TOKYO (AP) -- Workers entered one of the damaged reactor buildings at Japan's stricken nuclear power plant Thursday for the first time since it was rocked by an explosion in the days after a devastating earthquake, the country's nuclear safety agency said.
The plant's operator, Tokyo Electric Power Co., said workers are connecting ventilation equipment in Unit 1 in an attempt to absorb radiation from the air inside the building. The work is expected to take about four or five days.
The utility must lower radiation levels inside the reactor before it can proceed with the key step of installing a cooling system that was knocked out by the March 11 quake and subsequent tsunami that left more than 25,000 people dead or missing along Japan's northeastern coast.
Workers have not been able to enter the reactor buildings at the Fukushima Daiichi nuclear power plant, about 140 miles northeast of Tokyo, since the first days after the tsunami. Hydrogen explosions at four of the buildings at the six-reactor complex in the first few days destroyed some of their roofs and walls and scattered radioactive debris.
In mid-April, a robot recorded radioactivity readings of about 50 millisieverts per hour inside Unit 1's reactor building -- a level too high for workers to realistically enter.
The decision to send the workers in was made after robots last Friday collected fresh data that showed radiation levels were safe enough for workers to enter some areas, said Taisuke Tomikawa, a spokesman for TEPCO.
Two workers entered the reactor building around 11:30 am (0230GMT) for about 25 minutes. They were exposed to 2 millisieverts during that time, Tomikawa said.
A dozen workers split up into teams were expected to go into the building on a rotation for short periods to limit radiation exposure.
"This is an effort to improve the environment inside the reactor building," he said.
The workers were equipped with protective gear and a mask and air tank set similar to those used by scuba divers, according to an official at the Nuclear Safety and Industrial Agency.
Outside the reactor building, the utility erected a temporary tent designed to prevent radioactive air from escaping.
TEPCO has laid out a blueprint for bringing the bringing the plant into a cold shutdown within six to nine months.
Japanese authorities more than doubled the legal limit of radiation exposure for nuclear workers since the crisis began to 250 millisieverts a year. Workers in the U.S. nuclear industry are allowed an upper limit of 50 millisieverts per year. Doctors say radiation sickness sets in at 1,000 millisieverts and includes nausea and vomiting.
Radiation leaking from the Fukushima plant has forced 80,000 people living within a 12-mile (20-kilometer) radius to leave their homes. Many are living in gymnasiums and community centers.
Associated Press Writer Mari Yamaguchi in Tokyo contributed to this report.