Science Today: SSRL and Rocky Flats Plutonium Remediation
The Rocky Flats Environmental Technology Site (RFETS) is an environmental cleanup site located about 16 miles northwest of downtown Denver. Soils at RFETS are contaminated with actinide elements (Uranium, Plutonium, Americium) from improper storage of contaminated solvents and site operations. Until December 1989, the Rocky Flats Plant made components for nuclear weapons using various radioactive and hazardous materials, including plutonium, uranium and beryllium. In 1995 the site was designated an EPA Superfund cleanup site.
The DOE originally estimated site clean-up would cost $37 billion and take nearly 70 years. Independent contractor Kaiser-Hill and the DOE, working in close coordination with Rocky Flats stakeholders, devised an aggressive plan to complete the cleanup and closure of Rocky Flats by 2006 at an estimated cost between $6 billion and $8 billion.
The key to the aggressive clean-up strategy was to first understand the chemical and physical mechanisms controlling the transport of plutonium in the RFETS environment. The probability of release of plutonium from RFETS soils to the surrounding environment is governed by the solubility of its compounds in groundwater and surface waters, the tendency of plutonium compounds to be adsorbed, or stick to, minerals and organic materials in the soils, or be dispersed by wind. This information was key to choosing proper remediation strategies, the correct model for assessing public health risks, and aiding decisions for future land configuration and management.
Steven Conradson and co-workers from Los Alamos National Laboratory (LANL) used SSRL to measure x-ray absorption spectra (XAS) of samples of contaminated RFETS soils and concrete collected from the site. The XAS data, combined with ultrafiltration studies, showed that Pu was present dominantly as insoluble PuO2 that adheres to soil particles in the contaminated samples. This information provided a framework for decision makers to guide remediation efforts. Remediation efforts subsequently focused on removal of the layer of contaminated soil.
As a result of these studies, the clean-up was completed a year ahead of schedule in December 2005, likely saving the DOE and taxpayers billion of dollars. This study, conducted in 2002, played a crucial role because it was the first spectroscopic confirmation of the form of plutonium in soils at RFETS. XAFS showed unambiguously that plutonium in RFETS soils is insoluble. This recognition not only helped the Site develop the proper model to describe the transport mechanism of Pu and design a clean-up strategy, but significantly helped in gaining public trust.
David L. Clark