[en] The massive earthquake struck Onagawa nuclear power plant on March 11, 2011, and the nuclear reactor and the steam turbine was shut-down automatically. As a result of the post-earthquake steam turbine inspection, remarkable damages have been confirmed on the turbine blades and the bearing stands, and currently the restoration works have been conducted. This report introduces the damage status of the steam turbine caused by the earthquake, and activities of restoration work. (author)

[en] The main buildings for No. 2 plant in Onagawa Nuclear Power Station are constructed on the bedrocks about 14 m below the sea surface. Therefore, for the purpose of executing the works by shutting seawater off and dry work, the large scale impervious wall of about 500 m extension was installed underground. The feature of this impervious wall is the depth of embedment of about 3 m into the hard bedrocks having the uniaxial compressive strength of 2000 kg/cm² at maximum, carried out with the newly developed hard rock excavator. The outline of these construction works is reported. No. 2 plant in Onagawa Nuclear Power Station is the BWR plant of 825 MWe output. The construction works of the power station were began in August, 1989, and the rate of progress in civil engineering works as of the end of September, 1990 was 21.3%. The planning of the impervious wall, the geological features at the site, the method of shutting seawater off, the selection of wall materials, the design of the wall body, the investigation of the quantity of spring water, the execution of the construction and execution management, and the confirmation of the effect of the wall are reported. (K.I.)

[en] We have applied various countermeasures for worker dose reduction to plants in Onagawa Nuclear Power Station. 'Ni/Fe ratio control' has been applied to Unit 1. Due to the change of fuel cladding surface treatment, effect of Co-60 concentration reduction in the reactor water by Ni/Fe ratio control decreased. Considering such situation, 'ultra-low-crud high-nickel control' has been adopted to Unit 2. Main objective of ultra-low-crud high-nickel control is to suppress deposition of Co-60 onto primary circuit piping. Because we have obtained successful results in dose rate reduction in Onagawa unit 2, we decided to adopt this water chemistry control also in unit 3. In this paper, we describe water chemistry data under ultra-low-crud high-nickel control in unit 2 and unit 3. (author)

No abstract available

[en] A large earthquake occurred on March 11, 2011 and tsunami was generated following it. The East Japan suffered serious damage by the earthquake and tsunami. This is called the Great East Japan Earthquake. Onagawa Nuclear Power Station (NPS) is located closest to the epicenter of Great East Japan Earthquake. We experienced intense shake by the earthquake and some flooding from the tsunami, however, we have succeeded safely cold shutdown of the reactors. In this paper, we introduce summary of Great East Japan Earthquake response a Onagawa NPS and safety improvement measures which are based on both experience of Onagawa NPS and lesson from Fukushima Daiichi NPS accident. (author)

No abstract available

[en] Pipe wall thinning is an aging process that has a significant effect on the safety and safe operation of power plants and its appropriate management is an issue common to all electric power companies. Tohoku Electric Power Co., Inc. has been putting an emphasis on pipe wall thickness management since 2006, when a through-hole possibly due to droplet-impact erosion was found in the vent piping of high-pressure feedwater heater No.2 at Onagawa Nuclear Power Station unit 2. Following this incident, extensive revisions were made to the pipe wall thickness management program and we have been making efforts to improve the efficiency of the management. This report addresses such issues. (author)

[en] Onagawa-2 started commercial operation on 28 July 1995 and the first annual outage started 27 August 1996. The total man-Sv exposure during the first annual outage was about 0.15, which was quite low for a boiling water reactor. To obtain this reduction in man-Sv at Onagawa-2, various countermeasure were applied from the point of plant design. In addition, the 'Clean Plant Campaign' had been implemented extensively throughout the design, construction, start-up test period, and the first cycle. In this campaign, 'Ultra Low Crud Control' was adopted to keep the ionic Ni concentration high in the reactor water for the suppression of the incorporation of the activity on the piping. In this report, the performance results of the water quality control and the dose rate throughout the first cycle are reviewed. (author)

[en] At Onagawa Unit2, a piping hall accident caused by erosion was found. Taking it seriously, we revised our management of piping wall thickness and decided to introduce a new support system in order to manage it surely. Now we are planning to get our team (Tohoku-epco and our group company) ready so that main steps from measurement of wall thinning to evaluation of the data can be conducted on our own in the future. (author)

[en] Tohoku Electric Power Company promotes development of various power sources to provide a stable supply of electricity in the future, and nuclear power takes a leading part. In August 1989, construction of Onagawa nuclear power plant Unit No. 2 (825MW) was started, following Unit No. 1 (524MW) which went on line in 1984 as Tohoku Electric's first nuclear power plant unit. Unit No. 2 began commercial operation in July 1995 through satisfactory construction work such as RPV hydraulic test in March 1994, fuel loading in October 1994, and various startup tests in each power stage. The design and construction of Unit No. 2 reflect construction and operation experience gained from Unit No. 1, and the latest technology, including that of the LWR Improvement and Standardization Program, was adopted to enhance facility reliability, improve operation and maintenance performance, and reduce worker dosage. Features of the facility, construction techniques, and a description of preoperation of Onagawa nuclear power plant Unit No. 2 are described in this paper. (author)