Year
2001
Abstract
Swipe samples collected by the International Atomic Energy Agency (IAEA) inside nuclear facilities, including those taken in hot cells, are routinely submitted to the U.S. Department of Energy Network of Analytical Laboratories (NWAL) for uranium and plutonium mass spectrometry after clean laboratory destructive preparation. The use of delayed neutron activation has been investigated as a tool to rapidly screen these samples to determine the quantity of total fissile content. Having this information will significantly improve the sample preparation process by reducing the time and cost associated with sample preparation and by providing valuable information that will prevent the inadvertent contamination of a clean facility. The driving force for using this screening technique is that over the past year, samples containing uranium varied more than six orders of magnitude. The high end of this range completely renders Class 100 clean laboratories useless for analyzing important samples at the lower concentration range. The swipe samples provided to NWAL for analysis contain fissile materials composed of one or more of the following isotopes: 239Pu, 235U, and 233U. When exposed to an intense neutron field, these fissile materials undergo fission. Following irradiation, some fission elements emit neutrons designated “delayed neutrons” because they are emitted after a brief decay period. The counting of these delayed neutrons provides a simple method for determining the total fissile content in the swipe sample. The method is characteristically a very accurate technique; the pine needle standard (standard reference material, SRM 1575), which has a certified value of 20 ppb U, produced a value of 21 ppb U using the delayed neutron technique. Because neutron activation analysis (NAA) is a nuclear technique, the chemical bonding environment of a fissile atom is irrelevant. Likewise, the status of the electron cloud is of no importance. Therefore, NAA is virtually immune to matrix effects that arise from different chemistries. NAA analyzes pine needles, tree rings, soil, sediments, polymers, and biological materials equally well. All of the sources of error in the NAA-delayed neutron counting experiment are known. Although it is sometimes challenging to quantify these sources of error, there are no surprises. Most importantly, NAA is nondestructive and inexpensive for laboratories with an irradiation facility available, such as the Oak Ridge High Flux Isotope Reactor. A comparison of results from 35 IAEA samples will demonstrate the utility of the NAA technique and also will demonstrate the uranium concentration range from the low picograms to the milligram level.