Nuclear workers have entered the No. 1 reactor at the earthquake-damaged Fukushima Daiichi plant in Japan.
It is the first time anyone has gone in since its roof was ripped off by a hydrogen explosion following the Japan earthquake and tsunami.
High radiation levels inside the building have stopped workers entering to repair its cooling systems and finally bring the plant under control, a process plant operator Tokyo Electric Power (TEPCO) has said may take all year.
The two TEPCO managers and 10 contract staff entering the reactor will spend just 10 minutes each inside, connecting eight duct-pipes to ventilators to filter out radioactive material in the air, the company said.
Protective suits, masks and air tanks will be used by the workers as they enter through a special tent set up at the entrance to prevent radiation leaks.
TEPCO said two workers had entered the building first to measure radiation and 10 would follow to connect the pipes.
The two workers were exposed to about two millisieverts of radiation, TEPCO spokesman Yoshinori Mori 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 to 250 millisieverts in an emergency.
Radiation of up to 49 millisieverts per hour was detected inside the building on April 17 when the company sent in a robot.
TEPCO also said in a report issued to Japan's nuclear 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", in which the water cooling them is below 100 degrees Celsius and the reactors are considered stable.
The earthquake and tsunami had knocked out all the cooling systems at the Fukushima plant, 240 km 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 barred from returning home due to concerns about radiation levels.
U.S. Nuclear Regulatory Commission chairman Gregory Jaczko told lawmakers in Washington that Japanese authorities were struggling to control the damaged plant.
"While we have not seen or predicted 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.