L.R. Van Loon

Diffusion of HTO, 36Cl- and 125I- in Opalinus Clay samples from Mont Terri. Effect of confining pressure.

Diffusion coefficients (T=23 +/- 2 degrees C) and accessible porosities for HTO, 36Cl(-) and 125I(-) were measured on Opalinus Clay (OPA) samples from the Mont Terri Underground Rock Laboratory (URL) using the through-diffusion technique. The direction of transport (diffusion) was perpendicular to bedding. Special cells that allowed the application of confining pressure were designed and constructed. The pressures ranged from 1 to 5 MPa, the latter value simulating the overburden at the Mont Terri URL (about 200 m). The test solution used in the experiments was a synthetic version of the Opalinus Clay pore water, which has Na(+) and Cl(-) as the main components (I=0.42 M). The measured values of the effective diffusion coefficients (D(e)) and rock capacity factors (alpha) are: D(e)=1.2-1.5 x 10(-11) m(2) s(-1) and alpha=0.09-0.11 for HTO, D(e)=4.0-5.5 x 10(-12) m(2) s(-1) and alpha=0.05 for 36Cl(-) and D(e)=3.2-4.6 x 10(-12) m(2) s(-1) and alpha=0.07-0.10 for 125I(-). For non-sorbing tracers (HTO, 36Cl) the rock capacity factor alpha is equal to the diffusion-accessible porosity epsilon. The experimental results showed that pressure only had a small effect on the value of the diffusion coefficients. Increasing the pressure from 1 to 5 MPa resulted in a decrease of the diffusion coefficient of approximately 17% for HTO, approximately 28% for 36Cl(-) and approximately 30% for 125I(-). Moreover, the diffusion coefficients for 36Cl(-) and 125I(-) are smaller than for HTO, which is consistent with an effect arising from anion exclusion. The diffusion coefficients of HTO and 125I(-) measured in this study are in good agreement with recent measurements at three other laboratories performed within the framework of a laboratory comparison exercise. The values of the diffusion-accessible porosities show a larger degree of scatter.

PREFERRED ORIENTATIONS AND ANISOTROPY IN SHALES: CALLOVO-OXFORDIAN SHALE (FRANCE) AND OPALINUS CLAY (SWITZERLAND)

Anisotropy in clay-rich sedimentary rocks is receiving increasing attention. Seismic anisotropy is essential in the prospecting for petroleum deposits. Anisotropy of diffusion has become relevant for environmental contaminants, including nuclear waste. In both cases, the orientation of component minerals is a critical ingredient and, largely because of small grain size and poor crystallinity, the orientation distribution of clay minerals has been difficult to quantify. A method is demonstrated that relies on hard synchrotron X-rays to obtain diffraction images of shales and applies the crystallographic Rietveld method to deconvolute the images and extract quantitative information about phase fractions and preferred orientation that can then be used to model macroscopic physical properties. The method is applied to shales from European studies which investigate the suitability of shales as potential nuclear waste repositories (Meuse/Haute-Marne Underground Research Laboratory near Bure, France, and Benken borehole and Mont Terri Rock Laboratory, Switzerland). A Callovo-Oxfordian shale from Meuse/Haute-Marne shows a relatively weak alignment of clay minerals and a random distribution for calcite. Opalinus shales from Benken and Mont Terri show strong alignment of illite-smectite, kaolinite, chlorite, and calcite. This intrinsic contribution to anisotropy is consistent with macroscopic physical properties where anisotropy is caused both by the orientation distribution of crystallites and high-aspect-ratio pores. Polycrystal elastic properties are obtained by averaging single crystal properties over the orientation distribution and polyphase properties by averaging over all phases. From elastic properties we obtain anisotropies for p waves ranging from 7 to 22%.

