Tafel-analysis of the AP-CITROX decontamination technology of Inconel alloy 690

Safe, effective, cost-effective, easy feasible and low-waste decontamination technologies are fundamental importance from environmental and radiation protection aspects. In this study the effectiveness of AP-CITROX decontamination technology of Inconel alloy 690 was investigated. Non-radioactive representative metal samples were formed to test of decontamination technology and the clear-, the corroded-, the decontaminated layer were analysed electrochemically. The results indicate that the passivation step of the technology was not completed.

Alteration of vitrified intermediate level nuclear waste in alkaline media: effects of cementitious materials, pH and temperature

Alteration experiments involving intermediate level nuclear waste (ILW) glass in contact with hardened cement paste (HCP) were performed to assess its behavior under simulated repository conditions. Batch experiments were conducted at 20 °C and 50 °C in several artificial cement pore water (ACW) samples (pH from 10 to 13), in the presence of HCP (CEM-I, CEM-V and low pH), with a ratio of glass surface to volume of solution of 8000 m⁻¹ and a ratio of mass of HCP to volume of solution of 10 g L⁻¹. Glass alteration rates increase up to ∼4 × 10⁻² g m⁻² d⁻¹ with pH in contact with HCP, notably with CEM-I. This value decreases by 2 orders of magnitude in low pH cement solution and also for residual alteration rates. The effect of calcium on glass alteration was observed, mainly in Ca(OH)₂ saturated solution, with an incubation effect on the release of Si in solution. Experimental data were successfully modeled with the PhreeqC geochemical code. Glass and HCP samples were characterized via SEM/EDX and micro-Raman studies. This work showed that vitrified glass exhibits good performance in terms of low alteration rates (∼10⁻⁴ g m⁻² d⁻¹), the absence of secondary phases, and the formation of a gel layer at the surface, when in contact with low pH conditions (in the presence or absence of low pH HCP).

Alteration experiments involving intermediate level nuclear waste (ILW) glass in contact with hardened cement paste (HCP) were performed to assess its behavior under simulated repository conditions.

Vapor hydration of a simulated borosilicate nuclear waste glass in unsaturated conditions at 50 °C and 90 °C

Vapor hydration of a simulated typical French nuclear intermediate-level waste (ILW) glass in unsaturated conditions has been studied in order to simulate its behaviour under repository conditions before complete saturation of the disposal site.

Assignment of Raman-active vibrational modes of tetragonal mackinawite: Raman investigations and ab initio calculations

We report on the first assignment of the Raman-active vibrational modes of mackinawite using Density Functional Perturbation Theory and direct methods with BLYP + dispersion correction. Based on experimental data and calculation results, the Raman bands were assigned as 236 cm⁻¹ (B_1g), 256 cm⁻¹ (E_g), 376 cm⁻¹ (A_1g) and 395 cm⁻¹ (E_g).

Hydrotalcite Formed by Alteration of R7T7 Nuclear Waste Glass and Basaltic Glass in Salt Brine at 190°C

The R7T7 and synthetic basaltic glasses were submitted to corrosion in a saline MgCl2dominated solution at 190°C. For both glasses, the early alteration product is a hydrotalcite-like compound in which HPO42-, SO4-2and Cl-substitutes to CO32. The measured d003spacing is 7.68 Å for the hydrotalcite formed from R7T7 glass and 7.62 Å for the hydrotalcite formed from basaltic glass which reflect the high aluminium content. Chemical microanalyses show that the hydrotalcite is subsequently covered by a silica-rich gel which evolves into saponite after few months.

Vitrification of High-Level Radioactive Waste by Sintering Under Pressure

Sintered silicate glass waste forms with a continuous glass phase and embedded but not completely dissolved waste particles were prepared by powder technology. We used a typical defense waste compositions, containing 38 wt.% of ZrO2 and prepared sinter glass samples with 30, 35, and 40 wt.% waste loading at 830°C and 27.6 MPa, by hot isostatic pressing. The glass former was amorphous silica powder. Simulated waste was added as calcine. Samples were characterized using scanning and transmission electron microscopy. The results show that extensive sintering took place, particularly with the 40 wt.% samples. Waste components, such as Na2O, CaO, MnO2, La203, Fe2O3, NiO, Cr2O3, and P2O5, dissolved completely in the continuous glass phase. ZrO2 was partly dissolved and occurred as aggregates of tiny baddeleyite crystals. The porosity of the samples was 2-3%, indicating that the optimum sintering conditions have not been met to reach porosities <1%.

