Single-pass Flow-through Test Research Articles

Determination of the maximum dissolution rates of the Belgian reference glasses at very alkaline pH and 30 °C

To assess the chemical durability of the Belgian reference glasses SON68, SM513, and SM539 under hyper-alkaline conditions, maximum glass dissolution rates were measured at 30 °C and pH 13.5 by performing Single-Pass Flow-Through tests with a KOH solution and a synthetic young cementitious water containing Ca (YCWCa). In both leachants, the highest dissolution rate was determined for the Al-rich glass SM539, while similar rates were found for SM513 and SON68 glasses, whose compositions are comparable. For all glasses, the dissolution rates in YCWCa were lower than in KOH due to the presence of Ca, which leads to the formation of a slightly protective calcium silicate layer. The dissolution rates in YCWCa were similar to those determined from glass monolith mass losses in static tests carried out at 30 °C in YCWCa in the presence of hardened Ordinary Portland Cement paste powder with a cement-to-glass ratio of 1.Graphical abstract

Epsilon metal: A waste form for noble metals from used nuclear fuel

Epsilon metal (ε-metal) is the metallic phase that forms as inclusions at the grain boundaries in the UO2 fuel during reactor operation. This metal is composed of Pd, Mo, Rh, Ru, and Tc. These metallic inclusions are insoluble in strong acid and remnants of these metallic inclusions have been found in the UO2 matrix that remains from the natural reactors in Gabon that were active 1.8 billion years ago, therefore ε-metal should be an excellent waste form for the immobilization of the long-lived isotopes 107Pd (6.5 × 106 a) and 99Tc (2.13 × 105 a), with 99Tc being the isotope of interest for repository performance. Therefore, the chemical durability of this potential waste form is assessed in this study. Typically, corrosion rates for metallic materials are measured electrochemically because they are quick, inexpensive, and can reveal the mechanism by which a metal corrodes, at least initially. However, in a repository the waste form would be subjected to slowly flowing water without an applied electrical potential over long time periods. Therefore, the corrosion rates of ε-metal specimens were measured with both electrochemical tests and the single-pass flow-through test (SPFT). Potentiodynamic and potentiostatic polarization results suggest that a thin passive film exists on the alloy surface, which seems to be responsible for its high corrosion resistance. Additionally, X-Ray photoelectron spectroscopic results suggest that Pd oxides are significantly enriched in the passive film Results from the SPFT show that the dissolution rates were weakly dependent on pH. Only Mo and Re were found in solution and were used for the calculation of the dissolution rates. In general, the electrochemically determined corrosion rates agree reasonably well with the initial dissolution rate measured with the SPFT test, but they are about one or two orders of magnitude higher than the steady state rates. The causes for this discrepancy are discussed.

Investigating the Durability of Iodine Waste Forms in Dilute Conditions.

To prevent the release of radioiodine during the reprocessing of used nuclear fuel or in the management of other wastes, many technologies have been developed for iodine capture. The capture is only part of the challenge as a durable waste form is required to ensure safe disposal of the radioiodine. This work presents the first durability studies in dilute conditions of two AgI-containing waste forms: hot-isostatically pressed silver mordenite (AgZ) and spark plasma sintered silver-functionalized silica aerogel (SFA) iodine waste forms (IWF). Using the single-pass flow-through (SPFT) test method, the dissolution rates respective to Si, Al, Ag and I were measured for variants of the IWFs. By combining solution and solid analysis information on the corrosion mechanism neutral-to-alkaline conditions was elucidated. The AgZ samples were observed to have corrosion preferentially occur at secondary phases with higher Al and alkali content. These phases contained a lower proportion of I compared with the matrix. The SFA samples experienced a higher extent of corrosion at Si-rich particles, but an increased addition of Si to the waste led to an improvement in corrosion resistance. The dissolution rates for the IWF types are of similar magnitude to other Si-based waste form materials measured using SPFT.