Effect of confining pressure on the diffusion of HTO, 36Cl− and 125I− in a layered argillaceous rock (Opalinus Clay): diffusion perpendicular to the fabric

Abstract The through- and out-diffusion of HTO, 36Cl− and 125I− in Opalinus Clay, an argillaceous rock from the northern part of Switzerland, was studied under different confining pressures between 4 and 15 MPa. The direction of diffusion and the confining pressure were perpendicular to the bedding. Confining pressure had only a small effect on diffusion. An increase in pressure from 4 to 15 MPa resulted in a decrease of the effective diffusion coefficient of ∼20%. Diffusion accessible porosities were not measurably affected. The values of the effective diffusion coefficients, De, ranged between (5.6±0.4)×10−12 and (6.7±0.4)×10−12 m2 s−1 for HTO, (7.1±0.5)×10−13 and (9.1±0.6)×10−13 m2 s−1 for 36Cl− and (4.5±0.3)×10−13 and (6.6±0.4)×10−13 m2 s−1 for 125I−. The rock capacity factors, α, measured were circa 0.14 for HTO, 0.040 for 36Cl− and 0.080 for 125I−. Because of anion exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than the uncharged HTO. Unlike 36Cl−, 125I− sorbs weakly on Opalinus Clay resulting in a larger rock capacity factor. The sorption coefficient, Kd, for 125I− is of the order of 1–2×10−5 m3 kg−1. The effective diffusion coefficient for HTO is in good agreement with values measured in other sedimentary rocks and can be related to the porosity using Archies Law with exponent m=2.5.

Degradation of cellulosic materials under the alkaline conditions of a cementitious repository for low and intermediate level radioactive waste. Part I : Identification of degradation products

Abstract In order to assess the potential role of cellulose degradation products as metal-binding chelates in a repository for radioactive waste, different cellulosic materials (pure cellulose, cotton, tissues and recycling paper) were degraded under the chemical conditions of cement pore water (pH 13.3). The degradation products formed were characterised using different separation techniques (HPIEC, HPAEC, GC-MS, MS/MS) and by high resolution 1H-NMR spectroscopy. The main degradation products of pure cellulose are the diastereomeric α-isosaccharinic acid (α-ISA) and β-isosaccharinic acid (β-ISA), which accounted for ∼80% of total dissolved organic carbon formed. Minor components are short chain aliphatic acids such as formic acid, acetic acid, lactic acid and threonic acid comprising less than 10% of total dissolved organic carbon. The degradation of the other cellulosic materials resulted in lower amounts of α-ISA and β-ISA and in higher yields of small organic acids (acetic acid) and of unidentified compounds. The combination of the different MS and NMR techniques unequivocally shows the structural identity of β-ISA.

Review of the kinetics of alkaline degradation of cellulose in view of its relevance for safety assessment of radioactive waste repositories

The degradation of cellulose (a substantial component of low- and intermediate-level radioactive waste) under alkaline conditions occurs via two main processes: a peeling-off reaction and a basecatalyzed cleavage of glycosidic bonds (hydrolysis). Both processes show pseudo-first-order kinetics. At ambient temperature, the peeling-off process is the dominant degradation mechanism, resulting in the formation of mainly isosaccharinic acid. The degradation depends strongly on the degree of polymerization (DP) and on the number of reducing end groups present in cellulose. Beyond pH 12.5, the OH- concentration has only a minor effect on the degradation rate. It was estimated that under repository conditions (alkaline environment, pH 13.3-12.5) about 10% of the cellulosic materials (average DP = 1000-2000) will degrade in the first stage (up to 105 years) by the peeling-off reaction and will cause an ingrowth of isosaccharinic acid in the interstitial cement pore water. In the second stage (105-106 years), alkaline hydrolysis will control the further degradation of the cellulose. The potential role of microorganisms in the degradation of cellulose under alkaline conditions could not be evaluated. Proper assessment of the effect of cellulose degradation on the mobilization of radionuclides basically requires knowing the concentration of isosaccharinic acid in the pore water. This concentration, however, depends on several factors such as the stability of ISA under alkaline conditions, sorption of ISA on cement, formation of sparingly soluble ISA-salts, etc. A discussion of all the relevant processes involved, however, is far beyond the scope of the presented overview.

