Workers entered one of the damaged reactor buildings at Japan's stricken nuclear power plant Thursday, may 4, 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.

The decision to send the workers in was made after robots last Friday collected fresh data that showed radiation levels had fallen in some areas of the reactor, said Taisuke Tomikawa, a spokesman for TEPCO.

Two workers entered the reactor building around 11:30 am (0230GMT). Due to the high radioactivity, teams were expected to go into the building on a rotation for short periods, Tomikawa said, according to the Associated Press.