Japanese workers entered the No.1 reactor building at the crippled Fukushima Daiichi nuclear plant on Thursday for the first time since a hydrogen explosion ripped off its roof a day after a devastating March 11 earthquake and tsunami.
High radiation levels inside the building have prevented staff from entering to start installing a new cooling system to finally bring the plant under control, a process plant operator Tokyo Electric Power (TEPCO) has said may take all year.
The magnitude 9.0 quake and massive tsunami killed about 14,800 people, left some 11,000 missing and destroyed tens of thousands of homes.
It also knocked out the cooling systems at the Fukushima plant, 240 km (150 miles) north of Tokyo, leading to the greatest leak of radiation since the 1986 Chernobyl disaster.
Two TEPCO staff and 11 contractors with protective suits, masks and air tanks worked for 1-1/2 hours, moving in and out in small groups to connect duct pipes to ventilators that will filter out 95 percent of the radioactive material in the air, a company spokesman said.
"Things are moving forward steadily, one by one. Our final goal is to bring them (the reactors) to a cold shutdown. As a first step towards that, we were able to go inside the building and this is a major point," TEPCO official Junichi Matsumoto told a news conference.
"While they were going inside the reactor building for the first time since the hydrogen explosion, we already knew the situation inside and the radiation by using robots, so they did not go inside as so-called suicide squads," he said.
Cold shutdown means the water cooling the nuclear fuel rods inside reactors is below 100 degrees Celsius and the reactors are considered stable.
The ventilator system were now running and they w ould be operated for two or three days. If the radiation level drop ped after that, workers w ould start to install the cooling system, TEPCO officials said.
TEPCO said preliminary checks showed that the workers who went inside, most of them in their 40s and 50s, were exposed to up to 2.8 millisieverts of radiation each.
Under Japanese law, nuclear plant workers cannot be exposed to more than 100 millisieverts over five years, but to cope with the Fukushima crisis, the Health Ministry raised the legal limit on March 15 to 250 millisieverts in an emergency.
Radiation of 10 to 93 millisieverts per hour was detected inside the building when two workers went inside ahead of the pipe installation works to measure radiation.
Report rules out new explosion
TEPCO also said in a report to the Nuclear and Industrial Safety Agency that there was no possibility of another hydrogen explosion at the No.1 reactor due to progress in filling the containment vessel, an outer shell of steel and concrete that houses the reactor vessel, with water.
Workers have been trying to fill the reactors with enough water to bring the nuclear fuel rods inside to a cold shutdown and if the agency approves the report, TEPCO would increase the rate at which it is pumping in water to speed up the process.
Meanwhile, the temperature of the No.3 reactor, currently about 240 C, has been rising and need ed to be watched, said Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency.
"We do not need to be suddenly worried, but we need to be careful if it continues to rise," Nishiyama told a news conference, adding that TEPCO has increased the amount of water it is pumping into the reactor.
Sea s ediment with radioactivity 10,000 times higher than usual was found near the plant, Nishiyama said.
"The possibility of a new leak (into the sea) is not zero, but we do not have firm data," he said.