Elsa A. Rodriguez

Dissolution Kinetics of Pyrochlore Ceramics for the Disposition of Plutonium.

Abstract Single-pass β ow-through (SPFT) experiments were conducted on a set of non-radioactive Ti-based ceramics at 90 °C and pH = 2 to 12. The specimens contained 27.9 to 35.8 wt%CeO2 as a surrogate for UO2 and PuO2. Compositions were formulated as TiO2-saturated pyrochlore (CeP1) and pyrochlorerich baseline (CePB1) ceramic waste forms. Pyrochlore + Hf-rutile and pyrochlore + perovskite + Hf-rutile constituted the major phases in the CeP1 and CePB1 ceramics, respectively. Results from dissolution experiments between pH = 2 to 12 indicate a shallow pH-dependence with an ill-defined minimum. Element release rates determined from experiments over a range of sample surface areas (S) and β ow rates (q) indicate that dissolution rates become independent of q/S values at 10.8 to 10.7 m/s. Dissolution rates dropped sharply at lower values of q/S, indicating rates that are subject to solution saturation effects as dissolved constituents become concentrated. Forward dissolution rates were 1.3(0.30) x 10-3 and 5.5(1.3) x 10-3 g/m2·d for CeP1 and CePB1 ceramics, respectively. Dissolution rates obtained in other laboratories compare well to the findings of this study, once the rate data are placed in the context of solution saturation state. These results make progress toward an evaluation of CeO2 as a surrogate for UO2 and PuO2 as well as establishing a baseline for comparison with radiation- damaged specimens.

Aluminoborosilicate waste glass dissolution under alkaline conditions at 40 ◦ C: implications for a chemical affinity-based rate equation

Environmental context. The production of nuclear materials has generated a very large amount of highly radioactive wastes that need to be disposed of in a manner that will keep them from posing a danger for millions of years until the radioactivity decays. The process being considered for this daunting task is to contain the wastes in glass. Although studies with ancient and natural glass suggest the weathering of glass is slow, experiments are being conducted to determine the impact of this material on the natural environment and attempt to predict its long-term behaviour. The present paper briefly discusses three models that are being considered for implementing this process and the one that appears to hold the most promise. Abstract. Single-pass flow-through experiments were conducted with aluminoborosilicate waste glasses to evaluate how changes in solution composition affect the dissolution rate (r) at 40°C and pH (23°C) = 9.0. The three prototypic low-activity waste (LAW) glasses, LAWE-1A, -95A and -290A, used in these experiments span a wide range covering the expected processing composition of candidate immobilised low-activity waste (ILAW) glasses. Results suggest incongruent release of Al, B, Na, and Si at low flow-rate (q) to sample surface area (S), in units of (m s–1), (log10(q/S) –7.9). Dissolution rates increase from log10(q/S) ≈ –9.3 to –8.0 and then become constant at log10(q/S) > –7.9. Forward (maximum) dissolution rates, based on B release, are the same irrespective of glass composition, evident by the dissolution rates being within the experimental error of one another (r1A = 0.0301 ± 0.0153 g m–2 day–1, r95A = 0.0248 ± 0.0125 g m–2 day–1, and r290A = 0.0389 ± 0.0197 g m–2 day–1). The results also illustrate that as the activity of SiO2(aq) increases, the rate of glass dissolution decreases to a residual rate. The pseudo-equilibrium constant, Kg, (log10(Kg) = –3.7) predicted with these results is slightly lower than the K for chalcedony (log10(K) = –3.48) at 40°C. Finally, these results support the use of a chemical affinity-based rate law to describe glass dissolution as a function of solution composition.

Laboratory Testing of Bulk Vitrified and Steam Reformed Low-Activity Waste Forms to Support A Preliminary Risk Assessment for an Integrated Disposal Facility

Laboratory testing was conducted on bulk vitrified and steam reformed waste forms to supply the input parameters needed for reactive chemical transport calculations with the Subsurface Transport Over Reactive Multiphases (STORM) code. This same code was used to conduct the 2001 ILAW performance assessment. The required input parameters for both waste forms are derived from a mechanistic model that describes the effect of solution chemistry on contaminant release rates. The single-pass flow-through test was the principal method used to obtain these input parameters, supplemented by product consistency test measurements and physical property measurements.

