The Fukushima I Nuclear Power Plant comprised six separate boiling water reactors originally designed by General Electric (GE) and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the Tōhoku earthquake on 11 March 2011, reactors 4, 5 and 6 were shut down in preparation for re-fueling.[23] However, their spent fuel pools still required cooling.[24]
Immediately after the earthquake, the electricity-producing reactors 1, 2 and 3 automatically shut down their sustained fission reactions by inserting control rods in a legally-mandated safety procedure referred to as SCRAM, which ceases the reactors' normal running conditions. As the reactors were unable to generate power to run their own coolant pumps, emergency diesel generators came online, as designed, to power electronics and coolant systems. These operated nominally until the tsunami destroyed the generators for reactors 1–5. The two generators cooling reactor 6 were undamaged and were sufficient to be pressed into service to cool the neighboring reactor 5 along with their own reactor, averting the overheating issues that reactor 4 suffered.[24] The largest tsunami wave was 13 meters high and hit 50 minutes after the initial earthquake, overwhelming the plant's seawall, which was 10 m high.[6] The moment of impact was recorded by a camera.[25] Water quickly flooded the low-lying rooms in which the emergency generators were housed.[26] The flooded diesel generators failed soon afterwards, resulting in a loss of power to the critical coolant water pumps. These pumps needed to continuously circulate coolant water through a Generation II reactor for several days to keep the fuel rods from melting, as the fuel rods continued to generate decay heat after the SCRAM event. The fuel rods would become hot enough to melt during the fuel decay time period if an adequate heat sink was not available. After the secondary emergency pumps (run by back-up electrical batteries) ran out, one day after the tsunami, 12 March,[27] the water pumps stopped and the reactors began to overheat. The insufficient cooling eventually led to meltdowns in reactors 1, 2, and 3, where the resulting corium is believed to have melted through the bottom of each reactor pressure vessel. Meanwhile, as workers struggled to supply power to the reactors' coolant systems and restore power to their control rooms, a number of hydrogen-air chemical explosions occurred, the first in Unit 1, on 12 March and the last in Unit 4, on 15 March.[27][28][29] It is estimated that the hot zirconium fuel cladding-water reaction in reactors 1–3 produced 800 to 1000 kilograms of hydrogen gas each. The pressurized gas was vented out of the reactor pressure vessel where it mixed with the ambient air, and eventually reached explosive concentration limits in units 1 and 3. Due to piping connections between units 3 and 4, or alternatively from the same reaction occurring in the spent fuel pool in unit 4 itself,[30] unit 4 also filled with hydrogen, resulting in an explosion. In each case, the hydrogen-air explosions occurred at the top of each unit, that was in their upper secondary containment buildings.[31][32] Drone overflights on 20 March and afterwards captured clear images of the effects of each explosion on the outside structures, while the view inside was largely obscured by shadows and debris.[1]
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