It's the first time they've searched within a six-mile radius of the leaking facility. Falling radiation levels finally allow them to get closer to the complex.
Police believe the zone may contain the bodies of about 1,000 missing people.
Meantime, the emperor and empress of Japan met with more survivors on Thursday.
This was the royal couple's first trip into the disaster zone. Emperor Akihito and Empress Michiko visited two evacuation shelters in Asahi city, where they spoke quietly with evacuees sitting on mats in the community centers that have become their temporary homes.
Asahi, about 54 miles east of Tokyo near the Pacific coast, is one of the southernmost areas to be heavily affected by the March 11 natural disasters.
So far, only about 13,500 bodies have been found in the disaster, although nearly twice that number are believed dead.
Overall, nearly 140,000 people are still living in shelters after losing their homes or being advised to evacuate because of concerns about radiation leaking from the Fukushima Dai-ichi nuclear plant.
The Associated Press contributed to this report.