Chemical durability tests were carried out with sintered glass with 35 wt.% waste loading in deionized water at 90°C under static conditions, showing that the glass is at least as chemically durable as a borosilicate glass with 31 wt.% melted at 1350°C.

Precipitation of technetium by subsurface sulfate-reducing bacteria

To study the interaction between Tc and subsurface bacteria, we conducted batch experiments with soil and groundwater or sterilized deionized water. The system water/soil was amended with lactate and phosphate for bacterial growth. Nitrate and sulfate were added to stimulate the growth of indigenous denitrifying and sulfate-reducing bacteria. During denitrification Tc-concentration did not change with time. In the presence of sulfate-reducing bacteria, Tc-concentrations decreased in reacted waters which could be attributed to Tc(VII) reduction and precipitation of TcO2and/or TcS2. Coprecipitation with newly formed iron sulfide is expected to contribute to Tc removal. Additional experiments with U and Tc showed that these elements were simultaneously reduced by sulfate-reducing bacteria. This work shows that 1) subsurface mixed cultures of denitrifying bacteria do not remove Tc from solution, this is different from uranium and 2) sulfate-reducing bacteria reduce and remove Tc from aqueous solutions and thus in situ bioremediation of subsurface waters and soils may be possible with such ubiquitous bacteria.

Discrepancies in thorium oxide solubility values: a new experimental approach to improve understanding of oxide surface at solid/solution interface

The solubility of ThO2(cr) was studied since many years but a large discrepancy in solubility values is noticed in the literature. The present work suggests that this discrepancy is related to differences in the surface properties of thorium oxide.

To understand the role of surface properties on solubility values, we conducted experiments with ThO2(cr) spheres with reproducable surface properties. Batch dissolution experiments were conducted in 0.01 M NaCl solution at pH=3.0 and 4.0 for periods of time up to 270 days. The solutions were spiked with229Th to determine precipitation (sorption) rates of thorium, while dissolution rates were determined by measuring232Th released from ThO2(cr) spheres. We assume that229Th atoms are exchanged only with active sites involved in Th-dissolution. Using229Th as local sensor of attachement and detachment processes at the ThO2(cr) surface under close-to-equilibrium conditions, allows to assess surface reactivity of the solid during solubility experiments.

Oxidation and dissolution rates of UO2(s) in carbonate-rich solutions under external alpha irradiation and initially reducing conditions

This paper describes the effects of aqueous carbonate concentrations on the UO2 oxidation and dissolution rates under an alpha beam irradiation in a cyclotron. As solid samples, UO2 colloids were synthesized by nano-particle precipitation. The specific surface area obtained is 85.3 m2/g. All aqueous solutions were buffered in the pH range 8<pH<9.

The observation of particles under TEM and X-ray diffraction showed well-crystallized UO2. Molecular radiolysis products such as H2O2 and carbonate degradation products where analysed. Solution analysis of dissolved uranium and of radiolytically generated H2O2 allowed to deduce dissolution rates of UO2 as well as H2O2 consumption rates (deduced from the H2O2 concentrations in presence of UO2 colloids and those in irradiated blank solutions). In absence of carbonate, the dissolution rates of UO2 are much lower than the H2O2 consumption rates. The difference is explained by the oxidation of uranium(IV) at the UO2/solution interface (e.g. formation of U3O7). In contrast, in the presence of carbonate (up to 4×10-2 mol/L), the dissolution rates of UO2 are similar to H2O2 consumption rates. In this case the oxidized uranium is complexed with carbonate ions resulting in highly soluble species as indicated by the geochemical model PHREEQC calculations [UO2(CO3)3 4 -]. The high carbonate concentration did not affect the H2O2 formation/consumption rates. Radiolytic decomposition of carbonate ions and formation of organic radiolytic degradation products were negligible.

The results indicate that under the present experimental conditions, UO2 oxidation by H2O2 and UO2 dissolution are connected as a sequential reaction: for a given H2O2 concentration, UO2 oxidation occurs at a constant rate. At low carbonate concentration, the dissolution of oxidized UO2 is slower than the oxidation rate (hence oxidation is not rate limiting), while at high carbonate concentrations all oxidized UO2 becomes immediately dissolved and UO2 oxidation becomes rate limiting for UO2 dissolution. This mechanism is in agreement with that proposed in the Matrix Alteration Model (MAM).

