Tokyo Electric Power Co. said Thursday it will shut down the No. 1 reactor at the Fukushima No. 1 nuclear plant to investigate a leakage of fluids containing radioactive tritium found July 28 in the turbine building.

Tepco began procedures at 6 p.m. Thursday to shut down the reactor, which is expected to come to a complete halt early Friday. The 34-year-old reactor, in the town of Okuma, Fukushima Prefecture, is the oldest of all nuclear reactors owned by Tepco and had just resumed operations in July.

The shutdown comes after Japan Atomic Power Co. had to manually suspend operations at its Tokai No. 2 nuclear power plant in Tokai, Ibaraki Prefecture, on Monday and Wednesday. The operator claimed the halts were due to a nonradioactive steam leakage and a faulty pump valve.

Tepco said in a statement Thursday that it would be difficult to inspect the reactor while it is running because radiation is strong near the area of the leak.

Tepco, Japan's largest utility, has yet to identify where the leak originated and it is unknown when the reactor will be restarted.

The leaked fluid contained about 8,000 times the normal amount of tritium in seawater and was found to have seeped from beneath the floor near a pump at a system to condensate steam from the turbine back into water, according to Tepco.

The utility claimed the leak posed no threat to the external environment as no radioactive substance was detected in water outside the turbine building.

Tepco said it would thoroughly investigate underground equipment and the turbine building structure as it is possible that water that leaked from beneath the pump may have mixed with groundwater that penetrated the building.