Residual Waste from Hanford Tanks 241-C-203 and 241-C-204. 2. Contaminant Release Model

Abstract

Release of U and 99Tc from residual sludge in Hanford waste tanks 241-C-203 and 241-C-204 atthe U.S. Department of Energy's (DOE) Hanford Site in southeastern Washington state was quantified by water-leaching, selective extractions, empirical solubility measurements, and thermodynamic modeling. A contaminant release model was developed based on these experimental results and solid-phase characterization results presented elsewhere. Uranium release was determined to be controlled by two phases and occurred in three stages. In the first stage, U release is controlled by the solubility of tejkaite, which is suppressed by high concentrations of sodium released from the dissolution of NaNO3 in the residual sludges. Equilibrium solubility calculations indicate the U released during this stage will have a maximum concentration of 0.021 M. When all the NaNO3 has dissolved from the sludge, the solubility of the remaining cejkaite will increase to 0.28 M. After cejkaite has completely dissolved, the majority of the remaining U is in the form of poorly crystalline Na2U2O7 [or clarkeite Na[(UO2)O(OH)](H20)0-1]. In contact with Hanford groundwater this phase is not stable, and becquerelite becomes the U solubility controlling phase, with a calculated equilibrium concentration of 1.2 x 10(-4) M. For Tc, a significant fraction of its concentration in the residual sludge was determined to be relatively insoluble (20 wt % for C-203 and 80 wt % for C-204). Because of the low concentrations of Tc in these sludge materials, the characterization studies did not identify any discrete Tc solids phases. Release of the soluble fraction of Tc was found to occur concomitantly with NO3-. It was postulated that a NaNO3-NaTcO4 solid solution could be responsible for this behavior. The Tc release concentrations for the soluble fraction were estimated to be 2.4 x 10-6 M for C-203 and 2.7 x 10(-5) M for C-204. Selective extraction results indicated that the recalcitrant fraction of Tc was associated with Fe oxides. Release of the recalcitrant fraction of Tc was assumed to be controlled by dissolution of Fe oxide in the form of ferrihydrite. Based on this assumption and measured values for the ratio of recalcitrant Tc to total Fe in each bulk sludge, the release concentration of the recalcitrant fraction of Tc was calculated to be 3.9 x 10(-12) M for C-203 and 10.0 x 10(-12) M for C-204.

Becquerelite mineral phase: crystal structure and thermodynamic and mechanical stability by using periodic DFT

“…21,22 Becquerelite phase has been observed as alteration product of spent fuel in cooling basins at the Hanford, Washington site. [23][24][25][26] A contaminant release model 23 was developed to evaluate the release of uranium and other contaminants in the residual sludge of Hanford waste tanks based on the experimental results of waterleaching, selective extractions, empirical solubility measurements, thermodynamic modeling and solid phase characterization. These studies highlighted the importance of the availability of accurate thermodynamic data for the secondary phases of spent nuclear fuel as becquerelite.…”

The Application of Periodic Density Functional Theory to the Study of Uranyl-Containing Materials: Thermodynamic Properties and Stability

“…The uranyl oxyhydroxides also begin to appear soon from the alteration of uranium dioxide [42][43][44][45][46][47][48]. Studtite, schoepite, metaschoepite, and becquerelite phases have been observed as alteration products of spent fuel in cooling basins at the Hanford Site (Washington) [112][113][114][115][116] and on Chernobyl "lava" formed during the nuclear accident that occurred in 1986 [117]. The next mineral phases appearing in this sequence are uranyl silicates and, less frequently, uranyl phosphates [42][43][44][45][46][47][48].…”

Raman spectroscopic study of the uranyl carbonate mineral čejkaite and its comparison with synthetic trigonal Na4[UO2(CO3)3]

“…Catalano and Brown [19] analysed syntheticčejkaite by EX-AFS spectroscopy. Krupka et al [20] and Cantrell et al [21] studied the behaviour ofčejkaite in residual waste from Hanford tanks. This paper is a part of vibrational spectroscopic investigation on supergene minerals originating from oxidation zone inclusive of uranyl minerals.…”