Microbial reduction of 99Tc in organic matter-rich soils

For safety assessment purposes, it is necessary to study the mobility of long-lived radionuclides in the geosphere and the biosphere. Within this framework, we studied the behaviour of (99)Tc in biologically active organic matter-rich soils. To simulate the redox conditions in soils, we stimulated the growth of aerobic and facultative denitrifying and anaerobic sulphate-reducing bacteria (SRB). In the presence of either a pure culture of denitrifiers (Pseudomonas aeruginosa) or a consortium of soil denitrifiers, the solubility of TcO(4)(-) was not affected. The nonsorption of TcO(4)(-) onto bacteria was confirmed in biosorption experiments with washed cells of P. aeruginosa regardless of the pH. At the end of denitrification with indigenous denitrifiers in soil/water batch experiments, the redox potential (E(H)) dropped and this was accompanied by an increase of Fe concentration in solution as a result of reduction of less soluble Fe(III) to Fe(II) from the soil particles. It is suggested that this is due to the growth of a consortium of anaerobic bacteria (e.g., Fe-reducing bacteria). The drop in E(H) was accompanied by a strong decrease in Tc concentration as a result of Tc(VII) reduction to Tc(IV). Thermodynamic calculations suggested the precipitation of TcO(2). The stimulation of the growth of indigenous sulphate-reducing bacteria in soil/water systems led to even lower E(H) with final Tc concentration of 10(-8) M. Experiments with glass columns filled with soil reproduced the results obtained with batch cultures. Sequential chemical extraction of precipitated Tc in soils showed that this radionuclide is strongly immobilised within soil particles under anaerobic conditions. More than 90% of Tc is released together with organic matter (60-66%) and Fe-oxyhydroxides (23-31%). The present work shows that ubiquitous indigenous anaerobic bacteria in soils play a major role in Tc immobilisation. In addition, organic matter plays a key role in the stability of the reduced Tc.

Porous bioactive glass and glass-ceramics made by reaction sintering under pressure

A glass and a rhenanite-wollastonite glass-ceramic were synthesized with the qualitative composition Na2O-CaO-SiO2-P2O5. Both materials were prepared by reaction sintering under isostatic pressure (RSIP) using powder mixtures. Solid state reactions were complete within a few hours at 950 degrees C under modest pressure. Formation of the glass and crystalline phases was driven by an intermediate, reactive, low viscosity Na2O-SiO2 phase. A reaction mechanism is suggested. Porous materials were obtained with two ranges of pore sizes: 100-200 microm and < or =5 microm in diameter. The glass and the glass-ceramic were corroded in simulated body fluid at 37 degrees C. The evolution of surface features was studied. Gel layers formed on both materials. Corrosion was fastest inside the pores. Microcrystals of apatite were identified by crystal structure analysis and by chemical analysis. During corrosion of the glass-ceramic, rhenanite most likely was converted into apatite. Comparison of these results with published information suggests that the glass and glass-ceramic are bioactive. We suggest that RSIP can be used (a) to control the surface porosity and pore size of bioactive implants, thereby increasing the stability of tissue/implant interfaces; (b) to make glasses and glass-ceramics with new properties; and (c) to make near net-shape materials.

Biological reduction of uranium in groundwater and subsurface soil

Biological reduction of uranium is one of the techniques currently studied for in situ remediation of groundwater and subsurface soil. We investigated U(VI) reduction in groundwaters and soils of different origin to verify the presence of bacteria capable of U(VI) reduction. The groundwaters originated from mill tailings sites with U concentrations as high as 50 mg/l, and from other sites where uranium is not a contaminant, but was added in the laboratory to reach concentrations up to 11 mg/l. All waters contained nitrate and sulfate. After oxygen and nitrate reduction, U(VI) was reduced by sulfate-reducing bacteria, whose growth was stimulated by ethanol and trimetaphosphate. Uranium precipitated as hydrated uraninite (UO2 x xH2O). In the course of reduction of U(VI), Mn(IV) and Fe(III) from the soil were reduced as well. During uraninite precipitation a comparatively large mass of iron sulfides formed and served as a redox buffer. If the excess of iron sulfide is large enough, uraninite will not be oxidized by oxygenated groundwater. We show that bacteria capable of reducing U(VI) to U(IV) are ubiquitous in nature. The uranium reducers are primarily sulfate reducers and are stimulated by adding nutrients to the groundwater.