Kinetics of oxyapatite [Ca2Nd8(SiO4)6O2] and powellite [(Ca,Sr,Ba)MoO4] dissolution in glass-ceramic nuclear waste forms in acidic, neutral, and alkaline conditions

Immobilization of chemically complex aqueous waste streams from used nuclear fuel reprocessing is achievable at higher waste loadings with glass ceramics as compared to borosilicate glasses. Additionally, crystalline phases with similar chemistry are more durable than their amorphous counterparts. However, during glass ceramic fabrication, mechanical stresses at crystal-glass interfaces, which are caused by thermal expansion mismatching during cooling, create locales where water is capable of accessing and reacting with the various phases in the glass ceramic, thus releasing radionuclides into the aqueous phase. In the present work, we build on previous chemical durability investigations of a glass-ceramic containing crystalline oxyapatite [Ca2Nd8(SiO4)6O2] and powellite [(Ca,Sr,Ba)MoO4] secondary phases. The individual crystalline and bulk glass phases have been fabricated separately and the corrosion behavior has been investigated with single-pass flow-through (SPFT) testing at 90 °C in buffered pH(RT) 4, 7, 9 and pH 11 solutions and with the static product consistency test (PCT). The results demonstrate the varying dissolution kinetics of the individual phases in the range of pH studies. The consequence of the varying dissolution kinetics are described with a conceptual model of glass-ceramic dissolution mechanisms.

Quantification of kinetic rate law parameters for the dissolution of natural autunite in the presence of aqueous bicarbonate ions at high concentrations

Uranium is a key contaminant of concern in the groundwater at U.S. Department of Energy (DOE) facilities within the United States and is a potential source of groundwater contamination and a risk to human health and the environment through discharges to surface water. Dissolved inorganic carbon (bicarbonate/carbonate) has a high affinity for complexing with uranium that is present as sorbed or unique uranium-bearing mineral phases within the sedimentary matrix. This process can result in the formation of soluble uranyl carbonate aqueous species, which are mobile under circumneutral pH conditions. This study was conducted to quantify the rate of release of uranium from the autunite mineral, (Ca[(UO2)(PO4)]2•3H2O), that was formed during polyphosphate injection to remediate uranium; the dissolution of uranium was studied as a function of the aqueous bicarbonate concentration, ranging from 25 to 100 mM. Experiments were carried out in the pH range from 7 to 11 in the temperature range of 23–90 °C via single-pass flow-through testing. Consistent with the results of previous studies (Gudavalli et al., 2013a, 2013b), the rate of uranium release from autunite exhibited minimal dependency on temperature, but was strongly dependent on pH and increasing concentrations of bicarbonate in the solution. Data obtained during these experiments were compared with results of previous experiments conducted using a low-concentration range of bicarbonate solutions (0.5–3.0 mM). An 8- to 30-fold increase in the rate of uranium release was observed in the presence of high bicarbonate concentrations at pH 7–8 compared to low bicarbonate values, while at pH 9–11, there was only a 5-fold increase in uranium rate of release with an increase in bicarbonate concentrations. The rate of uranium release was calculated to be between 5.18 × 10−8 and 1.69 × 10−7 mol m−2 s−1. The activation energy values at high and low bicarbonate concentrations were similar, with ratio values in the range of 0.6–1.0.

The dissolution behavior of borosilicate glasses in far-from equilibrium conditions

An area of agreement in the waste glass corrosion community is that, at far-from-equilibrium conditions, the dissolution of borosilicate glasses used to immobilize nuclear waste is known to be a function of both temperature and pH. The aim of this work is to study the effects of temperature and pH on the dissolution rate of three model nuclear waste glasses (SON68, ISG, AFCI). The dissolution rate data are then used to parameterize a kinetic rate model based on Transition State Theory that has been developed to model glass corrosion behavior in dilute conditions. To do this, experiments were conducted at temperatures of 23, 40, 70, and 90 °C and pH (22 °C) values of 9, 10, 11, and 12 with the single-pass flow-through (SPFT) test method. Both the absolute dissolution rates and the rate model parameters are compared with previous results. Rate model parameters for the three glasses studied here are nearly equivalent within error and in relative agreement with previous studies though quantifiable differences exist. The glass dissolution rates were analyzed with a linear multivariate regression (LMR) and a nonlinear multivariate regression performed with the use of the Glass Corrosion Modeling Tool (GCMT), with which a robust uncertainty analysis is performed. This robust analysis highlights the high degree of correlation of various parameters in the kinetic rate model. As more data are obtained on borosilicate glasses with varying compositions, a mathematical description of the effect of glass composition on the rate parameter values should be possible. This would allow for the possibility of calculating the forward dissolution rate of glass based solely on composition. In addition, the method of determination of parameter uncertainty and correlation provides a framework for other rate models that describe the dissolution rates of other amorphous and crystalline materials in a wide range of chemical conditions. The higher level of uncertainty analysis would provide a basis for comparison of different rate models and allow for a better means of quantifiably comparing the various models.

