Development of an in-situ fission track analysis for detecting fissile actinides in soils and sediments contaminated with actinides

Full text: A fission track analysis (FTA) and an alpha track analysis (ATA) were developed to identify hot particles in contaminated soil or sediment. With the FTA techniques, the track image of the grid coated with Th and fissile nuclides on the Lexan detector was so clearly recorded that the location of the fissile particles was easily identified in the soil or sediment contaminated with the actinides. With the ATA techniques, many of the hot particles contaminated with Pu were discriminated from the U on the track detector due to the different sensitivity of the 239Pu and 235U on the CR-39 detector. Conventional analytical methods used for determining concentrations and distributions of hot particles emitting alpha radiation involve a complex chemical processing and need intensive manpower. In recent years various FTA and ATA techniques with nuclear track detectors have been applied directly to highly contaminated soil without a radiochemical analysis. FTA and ATA techniques provide direct information for detecting nuclides with high fission cross sections such as 235U and 239Pu in contaminated environmental samples. For the FTA, thorium was electroplated onto the SEM grid with electrodeposition solution. The current was adjusted to 0.85 A and held for 1 hour. After the SEM grid electroplated with Th was put on the track detector, contaminated soil particles collected from BOMARC missile facilities (McGuire Air Force Base, New Jersey) were sprinkled on the Lexan detector and then taped together to immobilized the soil particles. The samples were irradiated with a thermal neutron flux of 2.92 x 1012 cm-2·s-1. After the samples were unpacked, the detectors were etched in a solution of 6 M NaOH at 70 ± 1 deg. C for 10 minutes. For the ATA, soil particles were sprinkled on the CR-39 detector and then taped for immobilizing the soil particles. The samples were exposed for a minimum of two weeks to create detectable alpha tracks. After the samples were unpackaged, the detectors were etched in a solution of 6.25 M NaOH at 75 ± 1 deg. C for 5 hours. Fission and alpha tracks were observed using an optical microscope. An image of the fission tracks by the grid electrodeposited with thorium was clearly recorded on the track detector. The image generated by the fissile nuclides can be distinguished from the image of the outline of the SEM grid. The fission tracks were not distributed randomly within the detector, but rather were arranged in a distinct 'star-burst' shape. The fission tracks from the highly fissile nuclides were correlated with the fission tracks from the grid coated with thorium. By using the fission track image, the fissile particles within the grid electroplated with thorium were easily identified and could be separated from the sample matrix for the analysis of speciation of the single particles. Alpha tracks of the contaminated BOMARC soil particles were recorded randomly on the CR-39 track detector. Various bundles of alpha tracks were recorded on the CR-39 detector, depending on the activity concentration of the fissile nuclides. The 'agglomerated drops' shape of the alpha tracks on the CR-39 is a little different from the 'star-burst' shape of the fission tracks on the Lexan. Most of the alpha tracks recorded on the CR-39 detector were likely generated from 239Pu not 235U, because the alpha tracks generated by 239Pu are more active on the CR-39 detector than those by 235U due to the different half-lives. This infers that many of the hot particles in the BOMARC soil may be contaminated with plutonium isotopes. This verification was supported from the results of the radiochemical analysis that the activity concentration of 239,240Pu in the BOMARC soil, approximately 4.56 Bq/g, was determined to be about two orders of a magnitude higher than the natural level of the 235U concentration. Also, for obtaining information of the speciation of the hot particles, the fissile particles can be further studied by microscale techniques such as SIMS (secondary ion mass spectrometry) and SXMA (synchrotron x-ray microprobe analysis), after separating the hot particles from the contaminated soil with the FTA techniques. (author)