Radioactive Waste Disposal In Japan Research Articles

Low-Level Radioactive Waste Disposal in Japan and Role of Cementitious Materials

Radioactive waste disposal is a very important issue when constructing a sustainable society with nuclear energy. To construct disposal facilities, cementitious materials are used for various purposes, such as structural elements, storage cells, tunnel plugs and engineered barriers. This technical report describes the history and current status of the disposal of low-level radioactive waste (LLW) in Japan, particularly near surface disposal of relatively low-level LLW (L2-LLW) and intermediate depth disposal of relatively high-level LLW (L1-LLW) as two major types of LLW. The classification of radioactive wastes and disposal methods and concept of design and long-term dose assessment are out-lined to demonstrate the importance of cementitious materials in nuclear waste management.

Corrosion resistance of a cast steel overpack for high‐level radioactive waste disposal in Japan

AbstractThe corrosion rate and stress corrosion cracking (SCC) susceptibility of a cast steel are studied for its application to metal containers (namely overpacks) for geological disposal of high‐level radioactive waste. Specimens for corrosion tests are cut from a prototype overpack manufactured by full‐scale casting. Casting defects are widely distributed in the prototype overpack; however, the flat‐bottom hole equivalent diameters for all defects detected by an ultrasonic test are 3.6 mm or less, which is relatively small. Forged steels and rolled steels are also tested for comparison of their corrosion properties with the cast steel. The corrosion rates are obtained by immersion tests in bentonite saturated with synthetic seawater under anaerobic conditions at 80°C for up to 1 year. The corrosion rate for the cast steel calculated by the weight loss during the experiments is close to that for the forged steels and rolled steels. The SCC susceptibility is examined using slow strain rate tests in a 1.5‐mol L−1 carbonate–bicarbonate solution, in which the occurrence of high‐pH SCC is often reported for carbon steels. The SCC susceptibility increased with the increase in the carbon content of the products; however, there are no clear differences between casting and forging.

Re-evaluation of the required thickness of the carbon steel overpack for high-level radioactive waste disposal in Japan based on the latest scientific and engineering knowledge

ABSTRACTThe required thickness of the carbon steel overpack for high-level radioactive waste geological disposal in Japan was re-evaluated based on the latest scientific and engineering knowledge. The re-evaluation of the required thickness was carried out for the concept of vertical emplacement in a hard rock system (which requires the greatest overpack thickness). The re-evaluation was carried out to meet design requirements for corrosion resistance, structural integrity and the prevention/reduction of detrimental radiation effects on corrosion resistance. The re-evaluated minimum thicknesses to assure a lifetime greater than 1000 years were 122 mm for the flat end plate and 91 mm for the cylindrical body. Therefore, the applicability of the reference thickness (190 mm) for the overpack was reconfirmed.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Diffusion Behavior of Organic Carbon and Iodine in Low-heat Portland Cement Containing Fly Ash

AbstractThe diffusion of radionuclides in cementitious materials used as an engineered barrier is an important parameter in the performance assessment of the sub-surface repository system used for low-level radioactive waste disposal in Japan. In particular, organic carbon-14 and iodine-129 would provide large contributions to the dose evaluation, because of their low ability to be adsorbed on cementitious materials. In this study, the diffusion of acetate and iodide in hardened cement pastes was examined by through-diffusion experiments. Low-heat Portland cement containing 30 wt% fly ash (FAC), which is a candidate cement material for the construction of the sub-surface repository, was prepared for the diffusion experiments. The effective diffusion coefficients, De, of the trace ions for hardened FAC cement pastes were estimated to be on the order of 10-13 m2 s-1 at the beginning of the diffusion experiments. Then, the rate of diffusion of the trace ions decreased over the experimental period of 1-15 months. This is probably due to the change in the microstructure of the FAC as the result of a pozzolanic reaction. After a few months, the values of De were estimated to be on the order of 10-14 m2 s-1. These results suggest that an engineered barrier made of FAC can act as an effective barrier inhibiting the diffusion of trace ions such as organic carbon and iodine.

