The Japan Atomic Energy Agency decontaminated schools, playgrounds, swimming pools, and houses in nonevacuated, less-contaminated areas in Fukushima for environmental restoration. A small, 150 m2 playground lot in the residential area was chosen for decontamination demonstration, which used routinely available tools and commodities to carry out the work. The surfaces of playground lot equipment, such as swings, slides, and horizontal iron bars, were completely decontaminated by brushing with water and/or detergent. Side gutters around the playground lot were cleaned by removing the mud and then brushed and washed with a high-pressure water jet (7 MPa). The air dose rate at the playground lot was dominated by radiation from the ground surface and adjacent surroundings, such as apartments and rice fields. Two or three centimeters of the surface soil contaminated with cesium was removed manually with shovels, hoes, and other gardening tools. This significantly reduced the average air dose rate of the entire playground lot from 1.5 μSv/h before decontamination to 0.6 μSv/h. These results showed that ground surface decontamination can contribute measurably to the reduction in air dose rate in relatively small areas in residential areas.
In the development of the above 5% enrichment fuel, which is one of the targeted concepts of the next-generation LWR, a reduction in power generation cost is expected. However, the introduction of the above 5% enrichment fuel has several adverse effects on the cost, such as SWU increase, newly required measures for criticality prevention, and increases in gamma, neutron and heat production levels. In addition, the power generation cost is greatly influenced by the uncertainty of uranium price; thus, the sensitivities of those parameters were analyzed. By sensitivity analysis, the range of parameters required to improve the generation cost was clarified. We assumed that the throughput of fuel fabrication is reduced by one third by employing the above 5% enrichment fuel and the spent fuel is reprocessed in the FBR reprocessing plant. In conclusion, it is demonstrated that the power generation cost can be reasonably reduced by increasing enrichment up to 7% or more in the case that the uranium price is less than ten times as much as the reference price and also the increase in fabrication cost is prevented by introducing Erbia credit.
The distribution and chemical state of the Ru element in the simulated high-level waste glass were examined by using the synchrotron radiation-based X-ray imaging technique. An X-ray CCD camera instead of an ion chamber was used to determine the transmitted X-ray intensity in the imaging measurement. Spectra of the X-ray absorption fine structure with position sensitivity were obtained by analyzing the gray-scale value of each pixel in the X-ray CCD image. We confirmed the position sensitivity in the imaging XAFS technique by analyzing a test sample, in which RuO2 and Ru metal powders were scattered at random. We successfully obtained information on the Ru distribution and the chemical state (oxide or metal) of each small Ru-rich spot in the sample. The imaging XAFS technique was applied to the simulated high-level waste glass samples. It was concluded from the analyzed imaging XAFS spectra that the Ru element scattered in the glass sample exists as oxide RuO2.
In order to understand the influences of boric acid and lithium hydroxide on the IGSCC of type 316 stainless steel, an oxide film was analyzed in simulated PWR primary water while varying the boric acid and lithium hydroxide concentrations. It was found that, although boric acid and lithium hydroxide did not affect the structure and chemical composition of the surface oxide film remarkably, a lower boric acid concentration or a higher lithium concentration produced an oxide film with a thicker surface. It was considered that the lower boric acid concentration and higher lithium hydroxide concentration caused a higher magnetite solubility at the surface of the material and that the higher magnetite solubility caused a higher iron concentration gradient, which promoted iron dissolution from the material and the formation of a thicker oxide film. It was found that the thicker oxide film caused a higher IGSCC susceptibility and that the corrosion was the dominant factor of the IGSCC mechanism. No significant change was found in the morphologies of crack tip oxide in different bulk water chemistry systems, thus producing CT specimens with similar crack growth rates.
A breach of the heat transfer tube in a steam generator (SG) of a sodium-cooled fast reactor results in sodium-water reaction (SWR) by direct contact between liquid sodium and water vapor, which generates a high-temperature reaction jet. This reaction jet causes damage to the surface of neighboring heat transfer tubes by thermal and chemical effects. Therefore, it is important to clearly understand the SWR for the safety assessment of SG. Regarding the chemical effect, the surface and gas-phase reaction models are considered. As for the surface reaction, the water vapor reacts with the liquid sodium, and the reaction occurs at their interface. Subsequently, the chemical reaction heating vaporizes the liquid sodium, causing the gas-phase reaction. However, there is limited knowledge on the surface reaction because of an extremely rapid reaction. Therefore, in this study, ab initio calculation was performed to evaluate the surface reaction path in SWR. The reaction path regarding the dissociation of a water molecule at the liquid sodium surface was obtained. In addition, we also confirmed the mechanism of hydrogen generation in the liquid sodium.
The Japan Atomic Energy Agency (JAEA) has carried out research and development of high-temperature gas-cooled reactors (HTGRs) since 1960s. The high-temperature engineering test reactor (HTTR) has attained full power in 2000. In this study, a failure probability calculation method for Japanese high-quality HTGR fuel particles is suggested for the design of Japanese HTGRs. Failure probability is strongly dependent on the irradiation characteristics of pyrolytic carbon (PyC) layers of HTGR fuel particles. However, it is very difficult to obtain new irradiation data of PyC layers with experiments. Therefore, in this study, we calculated failure probabilities using a report summarizing the irradiation data obtained at various irradiation temperatures of PyC layers with various as-fabricated characterisitics and the result of the failure probability measurement of the irradiation test of HTTR fuel. The characteristics of the coating layers of Japanese HTGR fuels, including future fuel types, would be very similar to those of HTTR fuel. In addition, some calculations were carried out with the suggested method. It was shown that the thickening of the SiC layer is more effective in reducing the failure probability than the thickening of the buffer layer. Thus, the thickening of the SiC layer is important for the modification of HTGR fuel for burnup extension, suppressing the increase in core volume.