failed when the building that housed them flooded. All AC power was lost to units 1–4. All DC power was lost on Units 1 and 2 due to flooding, while some DC power from batteries remained available on Unit 3. Steam-driven pumps provided cooling water to reactors 2 and 3 and prevented their fuel rods from overheating, as the rods continued to generate decay heat after fission had ceased. Eventually these pumps stopped working, and the reactors began to overheat. The lack of cooling water eventually led to meltdowns in Reactors 1, 2, and 3. Further batteries and mobile generators were dispatched to the site, but were delayed by poor road conditions; the first arrived at 21:00 11 March, almost six hours after the tsunami struck. Unsuccessful attempts were made to connect portable generating equipment to power water pumps. The failure was attributed to flooding at the connection point in the Turbine Hall basement and the absence of suitable cables. TEPCO switched its efforts to installing new lines from the grid. One generator at unit 6 resumed operation on 17 March, while external power returned to units 5 and 6 only on 20 March. Hydrogen explosions As workers struggled to supply power to the reactors' coolant systems and restore power to their control rooms, three hydrogen-air chemical explosions occurred, the first in Unit 1 on 12 March, and the last in Unit 4, on 15 March. It is estimated that the oxidation of zirconium by steam in Reactors 1–3 produced 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, Unit 4 also filled with hydrogen, resulting in an explosion. In each case, the hydrogen-air explosions occurred at the top of each unit, in their upper secondary containment buildings which in a boiling-water reactor (BWR), are constructed out of steel panels which are intended to be blown off in the event of a hydrogen explosion. 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. In Reactors 1, 2, and 3,