Workers at the Fukushima Daiichi nuclear power plant have entered one of the damaged reactor buildings for the first time since it was hit by an explosion days after a devastating earthquake, Japan's nuclear safety agency said.

The plant's operator, Tokyo Electric Power Company (Tepco), said workers were connecting ventilation equipment in Unit 1 to try to absorb radiation from the air inside the building. The work is expected to take about four or five days.

Radiation levels inside the reactor must be lowered before a cooling system can be installed. The previous cooling system was damaged by the 11 March quake and subsequent tsunami that left more than 25,000 people dead or missing along Japan's north-east coast.

Workers have not been able to enter the reactor buildings at the plant, about 140 miles north-east of Tokyo, since the first days after the tsunami. Hydrogen explosions in 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 levels of about 50 millisieverts an hour inside Unit 1's reactor building – a level too high for workers to safely enter.

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

Two workers entered the building at around 11.30am (3.30am BST). Due to the high radioactivity, teams were expected to go into the building on rotation for short periods, Tomikawa said.

"This is an effort to improve the environment inside the reactor building," he said.

Since the crisis Japanese authorities more than doubled the legal limit of radiation exposure for nuclear workers to 250 millisieverts a year. Workers in the US nuclear industry are allowed an upper limit of 50 millisieverts per year. Doctors say radiation sickness sets in at 1,000 millisieverts and leads to nausea and vomiting.

Radiation leaking from the Fukushima plant has forced 80,000 people living within a 12-mile (20km) radius to leave their homes. Many are living in gymnasiums and community centres.