Trace metal-humate interactions. II. The “conservative roof” model and its application

Abstract The modelling of metal-humate interactions has been a field of active research for more than 3 decades but despite all efforts there is still far from a consensus concerning humic binding models. The authors demonstrate that a synopsis of large sets of reliable experimental data for Ca, Co, Ni, Eu, Am, Cm, Np(V) and U(VI) reveals a consistent picture of the influence of metal concentration, pH and ionic strength on metal-humate interactions. However, this consistent behaviour cannot be interpreted with high numerical accuracy by simple binding models, the need for more adjustable parameters increases proportional to the width of the experimental parameter range to be fitted. This experience triggered the proposal of a pragmatic approach for performance assessment purposes. The “conservative roof” approach does not aim to accurately model all experimental data but rather allows estimates to be made of the maximum effects on metal complexation to be expected from humic substances. A specific “conservative roof” model is applied to situations generally to be expected in deep groundwater and selected cases of interest for planned Swiss repositories of radioactive waste are discussed in detail.

Equilibrium dialysis-ligand exchange: adaptation of the method for determination of conditional stability constants of radionuclide-fulvic acid complexes

Abstract The equilibrium dialysis-ligand exchange technique (EDLE) has been adapted to measure conditional stability constants for the complexation of metal ions with fulvic acid. Since fulvic acid permeates across the membrane during the experiment, the quantities involved have to be determined analytically and taken into account when calculating the conditional stability constants. It is shown here that the larger diffusion rates of fulvic acid compared to those of humic acid do not lead to enhanced errors in the results. Conditional stability constants (cKFA) of Laurentian Soil fulvic acid are determined at various pH values and ionic strengths for the binding of Co2+ and UO2+2. Over the whole pH range, at ionic strength of 0.2 M, log ( c K FA ) averages half a log unit lower than at ionic strength of 0.02 M. A comparison with the corresponding binding constants measured for Aldrich humic acid (cKHA) by the conventional EDLE technique shows that log ( c K FA ) values differ by not more than one order of magnitude from log ( c K HA ) values.

Equilibrium dialysis—ligand exchange: a novel method for determining conditional stability constants of radionuclide—humic acid complexes

Abstract The equilibrium dialysis—ligand exchange technique (EDLE) is a combination of two classical techniques and is designed to measure the conditional stability constants of radionuclide—humic acid complexes over broad pH and ionic strength ranges. The use of a reference ligand hinders the hydrolysis of the radionuclide and enables the stability constant to be determined under neutral and alkaline conditions. By measuring the distribution of the radionuclide between the reference ligand and the humic acid, the conditional stability constant can be calculated. The technique was tested with 60Co2+ and a commercial humic acid (HA) (Aldrich). It was possible to measure the conditional stability constants for the Co-HA complex for pH values between 5 and 10 in an ionic strength range between 0.02 and 0.2 M. The conditional stability constant increases non-linearly with increasing pH and decreases with increasing ionic strength.

Evidence for the existence of Tc(IV) - humic substance species by X-ray absorption near-edge spectroscopy

Summary The redox–sensitive fission product technetium–99 has been investigated in systems containing different reducing solid phases (pyrite, magnetite, ironsulphide and Gorleben sand) on the one hand and Gorleben groundwater, which contains a high amount of humic substances, on the other hand. Initially, technetium–99 was added to these systems as pertechnetate (Tc(VII)), which was reduced in presence and absence of humic substances with the aid of the different reducing surfaces (neutral to alkaline pH). Both in absence and presence of humic substances, Tc concentrations were observed which exceeded the TcO2 solubility limit, whereby the presence of humic substances had a 100 fold higher Tc concentration compared to its absence. Using the La–precipitation method, it was shown that Tc(IV) inorganic colloids and organic colloids were quantitatively precipitated. It was demonstrated for the first time by a combination of chemical speciation methods (La–precipitation method and gel permeation chromatography) and XANES spectroscopy of the humic substance containing supernatant solutions, that Tc(IV) species were formed in these systems, indicating an association between Tc(IV) and humic substances.

Complexation of Radionuclides with Humic Substance: The Metal Concentration Effect

A synopsis of experimental data concerning the complexation of radionuclides with humic substance from various laboratories using different experimental methods reveals a consistent pattern of the metal concentration effect: With decreasing radionuclide concentration (and all other parameters held constant) an increase in the complexation of the radionuclide with humic substance is observed. The implications of this effect for model calibration and practical applications are discussed.