Corrosion Behavior and Microstructure Influence of Glass-Ceramic Nuclear Waste Forms

Glass-ceramic waste forms present a potentially viable technology for the long-term immobilization of liquid nuclear wastes arising from used nuclear fuel reprocessing. Through control of chemistry during fabrication, such waste forms can have designed secondary crystalline phases within a borosilicate glass matrix. In this work, a glass-ceramic containing crystalline powellite and oxyapatite secondary phases was tested for its corrosion properties in dilute conditions using single-pass flow-through testing. Three glass-ceramic samples were prepared using different cooling rates to produce samples with varying microstructure sizes. In testing at 90°C in buffered pH(RT) 7 and pH 9 solutions, it was found that increasing solution pH and decreasing microstructure size (resulting from rapid cooling during fabrication) both led to a reduction in overall corrosion rate, indexed by boron release from the glass matrix. On the other hand, the corrosion rate of crystalline phase decreased with a decrease in pH. The...

Dilute condition corrosion behavior of glass-ceramic waste form

Borosilicate glass-ceramics are being developed to immobilize high-level waste generated by aqueous reprocessing into a stable waste form. The corrosion behavior of this multiphase waste form is expected to be complicated by multiple phases and crystal-glass interfaces. A modified single-pass flow-through test was performed on polished monolithic coupons at a neutral pH (25 °C) and 90 °C for 33 d. The measured glass corrosion rates by micro analysis in the samples ranged from 0.019 to 0.29 g m−2 d−1 at a flow rate per surface area = 1.73 × 10−6 m s−1. The crystal phases (oxyapatite and Ca-rich powellite) corroded below quantifiable rates, by micro analysis. While, Ba-rich powellite corroded considerably in O10 sample. The corrosion rates of C1 and its replicate C20 were elevated an order of magnitude by mechanical stresses at crystal-glass interface caused by thermal expansion mismatch during cooling and unique morphology (oxyapatite clustering).

Evidence of technetium and iodine release from a sodalite-bearing ceramic waste form

Sodalites have been proposed as a possible host of certain radioactive species, specifically 99Tc and 129I, which may be encapsulated into the cage structure of the mineral. To demonstrate the ability of this framework silicate mineral to encapsulate and immobilize 99Tc and 129I, single-pass flow-through (SPFT) tests were conducted on a sodalite-bearing multi-phase ceramic waste form produced through a steam reforming process. Two samples made using a steam reformer samples were produced using non-radioactive I and Re (as a surrogate for Tc), while a third sample was produced using actual radioactive tank waste containing Tc and added Re. One of the non-radioactive samples was produced with an engineering-scale steam reformer while the other non-radioactive sample and the radioactive sample were produced using a bench-scale steam reformer. For all three steam reformer products, the similar steady-state dilute-solution release rates for Re, I, and Tc at pH (25 °C) = 9 and 40 °C were measured. However, it was found that the Re, I, and Tc releases were equal or up to 4.5x higher compared to the release rates of the network-forming elements, Na, Al, and Si. The similar releases of Re and Tc in the SPFT test, and the similar time-dependent shapes of the release curves for samples containing I, suggest that Re, Tc, and I partition to the sodalite minerals during the steam reforming process.

Mineral assemblage transformation of a metakaolin-based waste form after geopolymer encapsulation

Mitigation of hazardous and radioactive waste can be improved through conversion of existing waste to a more chemically stable and physically robust waste form. One option for waste conversion is the fluidized bed steam reforming (FBSR) process. The resulting FBSR granular material was encapsulated in a geopolymer matrix referred to here as Geo-7. This provides mechanical strength for ease in transport and disposal. However, it is necessary to understand the phase assemblage evolution as a result of geopolymer encapsulation. In this study, we examine the mineral assemblages formed during the synthesis of the multiphase ceramic waste form. The FBSR granular samples were created from waste simulant that was chemically adjusted to resemble Hanford tank waste. Another set of samples was created using Savannah River Site Tank 50 waste simulant in order to mimic a blend of waste collected from 68 Hanford tank. Waste form performance tests were conducted using the product consistency test (PCT), the Toxicity Characteristic Leaching Procedure (TCLP), and the single-pass flow-through (SPFT) test. X-ray diffraction analyses revealed the structure of a previously unreported NAS phase and indicate that monolith creation may lead to a reduction in crystallinity as compared to the primary FBSR granular product.

