Radioactive water leaks were found Friday at Onagawa nuclear power plant in Miyagi prefecture after the 7.4-magnitude aftershock shook northeastern Japan Thursday night, Kyodo News Agency reported.

Radioactive water spilled from pools holding spent nuclear fuel rods at the power plant. It was observed inside the buildings at all three reactors at the Onagawa plant, which has suspended operations since the mega earthquake and tsunami on March 11, according to the nuclear safety agency.

As much as 3.8 liters of water leaked at one of them, with the highest level of a radioactive isotope -- 5,410 bequerels per kilogram -- found in the spilled water on the floor beside a spent fuel pool in the building housing the No. 1 reactor, Kyodo said.

The powerful aftershock knocked out three out of four external power supply units in Onagawa, operated by Tohoku Electric Power Co., leaving only one power source to cool the spent fuel.

The cooling operations at the plant stopped once, but then resumed quickly.

One of the crippled external power supply was restored on Friday morning, the operator said.

Onagawa was the nearest nuclear power plant from the focus of the March 11 earthquake and the aftershock on Thursday. It still has its emergency backup generator to fall back on.

The strong aftershock didn't cause any problem to the troubled Fukushima No. 1 nuclear plant. No changes in radiation readings have been observed at any of the nuclear facilities in Japan's northeastern Pacific coast after the temblor.