Plutonium weathering on Johnston Atoll

Johnston Atoll was contaminated with transuranic elements, particularly plutonium, by atmospheric nuclear weapons tests and aborted nuclear devices. Initial cleanup operations and and an extensive soil remediation program were performed. However, many areas contained a low-level continuum of activity, and subsurface contamination has been detected. Discrete hot particles and contaminated soil were characterized to determine whether the spread of activity was caused by weathering. Analytical techniques included gamma spectrometry, alpha spectrometry, and inductively coupled plasma-mass spectrometry to determine transuranic elemental and isotopic composition. Ultrafiltration and small-particle handling techniques were employed to isolate individual particles. Optical microscopy, scanning electron microscopy, analytical transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy were used to characterize individual particles. Analyses of the hot particles showed that they are aborted nuclear warhead fragments that been melted and weathered in the presence of water and CaCO3. It was concluded that the formation of aqueous ionic (Pu/Am)-CO3 coordinated complexes, during environmental exposure to large volumes of rainwater and carbonate-satured seawater, enhanced the solubility of transuranic elements. The (Pu/Am)-CO3 complexes sorbed onto colloidal CaCO3 and coral soil surfaces as they were exposed to rain and seawater. This mechanism led to greater dispersal of plutonium and americium than would be expected by physical transport of discrete hot particles alone