A. Lodding

In situ testing of the chemical durability of vitrified high-level waste in a Boom Clay formation in Belgium: discussion of recent data and concept of a new test

Abstract SCK·CEN has been conducting in situ experimental programmes on candidate high-level waste (HLW) glasses, as part of a global approach to evaluate the long-term chemical durability of the HLW glasses. The in situ tests are conducted in the underground research facility high activity disposal experimental site (HADES), located in the Boom Clay at 223 m below the SCK·CEN site. We briefly present the test concepts for three in situ tests: a first type of corrosion test, tests as part of the Control Experiment with Radiation of the BElgian Repository for Underground Storage (CERBERUS) test (involving γ-radiation sources), and the CORrosion of Active gLass in Underground Storage condition (CORALUS) test, that involves both γ-sources and glass samples doped with α -radionuclides (Np, Pu, Am). The glasses are corroded slightly less compared with the first in situ tests (where no γ-radiation was present). By using mass loss data and an extensive analysis of the reaction layer formed on top of the glasses, we managed to identify the main interaction processes: matrix dissolution and selective leaching. The four glasses studied reveal significant differences in dissolution behaviour. We further discuss the status of the CORALUS in situ corrosion test, which is in its first phase. An inactive CORALUS tube has been prepared and is presently in operation.

Nuclear waste glass interfaces after one year burial in STRIPA part 1: Glass/glass

Abstract Two simulated nuclear waste glasses were corroded in an in-situ experiment in the STRIPA mine up to one year at 90°C. Changes in compositional in-depth profiles for glass/glass interfaces were measured using SIMS. Both glasses showed a depletion of Na, Cs, and B, but for the more corrosion resistant glass, the lower depletion depth is ascribed to the formation of a thin (2.0 m) coherent and dense outer layer, enriched in Mg, Ca, Sr, Ba, Zn-Al, Fe, and Si, which impedes both the ion exchange and network attack of the bulk glass underneath.

Applications of SIMS in Interdisciplinary Materials Characterization

Although pioneering work on secondary ion mass spectrometry was performed already in the 1930-s, one may say that SIMS as a microstructural-microanalytical technique for materials research saw the light of day about 21 years ago1, 2. As a commercially available tool, SIMS is only some 14 years old3–6. Since about half that time, its typical assets, mechanisms and artifacts have been illuminated by extensive and thorough international discussion7, 8, and today SIMS is widely accepted as an off-age technique for very sensitive three-dimensional characterization of materials. Current trends9–11 are directed mainly towards perfected quantitation, towards efficient routines in daily applications, and towards apparative development for further improved detection sensitivity and spacial resolution.

Calibration methods for the ion probe determination of fluorine in mineralized tissue

SummaryAn ion micro-analyzer (Cameca IMS 300 with a TD 1000 computer unit) has been calibrated for quantitative point determination of fluorine in apatites, including tooth and bone material. Samples with macroscopically measured F concentrations were used as external standards. The samples were bombarded with O− primary ions. Secondary positive ion currents were registered as mass spectra containing mass numbers 19, 20, 31, 44, 59, 60 and 61. The latter mass numbers were used in isotopic correction to subtract oxide and hydroxide contributions from the fluoride 59 peak. For each bombarded spot, the ratios19F+/40Ca2+, respectively59CaF+/44Ca+, were plotted in a log-log graph against40Ca2+/44Ca+, the abscissa providing a measure of ionization efficiency. The plots yielded two sets of parallel straight lines, with ordinate values proportional to the respective F/Ca concentration ratios, in agreement with earlier theory. Each set of lines constitutes a calibration graph, where the fluorine concentration at an investigated spot is obtained when substituting the measured ion current ratios. With biological apatites, account must also be taken of effects due to, among other things, adsorbed humidity, other contaminations, and non-stoichiometry. When all precautions are taken, the accuracy of the determined F contents in the point analysis of an area some 100 μm in diameter can be expected to be better than 6% at 1500 ppm F, and better than 25% at 100 ppm F. The calibration is effective down to about 20 ppm F. Applications have been carried out on samples of human tooth enamel.

Ion Probe Study of Fluorine Gradients in Outermost Layers of Human Enamel

F concentrations in the outermost layers of human tooth enamel were studied with the aid of a secondary ion microanalyzer. Concentration profiles were recorded in continuous sputtering analysis from the surface down to a depth of about 0.3 micrometer. Samples previously subjected to topical fluoride treatment were compared with reference specimens. In some samples, the results were compared with those obtained at greater depths by macrascopic etching analysis.

