Yoshie Akai

Corrosion Performance of Metals for Supercritical Water, Oxidation-Utilized Organic Waste-Processing Reactors

Abstract Corrosion tests were conducted to select the structural material for a reactor to decompose organic wastes generated from nuclear power plants, utilizing the supercritical water oxidation (SCWO) process. Test conditions were based on the decomposition of chloroprene (C4H5Cl) and cation exchange resin, which generate hydrochloric acid (HCl) and sulfuric acid (H2SO4) in process fluids. In order to select candidate materials, short-period screening tests were carried out on various corrosion-resistant materials in HCl and H2SO4 solutions, using static test vessels at 300°C, 400°C, and 450°C. Subsequently, cyclic corrosion tests were carried out for Ta, Ti, and Ti alloys to estimate the corrosion rate and to evaluate their applicability for waste-processing plants. The results of these tests indicate that the corrosion resistance of Ti alloys is sufficient for them to be applied as reactor materials for organic waste decomposition systems using the SCWO process. In this paper, the stability of surf a...

Chemical thermodynamics consideration on corrosion products in supercritical-water-cooled reactor coolant

In order to elucidate the dissolution/deposition behavior of corrosion products in supercritical-water–cooled reactor (SCWR) coolant, chemical thermodynamics calculation was carried out, taking into account the electrochemical properties of supercritical water. The review of thermodynamics models revealed the applicability of the modified HKF model for ionic species in the supercritical region. Using the calculation results, potential-pH diagrams were drawn for metal/water systems, which exhibited the effect of temperature and pressure on the stability of oxide in the SCWR core region.

Ion-exchange resin decomposition in supercritical water

Abstract Using supercritical water oxidation, the cation exchange resin was decomposed fast and completely to water, carbon dioxide and sulfuric acid. While the resin decomposition yield increased with the reaction time and the amount of hydrogen peroxide added as oxidizing agent, it was constant in the resin concentration from 0.14 to 1.9 dry resin weight percent to water. More than 99% of the cation exchange resin was decomposed with hydrogen peroxide added in the amount of 7 times the stoichiometric value at 673 K and 30MPa for 30 minutes of the reaction time. The cation exchange resin is decomposed through two main reaction pathways. One has a rate controlling intermediate such as acetic acid whose decomposition rate was very slow, and the other does not have stable intermediates. The decomposition of the acetic acid is a significant factor for the complete decomposition of the resin, although it does not dominate the whole resin decomposition. A simple kinetic model that estimates the resin decomposition yield was developed.

Development of transuranium elements recovery from high-level radioactive liquid waste

The Transuranium Elements (TRUs) recovery process from High-Level Liquid Waste (HLLW) at a Purex reprocessing plant requires both simple processes and a high yield of TRUs recovery. This paper outlines a new TRUs recovery process which combines an aqueous process and a pyrochemical process. In the first step, TRUs and Rare Earth elements (REs) are precipitated from HLLW to separate alkali metal elements because the HLLW is an aqueous solution. It is difficult for TRUs to be separated simply from REs in the aqueous solution. A pyrochemical process is more effective to separate TRUs than an aqueous one. TRUs are separated from REs with an electrorefining process in molten salt in the final step. A tracer of Am was experimentally separated from a simulated HLLW at more than 99.5% yield in the oxalate precipitation. After the simulated HLLW was treated with the oxalate precipitation process, the precipitates were converted to their chlorides with hydrochloric acid and hydrogen peroxide below 100°C in 99.4% yield. Finally, Ce used to simulate TRUs was deposited on the cathode in order to study the separation from La used for REs with the electrorefining process in molten salt. The separation factor of TRUs from REs is estimated to be about 70.

Liquid Scintillation Counter Cocktail Decomposition in Supercritical Water

Using supercritical water oxidation, a liquid scintillation counter cocktail used for the analysis of radionuclides was decomposed quickly and completely to water and carbon dioxide. The decomposition yield of the liquid scintillation counter cocktail increased with the reaction time, temperature, and pressure. More than 99.96% of the liquid scintillation counter cocktail decomposed with hydrogen peroxide at 773 K and 30MPa for 3.8 min of reaction time. The liquid scintillation counter cocktail decomposed through two main reaction pathways: One had a rate-controlling intermediate whose decomposition rate was very slow and the other did not have stable intermediates. The decomposition of the rate-controlling intermediate was a significant factor in the complete decomposition of the cocktail although it did not dominate the entire decomposition process. A simple kinetic model that estimates the decomposition yield of the liquid scintillation counter cocktail was developed.