water to form free hydrogen gas. The reaction between zirconium and the coolant produces more heat, accelerating the reaction. In addition, zircaloy can react with uranium dioxide to form zirconium dioxide and uranium metal. This exothermic reaction together with the reaction of boron carbide with stainless steel can release additional heat energy, thus contributing to the overheating of a reactor. Accident Background At the time of the Tōhoku earthquake on 11 March 2011, Reactors 4, 5, and 6 were shut down. However, their spent fuel pools still required cooling. Initial effects of earthquake The 9.0 MW earthquake occurred at 14:46 on Friday, 11 March 2011, with the epicenter near Honshu, the largest island of Japan. It produced maximum ground g-forces of 0.56, 0.52, 0.56 at units 2, 3, and 5 respectively. This exceeded the seismic reactor design tolerances of 0.45, 0.45, and 0.46 g for continued operation, but the seismic values were within the design tolerances at units 1, 4, and 6. When the earthquake struck, units 1, 2, and 3 were operating, but units 4, 5, and 6 had been shut down for a scheduled inspection. 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 safety procedure referred to as a SCRAM, which ends the reactors' normal running conditions by closing down the fission reaction in a controlled manner. As the reactors were now 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 normally 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 the other reactors suffered. Arrival of tsunami The largest tsunami wave was 13–14 m (43–46 feet) high and hit approximately 50 minutes after the initial earthquake, overwhelming the plant's ground level, which was above the sea level. The moment of impact was recorded by a camera. Disabling of emergency generators The waves flooded the basements of the power plant's turbine buildings and disabled the emergency diesel generators at approximately 15:41. TEPCO then notified authorities of a "first-level emergency". The switching stations that provided power from the three backup generators located higher on the hillside