc. • Direct measurement of other gamma emitters in the waste that are not identified in the isotopic measurement (e.g., Cm). • Basic positional information to improve the accuracy of both the gamma and the neutron measurement. Active Neutron Interrogation provides: • Measurement of plutonium in the presence of interfering neutron emitters such as curium and californium. • Measurement of plutonium in the presence of (alpha, n) neutron interferences • Confirmation that the container is below fissile limits when the uranium content can not be definitively tied to the passive neutron measurement. • “Arbitration” when passive neutron and quantitative gamma results disagree. • Provides rapid screening for LLW wastes that are difficult to assay by means of the passive neutron or gamma modes. . • Measurement of uranium in the absence of plutonium.