César Maffiotte

Surface reactions kinetics between nanocrystalline magnetite and uranyl

Magnetite is the most important end member of iron corrosion products under reducing environment, which is the condition expected in a deep geological high level radioactive waste disposal. Nanocrystalline magnetite was synthesized in the laboratory and its physicochemical properties were analyzed in detail. The kinetics of the adsorption of U(VI) and the kinetics of the actinide reduction to a lower oxidation state, in presence of the oxide, were studied by means of batch sorption techniques and X-ray photoelectron spectroscopy (XPS) analysis. The results showed that the uranium sorption and reduction processes on the magnetite surface have very fast kinetics (hours), the reduction process being triggered by sorption. XPS measurements showed that the speciation of uranium at the surface does not show significant changes with time (from 1 day to 3 months), as well as the quantity of uranium detected at the surface. The surface speciation depended on the initial pH of the contact solution. Considering that the Eh of equilibrium between magnetite and the solution, under our experimental conditions, is slightly positive (50-100 mV), the uranium reduction would also be thermodynamically possible within the liquid phase. However, the kinetics of reduction in the liquid occur at a much slower rate which, in turn, has to depend on the attainment of the magnetite/solution equilibrium. The decrease of uranium in solution, observed after the uranyl adsorption stage, and particularly at acidic pH, is most probably due to the precipitation of U(IV) formed in the solution.

Experimental and modeling study of the uranium (VI) sorption on goethite

Acicular goethite was synthesized in the laboratory and its main physicochemical properties (composition, microstructure, surface area, and surface charge) were analyzed as a previous step to sorption experiments. The stability of the oxide, under the conditions used in sorption studies, was also investigated. The sorption of U(VI) onto goethite was studied under O(2)- and CO(2)-free atmosphere and in a wide range of experimental conditions (pH, ionic strength, radionuclide, and solid concentration), in order to assess the validity of different surface complexation models available for the interpretation of sorption data. Three different models were used to fit the experimental data. The first two models were based on the diffuse double layer concept. The first one (Model 1) considered two different monodentate complexes with the goethite surface and the second (Model 2) a single binuclear bidentate complex. A nonelectrostatic (NE) approach was used as a third model and, in that case, the same species considered in Model 1 were used. The results showed that all the models are able to describe the sorption behavior fairly well as a function of pH, electrolyte concentration, and U(VI) concentration. However, Model 2 fails in the description of the uranium sorption behavior as a function of the sorbent concentration. This demonstrates the importance of checking the validity of any surface complexation model under the widest possible range of experimental conditions.

Suppression of hydrogenated carbon film deposition by scavenger techniques and their application to the tritium inventory control of fusion devices

The well-known radical and ion scavenger techniques of application in amorphous hydrogenated carbon film deposition studies are investigated in relation to the mechanism of tritium and deuterium co-deposition in carbon-dominated fusion devices. A particularly successful scheme results from the injection of nitrogen into methane/hydrogen plasmas for conditions close to those prevailing in the divertor region of present fusion devices. A complete suppression of the a-C : H film deposition has been achieved for N2/CH4 ratios close to one in methane (5%)/hydrogen DC plasma. The implications of these findings in the tritium retention control in future fusion reactors are addressed.

Microstructure Study of NiCrAlY and FeCrAlY Exposed to Superheated Steam at 800 °C

Supercritical water-cooled reactor (SCWR) was chosen as Generation IV reactor concept in Canada to utilize Canada’s expertise and technical capabilities from past research and designs. The conceptual design of Canadian SCWR has a core outlet temperature of 650 C at 25 MPa and a peak cladding temperature as high as 800 C. Corrosion/oxidation resistance is an important factor in material selections and also coating considerations. Most of the reported supercritical water (SCW) test data have been obtained at temperatures up to 700 C as no autoclave exists that can operate above 700 C at supercritical pressures and temperatures. Superheated steam (SHS) is used as a surrogate fluid to SCW in this study to evaluate two coating materials, FeCrAlY and NiCrAl, at 800 C. The results showed that the FeCrAlY became discolored rapidly while NiCrAl still maintained some metallic sheen after 600 h. The weight change results suggest that more oxide formation took place on FeCrAlY than NiCrAl. In particular, grain boundary oxide (Al2O3) formed on FeCrAlY surface upon exposure to steam after 300 h. Further exposure caused more intragranular Al2O3 to form, in addition to magnetite formation on the grain boundary regions. For NiCrAl samples, NiO formed after steam exposure for 300 h. Spinel and (Cr,Al)2O3 were also found after 300 h along with very limited amount of Al2O3. After 600 h, Al2O3 became well developed on NiCrAl and the coverage of spinel and Cr2O3 on the surface reduced.

INFLUENCE OF CHANGES IN PRESSURE AND TEMPERATURE OF SUPERCRITICAL WATER ON THE SUSCEPTIBILITY TO STRESS CORROSION CRACKING OF 316L AUSTENITIC STAINLESS STEEL

The Supercritical Water Reactor (SCWR) is one of the Generation IV designs. The SCWR is characterized by its high efficiency, low waste production and simple design. Despite the good properties of supercritical water as coolant, its physicochemical properties change sharply with pressure and temperature in the supercritical region. For this reason, there are many doubts about how changes in these variables affect the behavior of the materials to some degradation processes like Stress Corrosion Cracking (SCC). Austenitic stainless steels are candidate materials to build the SCWR due to their good behavior in the Light Water Reactors (LWR). Nevertheless, their behavior under the SCWR conditions is not well known. In this work, an austenitic stainless steel 316 type L was tested in deaerated supercritical water at 673 K/25 MPa and 30 MPa and 773 K/25 MPa to determine how variations in properties of water influence its behavior to SCC and to make progress in the understanding of mechanisms involved in SCC processes in this environment. In addition to this, a selected oxide layer formed at 673 K/30 MPa/lt; 10 ppb O2 was analyzed to gain some insight into these processes.