Experimental determination of the dissolution kinetics of zero-valent iron in the presence of organic complexants

Environmental context. Iron metal is being considered as a material to be used for the treatment of groundwater contaminated with toxic metals and organics. Although time-dependant information is available, predicting the long-term behaviour of this material has been complicated by the build-up of rust or other alteration phases on the surface of Fe metal. In addition to the build-up of rust, changes to important environmental factors also complicate these types of predictions. The research discussed in this paper uses a non-traditional experimental technique to isolate the impact of specific environmental factors (i.e. pH, temperature) and organic complexants on the dissolution of Fe metal. Abstract. The geochemical cycling of iron, the reactivity of iron minerals and, more recently, the reactivity of zero valent iron (α-Fe), have been the subject of numerous investigations for over more than three decades. These investigations provide a wealth of knowledge regarding the effect of pH, temperature, chelating agents etc. on the reactivity and mechanism(s) of dissolution for α-Fe and iron oxide/oxyhydroxide minerals. However, most investigations have been conducted under static conditions that promote the formation of a partially oxidised surface film (e.g. passivating layer). In the presence of a passivating layer, the proposed dissolution mechanisms are vastly different and are based on the composition of the partially oxidised surface film. The objective of this study was to quantify the dissolution of α-Fe under conditions that maintain the pO2 at a relatively constant level and minimise the formation of a passivating layer on the metal surface. Single-pass flow-through tests were conducted under conditions of relatively constant dissolved O2 [O2(aq)] over the pH(23°C) range from 7 to 12 and temperature range from 23 to 90°C in the presence of ethylenediamine tetraacetic acid (EDTA) and ethylenediamine di-O-hydroxyphenylacetic acid (EDDHA) to maintain dilute conditions and minimise the formation of a partially oxidised surface film and Fe-bearing secondary phase(s) during testing. Although more information is needed, these results suggest the adsorption of EDTA and EDDHA, or the diffusion of the oxidised Fe–organic complex from the surface of α-Fe, is the rate-limiting step in the dissolution reaction. Results also suggest that the rate of dissolution is independent of pH, has a non-linear dependence on the concentration of organic complexant, and the forward dissolution rate for α-Fe is as much as three orders of magnitude greater than when a passive film and corrosion products are present.

Interim Report: Uranium Stabilization Through Polyphosphate Injection - 300 Area Uranium Plume Treatability Demonstration Project

This report presents results from bench-scale treatability studies conducted under site-specific conditions to optimize the polyphosphate amendment for implementation of a field-scale technology demonstration to treat aqueous uranium within the 300 Area aquifer of the Hanford site. The general treatability testing approach consists of conducting studies with site sediment and under site conditions, in order to develop an effective chemical formulation for the polyphosphate amendments and evaluate the transport properties of these amendments under site conditions. Phosphorus-31 (31P) NMR was utilized to determine the effects of Hanford groundwater and sediment on the degradation of inorganic phosphates. Static batch tests were conducted to optimize the composition of the polyphosphate formulation for the precipitation of apatite and autunite, as well as to quantify the kinetics, loading and stability of apatite as a long-term sorbent for uranium. Dynamic column tests were used to further optimize the polyphosphate formulation for emplacement within the subsurface and the formation of autunite and apatite. In addition, dynamic testing quantified the stability of autunite and apatite under relevant site conditions. Results of this investigation provide valuable information for designing a full-scale remediation of uranium in the 300 aquifer.

Laboratory Testing of Bulk Vitrified Low-Activity Waste Forms to Support the 2005 Integrated Disposal Facility Performance Assessment

The purpose of this report is to document the results from laboratory testing of the bulk vitri-fied (BV) waste form that was conducted in support of the 2005 integrated disposal facility (IDF) performance assessment (PA). Laboratory testing provides a majority of the key input data re-quired to assess the long-term performance of the BV waste package with the STORM code. Test data from three principal methods, as described by McGrail et al. (2000a; 2003a), are dis-cussed in this testing report including the single-pass flow-through test (SPFT) and product con-sistency test (PCT). Each of these test methods focuses on different aspects of the glass corrosion process. See McGrail et al. (2000a; 2003a) for additional details regarding these test methods and their use in evaluating long-term glass performance. In addition to evaluating the long-term glass performance, this report discusses the results and methods used to provided a recommended best estimate of the soluble fraction of 99Tc that can be leached from the engineer-ing-scale BV waste package. These laboratory tests are part of a continuum of testing that is aimed at improving the performance of the BV waste package.

