Indirect neutron radiography by transferring radiation from an activated metal foil to a film is an effective technique under a high gamma-ray field. By adopting an X-ray imaging plate (IP), the sensitivity characteristics of indirect neutron radiography were quantitatively evaluated through experiments using cyclotron accelerator neutrons. The wider dynamic range and higher sensitivity of the IP method than the conventional film method were shown through parametric tests of neutron flux, exposure time, and transfer time. It was confirmed that the X-ray IP-based transfer exposure method is a promising neutron radiography method under neutron/gamma mixed fields.
OPECST (Office parlementaire d’evaluation des choix scientifiques et technologiques) helps decision making of the French Parliament by assessing options of scientific and technological policy. In this process, OPECST holds public hearings and gathers opinions from various participants: experts, industry, citizens, and so on. It held many public hearings and assessments on nuclear energy policy in the 1990s, when French people started to demand more transparency and independence of the nuclear safety regime than before. So we can assume that OPECST helped the reform in the 1990s and in the 2000s that finally established the Law on Transparency and Security in the Nuclear Field. This research aims to precisely clarify its function through a survey of all the reports of OPECST on nuclear safety policy published in this period and of political decisions related to them. As a result, it is shown that the function of OPECST consists of three elements: it defines problems, elaborates policy recommendations from various opinions, and accumulates its survey results in the form of reports. Nowadays, the form of the discussion carried out by OPECST is changing, so we have to learn both its history and recent activities to make a policy-making system that would be suitable in Japan.
Concrete debris contaminated with radioactive cesium and other nuclides has been generated by the accident in the Fukushima Dai-ichi nuclear power plant. Moreover, there is concern that a large amount of radioactive concrete waste will be generated by the decommissioning of nuclear power plants in the future. Although conventional techniques are effective in decontaminating concrete with flat surfaces such as floors and walls, it is not clear what techniques to apply for decontaminating radioactive concrete debris. In this study, focusing on a pulsed power discharge technique, fundamental experimental work was carried out and the applicability of the technique to decontaminating radioactive concrete debris associated with the accident in the Fukushima Dai-ichi nuclear power plant was evaluated. The decontamination of concrete by applying the aggregate recycling technique using the pulsed power discharge technique was evaluated by measuring the radioactivity concentration of the divided aggregate and sludge from the contaminated concrete using a Ge-semiconductor detector. It indicated a reduction of the radioactivity concentration in the recovered aggregate and an increase in the radioactivity concentration in the sludge. These findings suggest that the division of the contaminated concrete debris into aggregate and sludge could result in the decontamination and reuse of the aggregate, which would reduce the amount of contaminated concrete debris.
The Periodic Safety Review (PSR) is a continuous safety enhancement process to improve overall plant safety. The report on the Fukushima Daiichi accident issued by the IAEA indicated that the approach to PSR in Japan was not fully in line with guidance from the IAEA. PSR was originally an efficient means of improving the safety level of installations, and to make a decision on the continuation of the reactor operation for the next decade. In order to achieve the objectives of PSR, plant operators need to review their operating experience as well as reassess their past review and identify findings which lead to improvements of plant safety. It is also important that regulators participate in the preparatory work of the review to identify the issues and the outcome of the review. To improve the efficiency and effectiveness of this activity, the regulators need to develop a technical information base reflecting the safety inspection carried at the sites to identify issues. The plant operator also needs to reassess the past PSR to verify the validity of the review. Regulators and plant operators should continue to make bilateral efforts through PSR and deliver improvements of safety throughout the plant life.
In severely accident-damaged nuclear power plants, fuel rods may melt through the reactor vessel wall and drop to the floor inside the primary containment vessel (PCV). The PCV and the reactor building may also be damaged by a severe accident. Even in such a case, the three fundamental safety functions, i.e., stopping the reactor (preventing recriticality), cooling the reactor, and confining radioactive materials, must be maintained so that the uncontrolled release of radioactive materials to the outside can be minimized during decommissioning. However, the success of this strongly depends on the availability of the cooling function of the reactor cooling system, which can maintain the physical form of the fuels and fuel debris by water cooling and therefore prevent their recriticality and the uncontrolled release of radioactive materials to the outside. In other words, the cooling function before completing the retrieval of fuels and fuel debris is very important for safety. Thus, we focused on the reactor cooling system of an accident-damaged boiling water reactor (BWR) during decommissioning, evaluated its reliability, and then considered how the evaluation results should be used in the risk management during the decommissioning of an accident-damaged BWR.