Low Leaching Rate Research Articles

Immobilization of stimulated high level wastes into AASC waste form

Alkali-activated slag cement (AASC) was studied as a host matrix for the immobilization of simulated high-level wastes. Experimental results show that, with a waste loading as high as 25%, AASC waste form presents high compressive strength (65–100 MPa), low porosity (less than 10%) and low leach rates (10 −5 and 10 −6 g/cm 2d for Cs and Sr, respectively). The mechanism of encapsulation of nuclides in AASC waste form was also studied.

Geochemistry of Dissolved and Particulate Elements in the Major Rivers of China (The Huanghe, Changjiang, and Zhunjiang Rivers)

The relationship between the geochemistry of dissolved and particulate materials and geographic conditions was investigated. Samples of water and suspended particulate matter were collected from five locations in three of the major rivers of China (the Huanghe, Changjiang, and Zhunjiang rivers). Because these rivers generally flow parallel to latitudes and flow through diverse geologic and climate zones, they provide excellent opportunities for comparisons of solute transport. The geochemistry of these rivers is influenced strongly by climate. The low discharge of the Huanghe River influences the character of the major ionic materials (Cl−+SO42− and Na++K+) as well as the high degree of mineralization within the system. Dissolved concentrations of both major ions and trace elements are lower in the southern reaches of the rivers. The highly mobile ions, such as, Na+ and Ca2+, are depleted from the suspended particulate material in the southern regions, while the relatively immobile ions of Al, Fe, Ti, Mn, and trace metals are concentrated within the suspended mateiral. The relative mobility of some elements as measured by the Dissolved Transport Index (DTI) changes with climate. The geology of the area drained by the rivers has a major influence on the geochemistry in areas of similar climate. Lowest leaching rates in the southern climates occur in areas dominated by granite, which is resistant to weathering. The composition of both cations and anions among the three tributaries of the Zhujiang River are dependent on the rocks that dominate each eregion. Some particulate forms of the rare earth elements are present in the highest concentrations in regions dominated by granite. The DTI calculated for the major rivers in China are much smaller than those computed for other major rivers of the world. The low concentration of heavy metals in the particulate material suggests that pollution in the rivers of China is less serious than in rivers of other industrialized countries.

Formation and evolution of soils from an acidified watershed: Plastic Lake, Ontario, Canada

The Plastic Lake watershed contains podzols developed on glacial tills deposited 12,000 years ago. Present-day, cationic fluxes from the soils are greater by a factor of 2 than long-term fluxes averaged over the age of the tills. The high rates of present-day chemical weathering may be a result of increased input of anthropogenic acids into the Plastic Lake watershed. Time-averaged proportions of cations leached from the soils are strikingly different from the proportions of cations now being leached, indicating that the character of chemical weathering has changed over time. Weathering was and is dominated by mineral dissolution, but cation exchange has become increasingly important as the soils have matured. The large amount of Ca now released to the soil solutions probably is derived from exchange substrates, promoted by preferential uptake of Al onto the substrates. Aluminum is derived primarily through dissolution of feldspars. Bulk compositional analyses of soil profiles demonstrate that feldspars of the AE horizon release base cations (Na, K, Ca) and Al to solution in near-stoichiometric proportions, just as is observed experimentally for feldspar dissolution in acidic solutions. Furthermore, surface area normalized dissolution rates of plagioclase, K-feldspar, quartz, and micas of Plastic Lake soils are similar to rates obtained experimentally. The favourable comparison between natural and experimental dissolution rates, and near-stoichiometric release of base cations and Al from feldspars to acidic solutions in both natural and experimental settings, suggests that recent laboratory release rates can be applied to dissolution of feldspars within the AE horizon of the Plastic Lake Podzols. Surface area-normalized, time-averaged, dissolution rates of primary minerals of Plastic Lake soils are significantly greater than present-day rates measured for mature soil profiles. Rapid release from Plastic Lake soils probably results from reaction of acidic soil solutions with highly reactive biotite and ultrafine grains of feldspars produced by comminution during glaciation. Exhaustion of these reactive phases may well explain the low leach rates observed for mature profiles. After just 12,000 years of weathering, biotite has been effectively exhausted in the Plastic Lake soils and much of the fine-grained feldspar probably has been dissolved. Time-averaged concentrations of elements removed from the soil, recast into essential mineralogy, indicate that vermiculite weathers most rapidly, followed closely by plagioclase. K-feldspar weathers less rapidly than plagioclase but more rapidly than quartz or hornblende. Al-silicate, possibly imogolite, and goethite are added to the soils during weathering. Plagioclase dissolution consumes more acid than all other reactions, accounting for 60% of the total acid consumed. Combined with K-feldspar, they account for more than 80% of the total acid neutralized. The dissolution rate of quartz is one-half that of plagioclase; furthermore, the rate is consistent with the experimentally determined rates. The combination of high abundance and dissolution rate results in large amounts of quartz being dissolved from the soils of Plastic Lake; it may also contribute significantly to total elemental fluxes from other catchments.

