SUCCESSFUL COLD SHUTDOWN OF ONAGAWA: THE CLOSEST NUCLEAR POWER STATION TO THE MARCH 11, 2011, EPICENTER

摘要

This paper describes the emergency response of the Onagawa nuclear power station (NPS) on March 11, 2011, and the primary factors that allowed the Onagawa NPS to reach a state of cold shutdown, even though it suffered the highest ground acceleration and tsunami, comparable to those at the Fukushima Daiichi NPS. There was no release of radioactive material to the environment despite damage to several pieces of equipment, such as the toppling of a heavy oil tank, short-circuiting of non-safety-related high-voltage metal-clad switchgear, and internal flooding of the reactor auxiliary building. While we conducted the plant control, people who lived in the neighborhood of the NPS, whose residences had been damaged by the tsunami, came to the plant seeking shelter and help with evacuation. We accommodated them in the on-site gymnasium and provided necessities such as food and blankets. Within several days, the number of evacuees increased and surpassed 360, and we lived with them for nearly 3 months. The key points for safe cold shutdown were first, the plant site grade was higher than the maximum tsunami height and, second, an emergency diesel generator for each unit and one of the off-site electrical power lines remained available. In addition to these factors, preparedness (such as seismic reinforcements for all units, updating of tsunami predictions where appropriate, and regular fire drills and simulator training for loss of off-site-power) contributed greatly. However, we must still achieve higher standards of safety. First, we must conduct a detailed evaluation of the March 11 earthquake and tsunami and take necessary actions based on this evaluation. Second, we have to take proper countermeasures against severe accidents. We have learned many lessons from the Fukushima Daiichi accident, and we will continue to make efforts in order to avoid a similar severe accident again.