TOKYO -- Workers entered one of the damaged reactor buildings at the stricken nuclear power plant yesterday for the first time since it was rocked by an explosion in the days after a devastating earthquake, the plant's operator said.
Tokyo Electric Power Co. said workers connected ventilation and air filtration equipment in Unit 1 in an attempt to reduce radiation levels in the air inside the building.
The utility must lower radiation levels before it can proceed with the key step of replacing the cooling system that was knocked out by the March 11 quake and tsunami that left more than 25,000 people dead or missing along Japan's northeastern coast.
Hydrogen explosions at four buildings in the six-reactor complex in the first few days destroyed some roofs and walls and scattered radioactive debris.
Power company spokesman Junichi Matsumoto called yesterday's development "a first step toward a cool and stable shutdown," which the utility hopes to achieve in six to nine months.
In mid-April, a robot recorded radioactivity of about 50 millisieverts per hour inside Unit 1's reactor building, too high for workers to enter.
Readings taken later in April in another part of the building were as high as 1,200 millisieverts.
The decision to send the workers in came after robots collected fresh data last Friday that showed radiation levels in some areas in the building were safe enough for workers to enter, said Taisuke Tomikawa, another company spokesman.
Two utility workers, wearing a mask and air tank, entered the building for about 25 minutes to check radiation levels. They were exposed to 2 millisieverts in that time, Tomikawa said. Outside, a temporary tent was erected, designed to prevent radioactive air from escaping.
Later, 11 other workers wearing similar gear went in to install ducts for the air filtering equipment. Twenty other workers outside provided help.
Doctors say radiation sickness sets in at 1,000 millisieverts and includes nausea and vomiting.