Aging Studies of Pu-238 and -239 Containing Calcium Phosphate Ceramic Waste-forms

ABSTRACTA calcium phosphate ceramic waste-form has been developed at AWE for the immobilisation of chloride containing wastes arising from the pyrochemical reprocessing of plutonium. In order to determine the long term durability of the waste-form, aging trials have been carried out at PNNL. Ceramics were prepared using Pu-239 and -238, these were characterised by PXRD at regular intervals and Single Pass Flow Through (SPFT) tests after approximately 5 yrs.While XRD indicated some loss of crystallinity in the Pu-238 samples after exposure to 2.8 x 1018 α decays, SPFT tests indicated that accelerated aging had not had a detrimental effect on the durability of Pu-238 samples compared to Pu-239 waste-forms.

Single-pass flow-through test elucidation of weathering behavior and evaluation of contaminant release models for Hanford tank residual radioactive waste

Contaminant release models are required to evaluate and predict long-term environmental impacts of even residual amounts of high-level radioactive waste after cleanup and closure of radioactively contaminated sites such as the DOE’s Hanford Site. More realistic and representative models have been developed for release of uranium, technetium, and chromium from Hanford Site tanks C-202, C-203, and C-103 residual wastes using data collected with a single-pass flow-through test (SPFT) method. These revised models indicate that contaminant release concentrations from these residual wastes will be considerably lower than previous estimates based on batch experiments. For uranium, a thermodynamic solubility model provides an effective description of uranium release, which can account for differences in pore fluid chemistry contacting the waste that could occur through time and as a result of different closure scenarios. Under certain circumstances in the SPFT experiments various calcium rich precipitates (calcium phosphates and calcite) form on the surfaces of the waste particles, inhibiting dissolution of the underlying uranium phases in the waste. This behavior was not observed in previous batch experiments. For both technetium and chromium, empirical release models were developed. In the case of technetium, release from all three wastes was modeled using an equilibrium Kd model. Formore » chromium release, a constant concentration model was applied for all three wastes.« less

Deep-UV Raman spectroscopic analysis of structure and dissolution rates of silica-rich sodium borosilicate glasses

As part of ongoing studies to evaluate relationships between structure and rates of dissolution of silicate glasses in aqueous media, sodium borosilicate glasses of composition Na 2O·xB 2O 3·(3 − x)SiO 2, with x ≤ 1 (Na 2O/B 2O 3 ratio ≥ 1), were analyzed using deep-UV Raman spectroscopy. Results were quantified in terms of the fraction of SiO 4 tetrahedra with one non-bridging oxygen (Q 3) and then correlated with Na 2O and B 2O 3 content. The Q 3 fraction was found to increase with increasing Na 2O content, in agreement with studies on related glasses, and, as long as the value of x was not too high, this contributed to higher rates of dissolution in single pass flow-through testing. In contrast, dissolution rates were less strongly determined by the Q 3 fraction when the value of x was near unity, and appeared to grow larger upon further reduction of the Q 3 fraction. Results were interpreted to indicate the increasingly important role of network hydrolysis in the glass dissolution mechanism as the BO 4 tetrahedron replaces the Q 3 unit as the charge-compensating structure for Na + ions. Finally, the use of deep-UV Raman spectroscopy was found to be advantageous in studying finely powdered glasses in cases where visible Raman spectroscopy suffered from weak Raman scattering and fluorescence interference.