Properties of Y<sub>2</sub>O<sub>3</sub>-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Glasses as a Model System of Grain Boundary Phase of Si<sub>3</sub>N<sub>4</sub> Ceramics (Part 2)

The leaching behavior of Y2O3-Al2O3-SiO2 glasses was studied in connection with the hydrothermal corrosion of Si3N4-Y2O3/Al2O3 ceramics. Leaching tests were conducted with the glass blocks under hydrothermal conditions at 300°C for 1 to 10 days. It was found that the weight loss of the glasses is less than 0.8mg/cm2 after 10 days and the durability of the glasses is higher than that of SiO2 glass, Corning-1723 glass and Si3N4-Y2O3/Al2O3 ceramics. It was considered that the higher durability of the glass under hydrothermal conditions is due to the lower leach rates of Y2O3 and Al2O3.

Leaching of Radionuclides from Decaying Blueberry Leaves: Relative Rate Independent of Concentration

AbstractLeaching of radionuclides from decaying vegetation has not been extensively investigated, especially for radionuclides other than 137Cs. We obtained leaves of blueberry (Vaccinium angustifolium × V. corymbosum) that contained over 25‐fold ranges in Se, Cs, and I concentrations, as well as a small quantity of leaves containing detectable U. All were contaminated by way of root uptake. Leaching took place for a period of 1 yr in the laboratory, using leach water from forest litter. Monthly, measurements were made of the radionuclide contents and decaying leaf dry weights. The data conformed to an exponential decay model with two first‐order components. In no case did the relative loss rates vary systematically with the initial tissue radionuclide concentrations. Loss rates decreased in the order Cs > I > U > dry wt. > Se. Because of the low leaching rate of Se relative to the loss of dry weight, decaying litter may actually accumulate elements such as Se. Accumulation of radionuclides in litter could have important implications for lateral transport, recycling, and direct incorporation into edible mushrooms.

Ｔｈｉｏｂａｃｉｌｌｕｓ ｆｅｒｒｏｏｘｉｄａｎｓによる黄銅鉱精鉱浸出の挙動とその機構

Leaching behavior of a chalcopyrite concentrate by Thiobacillus ferrooxidansstrain, isolated from an acid mine water, was considered on the basis of the results of batch shake flask experiments. Then, its leaching mechanism and rate controlling factors were also considered.It was found that under the same pulp density, there was a linear relation between the initial specific surface area and the upper limit of copper concentration attainable with a high leaching rate (Fig. 3).Leaching proceeded in two stages (Fig. 6-9). The first stage was characterized by a high growth rate, a high demand of hydrogen ion and a high leaching rate of copper. The second stage was characterized by a generation of hydrogen ion, a low leaching rate of copper and a low growth rate. It was observed that the second stage of leaching was followed by a leaching behavior similar to the first stage, then by another second stage behavior.The leaching mechanism of the first stage is mainly indirect (Eq. 6 and Eq. 9), and the reactions proceed by depositing elemental sulphur on the mineral surface (Eq. 11-15) until reaching a particular thickness. In the second stage, the deposited elemental sulfur is dissolved by bacterial catalytic action and sulfuric acid is produced.The main factor affecting the rate in the leaching of a chalcopyrite concentrate by Thiobacillus ferrooxidans is the oxidation of sulfide to sulfate. Accordingly, for improvement of the leaching rate, it is necessary to accelerate this oxidation reaction.

