Cladding tubes of water-cooled nuclear reactors are usually made of Zircaloy and are an important... more Cladding tubes of water-cooled nuclear reactors are usually made of Zircaloy and are an important retaining element for radionuclides present in the fuel both during predisposal activities such as reloading of fuel assemblies from interim storage casks to final disposal casks and during final disposal in the case of canister breaching. However, cladding integrity is affected by various processes during reactor operation and beyond, e.g. fuel cladding chemical interaction and fission product precipitation onto the inner cladding surface. Using experimental and modelling methods, the radionuclide inventory of an irradiated Zircaloy-4 plenum section is analyzed. Quantities of 235/238 U, 237 Np, 238/239/240/241/242 Pu, 241/243 Am, 243/244 Cm besides 14 C, 55 Fe, 125 Sb, 154 Eu, and 134/137 Cs were (radio-)chemically determined in digested Zircaloy-4 subsamples. Measured inventories of activation products in the Zr-alloy are in good agreement with calculated values. However, amounts of actinides and fission products exceed the calculated inventory by factor ∼57 (minor actinides and non-volatile fission products) and ∼114 ( 137 Cs). Excess of minor actinides and part of enhanced Cs inventory originate from fuel residues deposited on the inner cladding surface during fuel rod fabrication, whereas vast amount of cesium is volatilized from subjacent fuel pellets and transported to the plenum.
ABSTRACT Green rust (GR) forms where pH is neutral to basic, iron concentration is high and oxida... more ABSTRACT Green rust (GR) forms where pH is neutral to basic, iron concentration is high and oxidation potential provides a small amount of Fe(III). GR is best known from metallic iron corrosion but it has also been reported in soil. It typically forms nano-particles, so surface area is high. It has a layered structure and is reactive, adsorbing species on its surface, providing exchange of interlayer ions, and allowing reaction of redox active species. Corroding stainless-steel canisters in a concrete and steel radioactive waste repository would offer geochemical conditions for GR formation. We used surface-sensitive and high resolution techniques (atomic force microscopy, AFM, transmission electron microscopy, TEM, X-ray photoelectron spectroscopy, XPS) to supplement data from traditional methods (X-ray diffraction, XRD, and wet chemistry). The purpose was to refine structural and compositional parameters for green rust sulfate; to define trace component uptake mechanisms; and to assess potential mobility of GR colloids and thus, sorbed radionuclides. Green rust reduced dissolved Np(V), Cr(VI) and Se(VI), rapidly decreasing solution concentration. High resolution TEM and AFM images showed that chromate penetrates GR interlayers to a distance of about 100 nm from crystal edges. It reduces to Cr(III), blocking further movement and GR transforms topotactically to Cr- goethite, thus immobilising the contaminant in a phase significantly less soluble than pure goethite. Further oxidation results in dissolution of GR and growth of more Cr-goethite. In-situ AFM imaging showed that GR can nucleate and grow both in solution and on minerals typical of fractures in granite, i.e. graphite, muscovite, biotite, quartz and amorphous silica. Particles are more likely to stick to each other or to a substrate than to remain monodispersed.
The current view of charging of oxide electrolyte interfaces is direct protonation and/or deproto... more The current view of charging of oxide electrolyte interfaces is direct protonation and/or deprotonation of surface functional groups. We have performed various experiments to study the charging of several crystal planes of sapphire. All measurements were performed carefully to exclude impurities both inorganic (carbonate, silicate lowering the isoelectric point) and organic (organic films giving rise to a hydrophobic surface). Streaming potential/current measurements were performed at several crystal planes of sapphire (a-Al2O3) using the Microslit Electrokinetic Set-up. Zeta potentials (pertaining to a plane of small but unknown separation from the surface plane) were derived from those measurements using the Smoluchowski equation. In agreement with previous reports in the literature, isoelectric points around pH 4 were obtained. We compare those results to zeta potentials obtained for interfaces between aqueous electrolyte solutions and hydrophobic surfaces and find agreement between the data. Both the isoelectric points and the absolute values of the zeta potentials as a function of pH at a given ionic strength coincide. Surface potential measurements were also performed at the crystal planes using the Single-Crystal-Electrode approach. Surface potentials (pertaining to the surface plane) are nearly zero over a wide pH range. For the 001 plane of sapphire, this can be explained in terms of surface functional groups on this plane. High zeta potentials are incompatible with this interpretation. Consequently, the zeta potentials must arise from another charge building mechanism. Phenomenologically, we explain the observations by an (on average) oriented film of water molecules at the oxide surface, which will cause a similar preference for hydroxide adsorption as is the case for hydrophobic surfaces. Data on other crystal planes of sapphire show similar behaviour. Our interpretation of the experimental data would be a strong challenge for the current view of oxidic surfaces, where the surface charge and potential in electrolyte solutions is entirely attributed to direct (chemical) adsorption or desorption of protons (or hydroxide ions) to surface functional groups.
