SUCCESSFUL COLD SHUT DOWN OF ONAGAWA: THE CLOSEST NPS TO THE 3/11/'11 EPICENTER

摘要

This paper presents the primary factors that Onagawa nuclear power station (NPS) reached a state of cold shut down, even though it suffered the highest ground acceleration and tsunami which was comparable to that of Fukushima Daiichi NPS. The key points for safety cold shutdown was, first, plant site level was higher than the maximum tsunami height level; next, emergency diesel generator for each unit and one of the off-site electrical power lines remained available. In addition to the above factor, preparedness (such as seismic reinforcements for all three units, update the tsunami prediction where appropriate, and continuous drills for fire and simulator trainings for loss of off-site-power) contributed greatly. However, we must achieve higher standards of safety. From Fukushima Daiichi accident, we learned that it is important to protect from flooding, to enhance electric power supply system and confinement system. We have already taken some safety measures such as the construction of levee whose height is 17m above sea level, and the installation of air-cooled diesel generator. We will continue to implement various enhancements in order to avoid a similar severe accident again. Moreover, as further steps, we are planning to set up the mitigation system such as a filtered primary containment vessel (PCV) venting system.