TOKYO (Reuters) - 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 that followed on killed about 14,800 people, left some 11,000 missing and destroyed tens of thousands of homes.
It also knocked out all 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 10 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.
"We will operate the (ventilators) for about two or three days. After that we plan to start work on actually installing the cooling system," spokesman Naoyuki Matsumoto said.
The nuclear safety agency later said the ventilator system and filters were running.
TEPCO said the workers who did the work were expected to have been exposed to about 3 millisieverts of radiation each in the operation.
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 up to 49 millisieverts per hour was detected inside the building on April 17 when TEPCO sent in a robot to survey conditions.
Matsumoto could not give details of the 10 contractors' employment conditions, but he said they were bound by the same 250-millisievert limit for exposure to radioactivity as all other staff at the beleaguered plant.
TEPCO also said in a report issued to the nuclear safety agency on Thursday 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", in which the water cooling them is below 100 degrees Celsius and the reactors are considered stable.
If the nuclear safety agency approves the TEPCO report, the company said it would increase the rate at which it is pumping in water to speed up the process.
After the disaster nearly two months ago, people living within a 20 km (12 mile) radius of the plant were evacuated and barred from returning home on April 21 because of concern about radiation levels.
The government and TEPCO have come under fire both at home and abroad for their handling of the crisis.
Families at an evacuation centre shouted at TEPCO President Masataka Shimizu when he visited on Wednesday, telling him to kneel down and apologise.
"I could live with this if it was all caused by the natural disaster, but this is a man-made disaster and we have to pay for it," one man said in exchanges shown on television.
"You told us for years that nuclear energy was safe. We believed you. Now look where we are," said another.
U.S. Nuclear Regulatory Commission chairman Gregory Jaczko told legislators in Washington on Wednesday that Japanese authorities were struggling to control the damaged plant.
"While we have not seen any new significant challenges to safety at the site, we have only seen incremental improvements towards stabilising the reactors and spent fuel pools," Jaczko said.
Additional reporting by Yoko Kubota in TOKYO, Roberta Rampton and Ayesha Rascoe in WASHINGTON; Editing by Robert Birsel