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The problem at the coastal plant in Shizuoka Prefecture was caused by seawater that leaked into steam condenser coils at the reactor. The condenser is a device used to cool the steam as it comes out of the turbines and turn it back into water.
The problem prompted the utility to switch to another system to cool and stabilize the reactor, which achieved cold shutdown status shortly past noon, it said.
The No. 5 unit was the last active reactor at the plant, which was built on a major active fault line that runs through the city of Omaezaki.
To achieve a cold shutdown, the internal temperature of the reactor must be brought below 100 degrees.
The utility, which serves the roughly nine-prefecture Chubu region southwest of Tokyo, closed its only nuclear facility on Saturday after receiving an unprecedented request from the government, which is concerned that a large quake widely expected to hit the region could create another nuclear disaster.
The ongoing nuclear crisis at Tokyo Electric Power Co.'s Fukushima No. 1 power plant in the Tohoku region northeast of Tokyo was triggered by the devastating March 11 mega-quake and tsunami.
The Nagoya-based firm said the problem in the reactor's cooling system was found Saturday evening after a gauge indicated that around 400 tons of seawater had flowed into the condenser coils at around 4:30 p.m., most likely because of a piping problem.
The water also found its way into the reactor, making it necessary to desalinate it, the company said.
The shutdown of unit 5 completed Hamaoka's shutdown. The No. 4 reactor was halted Friday and put into cold shutdown the following day.
The No. 3 reactor was already suspended for regular checks. Reactors No. 1 and 2 were already shut down and are set to be decommissioned.
In a related development, Katsuya Okada, secretary general of the ruling Democratic Party of Japan, said Sunday that any nuclear power plants that are still closed or were shut down after March 11 should be restarted if they meet tougher safety standards.