Current Status of High Level Radioactive Waste Disposal in Japan and Foreign Countries

Current Status of High Level Radioactive Waste Disposal in Japan and Foreign Countries

Plutonium Determination in Compacted Bentonite by Using PERALS

The determination of plutonium activities at low levels was occurred during study on diffusion of plutonium onto compacted bentonite, a candidate buffer material of multi-barrier system suggested for high-level radioactive waste disposal in Japan. It was performed by using photo/electron-rejecting alpha liquid scintillation (PERALS) spectrometry, model 8100AB (ORDELA INC.), followed by plutonium extraction from slices of the bentonite by hydrochloric acid (HC1). Pulse shape spectrum was chosen because of a high counting efficiency and only one alpha emitter existing. For an accurate determination, the optimum pulse shaped discrimination (PSD) setting was obtained. Effects resulted from HC1 concentration used for plutonium extraction and sample-loading (SL) were also investigated by PERALS spectrometry from the view of its application. The results showed that peak region in pulse shape spectrum shifted with changes of the HC1 concentrations and sample-loading. Pulse shape spectrum of background blank (liquid scintillator, Ultima Gold AB) showed an obvious peak located on the left side of alpha region. This peak influenced seriously plutonium counting by means of peak overlap when total plutonium activity in sample was less than several Bq. The low limit of detective concentration was estimated to be 0.79 Bq/g-bentonite.

自然から学ぶ放射性廃棄物処分の知恵

"Natural analogues" can be defined as the processes or materials analogous to those operating in the geological disposal system of radioactive waste. Natural analogue studies provide the only means by which long-term data can be obtained under the real natural conditions, and also the most convincing support to the long-term performance assessment of the geological disposal system.The framework of our natural analogue studies concerning the stability of the engineered barrier materials for geological disposal system of high-level radioactive waste is reviewed. One of the results is that the volcanic glass included in a clay bed did not alter during the past one million years.The Tono Uranium Deposits are studied as geochemical analogues of radioactive waste disposal in Japan. We conclude that although the deposits have been subjected to a variety of geological processes and events such as faulting, erosion and uplift/subsidence, the reducing condition has been maintained and uranium has not migrated for at least the past ten million years.Application and further development of the natural analogue studies are also discussed.

Overview of the stability and barrier functions of the granitic geosphere at the Kamaishi Mine: relevance to radioactive waste disposal in Japan

The Kamaishi in situ studies were carried out in order to provide a technical basis for the studies of the geological disposal of High-level Radioactive Waste (HLW) in Japan. The studies were conducted in drifts located at ca. 300m and ca. 700m below the surface in the former Kamaishi iron ore mine, situated in the Kurihashi granodiorite, an early Cretaceous granitic rock (110∼120Ma) in the Kitakami area, Iwate Prefecture, northeastern Japan. The fractures identified in the Kurihashi granodiorite were mainly formed during the early stages of the cooling process. The mineralogical and dating data suggest that almost no new fractures have been generated since 70∼60Ma ago. The fractures have high-porosity fracture fillings with relatively high sorption values that imply the fractured granitic rock could have a significant retardation capability for contaminant transport through the granite. Monitoring of hydrological and geochemical conditions of groundwater for 10years showed that even fractured granitic rock close to the drift is not thoroughly oxidized, and there are only negligible changes of hydrological conditions and groundwater geochemistry due to earthquakes. This information is of considerable importance for the practical design of a HLW repository and to assess any impact of the pre-closure conditions on the post-closure performance of the geological repository. Finally, the results of the study methodology and tools developed help increase confidence in the technical feasibility of geological disposal of HLW in Japan.

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

ABSTRACTCarbon 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.

Geologic problems of radioactive waste disposal in Japan

Geologic problems of radioactive waste disposal in Japan