Radon diffusion in soils has been studied over the years by many researchers. The application of such studies to the evaluation of radiation exposure caused by radon from uranium-bearing wastes disposed in a shallow land site is very important. The present paper surveyed closely relevant studies and elucidated the inherent nature of radon diffusion in terms of the definition of radon diffusion coefficients. Then, basic features of measurement methods for determining radon diffusion coefficients in soils were explained. Furthermore, theoretical aspects of radon diffusion in soils were discussed in terms of microscopic radon diffusion in soils and large-scale radon diffusion through cover soil defects for uranium mill tailings. Finally, in order to apply the radon diffusion studies to uranium-bearing waste disposal in shallow land sites, new challenges were presented: elucidation of radon diffusion in uranium-bearing wastes and cover-soil cracks, and demonstration of the validity of applying only radon diffusion in the evaluation of radiation exposure caused by radon, which would come through Japanese cover soils for uranium-bearing waste disposal.
The discussion on an appropriate level of physical protection has not been elaborated so far because of the confidentiality of its nature, thus resulting in a lack of consensus on this issue. In view of this context, a new system for the evaluation of antisabotage physical protection systems is proposed in this paper, in which we introduce openness to a certain extent in the process of the evaluation. The proposed system is composed of the following three elements; (1) establishment of an evaluation basis threat (EBT), which should be less strong but more likely to occur than the design basis threat (DBT); (2) employment of realistic standard scenarios in the process of evaluation; (3) disclosure of results of evaluation implemented based on the above EBT and standard scenarios. It is expected that this considerably open system will foment peace of mind among citizens as well as create a deterrent effect that would minimize the occurrence of sabotage on nuclear facilities.
The establishment of effective anti-insider measures is a key issue with regard to the physical protection system. The two-man rule, which obligates workers to act always in pairs inside the protected area, is one of the important measures for the anti-insider physical protection system. However, the effectiveness of this method has not been examined in a quantitative manner taking into account the circumstances of Japanese society, which may influence the effectiveness of this method. In this paper, although with several hypotheses, the effectiveness of the two-man rule is confirmed based on the percentage of criminals in the Japanese population.
Interest on the development of the Very High Temperature Gas-Cooled Reactor (VHTR), of which the reactor outlet temperature is 950°C or much higher, has recently been increasing worldwide, and it was selected as one of the candidate reactor types in the Generation IV Reactor System International Forum. The Japan Atomic Energy Agency (JAEA) has already initiated R&D efforts on the electricity and hydrogen cogeneration plant with the VHTR system, GTHTR300C. Although the technical feasibility of the VHTR using pin-in-block fuel, which has already been used in the High-Temperature Engineering Test Reactor (HTTR), has been shown fundamentally, more improvements in core performances, such as a decrease in the occupational exposure doses during plant maintenance, are desired. This report presents the results of the conceptual core design study using multihole-type fuel and the study on the occupational exposure doses. The latter results show much better plant maintainability compared with the previous results on the GTHTR300.
The accelerator-driven nuclear transmutation system aims at transmuting minor actinides (MAs) and long-lived fission products to stable or short-lived nuclei. A feasibility design study of the proton beam window, which is an interface component between an accelerator and a nuclear reactor, needs a method to be able to evaluate heat transfer characteristics of the beam window under flowing lead bismuth. However, the validity of the numerical simulation model of the beam window has not been proven yet and a database on heat transfer characteristics has also not been established. Two experiments were conducted: one was particle image velocimetry measurement around the beam window in flowing water and the other was temperature measurement at the beam window under flowing lead bismuth. Numerical simulation was also done to validate the beam window model for design work. Results show that heat transfer characteristics of the beam window averaged in space and time under flowing lead bismuth was formulized by the experimental equation. The numerical simulation model can estimate the mean heat transfer coefficient. However, the local heat transfer coefficient was not stable: it fluctuates with time and even in space, especially around the stagnation point. The reason is discussed in detail.
Water electrolysis utilizing sulfur dioxide is attractive for low-electricity-consumption hydrogen production and has been investigated for the electrolytic and thermochemical hybrid hydrogen production process utilizing the heat generated by a nuclear power plant. In this study, the hydrogen production performance of the water electrolysis cell utilizing sulfur dioxide gas and the sulfur dioxide crossover prevention performance of several polymer electrolytes were investigated experimentally.
The effects of specimen size and strain rate up to nearly ε = 103/s on the compressive properties of isotropic nuclear-grade graphite IG-11 were investigated experimentally at room temperature. Solid cylindrical specimens with two different slenderness (length/diameter) ratios of 0.5 and 1.0 were used in the impact and static compression tests, respectively. The specimen's diameter was varied from 6 to 15 mm, keeping the slenderness ratio constant. The high strain-rate compressive stress-strain curves up to failure were determined using the conventional split Hopkinson pressure bar (SHPB). The corresponding static and intermediate strain-rate stress-strain curves up to failure were measured with an Instron testing machine. The two parameters of Weibull distribution were used to evaluate the ultimate compressive strength (nominal maximum compressive stress) at three different strain rates. It was found that the ultimate compressive strength and corresponding strain (or failure strain), and the absorbed energy up to failure increased substantially at strain rates of over 10−1/s, and were independent of the specimen size tested over a strain rate range from 10−3/s to 103/s. Macro- and micro-scopic examinations revealed that there was a marked difference in the failure modes between the static and impact compression specimens.
Since the beginning of the 1980's, in the United States, there have been many licensee event reports (LERs) involving setpoint drift in safety or safety/relief valves. The United States Nuclear Regulatory Commission (NRC) has issued a lot of generic communications on this issue and the industry has made efforts to resolve the issue. However, the NRC staff recently highlighted that over 70 LERs involved instances where safety or safety/relief valves failed to meet the allowed setpoint tolerance from 2001 through August 2006. In the present study, we analyzed the U.S. experience with setpoint drift in safety/relief valves (SRVs) at BWRs, pressurizer safety valves (PSVs), and main steam safety valves (MSSVs) at PWRs by reviewing approximately 90 LERs from 2000 to 2006 and examined the trend focusing on causes and setpoint deviation ranges. This study indicates that for SRVs and MSSVs, disc-seat bonding is a dominant cause of the setpoint drifting high and has a tendency to result in a relatively large deviation of the setpoint. This means that disc-seat bonding might be a safety concern from the view point of overpressure protection. For PSVs, the deviation of setpoints is generally small, although its causes are not specified in many instances.