Environ. Sci. Technol. 2005, 39, 5512-5516
Activities of Pu and Am Isotopes and Isotopic Ratios in a Soil Contaminated by Weapons-Grade Plutonium M. H. LEE* Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Post Office Box 150, Daejon, Yusong, Republic of Korea S. B. CLARK Washington State University, Department of Chemistry and Nuclear Radiation Center, Post Office Box 644630, Pullman, Washington 99163-4630
An accident and fire at the former McGuire Air Force Base and Boeing Michigan Aeronautical Research Center (BOMARC) site in New Jersey resulted in dispersion of weapons-grade plutonium in particulate form to the local environment. Soil samples collected at the BOMARC site were measured for their activities and isotopic ratios of Pu and Am isotopes by radioanalytical techniques. The activities of the Pu and Am isotopes in the BOMARC soil were markedly higher than fallout levels, and they decreased nearly exponentially with increasing particle size of the soil. The measured 241Am activity was compared to calculated values based on decay of 241Pu. The activity ratios of 238Pu/239,240Pu, 241Pu/239,240Pu, and 241Am/239,240Pu observed in the BOMARC soil were much lower than those attributed to nuclear reprocessing plants and Chernobyl fallout. From the activity ratios of 241Pu/239,240Pu and 241Am/239,240Pu, the origin of the Pu isotopes was identified as weapons-grade and the time since production of the material was estimated. Furthermore, the atomic ratio of 240Pu/239Pu in the BOMARC soil was remarkably lower than the fallout value influenced by nuclear weapons testing and the Chernobyl accident. The atomic ratio of 240Pu/239Pu was very close to the value of the weaponsgrade Pu detected from the Thule accident in Greenland. This work demonstrates the utility of radioanalytical measurements and decay calculations for defining characteristics of the source term and discriminating multiple processes that contribute to a source. Such an approach would also be needed to respond to a terrorist event involving an improvised nuclear device or radiological dispersal device.
Introduction Artificial radionuclides have been released into the environment as a consequence of energy production, weapons production, other industrial processes, and accidents (1). More recently, concern has grown over the threat of a radiological attack involving either an improvised nuclear * Corresponding author phone: +82-42-868-4728; +82-42-868-8148; e-mail:
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 15, 2005
device or a radiological dispersal device (2), which would release radionuclides in an attempt to terrorize and/or kill. Whether releases of nuclear materials are intentional or accidental, radioanalytical measurements and radioactive decay relationships are useful in characterizing the source material and the nature of the event and in discriminating among possible sources of the radioactivity. Plutonium is a transuranic element that is present in the environment as a result of fallout from above-ground weapons testing during the late 1940s through the early 1960s and accidental releases due to production of defense materials and military mishaps. The environmental behavior of Pu and the other actinides is complex and depends on the physicochemical properties of the soil, the chemical forms of these nuclides, and the biological-chemical processes in a given environment. The level of Pu contamination in soils or sediments due to nuclear weapon tests or accidents at nuclear facilities varies with the geographic location, with the Pu being inhomogeneously distributed in the local environment (3). Plutonium migration in the environment can occur via the transport of soluble dissolved species and/ or colloids. Often, however, a significant fraction of the total Pu in a given system remains strongly sorbed to the soil particles (and hence immobile). Thus, soil particles are a very important reservoir for plutonium and other actinides in the terrestrial environment. Many parameters such as the grain size of the soil particles, organic substances, and pH can influence the distribution of these isotopes in the soils and sediments. In 1960, a military accident occurred at McGuire Air Force Base in New Jersey, when a Boeing Michigan Aeronautical Research Center (BOMARC) missile caught fire and the warhead was partially melted by the fire (4). While the warhead did not explode, heat from the fire and firesuppression activities and the turbulent local atmospheric conditions during the fire contributed to dispersion of weapons-grade plutonium (WGP) into the local environment. As a result, a significant quantity of plutonium particles with