After the Tsunami Attack
Fukushima Daiichi was hit by the first enormous tsunami at 15:27 on March 11, and the next enormous wave was around 15:35. The license for the establishment of nuclear reactors at the Fukushima Daiichi was based on the assumption that the maximum design basis tsunami height expected was 3.1 m. The assessment in 2002 based on the “Tsunami Assessment Method for Nuclear Power Plants in Japan” proposed by the Japan Society of Civil Engineers indicated a maximum water level of 5.7 m, and TEPCO raised the height of its Unit 6 seawater pump in response to this assessment. However, the height of the tsunami this time was 14 to 15 m, and all EDGs and the power panels installed in the basements of the reactor buildings and turbine buildings, except one air-cooled diesel generator for Unit 6, were inundated and stopped functioning (Table 3.2): this resulted in a station blackout (SBO) event for Units 1 through 4. All station DC powers (batteries) were also lost at Units 1, 2, and 4 because of the tsunami, but some DC power survived initially at Unit 3. The tsunami also damaged the coolant intake structures, the seawater pumps for
Table 3.2 Summary of damage after tsunami attack
auxiliary cooling systems, and turbine and reactor buildings, resulting in a loss of ultimate heat sink for all six units. It took 9 days to restore offsite power to the site. TEPCO's operators followed their manuals for severe accidents and attempted to secure power supplies to recover equipment within the safety systems, such as corecooling and water-injection systems, which had automatically started up. However, ultimately power supplies could not be recovered. Because the core-cooling functions using AC power were lost in Units 1 through 3, core-cooling systems without need of AC power were put into operation: the isolation condenser (IC) in Unit 1, reactor core isolation cooling system (RCIC) in Unit 2, and RCIC and high-pressure injection system (HPCI) in Unit 3. Schematics of IC and RCIC systems are shown in Figs. 3.1 and 3.2, respectively. These core-cooling systems, which do not need AC power, stopped functioning thereafter, and were switched to alternative injections of freshwater or seawater by fire-extinguishing lines, using fire engine pumps. In Units 1 through 3, because water injection to each reactor core was impossible to continue for several hours, the nuclear fuels were not covered by water but were exposed to the steam, leading to a core melt situation. It is believed that part of the melted fuel stayed at the bottom of the RPV. The main causes of the damage during the accident are illustrated in Fig. 3.3. More details on the accident sequences of Units 1 through 3 are described in the following sections .