Tohoku Electric Power Co., Ltd. plans to continuously improve its safety, based on high-quality risk management. Tn the risk management framework, quantification of risk by PRA plays an important role. Here, we introduce the status of our efforts to improve our PRA method, such as refined internal event level l PRA models, improved data, and the expanded scope of PRA....
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Tohoku Electric Power's efforts to improve PRA method

Process computers are essential to power stations; they gather a plenty of plant data from plants, perform complicated calculation and give plant information to aid operators in monitoring and assisting plant operation. The process computer of the Higashidori Nuclear Power Station has been updated because the maintenance period expired after about 13 years since the start of operation. In updating, on purpose to improve the reliability and maintainability of facilities, each server was integrated into three...
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Renewal Work of the Process Computer

When we found leakage from the leak detection piping installed behind the bottom of the storage pool in Onagawa NPP’s on-site bunker building, we once had judged there should be some damaged points causing the leakage on the bottom of the storage pool. Therefore we planned to inspect some defective points on the bottom of the storage pool after displacing the radioactive waste and draining off from the storage pool in order to repair the defective points. However, it turned out that there had been leakage f...
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The action to leakage from the on-site bunker pool storing radioactive waste of Onagawa NPP

Among the severe accident countermeasure works of Onagawa NPS unit-2, it is one of the characteristics to install filtered containment venting system (FCVS) facilities in the existing reactor building. Therefore, various considerations are taken about bringing in facilities, installing of facilities and future maintenance management. Most of the severe accident countermeasure works are laying of the piping or cable way, and therefore it is necessary to set penetrations in the building. Particularly, penetra...
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Summary of prevention from overpressurization damage of containment vessel in Onagawa NPS unit-2

By the vibration diagnosis, which is conducted for the purpose of condition monitoring (CM) and condition-based maintenance (CBM) in Onagawa NPS unit-2, vibration increasing of a motor for a condensate transfer pump was confirmed. We introduce the case where treatment was performed to the motor before loss of function because the diagnosis data could detect abnormal condition early and cause factor estimation....
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Response to the vibration increasing event of a motor for a condensate transfer pump in Onagawa NPS unit-2

At Onagawa Nuclear Power Station Unit 2, Tohoku Electric Power Co., Inc., many safety measures are implemented to further improve the safety level, giving top priority to ensuring safety. In addition, on November 18, 2020, we received prior approval from the local governments that are located nuclear power plant regarding the application for conformity assessment to the new regulatory standards based on the safety agreement, and we were able to make great progress toward restarting Onagawa Unit 2. This manu...

In this paper, we outline the details of inspection for ducts of main control room air-conditioning system we conducted last year. Then, we describe the measures based on results of inspection....
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Inspection for air-conditioning duct and the measures

Since June 2014, the Higashidori Nuclear Power Station had been taking the assessment of the on-site fault in the conformity assessment of the new regulatory standard. After that, although the assessment of the fault located directly under key facilities ended in 2018, We are taking an assessment of active fault examination considered as an epicenter at present. This document is a summary of the supplementary survey including geology survey such as reflection survey and boring exploration for active fault a...
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Supplemental investigation on faults in site of Higashidori Nuclear Power Plant Unit 1

The dominant cause of the Fukushima accident is underestimation of natural hazards such as earthquake and tsunami. Therefore, when we evaluate safety at nuclear power station, we need to consider uncertainties of natural events. 2011 Tohoku Region Pacific Coast Earthquake (3.11 earthquake) on March 11, 2011 generated violent shake and massive tsunami, but all units of Onagawa nuclear power station achieved the cold shutdown avoiding major accident because of a lot of efforts to improve safety at the site. I...
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Safety Requisites’ Re-definitions in accordance with Fukushima Lessons - Design-base Tsunami for Onagawa Site -

The core shroud specimens (316L stainless steel) were used to characterize the surface hardening layer introduced by machining and to evaluate the SCC susceptibility in the simulated BWR coolant environment. Based on the results obtained, the effect of the surface hardening layer on the SCC susceptibility and its mechanism are discussed. It is suggested that the wall part has clearly higher SCC susceptibility than the support ring part when compared between the as-received materials (with surface hardening ...
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Evaluation of SCC susceptibility of austenitic stainless steel with surface hardening layer in high-temperature water