The plant operator says the water had cleared tests for radioactivity.Tokyo Electric Power Company says the rainwater had accumulated in barriers surrounding the tanks used to store radioactive water.
TEPCO checked the water on Tuesday and officials say it cleared the standards set by the Nuclear Regulation Authority.
The utility says it released about 40 tons of rainwater on Wednesday morning.Officials say the water underwent 5 radioactivity checks including those for cesium-134 and strontium-90.
They say the level of tritium, which takes longer than other substances to measure, also likely meets standards for radioactive wastewater normally released into the sea.
They say the deduction is based on past measurements.
TEPCO has increased the number of tanks, personnel and patrols dedicated to controlling contaminated water since a storm earlier this month.
Radioactive water overflowed at that time from a barrier and from an overfilled tank.
Oct. 16, 2013 - Updated 02:56