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Plutonium Segregation

Concern has been expressed by the Defense Nuclear Facility Safety Board (DNFSB) that plutonium stored in underground tanks at Hanford Site tank farms is present as particles that could segregate from other waste components, including neutron absorbers—such as iron—and thus pose a nuclear criticality safety hazard in the waste storage tanks and in process vessels at the Hanford Waste Treatment and Immobilization Plant (WTP). PNNL staff collaborated with Hanford Site and national experts in plutonium chemistry and Hanford process operations to evaluate this potential hazard. Considerations included determining the quantities, chemical forms, particle sizes, and tank-by-tank dispositions of plutonium disposed to the tank farms over the approximately 60 years of Hanford Site operations history. The chemical and physical behaviors of plutonium after it reached the tank farms were also addressed by the panel.

Plutonium dioxide

Plutonium dioxide from plutonium oxalate calcined at the Hanford Site Plutonium Finishing Plant

PNNL staff provided both Hanford Site plant operations and plutonium chemistry expertise. Contributions by PNNL staff to this 6-month study included examination of critical mass laboratory and hot semi-works historical operations, and evaluation of the accuracy of analytical methods used to determine plutonium concentrations in waste over Hanford Site operations history. PNNL staff also considered questions on plutonium disposal form and the resulting plutonium chemistry in tank waste. The disposal forms included alkalitreated acidic waste, solid plutonium fluoride and oxalate compounds, organic-rich interfacial cruds from solvent extraction operations, and particulate plutonium from burnt plutonium metal.

The overall conclusion reached was that the micron-scale (ìm) plutonium-rich particulates present in the tank waste of concern to criticality safety were introduced to the waste as solids, primarily in the form of plutonium dioxide and a much smaller quantity of unburned plutonium metal, and constitute only about 5% of the total plutonium inadvertently discarded to the tanks. The ~95% remainder was discharged primarily in the form of alkali-treated acid solutions and small quantities of solid interfacial cruds and plutonium fluorides and oxalates. The fluoride, oxalate, and crud solids would metathesize by interaction with the alkali to form miniscule and readily suspended particles of plutonium hydrous oxide, PuO2.xH2O. The plutonium lost via alkali-treated acidic process solutions likewise would have formed miniscule sub-micron particles of PuO2.xH2O.

The DNFSB also raised questions whether the sub-micron PuO2.xH2O could subsequently undergo sufficient crystal growth through Ostwald ripening in the alkaline tank waste to become potentially separable from neutron absorbing constituents by settling or sedimentation. However, PNNL staff determined by both thermodynamic and experimental evidence that even if pure plutonium nitrate acid solution had entered the alkaline tank waste and precipitated through neutralization, it would be initially present as 0.002- to 0.003-ìm scale PuO2.xH2O crystallite particles and would grow from that point at exceedingly slow rates, posing no risk to physical segregation. Under actual tank waste conditions, in which the plutonium is intimately mixed with vastly larger quantities of other metal ion compounds including uranium, chromium and principally iron, the opportunity for PuO2.xH2O crystal growth is even lower.

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