The operator of the Fukushima Daiichi nuclear plant has released into the ocean rainwater that had accumulated at the facility.
Tokyo Electric Power Company officials say radiation levels in the water were below government-set standards.
Heavy rain brought by tropical storm Man-yi led water levels to rise inside containment barriers that surround storage tanks.
Workers took water samples from the barriers on Monday to measure the amount of beta-ray emitting radioactive particles. The samples yielded between 3 and 24 becquerels per liter.
TEPCO officials concluded that the levels of radioactive strontium in the samples were below the government-set limit of 30 becquerels.
The utility says it started discharging excess water from the barriers into the ocean at around 12:40 PM on Monday.