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 225 kilometres 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 had fallen in some areas of the reactor, said Taisuke Tomikawa, a spokesman for TEPCO.

Two utility workers, wearing a mask and air tank similar to those used by scuba divers, entered the reactor building for about 25 minutes to check radiation levels. They were exposed to two millisieverts during that time, Tomikawa said. Outside the building, the utility erected a temporary tent designed to prevent radioactive air from escaping.

Later, 11 other workers — two from TEPCO and nine from its subcontractors — wearing similar gear went into the reactor building to install ducts for the air filtering equipment. Twenty other workers provided help from outside.

The utility hopes to start allowing workers into the building to set up a cooling system around mid-May. In addition to reducing radioactivity with the new air filtering system, it hopes to reduce it further by removing or covering up contaminated debris inside the building, Matsumoto said.

TEPCO is proceeding with a plan to fill the Unit 1 containment vessel with water to soak the core and cool it, and also plans to install big fans as an external cooling system, he said. TEPCO hopes to take similar steps at Units 2 and 3 but is struggling with tougher obstacles such as contaminated water leaks and debris.

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

In Vienna, International Atomic Energy Agency expert Hartmut Nies predicted traces of Cesium 134 and Cesium 137 leaking from the plant will be carried by the Pacific's Kuroshio current to the North American coast within two years.

"We expect that in one or two years it might be measured at the coast of Canada or California," Nies told reporters.

But the detected levels would be "very low" and there were no specific concerns, said Denis Flory, an IAEA deputy director general.

"It will be detected because radioactivity, as you know, is very easily detectable at very low levels," Flory said.