USE OF ACETIC ACID IN SOXHLET WEATHERING PROCEDURES

Modified soxhlet extractors using 0.3 M acetic acid were used to leach parent material from a sandy soil. Three extractors were run at a low leaching rate of 2.2 L d−1 and three others at a high leaching rate of 4.3 L d−1 for a total of 8 wk. Soil residues, leachates, suspended sediments and adhered precipitates on the sides of the collection flasks were analyzed for basic cations and Fe, Al and Si. Leaching reduced original sample weights 2.1% and 6.0% for the low and high rates, respectively, and resulted in podzolic weathering for both treatments. Although the low and high leaching rates did not differ in their ability to remove Mg and Fe from the soil samples, the extract-ability of these elements was greater in residues for the low versus the high leaching-rate treatments. It was concluded that extractable measures may not always predict the teachability of elements. In conjunction with routine soil chemical analyses, this acetic acid procedure shows promise for predicting behavior of soil materials that are to be placed in podzolic soil environments where fluctuating water tables are present. Key words: Soxhlet, podzolization, soil weathering, leaching studies

The corrosion of spent UO 2 fuel in synthetic groundwater

Leaching of high burnup BWR fuel for up to 3 years showed that both U and Pu attain saturation rapidly at pH 8.1, giving values of 1–2 mg/ l and 1 μg/ l respectively. The leaching rate for Sr-90 decreased from about 10 −5/d to 10 −7/d but was always higher than the rates for U, Pu, Cm, Ce, Eu and Ru. Congruent dissolution was only attained at pH values of about 4. When reducing conditions were inposed on the pH 8.1 groundwater by means of H 2/Ar in the presence of a Pd catalyst, significantly lower leach rates were attained. The hypothesis that alpha radiolytic decomposition of water is a driving force for UO 2 corrosion even under reducing conditions has been examined in leaching tests on low burn-up (low alpha dose-rate) fuel. No significant effect of alpha radiolysis under the experimental conditions was detected. Thermodynamically the calculated uranium solubilities in the pH range 4–8.2 generally agreed well with the measured ones, although assumptions made for certain parameters in the calculations limit the validity of the results.

Carbon-14 releases from solidified IRN-150 moderator resin wastes

Carbon-14 releases from unsolidified and solidified IRN-150 ion exchange resin wastes have been investigated to determine the matrix with the lowest leaching rates. After ninety-three days leaching, 14C cumulative releases from Portland Type 30 cement/epoxy resin matrix were lower than the releases from unsolidified ion exchange resins and those solidified with water extendible polyester polymer. Long-term 14C releases from the waste forms were estimated by a semi-infinite diffusion model that accounts for diffusion and non-diffusion controlled processes. The diffusivity coefficients calculated by this model ranged from 1.2 × 10 −13 cm 2/s to 7.3 × 10 −13 cm 2/s for Portland Type 30 cement/epoxy resin waste products and from 1.0 × 1.0 −10 cm 2/s to 8.0 × 10 -15 cm 2/s for water extendible polyester samples. After five hundred years leaching 14C releases are estimated at 6.5% for Portland Type 30 cement/epoxy resin samples. In the case of water extendible polyester waste forms, the data show all original radioactivity would be released by the end of this leaching period.

The chemical durability of tektites—A laboratory study and correlation with long-term corrosion behavior

Leach tests carried out on tektite specimens (indochinites and australites) under high-dilution conditions show a common behavior characterized by low leach rates (1.8 × 10 −5 g · m −2 · d −1 or 7.2 × 10 −12 m · d −1 at 23°C) and an activation energy of (79.6 ± 0.7) × 10 3 J · mol −1. The extent of selective leaching is very small, of the order of 10 −8 m. Extrapolation of test results over the lifetime of the tektites gives an excellent agreement with field observations on the extent of corrosion, and this is an important step in establishing the validity of laboratory tests as a basis for the development of models and predictions concerning long-term durabilities at least in the limiting case of high dilution or rapid flow. The results are also shown to be in agreement with various previous observations on the corrosion resistance of tektites. The chemical durability of tektites is observed to be consistent with their composition, highlighting requirements for high corrosion resistance in glasses; these requirements include a silica content in excess of 67 mol%, an extremely low water content and an alkali content which is low both absolutely and relative to the di- and poly-valent metal oxide levels. It is shown that artificial glasses which fulfil these criteria are no less corrosion-resistant than the corresponding natural glasses. These conclusions have bearing on the development as well as on the evaluation of glasses intended for very long service, such as radioactive waste vitrification media.

