Non Destructive Assay Box Counter

ABSTRACT

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 60Co 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 high-efficiency 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 ¹. 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 ^{2,3,4,5,6,7,8}. 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.