[en] In the final step of decommissioning the Japan Power Demonstration Reactor, procedures of decontamination and final survey of radioactivity were studied to release the site from radiation control. Then the procedures were applied to the decontamination and final radioactivity survey. Data and experiences were obtained during these activities. Some parts of controlled area in the JPDR facilities were released

[en] This paper summarized the features of decommissioning work and the methods how to develop human resources. The general flow of decommissioning includes the following steps: (1) evaluation of facility characteristics, (2) planning, (3) decontamination and disassembly of equipment and structures contaminated with radioactivity, (4) radioactivity measurement, (5) treatment and disposal of radioactive waste, and (6) release from legal restrictions (termination of decommissioning). For this purpose, techniques in various fields are required. In the evaluation of facility characteristics, radiation measurement and calculation of activation amount in the core part are required. In decontamination and dismantling, cutting technology (mechanical cutting, thermal cutting, etc.), decontamination technology, and remote control technology are required. In the nuclear power education in the past, the fields related to design, construction, operation, and maintenance among the plant life cycle were the main parts. Much attention was not payed to decommissioning and the treatment/disposal of radioactive waste in the second half of life cycle. As university education, Hokkaido University and Fukui University have lectures on decommissioning. Furthermore, the education and research for students are proceeding at seven universities, with a focus on common reactors including those of Fukushima Daiichi Power Station. It is a key for promoting decommissioning, to incorporate project management, risk analysis, cost evaluation, and decision making into education, and to foster human resources heading toward challenging problems including social problems. (A.O.)

[en] It is required to select an optimal decommissioning scenario by evaluating its characteristics since there are various kinds of decommissioning options of nuclear facilities which ceased operation. A methodology was studied on evaluation of an optimal scenario from the viewpoints of radiological risk reduction and easiness of work activities together with preparing several scenarios based on the IAEA proposal options (immediate and deferred dismantlings, and entombment). In this study, the basic decommissioning work activities are classified into facility dismantlement, site remediation and waste management, then these are further disassembled to be small categories by using hierarchically form. The computer system was developed to build hierarchically decommissioning work activities on a computer screen, then the scenarios are compared by using the analytical hierarchy process. The developed system was applied to the evaluation of decommissioning scenarios of Fukushima-Daiichi nuclear power plant of Tokyo Electric Power Company as a case study. It was cleared that the facility entombment option scored the highest value of relative importance from the viewpoint of radiological risk reduction; otherwise the safe enclosure (deferred dismantling) was scored the highest value if the easiness of work activity was taken as evaluation viewpoint. (author)

[en] In this study, basic strategies for the decommissioning and site remediation of the Fukushima Daiichi Nuclear Power Station (FDNPS) were investigated. Six scenarios were formulated based on two of the three decommissioning strategies of nuclear power plants defined by the International Atomic Energy Agency (IAEA): immediate dismantling and deferred dismantling. A multicriteria decision analysis was performed to analyze the preferences of the options from the viewpoints of the timeframe to complete decommissioning, the resulting waste, the site usability, and the availability of the radioactive waste disposal route. The same six scenarios were applied to both the FDNPS and the nuclear power plants that ceased operation after a normal plant life cycle for comparison. For the FDNPS, the decommissioning project involved fuel debris retrieval, dismantling, and site remediation. The analysis results suggest that the balance between the amount of waste and the time to achieve the end state may be one of the most critical factors to consider when planning the decommissioning and site remediation of the FDNPS

[en] Decommissioning and dismantling (D and D) of nuclear facilities are getting to be one of important issues of nuclear energy development in the world. The cooperative program on decommissioning has provided the chairing to exchange information on decommissioning and dismantling projects among the member countries of the Nuclear Energy Agency of Organization for Economic and Development (OECD/NEA) since 1985. The study on decommissioning cost was conducted by OECD/NEA in terms of cost items and variability of cost estimations among the D and D projects in 1990s. In progress of various D and D projects, it has been recognized that appropriate provisions have to be made in terms of policy, financing, and of management of the relatively large volume of materials arising from decommissioning. Under the circumstance, two groups have been set up by OECD/NEA to study recent problems; one group for cost and strategies, the other for safety and material management. These studies and information exchange on D and D projects are expected to be useful for D and D studies in Japan. (author)