Dissolution of Columbia River Basalt under mildly acidic conditions as a function of temperature: Experimental results relevant to the geological sequestration of carbon dioxide

Abstract Increasing attention is being focused on the rapid rise of CO2 levels in the atmosphere, which many believe to be the major contributing factor to global climate change. Sequestering CO2 in deep geological formations has been proposed as a long-term solution to help stabilize CO2 levels. However, before such technology can be developed and implemented, a basic understanding of H2O–CO2 systems and the chemical interactions of these fluids with the host formation must be obtained. Important issues concerning mineral stability, reaction rates, and carbonate formation are all controlled or at least significantly impacted by the kinetics of rock–water reactions in mildly acidic, CO2-saturated solutions. Basalt has recently been identified as a potentially important host formation for geological sequestration. Dissolution kinetics of the Columbia River Basalt (CRB) were measured for a range of temperatures (25–90 °C) under mildly acidic to neutral pH conditions using the single-pass flow-through test method. Under anaerobic conditions, the normalized dissolution rates for CRB decrease with increasing pH (3 ⩽ pH ⩽ 7) with a slope, η, of −0.15 ± 0.01. Activation energy, Ea, has been estimated at 32.0 ± 2.4 kJ mol−1. Dissolution kinetics measurements like these are essential for modeling the rate at which CO2-saturated fluids react with basalt and ultimately drive conversion rates to carbonate minerals in situ.

Dissolution kinetics of meta-torbernite under circum-neutral to alkaline conditions

Environmental context. Uranium-phosphate minerals have been identified as a long-term controlling phase that limit the mobility of uranium to groundwater in many contaminated subsurface environments. Complex, coupled processes confound the ability to isolate the rates attributed to individual processes. Results of this investigation provide the necessary information to refine current prediction on the release and long-term fate of uranium in subsurface environments. Abstract. The purpose of this investigation was to conduct a series of single-pass flow-through (SPFT) tests to (1) quantify the effect of temperature (23–90°C) and pH (6–10) on meta-torbernite dissolution; (2) compare the dissolution of meta-torbernite to other autunite-group minerals; and (3) evaluate the effect of aqueous phosphate on the dissolution kinetics of meta-torbernite. Results presented here illustrate meta-torbernite dissolution rates increase by ~100× over the pH interval of 6 to 10, irrespective of temperature. The power law coefficient for meta-torbernite, η = 0.59 ± 0.07, is greater than that quantified for Ca-meta-autunite, η = 0.42 ± 0.12. This suggests the stability of meta-torbernite is greater than that of meta-autunite, which is reflected in the predicted stability constants. The rate equation for the dissolution of meta-torbernite as a function of aqueous phosphate concentration is log rdissol (mol m–2 s–1) = –4.7 × 10–13 + 4.1 × 10–10[PO43–].

An experimental study of the dissolution rates of simulated aluminoborosilicate waste glasses as a function of pH and temperature under dilute conditions

Abstract Single-pass flow-through tests were conducted to determine the pH (7–12) and temperature (23–90 °C) dependence of kinetic rate law parameters; ko, η, and Ea, for the dissolution of glass in aqueous solution. Experiments were performed with three prototypic nuclear waste glasses that span a wide compositional range, which covers, with high probability, the expected processing composition range for candidate immobilized low-activity waste (ILAW) glasses. Comparison of the B to Na release rates for one glass was incongruent at 23 and 40 °C, and pH (23 °C) = 7.0 and 8.0, suggesting two distinct mechanisms are responsible for the Na+ release, namely Na+–H+ ion-exchange and matrix dissolution. Matrix dissolution became the dominant dissolution mechanism for all glasses at the forward rate and pH values greater than 9.0 as evident by the congruent release of Al, B, Na and Si to solution. By combining the results collected for each ILAW glass at pH values greater than 9.0, pH and temperature-dependent rate law parameters were determined for Al, B, Na and Si release. A comparison of the pH power-law coefficient for Al, B, Na and Si at each temperature suggest that η does not depend on temperature within experimental error and suggests the release of these elements into solution is controlled by the same dissolution mechanism at the forward rate of reaction. The activation energies (Ea), based on B release, range from 52 ± 4 to 56 ± 6 kJ/mol which suggest that dissolution is a surface-controlled reaction mechanism. The data presented in this manuscript suggest that for these three ILAW glasses the chemical durability for each glass is similar under these test conditions. A lack of compositional dependence on the forward dissolution rate is observed even though there is as much as a 39 kJ/mol difference in the free energy of hydration (ΔGhyd) among the borosilicate waste glasses tested. This similarity in the forward dissolution rate despite the large ΔGhyd difference is almost certainly because these glasses have similar, if not identical, polymerization states. This is evident from the almost identical 29Si chemical shifts for each of these glasses. The polymerization state is an indication of the number of framework SiO4 linkages contained in the glass network. In general, the greater the number of framework SiO4 linkages the more durable the glass. Finally, in agreement with previous work, these results suggest breakage of the Si–O bond is the rate-determining dissolution mechanism under alkaline conditions [pH (23 °C) > 9.0] far from saturation with respect to an alteration phase or phases.

