Distribution of Pu and Am isotopes in BOMARC missile site soil

Full text: The activity concentrations as well as the activity and atomic ratios of the Pu and Am isotopes in different sizes of the soil sampled around BOMARC Missile Site were obtained by radiochemical analysis. The association pattern between the actinides and soil particles was investigated using a spherical model of a particle size with a variable radius. From the activity and atomic ratios of Pu and Am, the origin of the Pu and Am isotopes was identified in the BOMARC Missile Site soil. Plutonium is one of the transuranic elements which is primarily present in the environment as a result of human activity, namely as the fallout from nuclear weapon testings during the late 1940s through to the early 1960s, and accidental releases due to military mishaps. One particular mishap occurred in 1960 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. Although the missile did not explode, subsequent fire fighting activities contributed to the dispersion of weapons grade plutonium into the local environment. Soil samples around BOMARC site were taken to a depth of 2 inches by the U.S. Air Force Institute for Environment. The soil samples were blended and homogenized in a soil tumbler, and subdivided into approximately 20-gram samples. Grain size fractions were determined with sieves. Determination of 239,240Pu, 238Pu, 241Pu, 241Am and 238U was performed using a radiochemical method. After adding 242Pu, 243Am and 232U tracers, a total of a 2 g ashed soil sample was dissolved with concentrated HNO3 and HF and evaporated to dryness. Dissolution in HNO3/HF was repeated and again evaporated to dryness. The residue was dissolved in 9 M HCl. After filtration, the solution was passed over an anion exchange column (chloride form) to which the Pu was sorbed. The columns were washed with 9 M HCl followed by 8 M HNO3. This effluent was evaporated to dryness and reserved for subsequent separation of the Am and U isotopes. Pu was eluted with 0.36 M HCl/0.01 M HF. The Am and U residue was dissolved in 2 M HNO3. Samples were loaded onto TRU Spec columns, after the TRU columns were conditioned with 2 M HNO3. The columns were washed with 2 M HNO3 and 9 M HCl. The Am fraction was eluted with 4 M HCl. U was eluted with 0.1 M ammonium oxalate solution. The Am fraction was purified with anion exchange resin that had been previously conditioned with 1 M HNO3 - 93% CH3OH. The purified Pu, Am and U isotopes were electroplated on stainless steel platelets and measured by an alpha spectrometer. For measuring the 241Pu, Pu isotopes electrodeposited onto the stainless steel planchet were dissolved in 8 M HNO3 and purified with an anion exchange column. The purified Pu isotopes were subjected to the 241Pu measurement by a low background liquid scintillation counter. Also, the atomic ratios of 240Pu/239Pu were measured by a high resolution inductively coupled plasma mass spectrometry. The activities of 239,240Pu and 241Am were apparently increased with a decreasing particle size of soil due to an increasing surface area, and the fitted values for these isotopes were correlated well with actual measured values (239,240Pu; 0.994, 241Am; 0.992). This result suggests that most of the Pu and Am isotopes are fixed to the soil as a surface coating. The fitted values of 238U did not correlated well with the measured activities of 238U (R2 ; 0.588), which implies that a significant fraction of the U may be partitioned into the soil matrix rather than sorbed onto the particle surfaces. The activity ratios of 238Pu/239,240Pu in different particle sizes of the BOMARC soil were found to be in the range of 0.018 to 0.033 with a mean value of 0.024, which is lower than the reported value (0.04) influenced by the SNAP-9A satellite accident in 1964. The activity ratios of 241Pu/239,240Pu and 241Am/239,240Pu were measured in the range of 2.87 to 3.52 an d 0.15 to 0.24, respectively. These values are a little lower than the recently reported activity ratios influenced by the fallout from the nuclear weapons test. Also, the atomic ratios of 240Pu/239Pu in the BOMARC soil were found to be in the narrow range of 0.0559 to 0.0594 with a mean value of 0.0576, which is close to the value (0.05) of the weapon grades plutonium rather than the fallout value (0.18) influenced by the nuclear weapons test. Therefore, as a result of the activity ratios 238Pu/239,240Pu of 241Pu/239,240Pu and 241Am/239,240Pu as well as the atomic ratios of 240Pu/239Pu, the Pu and Am isotopes detected in the BOMARC soil originated from the weapons grade plutonium rather than the nuclear weapon testings. (author)