Trygve E. Eriksen

Diffusion of 22Na+, 85Sr2+, 134Cs+ and 57Co2+ in bentonite clay compacted to different densities: experiments and modeling

Summary The diffusion of radionuclides in water-saturated porous media, such as compacted bentonite, is traditionally modeled assuming diffusion in the pore water and immobilization by adsorption on the clay surface. In reality there are several sorption mechanisms acting in the clay-water system. We have therefore carried out a careful diffusion study of the cations Na+, Sr2+, Cs+, which are predominantly sorbed by electrostatic interaction, and Co2+, for which the dominant sorption mechanism at pH > 7 is inner-sphere complexation and/or surface precipitation. The study has been performed at different densities of clay compaction (1.8, 1.6, 1.2, 0.8, 0.4 g cm−3) using a through diffusion technique. In each experiment, the apparent diffusivity and transport Kd were obtained by computer simulation of the experimentally measured cumulative flux through a plug of compacted bentonite and the concentration profile within the bentonite. Batch sorption experiments with Na+, Sr2+, Cs+ and Co2+ were also carried out. Measured batch Kd values were compared to diffusion-derived Kd values using the computer code ANADIFF. The obtained diffusivities of Cs+ and Sr2+ decreased as the density of bentonite increased. The diffusivity of Cs+ decreased from 1.5×10−7 to 1.1×10−8 cm2/s as the clay dry bulk density increased from 0.4 to 1.8 g/cm3 and the diffusivity for Sr2+ varied from 4.3×10−7 to 9.0×10−8 cm2/s. The diffusivities of Co2+ also decreased from 3.0×10−9 to 1.0×10−10 cm2/s in the same range of clay density. An interpretation of the diffusion data is proposed based on differences in porosity and tortuosity of the bentonite compacted to different densities, and on differences in binding energy, hydration energy, and hydrolysis constants for the cations studied.

On the acid-base equilibrium of the carbonate radical

Abstract Using pulse radiolysis and coupled optical detection the pKa of the HCO3 radical is shown to lie between 7.0 and 8.2. The rate constant of self-recombination of CO-3 has a negative apparent activation energy which indicates a composite process.

Dissolution of irradiated fuel: a radiolytic mass balance study

We have studied the production of H2,02 and H202 by radiolysis of the leach solution in a closed system containing fragments of irradiated PWR fuel and distilled water purged with argon. The experimental data is not reflected in the release of U(VI) to the solution, clearly indicating that most of the oxidant production has been taken up by the U02 spent fuel surface. This proves that the U02 surface constitutes a major redox buffer capacity to prevent radiolytic oxidation under repository conditions.

Anion diffusion pathways in bentonite clay compacted to different dry densities

Diffusion of the anions Cl- and I- in MX-80 compacted bentonite has been studied at different ionic strengths (0.01, 0.1 M NaClO4) and clay density (0.4, 0.8, 1.2, 1.8 g cm(-3)) at the buffered pH ...

Sorption effects on cation diffusion in compacted bentonite

Diffusion of Na+. Cs+, Co2+ and Sr2+ in bentonite compacted to a dry density of 1800 kg m(-3) and saturated with groundwaters and aqueous solutions of differing ionic strength have been studied exp ...

THE REACTIVITY OF SUPEROXIDE (O2‐) and ITS ABILITY TO INDUCE CHEMILUMINESCENCE WITH LUMINOL

Abstract— The quantum yield and the kinetics of O‐induced luminol chemiluminescence was investigated in a broad pH interval at varying luminol and concentrations. It is suggested that the weak chemiluminescence observed is mediated via a luminol‐superoxide‐adduct proposed to be an a‐hydroxyperoxyl radical. At pH 7 the maximum quantum yield of chemiluminescence per initial percent was determined to be 4 times 10‐8. The degree of involvement in phagocytosis and related processes should be viewed against this maximum limit.

On the corrosion of copper in pure water

Abstract Due to a recent publication questioning well known thermodynamic data the corrosion of copper in distilled water has been studied. No hydrogen evolution was observed during an exposure period of 61 days using a gaschromatographic technique. Cu 2 O was the only corrosion product detected by means of ESCA and cathodic reduction. The corrosion rates obtained for two different copper qualities are much lower than the corrosion rate reported by Hultquist and are ascribed to the reaction between the copper foils and residual oxygen initially present in the water. In conclusion the present investigation confirmed well established thermodynamics, which means that oxidation of copper by pure deoxygenated water under the formation of hydrogen as proposed by Hultqvist is not thermodynamically feasible.

Experimental determination and chemical modelling of radiolytic processes at the spent fuel/water interface

The spent fuel matrix constitutes a dynamic redox system, due to the time-dependent generation of oxidants and reductants at the spent fuel/water interface by alpha, beta and gamma radiolysis. In this context it is critical to understand the main processes and mechanisms that control the radiolytic production of redox components at this interface and their impact on the stability of the UO2 matrix and in radionuclide release. In order to achieve this, a series of carefully controlled experiments have been carried out in order to determine the time dependence of the radiolytic H2, O2 and H2O2 production and radionuclide release (U, Pu, Np, Tc, Mo, Sr and Cs). These tests have been performed by using PWR Ringhals spent fuel fragments in contact with 10 mM NaHCO3 solutions in an initially anoxic and closed system. The results of these experiments indicate quite a consistent and reproducible behaviour of the radiolytically generated reductants and oxidants. The results indicate that it is possible to define an equilibrium redox potential for the main redox pairs in the bulk system and that the experimentally determined radionuclide release can be rationalised in terms of the system redox potential.

The Solubility of TcO2 · nH2O in Neutral to Alkaline Solutions under Constant pco2

We have investigated the solubility of Tc0 2 • n H 2 0 in aqueous solution as a function of pH and pco2· The experimental data collected shows that the solubility of Tc02 · n H 2 0 increases with increasing pH and hydrocarbonate concentration. By studying the dependence of the solubility on the pH and Pco2 w e have determinated the stoichiometry and stability of the predominant aqueous complexes: TcO(OH)2 (aq), TcO(OH)J, Tc(0H)2C03 and Tc(OH)3CO;\ The behaviour of Tc(IV) in natural water systems is discussed in the light of these new data.

Swedish-German actinide migration experiment at ASPO hard rock laboratory.

Within the scope of a bilateral cooperation between Svensk Kärnbränslehantering (SKB) and Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung (FZK-INE), an actinide migration experiment is currently being performed at the Aspö Hard Rock Laboratory (HRL) in Sweden. This paper covers laboratory and in situ investigations on actinide migration in single-fractured granite core samples. For the in situ experiment, the CHEMLAB 2 probe developed by SKB was used. The experimental setup as well as the breakthrough of inert tracers and of the actinides Am, Np and Pu are presented. The breakthrough curves of inert tracers were analyzed to determine hydraulic properties of the fractured samples. Postmortem analyses of the solid samples were performed to characterize the flow path and the sorbed actinides. After cutting the cores, the abraded material was analyzed with respect to sorbed actinides. The slices were scanned optically to visualize the flow path. Effective volumes and inner surface areas were measured. In the experiments, only breakthrough of Np(V) was observed. In each experiment, the recovery of Np(V) was < or = 40%. Breakthrough of Am(III) and Pu(IV) as well as of Np(IV) was not observed.