Accident Sequence of Unit 3
In Unit 3, RCIC was operated using surviving DC power at 16:03 on March 11 after SBO. This RCIC was stopped at 11:36 on March 12. HPCI was automatically activated by the signal of “RPV low water level” at 12:35 on March 12. After the HPCI operation the PRV pressure, which once reached about 7.5 MPa, decreased to 4.8 MPa at 12:05, and 3.5 MPa at 14:25, 0.8 MPa at about 20:00, and 0.58 MPa at 2:42 on March 13 when HPCI was stopped (Fig. 3.6). The RPV pressure increased again to 4.0 MPa at 1 h and 7.38 MPa at 2 h after HPCI stopped. The RPV pressure rapidly decreased from 7.3 to 0.46 MPa at 8:55 on March 13 from opening of safety relief valves. At the same time, D/W pressure rapidly increased to 0.537 MPa, which is nearly the same as the RPV pressure, because of the inflow of hightemperature and high-pressure coolant. Because the D/W pressure was higher than the design pressure of the containment, 0.427 MPa, containment S/C venting was immediately conducted. D/W pressure decreased rapidly, but the opening of the venting was unstable and the opening began closing in a short time. Opening and closing of the venting were thus iterated five times. During this period RPV pressure and D/W pressure rapidly increased and decreased. Rapid increase of RPV pressure indicates a large amount of steam generation, which implies the existence of the core melt. The rapid decrease of both RPV pressure and D/W pressure clearly
Fig. 3.6 Accident sequence of Unit 3
indicates the effectiveness of the S/C containment venting for the depressurization of the containment. In contrast to Unit 2, the containment pressure of Unit 3 did not remain above 0.6 MPa for a long time because of the multiple depressurizations. Just after the fourth venting at 11:01 on March 14, the hydrogen explosion occurred at the reactor building. This explosion indicates that a large amount of hydrogen along with radioactive materials and steam was released to the reactor building, and the radioactive materials were released to the environment.
Spent Fuel Pools
In Unit 4, all core fuels had been transferred to the spent fuel pool for periodic inspection before the earthquake. The urgent task at the site, along with recovery of the power supply and the continuation of water injection into reactor vessels, was injection of water into the spent fuel pools. In the spent fuel pool in each unit, the water level continued to drop because of evaporation of the water caused by the heat of the spent fuel in the absence of the pool water cooling caused by the loss of power supply. Water injection to the spent fuel pool was conducted by the Self-Defense Forces, the Fire and Disaster Management Agency, and the National Police Agency, using helicopters and water cannon trucks. Concrete pump trucks were ultimately utilized, which led to stable water injection using freshwater from nearby reservoirs after the initial seawater injection. It is confirmed that the water level was never lower than the top of the fuel in any of the pools, and none of the pools suffered any significant structural damage , in contrast to concerns raised during the weeks following the accident. The spent fuel pool, storing 1,500 fuel assemblies, was designed such that the fuel elements would retain integrity for 30 days without active cooling.