Dissolution Kinetics of Titanate-Based Ceramic Waste Forms: Results from Single-Pass Flow Tests on Radiation Damaged Specimens

This report is a summary report in which the data from the single-pass flow-through test on resintered 238Pu-bearing ceramics are reported. These results show that radiation damage has litte effect on the dissolution kinetics of candidate titanate ceramics for plutonium immobilization.

Dissolution Kinetics of Titanium Pyrochlore Ceramics at 90°C by Single-Pass Flow-Through Experiments

Corrosion resistances of titanium-based ceramics are quantified using single-pass flow-through (SPFT) experiments. The materials tested include simple pyrochlore group (B2Ti2O7, where B=Lu^3+ or Gd^3+) and complex multiphase materials that are either pyrochlore- (PY12) or zirconolite-dominated (BSL3). Experiments are conducted at 90?C over a range of pH-buffered conditions with typical duration of experiments in excess of 120 days. Apparent steady-state dissolution rates at pH=2 determined on the Gd2Ti2O7 and Lu2Ti2O7 samples indicate congruent dissolution, with rates of the former (1.3x10^-3 to 4.3x10^-3) slightly faster than the latter (4.4x10^-4 to 7.0x10^-4 g m^-2 d^-1). Rates for PY12 materials into pH=2 solutions are 5.9x10^-5 to 8.6x10^-5 g m^-2 d^-1. In contrast, experiments with BSL3 material do not reach steady-state conditions, and appear to undergo rapid physical and chemical corrosion into solution. At faster flow-through rates, dissolution rates display a shallow amphoteric behavior, with a minimum (4.6x10^-5 to 5.8x10^-5 g m^-2 d^-1) near pH values of 7. Dissolution rates display a measurable increase (~10X) with increasing flow-through rate indicating the strong influence that chemical affinity asserts on the system. These results step towards an evaluation of the corrosion mechanism and an evaluation of the long-term performance of Pu-bearing titanite engineered materials in the subsurface.

Origins of Discrepancies Between Kinetic Rate Law Theory and Experiments in the Na2O-B2O3-SiO2 System

Discrepancies between classical kinetic rate law theory and experiment were quantitatively assessed and found to correlate with macromolecular amorphous separation in the sodium borosilicate glass system. A quantitative reinterpretation of static corrosion data and new SPFT data shows that a recently advanced protective surface layer theory fails to describe the observed dis-solution behavior of simple and complex silicate glasses under carefully controlled experimental conditions. The hypothesis is shown to be self-inconsistent in contrast with a phase separation model that is in quantitative agreement with experiments.

Experimental Determination of Dissolution Kinetics of Zr-Substituted Gd-Ti Pyrochlore Ceramics: Influence of Chemistry on Corrosion Resistance

The corrosion resistance of a series of zirconium-substituted gadolinium pyrochlore, Gd 2 (Ti 1-x Zr x ) 2 O 7 , where x = 0.0, 0.25, 0.50, 0.75, and 1.00, were evaluated using single-pass flow-through (SPFT) apparatus at 90°C and pH = 2. The zirconate end-member, Gd 2 Zr 2 O 7 , has a defect fluorite structure, which distinguishes it from the face-centered cubic structure of the true pyrochlore specimens. In addition to the chemical variation, the samples include annealed, un-annealed, and ion-bombarded monoliths. In the case of the titanate end-member, Gd 2 Ti 2 O 7 , the annealed specimen exhibited the least reactivity, followed by the un-annealed and ion-bombarded samples (2.39×10 -3 , 1.57×10 -2 , and 1.12×10 -1 g m -2 d -1 , respectively). With increasing zirconium content, the samples displayed less sensitivity to processing or surface modification with the zirconate end-member exhibiting no difference in reactivity between annealed, un-annealed, and ion-bombarded specimens (rate = 4.0×10 -3 g m -2 d -1 ). In all cases, the dissolution rate decreased with increasing zirconium content to the Gd 2 (Ti 0.25 Zr 0.75 ) 2 O 7 composition (1.33x10 -4 g m -2 d -1 ), but the zirconate end-member yielded rates nearly equal to that of the titanate end-member. These results demonstrate that to achieve the greatest radiation and corrosion resistance in this series, the Gd 2 (Ti 0.25 Zr 0.75 ) 2 O 7 composition should be considered.