Application of autoradiographic techniques for contaminant distribution studies in soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gt54kr29n

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  • In order to determine the physical location of contaminants in soil, two solidified soil "thin" sections, which preserve the undisturbed structural characteristics of the original soil, containing weapons-grade plutonium from the Rocky Flats Environmental Test Site were prepared. Two autoradiographic methods were used in radionuclide mapping: contact autoradiography using CR-39® plastic alpha track detectors and neutron-induced autoradiography that produced fission fragment tracks in Lexan® plastic detectors. The combination of the two autoradiographic methods distinguished alpha-emitting particles of natural U, from ²³⁹⁺²⁴⁰Pu and non-fissile alpha-emitters. The locations of 990 alpha "stars" caused by plutonium "hot particles" in two soil sections were recorded, particles were sized, their size-frequency and depth distributions were analyzed. Several large colloidal conglomerates of "hot particles" were found in soil profiles. One such conglomerate with a geometric size of about 500 μm contained over 94% of the total recorded contaminant alpha activity in the sample. It was found that the upper 6.5 cm of soil contained 20% of all recorded particles (mean equivalent size 0.35 μm). The deeper portion of the surface 6.5 cm soil layer contained 80% of the particles (with mean equivalent size 0.25 μm). The average specific activity (SA) for 989 hot particles (with the conglomerate of the particles excluded) with equivalent diameters over 80 nm was found to be greater than 23.9 Bq g⁻¹ (about 90% of the overall average SA). For dissolved and defragmented (below 80 nm of equivalent diameter) actinide particles, SA was found smaller than 2.9 Bq g⁻¹. Over 99% of the total actinide contaminant activity in the analyzed soil sample (with the conglomerate of the particles included) was found in particles with equivalent diameters over 0.08 μm. it suggests that larger particles of plutonium oxide moved down more slowly than smaller ones and no significant breakup of plutonium oxide particles occurred since the original plutonium soil deposition. Both profiles of the depth contaminant activity and number of actinide particles have distinctive peaks at the same depth, about 10 cm. Independent from nuclear track analysis, this pattern of the actinides depth distribution was observed in the measured gamma activity depth profile of ²⁴¹Am (daughter product of ²⁴¹Pu beta decay) in solidified soil blocks.
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