TEPCO, the utility that runs the earthquake-crippled reactors, told The New York Times the contract worker, a man in his 60s, collapsed after carrying heavy equipment in a waste disposal building. It said he was wearing protective clothing and a mask, and his body did not show unsafe radiation levels.
A doctor told Kyodo News the likely cause was a heart attack.
The reactor buildings still hold tons of contaminated water, and Saturday, Japan's Nuclear and Industrial Safety Agency said more than 12 feet of water was found in the basement of Unit 1.
Meanwhile Saturday, Chubu Electric Power Co. said it had finished shutting down the Hamaoka nuclear plant at Prime Minister Naoto Kan's request.
Hamaoka sits on a major fault line and is only 120 miles west of Tokyo.