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.

Fluidized Bed Steam Reformed (FBSR) Mineral Waste Forms: Characterization and Durability Testing

AbstractFluidized Bed Steam Reforming (FBSR) is being considered as a potential technology for the immobilization of a wide variety of high sodium low activity wastes (LAW) such as those existing at the Hanford site, at the Idaho National Laboratory (INL), and the Savannah River Site (SRS). The addition of clay, charcoal, and a catalyst as co-reactants with the waste denitrates the aqueous wastes and forms a granular mineral waste form that can subsequently be made into a monolith for disposal if necessary. The waste form produced is a multiphase mineral assemblage of Na-Al-Si (NAS) feldspathoid minerals with cage and ring structures and iron bearing spinel minerals. The mineralization occurs at moderate temperatures between 650-750°C in the presence of superheated steam. The cage and ring structured feldspathoid minerals atomically bond radionuclides like Tc-99 and Cs-137 and anions such as SO4, I, F, and Cl. The spinel minerals stabilize Resource Conservation and Recovery Act (RCRA) hazardous species such as Cr and Ni. Granular mineral waste forms were made from (1) a basic Hanford Envelope A low-activity waste (LAW) simulant and (2) an acidic INL simulant commonly referred to as sodium-bearing waste (SBW) in pilot scale facilities at the Science Applications International Corporation (SAIC) Science and Technology Applications Research (STAR) facility in Idaho Falls, ID. The FBSR waste forms were characterized and the durability tested via ASTM C1285 (Product Consistency Test), the Environmental Protection Agency (EPA) Toxic Characteristic Leaching Procedure (TCLP), and the Single Pass Flow Through (SPFT) test. The results of the SPFT testing and the activation energies for dissolution are discussed in this study.

Elemental dissolution study of Pu-bearing borosilicate glasses

Single-pass flow-through tests were conducted to study the effects of self-radiation damage from alpha decay on dissolution kinetics of three radiation-aged Pu-bearing (1 mass% PuO2) borosilicate glasses over a pH interval of 9–12 at 80–88°C. The chemical compositions of the glasses were identical except the 239Pu/238Pu isotopic ratio, which was varied to yield accumulated doses of 1.3×1016, 2.9×1017 and 2.6×1018 α-decays/g at the time of testing. Release of Al, B, Cs, Na, Si and U to solution increased with increasing pH, whereas Ca, Pu and Sr were invariant over the pH interval. Average dissolution rates, based on B release, were identical within experimental uncertainty for all three glass compositions and increased from 0.17±0.07 at pH(23°C) 9 to 10.6±2.7 (g/(m2d1)) at pH(23°C) 12. Release rates of Pu were 102- to 105-fold slower compared to all other elements and were not affected by isotopic composition, self-radiation damage sustained by the glass, or pH. These data demonstrate that self-radiation damage does not affect glass dissolution rates, despite exposure to internal radiation doses for >20 years.

Single-pass flow-through experiments on a simulated waste glass in alkaline media at 40°C.: II. Experiments conducted with buffer solutions containing controlled quantities of Si and Al

Additional single-pass flow-through (SPFT) experiments have been conducted with a complex, simulated waste glass at 40°C in moderately alkaline media. Results are compared to those obtained in the experiments described in Part I, and with published data obtained in long-term, static, batch dissolution experiments with this glass formulation. Dissolution rate laws for the glass must account for the rate influencing effects of both dissolved Si and Al species. These experiments have shown that on a mole per mole basis, dissolved Al has a more significant influence on the glass dissolution rate than dissolved Si under these experimental conditions. The very low ‘long-term’ dissolution rates reported in static batch dissolution experiments reflect near saturation conditions that are not attained in SPFT tests as a result of solution flow-through.