The situation at Tokyo Electric Power Co.'s No. 1 Fukushima nuclear power plant, damaged by the March 11 quake and tsunami, is worsening. Following hydrogen explosions in the No. 1 and No. 3 reactors Saturday and Monday, respectively, serious accidents occurred in the No. 2 and No. 4 reactors Monday and Tuesday.

The No. 2 reactor suffered a major loss of coolant, fully exposing its nuclear fuel rods for several hours. An explosion occurred Tuesday morning in the suppression pool at the bottom of its containment vessel. Water in the doughnut-shaped pool cools steam from the vessel and turns it into water when pressure inside the vessel goes up. The explosion points to the possibility that highly radioactive materials leaked through a crack in the suppression pool. Later Tuesday morning, a fire broke out in the No. 4 reactor's outer containment building, possibly caused by a hydrogen explosion. Another fire occured Wednesday. The function to cool spent nuclear fuel stored in the building with water may have been lost. If such fuel is not cooled, and if it is exposed, highly radioactive materials could leak outside.

Possible partial damage to the No. 3 reactor's containment vessel was reported Wednesday. At one point on Tuesday, radiation of 400,000 microsieverts was detected inside the plant site — close to a level that can cause acute health problems. Evacuation from areas within a 20-km radius from the plant has been carried out. The government told people within a 20- to 30-km radius to remain indoors.