Workers were set to enter the No.1 reactor building at Japan's crippled Fukushima Daiichi nuclear plant on Thursday for the first time since an explosion ripped its roof off a day after the devastating March earthquake and tsunami.

Tokyo Electric Power (TEPCO) said 12 staff equipped with protective suits, masks and air tanks would go through a special tent set up at the entrance to prevent radiation leaks.

They will then spend about 10 minutes each inside connecting eight duct-pipes to ventilators that will filter out the radioactive material in the air.

"Groups of four will go in one by one to install the ducts. They'll be working in a narrow space," TEPCO spokesman Junichi Matsumoto told reporters at a news conference.

The high radiation levels inside the building have so far prevented workers from entering to repair its cooling systems, and TEPCO has said it may take the rest of the year to bring the nuclear plant back under control.

The workers are expected to be exposed to about three millisieverts of radioactivity in the 10 minutes they will spend in the building, Matsumoto said.

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 situation. Radiation of up to 49 millisieverts per hour was detected inside the building on April 17 when the company sent in a robot.

The magnitude 9.0 quake and massive tsunami that followed on March 11 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. People living within a 20 km radius of the plant were evacuated and banned from returning home on April 21 due to concerns about radiation levels.

The Japanese government and TEPCO have come under fire both at home and abroad for their handling of the crisis. Angry families at an evacuation center shouted at TEPCO President Masataka Shimizu when he visited on Wednesday, telling him to kneel down and apologize.

"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.

"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 lawmakers in Washington on Wednesday that Japanese authorities were struggling to control the damaged Fukushima nuclear plant.

"While we have not seen or predicted any new significant challenges to safety at the site, we have only seen incremental improvements towards stabilizing the reactors and spent fuel pools," Jaczko said.