The company later sent in more workers to set up a ventilation system to filter radioactive material out of the air within the reactor building, the officials said.
"We are sending workers as a small group for a maximum of 10 minutes so
that radiation they will be exposed to can be limited," said Satoshi Watanabe, a spokesman for operator Tepco.
Other officials said that the workers were exposed to far lower levels of radiation than expected.
The plan is to reduce radiation to a 20th of the current level and then begin building a new cooling system outside the reactor in a bid to regulate temperatures inside.
Workers have been dousing the reactors and fuel rod pools with water to cool them and prevent a meltdown.
Tepco plans to complete construction of the new cooling system in late May or early June, local media said. Engineers aim to achieve stable "cold shutdowns" towards the end of the year.
Since the accident, sparked by the 11 March earthquake and tsunami, the reactor has been too dangerous for humans to enter and Tepco has so far used robots to gauge radiation and temperature levels in the reactor building, which was damaged by a hydrogen explosion.
Separately, Tepco said it planned to raise water levels inside the containment vessel of the number one reactor by adding another two tonnes of water to step up efforts to cool the entire atomic furnace.