Development of Nondestructive Measurement Techniques for Uranium-contaminated Waste in Containers

We have developed a new technique based on the passive gamma method for measuring a quantity of uranium in a uranium waste container (1200 liter volume). The measurement system consists of three NaI (Tl)detectors, one High Purity Germanium (HPGe) detector and a handling system. The measuring method uses two discrete gamma-ray energies emitted from the daughter nuclide Pa-234m (1001 keV and 766 keV). The technique is applicable even if distribution of density and radionuclides are not uniform. We developed a new technique to remove the influence of distances between radionuclides and detectors. This technique was named “the Facing Couple Method (FCM)”. If the influence of daughter nuclide is strong, the evaluation is carried out using data of detector to remove the influence. When counting rates are low, gross counting rates corrected the variation of background rate by container is adopted as an alternative method. We confirmed applicability of this technique and system by examination using natural uranium (50g 10kg) and the observation of container started from January 14, 2004 and 107 observations have been completed until August 23, 2004. INTRODUCTION Measurement evaluation of uranium content within wastes is necessary for waste management, waste treatment and disposal. The majority of uranium wastes in “Japan Nuclear Cycle Development Institute (JNC)” are packaged in 200 liter drums or 1200 liter rectangle containers. The quantity of uranium in drums has been measured by non-destructive assay (passive gamma method) in JNC (1) (2). By in contrast, the quantity of radionuclides in the container is difficult to evaluate. The measuring method of the waste drum with revolution cannot apply to the rectangle container. WM’05 Conference, February 27-March 3, 2005, Tucson, AZ One exception to this is a plutonium waste container. The container contained plutonium was measured by “the Waste Container Assay System (WCAS)” in JNC. The system utility of the WCAS is the passive neutron coincidence method (3). However, the technique such as WCAS cannot apply to the uranium waste, because the uranium waste releases little neutron. The estimation of quantity of uranium relied on waste information that composition of uranium and surface dose rate. For the reasons mentioned above, we have developed the new technique based on the passive gamma method for measuring the quantity of uranium in the uranium waste container. Waste and Container Characteristics The uranium waste containers are loaded with a large variety of waste materials contaminated with uranium and its daughter nuclides. For example, steel or PVC pipes, steel racks, pumps and etc. are packaged in a container. The distributions of density and radionuclides are not uniform. The following is a physical description of the container. The dimensions of a container are approximately 120 x 120 x 90 cm with each container likely to hold up to 1200 liter waste volumes. A wall thickness of container is 0.45cm and that of bottom is 0.6cm of steel (SS400). The weight of an empty container is 400 kg. System The measurement apparatus was manufactured in cooperation with Mitsubishi Heavy Industries, Ltd. The system was designed to assay containers with volume up to 1200 liter and weights up to 3000 kg in its standard configuration. The measurement system consists of three collimated NaI detectors, one collimated HPGe detector and a handling system which could move a container back and forth and turn it (Figure 1). The container was placed on the turntable which turns every 90 degree angle. WM’05 Conference, February 27-March 3, 2005, Tucson, AZ The NaI detectors measures two discrete gamma-ray energies emitted from the daughter nuclide Pa-234m (1001 keV and 766 keV) of U-238. The measurement of two gamma-rays was designed to correct the matrix attenuation losses. Three NaI detectors were placed in a line vertically. The one surface of container was divided into 12 regions like as meshes (3 x 4). The measurement was performed for four sides of a container (12 x 4). The count time was determined by the need to obtain reasonable count statistics in the two gamma-ray peaks of interest (1001 keV and 766 keV). Normally 900 sec count was adequate. The total measuring time, using 16 scans for each point is four hours. The weight of each container was registered. These data were then used to determine “activity / unit waste mass” values for defined waste components from which estimates of the total uranium content of filled containers has been established. The data acquisition system contains a PC-based MCA system, appropriate high voltage power supplies and amplifiers. Methods The measureme distributions of Fig. 1. System configuration nt was performed on the container divided in imaginary meshes because the density and radionuclides were not uniform. WM’05 Conference, February 27-March 3, 2005, Tucson, AZ We developed a new technique to remove the influence of distance between radionuclide and detector. This technique was named “the Facing Couple Method (FCM)”. These measurement results confirmed that the quantity of uranium in a large-sized container could be evaluated by passive gamma method (FCM). The direct radiations emitted by radionuclide in the same direction went through the materials that have the same density. Therefore the effective attenuation distance (ρt) could be evaluated by peak counting rate ratio of two direct radiations. This effective attenuation distance between radionuclide and detector revealed the quantity of uranium in container. In addition, the following relation could be used by the data which is measured at opposing point. Density of direction of detector (a) may not be the same as direction of detector (c) in the waste, but the distance (l+2∆) between detector (a) and the detector(c) is constant shown in Figure 2. Fig. 2. A conception diagram of FCM Distance between Source and Detector(a) Distance between Source and Detector(c) Addition If reliable measurement data at point (a) and ( radionuclide can be estimated except the influ below: (Eq.1) c) (or (b) and (d)) are provided, the quantity of ence of distance. The estimate process is given WM’05 Conference, February 27-March 3, 2005, Tucson, AZ