Inverse velocities of sputtered monatomic ions: Systematics and quantification

The emission of secondary ions of about fifteen different elements, sputtered from Ti-base metal specimens, has been studied by SIMS. Both positive and negative ion yields have been measured at different exit energies up to ca 350 eV. It is found that when the logarithm of ionizability is plotted versus the inverse of the exit velocity, each element suggests a straight line behavior at energies above ca 20 eV. The gradient of the straight line is related to the respective 1st ionization potential (for positive ions) or electron affinity (negative ions). This behavior gives considerable support to the premises of modern theory of ionization in sputtering. Furthermore, the straight line plots for different elements are seen to converge as exit velocity increases; the intercepts at zero inverse velocity are found to be proportional to the respective element concentrations. This in principle offers a means of quantification in elemental analysis by SIMS, a method that does not require any external standards. The usefulness of the new method is demonstrated for ten elements sputtered from two specified titanium-base alloy standards from NIST.

On the pH dependence of leaching of nuclear waste glasses

Abstract Two simulated nuclear waste glasses were leached for periods up to one month at 90°C in high-purity water, following standard MCC-1 test procedures. The changes in composition of the surface layers were determined using ESCA and SIMS, which analyses layers of different depths. The results are discussed with reference to the different pH values in the tests performed.

SIMS yields from glasses; secondary ion energy dependence and mass fractionation

SIMS studies of glasses indicate that calibration of positive monatomic ion yields via relative sensitivity factors (RSF) is significantly dependent both on the kinetic energyEk and on the massMt of the analyzed ions. Due to elemental differences in the energy distributions of the sputtered ions, relative emissivities at highEk are radically different from those at the tops of the distributions. While the RSF values of cations from glasses range within ca. 3 powers of ten, atEk above ca. 40 eV the range remains within a factor of ten or less, and further change of relative elemental sensitivities withEk is slow. At low exit energy the LTE formalism is reasonably well obeyed. At highEk, a trend is noted towards a relative suppression of the ion yields of lowvalent elements.Measurements of isotope fractionation in secondary ion yield were performed on 17 elements sputtered from glasses. The mass factorα (defined by the proportionality of the yield toMi−α) is found to range from near-zero toα≅2.5, dependent on the element and onEk. For most elements a drops steeply at lowEk, but generally a slow rise is noted at higher energies. The behaviour ofα appears to be to some extent connected with the shape of the energy distribution curve. The dependence ofα onEk and on elemental parameters can qualitatively be described in terms of a simple phenomenological model.

Atomic transport of trivalent impurities in silicon: diffusion, isotope effects, activation volumes

Tracer diffusion coefficients, and their dependence on temperature, pressure and isotope mass, have been determined for B and Ga as impurities in (111) oriented silicon single crystals. The measurements were evaluated with stable isotopes by means of secondary ion mass spectrometry. The diffusivities, measured between 850 and 1150 °C, are found to be represented, in cm2 s–1, by the expression: DB–Si= 33.7 exp[–(3.93 ± 0.10)eV/kBT] and DGa–Si= 6.5 exp[–(3.59 ± 0.10)eV/kBT]. The mean values of the isotope effects between 850 and 1150 °C are EB–Si= 0.39 ± 0.03 and EGa–Si= 0.51 ± 0.04. The relative volumes of activation are (V*/Ω)B–Si=+0.27 ± 0.13 (mean in the range 1050–1230 °C) and (V*/Ω)Ga–Si=–0.7 ± 0.1 (at 1053 °C). These results, and their comparison with earlier data on diffusion in elemental semiconductors, suggest a predominantly interstitial diffusion mode for group III impurities in Si. The behaviour is not in all parts reconcilable with a conventional interstitialcy mechanism, but rather suggests what has been introduced in the literature as a ‘kick-out’ mechanism.

Accelerated Leach Tests of SRL-165 High-Level Waste Glass in Deionized Water

Accelerated short-term leach tests in a laboratory are neccessary in order to estimate, with reasonable accuracy, the long-term leaching behavior of high-level waste glass. In the present study, static leach tests of an SRL-165 high-level waste glass were carried out in deionized water at two different glass-surface-area to solution-volume ratios (SA/V-ratio), namely 0.85 and 0.079 cm−1 at 90°C, and 0.85 cm−1 at 40°C. First, an equation was examined which related Si-concentrations with time, temperature and SA/V-ratio under the present static conditions. The parameter determined at 90°C, 0.85 cm−1 can be used to calculate the Si-concentration at 40°C, 0.85 cm−1. Second, at the low SA/V- ratio of 0.079 cm−1, the concentrations of Ca and Mg in the leachates peaked and then decreased a little. The equation used above does not explain the variation of the concentrations of Ca and Mg at a low SA/V-ratio. The precipitation of Ca and Mg onto the glass surface is probably caused by the adsorption efficiency of th...