varying sizes (submicrometer-sized hot particles or single large particles) containing 239,240Pu and 241Am were inhomogeneously dispersed. Among the Pu isotopes, 239Pu is a principal isotope of the WGP and is produced via thermal neutron capture by 238U, which then undergoes subsequent decay to 239Np and then to the long-lived 239Pu. As presented in Table 1, the WGP at the BOMARC missile site consisted of mainly 239,240Pu, with a small quantity of 241Pu and a negligible amount of 241Am at the time of manufacture (4). Note that 241Pu decays to 241Am by β emission with a half-life of 14.35 years; consequently, it is expected that the activity ratio of 241Am/239,240Pu should increase significantly since more than 40 years has elapsed since production of the WGP. At the same time, the activity ratio of 241Pu/239,240Pu is expected to decrease. Compared to other isotopes of Pu, relatively little has been reported in the literature on activity of 241Pu in the environment, likely due to the difficulty in its determination. At the same time, activities of 241Am are often reported without regard to its ingrowth from decay of 241Pu. Several investigations have characterized the distribution of 239,240Pu by R spectrometry and 241Am determined by γ spectrometry for soil from the BOMARC missile accident site (4-6). Because of the low intensity of the 241Am γ ray and the relatively high background activities at low γ energies (∼60 keV), it is difficult to estimate small quantities of 241Am contamination. This is due to γ self-absorption, even when a GM detector designed to compensate for this problem by using agar is employed. 10.1021/es0486115 CCC: $30.25
2005 American Chemical Society Published on Web 06/28/2005
TABLE 1. Isotopic Composition of Plutonium and Americium in the BOMARC Weapon (4) nuclide
half-life (years)
specific activity (Ci/g)
mass %
activity ratio to 239Pu
atomic ratio to 239Pu
239Pu
87.74 24 1000 6560 14.35 376 000 432.20
17.12 0.062 0.227 103.37 0.004 3.43
0.0099 93.70 5.60 0.47 negligible 0
0.029 1 0.22 8.36
0.0001 1 0.0598 0.0050
239Pu 240Pu 241Pu 242Pu 241Am
To our knowledge, there are very few published results of 241 Am and 241Pu activities in soils using a combination of alpha and beta spectrometric techniques after a radiochemical separation. This paper describes the distributions of 238Pu, 239,240Pu, 241Pu, and 241Am, obtained by radiochemical determination techniques, from different particle sizes of a soil sample collected from the BOMARC missile site. The activity ratios of 238Pu/239,240Pu, 241Pu/239,240Pu, and 241Am/239,240Pu as well as the atomic ratios of 240Pu/239Pu were also measured to characterize the source material and estimate the age of the WGP detected in the BOMARC soil. These results make it possible to contribute to the site-specific data for the BOMARC missile site remediation plan. In addition, such an approach could be useful in responding to nuclear terrorism, should an improvised nuclear device or radiological dispersal device be detonated.
Experimental Procedures Sampling Site and Sample Preparation. The BOMARC missile site is an inactive Air Force installation in the Plumstead Township of New Jersey. The site was an active nuclear missile defense site from 1958 to 1971. On June 7, 1960, a fire occurred in one of the shelters for missiles loaded with warheads, which partially consumed a missile and its warhead. The highly explosive materials in the weapon ignited but did not detonate. The fire melted the WGP, which was contained in the device. Turbulent local atmospheric conditions and the water applied during the fire contributed to the scattering of WGP over a 7-acre area in front of the shelter (4). The missile site is about 218 acres, of which there are approximately 8100 cubic yards of soil and debris with a total activity of less than 74 Bq/g and approximately 160 cubic yards of soil and debris with a total activity of more than 74 Bq/g. The Radiation Division of the Surveillance Directorate for the U.S. Air Force Institute for Environment, Safety and Occupational Health Risk Analysis collected soil samples at the BOMARC site in June, 2000. The soil samples were collected to a depth of 2 in., blended and homogenized in a soil tumbler, and subdivided into approximately 20-g samples. Organic matter content was determined by losson-ignition with an ashing temperature of 550 °C for 24 h (7). The organic content was 1.89% ( 0.02%. The pH measured with a glass electrode in a 1:5 suspension of soil and water (8) was 8.20 ( 0.10. Grain size fractions were determined with sieves. As shown in Figure 1, most of the BOMARC soil consisted of sand (>53 µm, 97.5%), with silt and clay (
