Characterization of Plutonium Contaminated Soils From the Nevada Test Site for Remediation Method Selection

Plutonium (239/240Pu) contamination in soils is an environmental concern at many U.S. Department of Energy (DOE) sites. Remediation actions have been attempted using different technologies, and clean-up plans have been implemented at several sites, such as the Nevada Test Site (NTS). During the 1950’s and early 1960’s, nuclear weapons testing at and near the NTS resulted in soil contaminated with plutonium particles. Clean-up efforts are continuing using conventional remediation techniques. However, the DOE desires to obtain technologies that can further reduce risks, reduce clean-up costs, and reduce the volume of contaminated soil for disposal. Low levels of plutonium contamination are distributed somewhat uniformly throughout the NTS soils and, as a result, it is difficult to obtain volume reductions above 70%. The subject of this research was to characterize the plutonium-contaminated soil from the Tonopah Test Range (TTR) north of the NTS. In order to select remediation methods, it is important to gain a better understanding of how plutonium is bound to the contaminated soil; thus, size separation, magnetic separation, and the sequential extraction (SE) methods were used for this purpose. The SE method consisted of targeting five operationally defined geochemical phases: ion exchangeable, bound to carbonates, bound to iron and manganese oxides (reducible), bound to organic matter, and resistant. Radiometric measurements were used to determine plutonium in each of these defined phases in the soil. Selected stable elements were also determined, to compare the operation of the SE method to other investigators. The SE experiments were performed with two types of samples: soil without heat treatment and soil with heat treatment. The MF treatment was used to destroy the organic content in the soil so as to further evaluate the SE procedure. Particle size analysis indicated that approximately 37% of the TTR soil by weight was larger than 300 micrometers and this fraction contained little plutonium, < 100 pCi/g. Thus, size separation may be useful as part of a remediation process. Magnetic separation tests showed that the magnetic fraction of the TTR soils is very small, and the non-magnetic fraction still contained the majority of the plutonium. Thus, a magnetic separation step in a treatment process would not be useful. Following SE, analysis results of the stable elements agreed with reported values. The SE results also indicated an association of plutonium with the organic and resistant defined phases. The main change in 239/240Pu distribution following heat treatment was an increase of plutonium recovery in the reducible phase. The SE results showed that fairly aggressive chemical treatment would be required if leaching were part of a remediation process.