[en] Fission product source terms at severe accident conditions have been studied vigorously since the accident at TMI unit 2. Among with the effort, some series of out-of-pile experiments have been performed to investigate fission product release behavior, exclusively, from fuel rods at the off-normal conditions. This report reviews the results of two major series of the source term tests, i.e. SASCHA (PNS Project, FRG) and HI-VI test (ORNL, USA). These tests have been the major sources for estimating fission product source terms in the primary cooling system, in the containment and subsequently to the environment at hypothetical Severe Accidents. Findings on the chemical forms of some important fission products in these tests are summarized. A diffusion model is introduced to represent volatile fission product release observed in the ORNL tests. The diffusion model described Kr and Cs releases better than the existing CORSON-M model does. In addition, experimental and analytical techniques used in these tests are also summarized, in order to clarify the reliability and problems of the test data. (author)

No abstract available

[en] The Code System for Management of JPDR Decommissioning (COSMARD) was developed for use for effective planning and management of reactor decommissioning. Efforts were made to develop ; (1) data-processing algorithm using a tree structure which reflects decommissioning Work Breakdown Structure (WBS), (2) a set of commands to describe WBS and specific working conditions as input data, and (3) models to evaluate various project management data using simple mathematical formulas and unit factors. In preparing these models, dismantling work activities related to the Japan Power Demonstration Reactor (JPDR) were analyzed to find out fundamental work items and make suitable formulations for evaluating the manpower needs in both manual and remote dismantling work. Manpower needs for dismantling the JPDR components were then estimated as a sample calculation. An outline of COSMARD, its methodology and calculation models are discussed. (author)

[en] In the NSRR experiments, it has been generally believed that an axial power distribution in a test fuel rod is almost flat due to the core length three times longer than a fuel stack region in a test fuel rod. However, the axial power distribution in a test rod is possible to change because of the effects of both control rod position and reflection of neutrons at edge regions of the fuel stack. Therefore, we measured the detailed axial distribution of γ-ray intensity in test fuel rods to characterize the power profile and to evaluate the effect of the power peaking on the fuel rod behavior. Based on the measurement of the γ-ray distribution in a fuel rod, the power was observed to be higher at the lower portion of a test fuel rod, and the effective power peaking was estimated to be 1.04-1.06 compared with the average in a fuel stack region. In the NSRR experiments, a fuel failure occurred usually at the lower portion when the fuel rod was subjected to the energy deposition of 260 cal/g UO₂ in axial average. On the other hand, reducing the peaking by insurting 5 % enriched UO₂ pellets at both edges of the fuel stack, a fuel failure did not occur even if the fuel rod was subjected to 260 cal/g UO₂ in axial average. This must indicate the strong effect of the peaking on the fuel rod failure behavior. As the relation was observed between fuel rod behavior and a local energy deposition in a fuel rod having a widely deviated power profile in axial direction, it must be better to evaluate the fuel rod failure behavior based on the local energy deposition. Researching the failed position among the test fuel rods subjected to around failure threshold energy of 260 cal/g UO₂, the local energy deposition was estimated to be around 274 cal/g UO₂ at failed position. (author)

[en] Decommissioning of reactor facilities was planned to perform progressive dismantling, decontamination and radioactive waste disposal with combination of required technology in a safe and economic way. This article outlined required technology for decommissioning as follows: (1) evaluation of kinds and amounts of residual radioactivity of reactor facilities with calculation and measurement, (2) decontamination technology of metal components and concrete structures so as to reduce worker's exposure and production of radioactive wastes during dismantling, (3) dismantling technology of metal components and concrete structures such as plasma arc cutting, band saw cutting and controlled demolition with mostly remote control operation, (3) radioactive waste disposal for volume reduction and reuse, and (4) project management of decommissioning for safe and rational work to secure reduction of worker's exposure and prevent the spreading of contamination. (T. Tanaka)