The workers went into the No. 1 reactor building shortly before noon to install air ducts for a ventilation system that will filter the air to reduce radioactive substances.
Once the radioactive contamination in the air is reduced, workers will be able stay in the building longer to install a cooling system that Tokyo Electric wants to use to do a cold shutdown of the reactor.
The company says it should take two to three days to bring down the contamination to levels suitable for workers to stay for extended periods when wearing protective suits, masks and air tanks.
Cooling systems at Fukushima Daiichi, about 240 km (150 miles) north of Tokyo, were knocked out by the massive tsunami that struck Japan's Pacific coast after a massive earthquake March 11.
The disaster triggered the worst nuclear accident since Chernobyl as the cores of reactors 1-3 overheated and spewed huge amounts of radioactive contamination across the surrounding area.
The buildings that house reactors 1 and 3 were blown apart by hydrogen explosions in the first days of the crisis. Another hydrogen buildup is believed to have ruptured a water reservoir beneath the No. 2 reactor.
In April, Tokyo Electric has laid out a six- to nine-month timetable for winding down the crisis and bringing the reactors to a complete shutdown.
The disaster has led to mandatory evacuations of about 78,000 people living within 20 km (12.5 miles) of the plant and orders to people living another 10 km away to remain sheltered, affecting another 60,000-plus.