The formation and stability of colloids has been analysed in LWR spent-fuel and HLW-glass leaching studies, in container corrosion tests and in precipitation experiments from oversaturated solutions. Experiments were carried out both in solutions of high and low ionic strength. As expected, the amounts of colloidal species determined by ultrafiltration were highest for the tetravalent elements. They were rather high in solutions of low ionic strength such as simulated granitic groundwaters and at pH-values higher than 7. Experiments carried out in saturated NaCl or MgCl2 rich brines show much lower concentrations of colloids. Corrosion of container material also contributes to the formation of colloidal species consisting mainly of Fe(II/III) species. Even though they are assumed to act as radionuclide carriers, their stability in brines has been found to be very low. At high ratios of the waste surface area to the solution volume, higher colloid concentration fractions have been detected, indicating a dependency of colloid formation on dissolution kinetics. Considerable concentrations of ultrafilterable fractions were found for some actinides also in solutions of very high ionic strength. These turned out to be unstable as the total concentration of dissolved and colloidal fractions in general decreased with time. From all these findings, colloid formation from waste forms in rock salt near-field conditions is not expected to be relevant.

Formation and stability of colloids under simulated near field conditions

This report evaluates the characteristics of oxidation and fracture of light-water reactor (LWR) spent fuel in dry air. It also discusses their effects on radionuclide releases in the anticipated high-level waste repository environment. A sphere model may describe diffusion-limited formation of lower oxides, such as U{sub 4}O{sub 9}, in the oxidation of the spent fuel (SF) matrix. Detrimental higher oxides, such as U{sub 3}O{sub 8}, may not form at temperatures below a threshold temperature. The nucleation process suggests that a threshold temperature exists. The calculated results regarding fracture properties of the SF matrix agree with experimental observations. Oxidation and fracture of Zircaloy may not be significant under anticipated conditions. Under saturated or unsaturated aqueous conditions, oxidation of the SF matrix is believed to increase the releases of Pu-(239+240), Am-(241+243), C-14, Tc-99, I-129, and Cs-135. Under dry conditions, I-129 releases are likely to be small, unlike C-14, in lower oxides; Cl-36, Tc-99, I-129, and Cs-135 may be released fast in higher oxides. 79 refs.

/pdf/dry-oxidation-and-fracture-of-lwr-spent-fuels-ycdzd3owro.pdf

Dry oxidation and fracture of LWR spent fuels

Multiple lines of evidence for performance of the canister and waste form in long-term nuclear waste disposal: Reviews

Abstract Colloid formation is discussed as a possible pathway for the radionuclide release from a nuclear waste repository. An assessment of the colloid relevance on radionuclide migration requires insight into the possible colloid generation mechanisms, their stability and mobility under given groundwater conditions. In various experiments dedicated to the investigation of nuclear waste form behaviour in contact with groundwater, colloidal species are observed mainly for the tri- and tetravalent actinides even in rather high ionic strength solutions where destabilization of colloids is expected. Experimental evidence of laboratory and field studies suggests colloid instability in saline groundwater with time. Groundwater of low ionic strength and high pH enhances colloid stability, as demonstrated in various laboratory and field experiments. The results of an in situ colloid characterization study at the Äspö hard rock laboratory in Sweden are discussed as an example. The mechanism of radionuclide interaction with colloids and notably the reversibility of the radionuclide-colloid binding are other key issues of colloid studies. Kinetic stabilization of the radionuclide binding to colloids may lead to a considerable enhancement of the colloid-mediated radionuclide migration. Substantial kinetic inhibition of actinide dissociation from humic colloids has been established by studying the behaviour of natural humic colloid borne U, Th and rare earth elements. Such behaviour might be explained by the incorporation of these elements into inorganic nanoparticles stabilized by humic coating. Spectroscopic evidence for the occurrence of actinide incorporation into colloidal structures is briefly discussed and the importance of considering the kinetics involved in all kinds of colloidal processes is emphasized. The enhanced migration of tri- and tetravalent actinide ions has been observed recently in various in situ dipole tests at the Grimsel hard rock laboratory in Switzerland. Such experimental findings underline the necessity of further studies on the colloid influence on actinide mobility. Moreover, an improved understanding of colloid-rock interaction mechanisms is required, which is essential for the description and prediction of colloid filtration processes.

Colloid influence on the radionuclide migration from a nuclear waste repository

Risk-informed assessment of radionuclide release from dissolution of spent nuclear fuel and high-level waste glass

This report evaluates continuous dissolution and colloid formation during spent-fuel performance under repository conditions in high-level waste disposal. Various observations suggest that reprecipitated layers formed on spent-fuel surfaces may not be protective. This situation may lead to continuous dissolution of highly soluble radionuclides such as C-14, Cl-36, Tc-99, I-129, and Cs-135. However, the diffusion limits of various species involved may retard dissolution significantly. For low-solubility actinides such as Pu-(239+240) or Am-(241+243), various processes regarding colloid formation have been analyzed. The processes analyzed are condensation, dispersion, and sorption. Colloid formation may lead to significant releases of low-solubility actinides. However, because there are only limited data available on matrix dissolution, colloid formation, and solubility limits, many uncertainties still exist. These uncertainties must be addressed before the significance of radionuclide releases can be determined. 118 refs.

/pdf/long-term-kinetic-effects-and-colloid-formations-in-15811juxlb.pdf

Long-term kinetic effects and colloid formations in dissolution of LWR spent fuels

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Long-term kinetic effects and colloid formations in dissolution of LWR spent fuels