The second-stage target to bring the nuclear disaster under control will involve achieving a cold shutdown state of the nuclear reactors at the Fukushima Dai-ichi power plant.

The shutdown of a nuclear reactor is when the state of the reactor is subcritical (this is when a nuclear fission reactor produces fission without achieving criticality i.e. when a small amount of fissile matter is still able to sustain a nuclear reaction).  A cold shutdown state is achieved when its coolant system is at atmospheric pressure and at a temperature below 95 degrees Celcius (200 degrees Fahrenheit); a temperature low enough where the water cooling the fuel rods does not boil when the reactor coolant system is depressurized.

Due to the damage to the reactor cores (where the fuels rods have melted), the Japanese government has so far failed to specify what a cold shutdown entails with the crippled reactors at the Fukushima Dai-ichi nuclear power plant.  The government is due to make this clear on Tuesday when it releases a revised plan to contain the nuclear accident.  It plans to define the term as bringing the reactor-bottom temperatures to about 100 degrees Celsius or lower, and substantially reducing the public's radiation exposure by controlling the release of radioactivity.



Achieving a cold shutdown state has been cited as one of the conditions for lifting the 20-kilometer no-entry zone around the nuclear plant.
  
It remains unclear, however, when the lifting would come, as the government still hasn't decided on benchmark levels of radiation that it deems safe enough for people to return to the restricted zone.