VALIDATION OF IQ3 MEASUREMENTS FOR HIGH-DENSITY LOW-ENRICHED-URANIUM WASTE DRUMS AT PELINDABA

ABSTRACT

The U.S. Department of Energy Office of International Safeguards and the Nuclear Energy Corporation of South Africa (NECSA) are collaborating on a measurement campaign to complete an inventory of nuclear materials contained in waste drums at Pelindaba. Containers with reportable quantities are declared to the International Atomic Energy Agency (IAEA). All known containers of high-enriched-uranium waste have been measured and declared. However, some 20,000 drums containing low-enriched- uranium (LEU) waste are currently being evaluated by the IQ3 drum waste assay scanner. The LEU inventory includes a large quantity of material that is composed of a high-density waste matrix that contains high concentrations of uranium. In some cases, the density or uranium quantity contained in the waste drums exceeds the current measurement calibration limits. Calibration of the IQ3 for these high-density, high-uranium-concentration containers is complicated by the fact that suitable standards are not available. Because this instrument is a dual-use system (i.e., both NECSA and IAEA personnel perform measurements using the IQ3), the calibration standards and techniques must be acceptable to both organizations. This paper provides an overview of the project, including techniques devised to compensate for the lack of suitable standards, as well as reconfiguration of the IQ3 to facilitate measurement of high-density, high-uranium- concentration drums to ensure that an accurate declaration of the LEU inventory can be completed.

INTRODUCTION

In 2001 the USDOE Office of International Safeguards (OIS) provided a nondestructive assay (NDA) system to the NECSA under a bilateral agreement between the United States and South Africa. The purpose of the system was to provide assay capability to help to resolve an anomaly in the safeguards inventory that had been declared to the IAEA and to characterize the legacy waste that had been generated by a uranium enrichment program ¹.

The system was delivered to the PelStore facility at the Pelindaba Nuclear Institute, the location of the former uranium enrichment program. As a part of the bilateral agreement, OIS provided a comprehensive support program, which included an upgrade to the system (originally built in 1995). Oak Ridge National Laboratory and Canberra Industries provided installation, setup, calibration, training, and development of measurement techniques. The system was an IQ3 drum waste assay system manufactured by Canberra Industries. The IQ3 has the capability of assaying 208-L drums using a six high-purity germanium (HP6e) detector configuration. The three planar detectors in a vertical plane on the rear are used for enrichment analysis, and the remaining three coaxial detectors in a vertical plane on the right-hand side of the counting chamber are used for quantification, with the capability of performing ¹³³Ba transmission‑corrected assays. The IQ3 drum-handling fixture was adapted to handle a variety of drum sizes and diameters encountered at PelStore ².

Prior work has involved developing calibrations and measurement techniques for a large number of container sizes, waste matrices ranging from very low density (< 0.1 g/cm ³) up to moderate densities (about 0.6 g/cm ³). Enrichments range from depleted to 93% enriched uranium, and mass loadings rang from less than 1 g of uranium up to more than kilogram quantities of uranium. The current measurement campaign focuses on high‑density, LEU containers. These containers have densities that can exceed 1.1 g/cm ³, with enrichment between depleted and <20%, and may contain kilogram quantities of uranium.

Because this is a safeguards application, the primary criterion of interest is ²³⁵U mass. In addition, the system is intended to be a shared‑use system between NECSA and the IAEA. Each organization has configured its own computer to control the IQ3 and has collaborated with U.S. experts to develop calibration and measurement methods that are acceptable to both organizations ³.