Tokyo Electric Power Co. said Tuesday it was manually shutting down for inspection one of six reactors at a power station in Fukushima Prefecture after discovering a radioactive steam leak.

There was no impact on the environment, according to the utility.

The shutdown came only half a day after Tepco restarted the reactor following regular safety checks.

The leak was in the No. 6 reactor of the company's Fukushima No. 1 nuclear power station, which straddles the towns of Okuma and Futaba in Fukushima Prefecture.

An engineer spotted steam escaping from a valve in the reactor's piping system at around 11 a.m. Tightening the valve's shaft seal had no effect on the leak, forcing the shutdown of the reactor, company officials said.

Tepco said it began shutting down the reactor manually at 1 p.m. The reactor was to be completely shut down later in the day, the officials said.

The utility began restarting the reactor at midnight Monday. It had been shut down for a scheduled inspection.

The No. 6 reactor, with an output capacity of 1.1 million kilowatts, is the biggest of the six at the power station, which has a combined capacity of 4.7 million kilowatts.