An explosion was heard early Tuesday morning at the troubled No. 2 reactor of the quake-hit Fukushima No. 1 nuclear power plant, the government said, about five hours after work resumed to inject seawater into it to prevent overheating of exposed fuel rods.
Shortly after the 6:10 a.m. incident, the radiation level exceeded the legal limit to reach 965.5 micro sievert per hour, and it is feared that the reactor’s pressure-suppression system was damaged, the Nuclear and Industrial Safety Agency.
Tokyo Electric Power Co (TEPCO), the plant’s operator, said it is evacuating workers from the plant, except for those necessary for the work to cool the reactor.
The reactor’s fuel rods were fully exposed for around two and a half hours Monday evening after water levels rapidly fell, and again late Monday night although seawater was being injected, prompting the utility to open some steam valves at 1:10 a.m. in order to resume pumping seawater.
As of 3 a.m., pressure inside the reactor’s pressure container had dropped and it was believed seawater had been successfully pumped in, but a rise in water levels has not been confirmed, the utility said.
Prime Minister Naoto Kan said earlier in the morning that the government and TEPCO will set up an integrated headquarters, headed by himself, to address issues at the Fukushima No. 1 plant.
With radiation levels around the facility up, TEPCO suspects the core of the No. 2 reactor has partially melted, a critical nuclear safety situation.
The development follows hydrogen blasts at both of the plant’s two other reactors whose cores are also believed to have partially melted, occurring Saturday at the No. 1 reactor and Monday at the No. 3 reactor.
‘‘A worrisome situation remains but I hope we can overcome this crisis,’’ Kan said of the nuclear power plant. ‘‘I will take all measures so that damage will not expand.’‘
At the headquarters set up at the TEPCO head office, with TEPCO president and the economy, trade and industry minister serving as its deputy chiefs, Kan confronted TEPCO officials about their delay in reporting recent blasts at the plant.
The No. 2 reactor automatically shut down after the magnitude 9.0 earthquake hit the region on Friday. Its reactor cooling function was lost on Monday and water levels rapidly dropped, fully exposing fuel rods for around two and a half hours from 6:30 p.m.
Seawater was injected and water levels were increased temporarily but late Monday night they started dropping, leading to full exposure of the rods again.
At 1:10 a.m. Tuesday, TEPCO opened some steam valves and resumed work to pump seawater and was considering opening more valves, according to the company.