In the MOX pellet fabrication process, the MOX powder is mixed, granulated, and pressed into MOX pellets and then finally sintered. In this process, a small amount of MOX powder disperses into the globe box and adheres to its wall and floor. This powder is recovered and stored, but rejected from further MOX pellet fabrication processes, because it does not meet the specifications. By the conventional method, this scrap MOX powder is pelletized once and dissolved with hot nitric acid. Then, plutonium is recovered by extraction. In this study, a new simple method of recovering plutonium was developed. We used pure MOX powder as a substitute for the scrap MOX powder in our examination. The scrap MOX powder is reacted with silicon carbide (SiC) and dissolved in nitric acid at room temperature. Nearly 70% of plutonium was recovered with uranium, although MOX powder without the above reaction cannot be dissolved in nitric acid at room temperature.
Radioactive materials might be released into air as a result of the accidental boiling of highly active liquid waste (HALW) in reprocessing plants. In such accidents, volatile radioactive nuclides, such as ruthenium, are released from the tanks into the atmosphere. In addition to the radioactive materials, nitrogen oxides are also released owing to the thermal decomposition of metal nitrates in HALW. The released nitrogen oxides transport volatile ruthenium and cause redox reactions associated with the composition or decomposition of volatile ruthenium. In this study, data of nitrogen oxide release were obtained by heating simulated HALW to 600℃. Data were also obtained for mixed lanthanide solution, mixed lanthanide solution containing zirconium dioxide, and mixed lanthanide solution containing ruthenium dioxide. As a result, the release of nitrogen oxides from the simulated HALW was observed from 200℃ to 600℃, and the main release of nitrogen oxides was observed at about 340℃. All the lanthanide nitrates were found to decompose in the simulated HALW, and the thermal decomposition temperature of the lanthanide nitrates decreased after the addition of ruthenium dioxide to the mixed lanthanide nitrate solution.
The Fukushima Daiichi nuclear accident has led to changes in the acceptance of nuclear power in many people. The authors conducted an opinion survey of 300 adult inhabitants of Tsuruga city in Fukui prefecture, Japan. The aim of this survey is to obtain people’s opinions concerning radiation and its risks. Authors classified Tsuruga inhabitants on the basis of responses to questions on the concept and knowledge of risk and the cognition of radiation by factor and cluster analyses of multivariable analysis. Using the results of these analyses, Tsuruga inhabitants have been assigned to five categories: “acceptance group,” “anxiety group,” and three intermediate groups.
To re-evaluate the design basis ground motions (DBGM) at nuclear power plants (NPPs) in Japan, the seismic probabilistic risk assessment (PRA)1~3) for fast breeder reactors (FBRs) has been carried out to confirm that the seismic safety is equivalent to that of light water reactors (LWRs). The seismic response on the reactor structure of FBRs causes seismic reactivity. The group motion of fuel assemblies is one of the typical seismic responses. Therefore, much attention has been paid to the mechanism of reactivity insertion due to the group motion of fuel assemblies and its consequence during an earthquake of the DBGM condition. The group motion of subassemblies is the phenomenon that almost all subassemblies move and collide simultaneously because the gaps between neighboring subassemblies are narrow. When each gap is reduced coherently and core compaction in the lateral direction occurs, positive reactivity insertion is introduced. We evaluate the gap reducing characteristic at the midplane of the core by using a correlation coefficient. As a result, we found that subassembly vibration modes depend on the excitation acceleration and frequency. The gap reduction exceeds that of the total deflection condition at the acceleration of 40 m/s2 and the frequency of 5 Hz. It is important to confirm that the seismic motion of such a condition does not cause a CDA, from the point of view of the reactor structure.
The concept of utilizing a Zr-Er alloy in high-burnup fuel claddings in light water reactors has been proposed. Er in a Zr alloy is expected to act as a burnable poison that reduces the excess reactivity. However, the required physical properties for practical realization of Zr-Er alloys have yet to be fully understood. Here, we prepared Zr-Er alloys with various Er contents and characterized the phase state, Young’s modulus, and thermal conductivity. The effect of Er content on the thermal and mechanical properties of the alloys was investigated.
Since there are many scenarios of the process from start to completion of a decommissioning project, it is important to study scenarios of decommissioning by evaluating such properties as safety, cost, and technology. An optimum scenario with the highest feasibility in accordance with the facility and environmental conditions should be selected on the basis of the results of the study. For analyzing a scenario of decommissioning, we prepared structured work packages by using the work breakdown structures (WBS) method together with qualitative evaluation of the technologies being applied to work packages located at the bottom (the third level) of the WBS. A calculation model was constructed to evaluate the feasibility of a scenario where fuzzy logic is applied to derive a score of technology performance and TOPSIS is applied for getting a feasibility grade of the scenario from technical performance scoring. As a case study, the model was applied to the debris removal scenario of Fukushima Daiichi Nuclear Power Plant to confirm its applicability. Two scenarios, underwater and in-air debris removal cases, were characterized by extracting the work packages with the lowest feasibility and by obtaining total average scores of the scenarios. It is confirmed that the methodology developed is useful for the scenario evaluation of decommissioning nuclear facilities.