Highly Dense Graphite Matrix: A New Material for the Conditioning of Radioactive Wastes

In continuation of 20 yr of experience with high-temperature gas-cooled reactor graphite fuel element fabrication (molded fuel spheres and monolithic block fuel elements), NUKEM introduced a new graphite-based material with an inorganic binder produced by molding. A mixture of nickel and sulfur is used as the inorganic binder for natural graphite powder. The fabrication is facilitated by the low temperature of 500/sup 0/ C in generating a chemically and thermally stable nickel sulfide. The newly developed material is suitable as a matrix for the fixation of high-active waste and as a corrosion-resistant layer of the steel supporting tube for final disposal of spent fuel elements. The material of natural graphite/nickel sulfide is distinguished by high density, high corrosion resistance, a low leaching rate, good thermal conductivity, an appropriate coefficient of thermal expansion, and high thermal stability due to the high melting point of nickel sulfide (790/sup 0/ C). Currently, a pilot plant for fabrication of containers for spent fuel elements is being erected. Additionally, development work is in progress to obtain basic design data for a high-level waste fixation facility.

GLASS SURFACES - 1982

A new category of glass surface is described. Type IIIB surfaces are composed of multiple layers of oxides, hydroxides, and hydrated silicates resulting from a sequence of solution-precipitation reactions between the glass surface and leaching solutions. Very low leach rates and then ion depletion depths of certain compositions of complex nuclear waste glasses are due to Type IIIB surfaces. The solubility limits that establish the equilibrium ionic concentrations for burial ground waters also establish the multiple barrier Type IIIB films, to protect nuclear waste glasses in contact with those ground waters.

In-Situ Leaching of Crownpoint, New Mexico, Uranium Ore: Part 2—Laboratory Study of a Mild Leaching System

Abstract This paper describes laboratory leaching studies involving Crownpoint uranium ore samples and a mild leaching system. Batch leach tests with sodium bicarbonate solution and either high-pressure oxygen or low-pressure hydrogen peroxide gave qualitative data used to estimate leach rate and potential recovery. Using pseudo-first-order rate constants derived from the batch test data, ore leachability was characterized as fast, intermediate, or slow. It was observed that leach rates varied by a factor of 50 for samples taken from different areas at Crownpoint; samples from the same ore trend often varied by a factor of 10. Packed-column and core-leach tests with oxygen at pressures up to 800 psig (5520 kPa) provided more quantitative estimates of leach rate and uranium recovery. Batch test results were correlatable with leach rates and uranium recoveries in packed-column or core tests. In ore samples where uraninite was the predominant uranium mineral, leach rates and recoveries were high. In samples containing coffinite, leach rates were generally lower than those with uraninite. Very low leach rates and recoveries were encountered where coffinite was intimately associated with carbonaceous material. However, the slow leaching rates are not caused by differences in reactivity of coffinite and uraninite. Mineralogical studies before and after leaching using electron microprobe analyses indicated that exposed coffinite crystals are dissolved easily, but finely disseminated coffinite crystallites persist after leaching if they are encapsulated in the carbonaceous matrix. Slow-leaching ores that did not respond to the mild oxidant system are called "refractory."

Fracture Appraisal of Large Scale Glass Blocks Under Realistic Thermal Conditions

The compositions of borosilicate glasses used to solidify fission product solutions are selected for their low leach rates. Fracturing of the glass blocks, caused primarily by thermal and residual stresses during cooling, increases the potential leaching surface area and the number of small particles, both of which are undesirable since long term container failure must be considered. It is therefore important to know the state of fracture of the glass occurring at different stages of its thermal history. A theoretical appraisal of the fracture state of the glass is presented, complimented by an experimental study using industrial scale glass blocks under realistic thermal conditions.