Cladding tubes of water-cooled nuclear reactors are usually made of Zircaloy and are an important... more Cladding tubes of water-cooled nuclear reactors are usually made of Zircaloy and are an important retaining element for radionuclides present in the fuel both during predisposal activities such as reloading of fuel assemblies from interim storage casks to final disposal casks and during final disposal in the case of canister breaching. However, cladding integrity is affected by various processes during reactor operation and beyond, e.g. fuel cladding chemical interaction and fission product precipitation onto the inner cladding surface. Using experimental and modelling methods, the radionuclide inventory of an irradiated Zircaloy-4 plenum section is analyzed. Quantities of 235/238 U, 237 Np, 238/239/240/241/242 Pu, 241/243 Am, 243/244 Cm besides 14 C, 55 Fe, 125 Sb, 154 Eu, and 134/137 Cs were (radio-)chemically determined in digested Zircaloy-4 subsamples. Measured inventories of activation products in the Zr-alloy are in good agreement with calculated values. However, amounts of actinides and fission products exceed the calculated inventory by factor ∼57 (minor actinides and non-volatile fission products) and ∼114 ( 137 Cs). Excess of minor actinides and part of enhanced Cs inventory originate from fuel residues deposited on the inner cladding surface during fuel rod fabrication, whereas vast amount of cesium is volatilized from subjacent fuel pellets and transported to the plenum.
ABSTRACT Green rust (GR) forms where pH is neutral to basic, iron concentration is high and oxida... more ABSTRACT Green rust (GR) forms where pH is neutral to basic, iron concentration is high and oxidation potential provides a small amount of Fe(III). GR is best known from metallic iron corrosion but it has also been reported in soil. It typically forms nano-particles, so surface area is high. It has a layered structure and is reactive, adsorbing species on its surface, providing exchange of interlayer ions, and allowing reaction of redox active species. Corroding stainless-steel canisters in a concrete and steel radioactive waste repository would offer geochemical conditions for GR formation. We used surface-sensitive and high resolution techniques (atomic force microscopy, AFM, transmission electron microscopy, TEM, X-ray photoelectron spectroscopy, XPS) to supplement data from traditional methods (X-ray diffraction, XRD, and wet chemistry). The purpose was to refine structural and compositional parameters for green rust sulfate; to define trace component uptake mechanisms; and to assess potential mobility of GR colloids and thus, sorbed radionuclides. Green rust reduced dissolved Np(V), Cr(VI) and Se(VI), rapidly decreasing solution concentration. High resolution TEM and AFM images showed that chromate penetrates GR interlayers to a distance of about 100 nm from crystal edges. It reduces to Cr(III), blocking further movement and GR transforms topotactically to Cr- goethite, thus immobilising the contaminant in a phase significantly less soluble than pure goethite. Further oxidation results in dissolution of GR and growth of more Cr-goethite. In-situ AFM imaging showed that GR can nucleate and grow both in solution and on minerals typical of fractures in granite, i.e. graphite, muscovite, biotite, quartz and amorphous silica. Particles are more likely to stick to each other or to a substrate than to remain monodispersed.
The current view of charging of oxide electrolyte interfaces is direct protonation and/or deproto... more The current view of charging of oxide electrolyte interfaces is direct protonation and/or deprotonation of surface functional groups. We have performed various experiments to study the charging of several crystal planes of sapphire. All measurements were performed carefully to exclude impurities both inorganic (carbonate, silicate lowering the isoelectric point) and organic (organic films giving rise to a hydrophobic surface). Streaming potential/current measurements were performed at several crystal planes of sapphire (a-Al2O3) using the Microslit Electrokinetic Set-up. Zeta potentials (pertaining to a plane of small but unknown separation from the surface plane) were derived from those measurements using the Smoluchowski equation. In agreement with previous reports in the literature, isoelectric points around pH 4 were obtained. We compare those results to zeta potentials obtained for interfaces between aqueous electrolyte solutions and hydrophobic surfaces and find agreement between the data. Both the isoelectric points and the absolute values of the zeta potentials as a function of pH at a given ionic strength coincide. Surface potential measurements were also performed at the crystal planes using the Single-Crystal-Electrode approach. Surface potentials (pertaining to the surface plane) are nearly zero over a wide pH range. For the 001 plane of sapphire, this can be explained in terms of surface functional groups on this plane. High zeta potentials are incompatible with this interpretation. Consequently, the zeta potentials must arise from another charge building mechanism. Phenomenologically, we explain the observations by an (on average) oriented film of water molecules at the oxide surface, which will cause a similar preference for hydroxide adsorption as is the case for hydrophobic surfaces. Data on other crystal planes of sapphire show similar behaviour. Our interpretation of the experimental data would be a strong challenge for the current view of oxidic surfaces, where the surface charge and potential in electrolyte solutions is entirely attributed to direct (chemical) adsorption or desorption of protons (or hydroxide ions) to surface functional groups.
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Papers by Dieter Schild