TEPCO said the radiation levels should drop over the next three months. It will take about six months for the reactors to achieve "cold shutdown" in which the temperature of the water inside the reactor is less than 100 degrees Celsius (212 F).
The current plan for cooling the reactors will mean injecting nitrogen into the reactor pressure vessel. All four damaged reactors experienced hydrogen explosions when water, heated by nuclear fuel, turned to steam and reacted with the zirconium alloy cladding of the fuel rods. Hydrogen, when exposed to oxygen, combusts. Nitrogen is an inert gas, so TEPCO hopes that it will prevent further explosions.