Jean-Francois Lucchini

Reduction of plutonium(VI) in brine under subsurface conditions

The redox stability of PuO22+ was investigated in brine under subsurface conditions. In simulated brines, when no reducing agent was present, 0.1 mM concentrations of plutonium(VI) were stable as regards to reduction for over two years, which was the duration of the experiments performed. In these systems, the plutonyl existed as a carbonate or hydroxy-chloride species. The introduction of reducing agents (e.g. steel coupons, and aqueous Fe2+) typically present in a subsurface repository, however, led to the destabilization of the plutonium(VI) complexes and the subsequent reduction to Pu(IV) under most conditions investigated. X-ray Absorption Near-Edge Spectroscopy (XANES) confirmed that the final oxidation state in these systems was Pu(IV). This reduction lowered the overall steady state concentration of plutonium in the brine by 3−4 orders of magnitude. These results show the importance of considering repository constituents in evaluating subsurface actinide solubility/mobility and provide further evidence of the effectiveness of reduced iron species in the reduction and immobilization of higher-valent plutonium species.

Actinide (III) solubility in WIPP Brine: data summary and recommendations

The solubility of actinides in the +3 oxidation state is an important input into the Waste Isolation Pilot Plant (WIPP) performance assessment (PA) models that calculate potential actinide release from the WIPP repository. In this context, the solubility of neodymium(III) was determined as a function of pH, carbonate concentration, and WIPP brine composition. Additionally, we conducted a literature review on the solubility of +3 actinides under WIPP-related conditions. Neodymium(III) was used as a redox-invariant analog for the +3 oxidation state of americium and plutonium, which is the oxidation state that accounts for over 90% of the potential release from the WIPP through the dissolved brine release (DBR) mechanism, based on current WIPP performance assessment assumptions. These solubility data extend past studies to brine compositions that are more WIPP-relevant and cover a broader range of experimental conditions than past studies.

Uranium(VI) solubility in carbonate-free WIPP brine

Abstract The solubility of uranium(VI) was determined in WIPP-relevant brines as a function of pCH+ and ionic strength, in the absence of carbonate. Carbonate concentration was below 2 × 105 M, measured using the gas chromatography method. In the absence of carbonate, the uranium(VI) solubilities were about × 106 M in GWB at pCH+≥7 and about × 108– × 107 M in ERDA-6 brine at pCH+≥8. Solubility of uranium(VI) was also measured in NaCl media at the same levels as in ERDA-6 brine. The data established a uranium solubility that was 10–100 times lower than published results from Diaz-Arocas and Grambow [13], and they are in good agreement with modeling results and other literature data [11,12]. In the absence of carbonate, hydrolysis was the main complexation and precipitation mechanisms for uranium(VI) solubility at high ionic strength and pCH+≥7. However, the effect of borate complexation was noticeable at pCH+∼8–9.

Uranium (VI) solubility in carbonate-free ERDA-6 brine

When present, uranium is usually an element of importance in a nuclear waste repository. In the Waste Isolation Pilot Plant (WIPP), uranium is the most prevalent actinide component by mass, with about 647 metric tons to be placed in the repository. Therefore, the chemistry of uranium, and especially its solubility in the WIPP conditions, needs to be well determined. Long-term experiments were performed to measure the solubility of uranium (VI) in carbonate-free ERDA-6 brine, a simulated WIPP brine, at pC{sub H+} values between 8 and 12.5. These data, obtained from the over-saturation approach, were the first repository-relevant data for the VI actinide oxidation state. The solubility trends observed pointed towards low uranium solubility in WIPP brines and a lack of amphotericity. At the expected pC{sub H+} in the WIPP ({approx} 9.5), measured uranium solubility approached 10{sup -7} M. The objective of these experiments was to establish a baseline solubility to further investigate the effects of carbonate complexation on uranium solubility in WIPP brines.

Iron Corrosion Observations: Pu(VI)-Fe Reduction Studies

Iron and Pu Reduction: (1) Very different appearances in iron reaction products were noted depending on pH, brine and initial iron phase; (2) Plutonium was associated with the Fe phases; (3) Green rust was often noted at the higher pH; (4) XANES established the green rust to be an Fe2/3 phase with a bromide center; and (5) This green rust phase was linked to Pu as Pu(IV).