The operator of the crippled Fukushima Daiichi nuclear plant says rainwater contaminated by an exhaust stack may be leaking into the ground.
The 120-meter tall chimney was heavily contaminated in the 2011 accident, and rainwater coming out of the duct contains high levels of radioactive substances.
The Tokyo Electric Power Company has installed a concrete box to collect contaminated rainwater.
The box is designed to automatically send the water to designated tanks when the water level exceeds 40 centimeters.
But TEPCO found that the water level falls even when rainwater is not discharged to the tanks.
The company suspects that's because the concrete box has holes from which the stored water is leaking.
TEPCO says there are no major changes in radioactivity in surrounding groundwater and that it has not confirmed any impact on the environment so far.
But the operator says it will consider measures to prevent leakage.