[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] A large amount of tritium are generated in the coolant water along with the operation of a nuclear reactor. Especially, more tritium are generated in a heavy water reactor. As for tritium included in the coolant water, the generation of the secondary contamination is thought by circulation process in a concrete surface. At the decommissioning of the nuclear facility, concrete wastes according to these secondary contamination are generated massively. It is demanded to measure tritium efficiently in these wastes for reasonable processing disposal. However, it is necessary for tritium to be extracted, separated from concrete etc. so that tritium emits the β ray of the low energy and to be measured. In the usual measurement method, accuracy of measuring tritium depends on extraction and separation from concrete, and then there are some problems such as requiring long time to extraction. Then, we focused on the measurement of contaminated concrete by a water immersion method. The condition of tritium which remained in the leaching sample of tritium was examined by the water immersion method. The water immersion method can efficiently measure the concrete specimen immersed in water for four hours. We expected that the water immersion method can be used to measure the clearance level of tritium in contaminated concrete. (author)

[en] The movable radioactivity measurement device was developed to evaluate a low-level radioactivity contamination distribution at the final stage of dismantling activities in decommissioning nuclear facilities. The detection system was designed to quantify only β-rays by separating from γ and β-rays emitted from ⁶⁰Co and others. The detector was structured in the form of collecting twelve unit plastic scintillation counters, to identify relatively small partial contaminations. It was found from the performance tests that the detection limit for Co-60 was about 0.1 Bq/cm² with 60 seconds of the measurement time, and that an external radiation such as from embedded pipes can be discriminated. The measurement device has capability to measure contamination distribution with 4-5m²/h in efficiency, which was three times faster than hands-on work using conventional detectors. (author)

[en] In the final stage of decommissioning nuclear facilities, building structures are demolished to be green field conditions if dismantling is chosen as a decommissioning strategy. Since building structures are generally made from massive reinforced concrete materials, it is not a rational way to treat all concrete materials arising from its demolition as radioactive waste. Segregation of radioactive parts from building structures and measurement of radioactivity are therefore indispensable before demolishing building structures. The procedure of decontamination and measurement of radioactivity were studied for demolition of building structures in consideration of treating waste as non-radioactive materials, on the basis of the concept published by Nuclear Safety Commission. This procedure was then implemented in the Japan Power Demonstration Reactor dismantling demonstration project. The data obtained in the project relating to decontamination and measurement of radioactivity showed that the procedure was reasonable, and that the experiences will be useful for future decommissioning of nuclear power plants in Japan. This report describes the procedure studied on the segregation of non-radioactive waste related to decommissioning nuclear facilities, and the results of its implementation in the Japan Power Demonstration Reactor dismantling demonstration project. (author)

[en] For recycle of dismantled metal wastes generated by the decommissioning of nuclear power plant, we examined a melting test for melting characterization of stainless steel scrap, designed the conceptual process to produce the recycle products, and developed a recycle cost evaluation code which is useful to make a rational planning for the waste management program (cost, determination of process, etc.) of these metal wastes. This report gives the summary of these development carried out from 2001 to 2005. This work was performed under the sponsorship of Ministry of Education, Culture, Sports, Science and Technology of Japan. (author)

[en] In general, neutron transport and activation calculation codes are used for residual radioactive inventory estimation; however, it is essential to verify calculations by measurement results because of geometrical complexity of the reactor and so on. A methodological study was made on radioactive inventory estimation in a reactor to be decommissioned. The comparison between measured and calculated radioactivity in the JPDR core components showed a relatively good agreement (factor of 2), and it was cleared that water content and weight ratio of steel bars to concrete materials significantly influenced the neutron flux distribution in the biological shield (factor of 2-10 error). The measured radioactivity inside of the reactor pressure vessel wall and at the inner part of the biological shield was compared in detail with the calculations to verify the methodology applied to calculations of radioisotope production. Then it was found that the radioactive inventory could be estimated accurately with combination of calculations and measurement of radioactivity in samples and dose rate distribution for planning of dismantling activities. (author)

[en] The technology for estimation of radioactive inventory was developed for decommissioning nuclear reactors, then it was practically applied to planning of dismantling activities in the JPDR decommissioning program. The accuracy of calculations (2-D neutron transport and Radioactivity calculations) was examined based on measured radioactivity data during the dismantling activities. The comparison between measured and calculated radioactivity in the JPDR reactor components and structures showed a relatively good agreement, and it also made clear that water content and weight ratio of steel bars to concrete materials significantly influenced the neutron flux distribution in biological shield. The measured radioactivity inside of reactor pressure vessel wall and at the inner part of biological shield were compared in detail with the calculations to verify the methodology applied to calculations of radioisotope production. The radioactive inventory could be estimated with sufficient accuracy by the developed techniques together with measurement of radioactivity in samples for planning of dismantling activities. (author)

[en] After dismantling the components in the facilities of Japan Power Demonstration Reactor (JPDR), decontamination on concrete surfaces and final survey of radioactivity have been started as the last step in the JPDR dismantling activities. At the first step for the decontamination on concrete surfaces and the final survey of radioactivity is as follows; The contamination on the concrete surfaces in the JPDR facility was characterized on the basis of radioactivity measurements of samples taken from the buildings. The contamination in the JPDR facility was categorized into two groups: fixed or removable; deep penetrative contamination was not found in the JPDR facility. The distribution map of the contamination was made based on the characterization. Decontamination activities were planned according to the distribution map of the contamination. The all buildings will be demolished and the site will be landscaped after finishing the final survey of radioactivity by March 1996. (author)

[en] In the final stage of dismantling activities for decommissioning a nuclear power plant, building structures have to be demolished to release the site for unrestricted use. Since building structures are generally made from massive reinforced concrete materials, it is not a rational way to treat all concrete materials arising from its demolition as radioactive waste. Segregation of radioactive parts from building structures is therefore indispensable. The rational procedures were studied for demolition of building structures by treating arising waste as non-radioactive materials, based on the concept established by Nuclear Safety Commission, then these were implemented in the following way by the JPDR dismantling demonstration project. Areas of the JPDR facilities are categorized into two groups : possibly contaminated areas, and possibly non-contaminated areas, based on the document of the reactor operation. Radioactivity on the building surfaces was then measured to confirm that the qualitative categorization is reasonable. After that, building surfaces were decontaminated in such a way that the contaminated layers were removed with enough margin to separate radioactive parts from non-radioactive building structures. Thought it might be possible to demolish the building structures by treating arising waste as non-radioactive materials, confirmation survey for radioactivity was conducted to show that there is no artificial radioactive nuclides produced by operation in the facility. This report describes the procedures studied on measurement of radioactivity and decontamination, and the results of its implementation in the JPDR dismantling demonstration project. (author)

No abstract available