The fuel rods create high temperatures which boil water then turn water to steam. During a disaster, when a power outage happens and diesel power generators which provide emergency power to the water pump are damaged by a tsunami or an earthquake, if no fresh water is being pumped to cool the fuel rods then the fuel rods continue to heat up. Once the fuel rods reach more than 1200 degrees Celsius, the zirconium tubes that contain the nuclear fuel will interact with the steam and split the hydrogen from the water. That hydrogen can then be released from the reactor core and containment vessel. If that hydrogen accumulates in sufficient quantities-concentrations of 4 percent or more in the air, then that hydrogen can explode, as has apparently occurred at Fukushima Daiichi reactors No. 1, 3, 4 but