Hirohisa Ishikawa

Effect of Compacted Bentonite on the Corrosion behavior of Carbon Steel as Geological Isolation Overpack Material

The corrosion rate of carbon steel in compacted bentonite was evaluated with regard to the test period length, temperature, chemicals of solution and bulk density of compacted bentonite. The average corrosion rate decreased gradually with increasing test period up to 180 days in immersion tests. The corrosion rate of carbon steel in compacted bentonite at a dry density of 1.32g/cm 3 was estimated to be about 0.01 mm/y which was one order of magnitude lower than that in bentonite slurry. No significant influence of temperature on corrosion rates was observed in compacted bentonite in the range of 50∼180 °C. Variation of kinds and concentration of anion(chloride, floride, sulfate, and carbonate)in aqueous solution did not have much influence on the corrosion rate of carbon steel. Immersion tests of carbon steel in compacted bentonite at a dry density of 0.69 ∼ 1.32 g/cm 3 , which was mixed with an aqueous solution(synthetic sea water and distilled water), were carried out. The corrosion rate in compacted bentonite decreased from 0.04 to 0.005mm/y as the density of bentonite increased. This result suggests that the corrosion rate of carbon steel in compacted bentonite is governed by the diffusivity of corrosive materials. In general, oxygen is the dominate factor affecting corrosion rate, therefore prediction of the average corrosion rate of carbon steel was carried out on the basis of the diffusion behavior of dissolved oxygen in compacted bentonite. The prediction agreed with experimental results.

Experimental studies on the interaction of groundwater with bentonite

Interactions of sodium bentonite with distilled water and two types of synthetic groundwater were studied by batch experiments. In the experiments, clay and pure minerals were reacted at room temperature under aerobic and anaerobic condition as a function of time and liquid/solid ratio. The clay and pure minerals used in the experiments were Kunigel-V1 (crude Na-bentonite), Kunipia F (purified Na-bentonite), purified Na-smectite (purified from Kunipia F), calcite and pyrite as accessory minerals. The chemical composition in the liquid phase was analyzed through centrifugation and ultrafiltration. Alteration of the distribution of exchangeable cation in the bentonite was analyzed by NH{sub 4}Ac and XRD. The results indicated that the interaction between bentonite (Kunigel-V1) and groundwater under aerobic condition was described by ion exchange reaction of smectite, dissolution of calcite and oxidation of pyrite. From these experimental studies, the model of the interaction of groundwater with bentonite proposed by Wanner was modified. The comparison between calculation and experimental results showed good agreement and indicated that this model could be adopted to predict porewater chemistry of bentonite for performance assessment of geological isolation system of high level waste.

Experimental Investigation of Passivation Behavior and Corrosion Rate of Carbon Steel in Compacted Bentonite

Carbon steel is one of the candidate materials for overpacks for high-level radioactive waste disposal in Japan. Passivation behavior and corrosion rate of carbon steel were investigated by electrochemical measurements under simulated repository conditions. The results of the anodic polarization measurements showed that carbon steel was hard to passivate in highly compacted bentonite. Therefore, general corrosion seems to be most probable in repository conditions. In order to monitor the in-situ general corrosion rate in compacted bentonite, the AC impedance of carbon steel was measured under aerated conditions. It was confirmed that the corrosion rate in saturated bentonite decreased with time and it was almost the same as that observed in deaerated aqueous conditions. The corrosion rate did not increase in the presence of corrosion products formed by external current supply.

A modeling study of general corrosion of copper overpack for geological isolation of high-level radioactive waste

This paper describes a modeling study for general corrosion of copper which is a candidate material for high-level radioactive waste overpacks. The model is a mixed-potential model combined with diffusive transport of reactants and reaction products. The rest potential and corrosion rate of copper in aerated solution were measured while controlling the thickness of a diffusive solution layer on the copper surface using a rotating-disk electrode. Experimental data were used for validation of the model.

Studies on material properties and structural mechanics of engineered barriers for HLW geological isolation

Abstract A massive engineered barrier system (EBS) composed of vitrified waste, carbon steel overpack and buffer material (compacted sodium-bentonite) can be expected to isolate hazardous radionuclides from the human environment. Corrosion, leaching and migration studies of EBS materials have supported the performance assessment of the system. Natural analogue is expected to be a method for the validation of the long-term durability of EBS materials. Geochemistry study of groundwater evolution in EBS supports the site generic performance assessment. Coupled thermo-hydro-mechanical process, mechanical stability and hydrogen gas behavior in EBS are also research items for more realistic evaluation of the EBS.

EFFECTS OF DISSOLVED OXYGEN CONTENT ON THE PROPAGATION OF LOCALIZED CORROSION OF CARBON STEEL IN SYNTHETIC SEA WATER

ABSTRACT Effects of dissolved oxygen content on the propagation of localized corrosion of carbon steel have been investigated both by immersion type corroion tests and electrochemical tests in synthetic sea water, which simulated the ground water environment in deep geological formations. Although the propagation rates of localized corrosion were reduced with the decrease of dissolved oxygen content, slight localized corrosion was recognized even in deaerated condition by blowing pure N2 gas.