TOKYO—Chubu Electric Power Co. President Akihisa Mizuno said Monday the company has decided to halt the No. 4 and No. 5 reactors at its Hamaoka nuclear power plant in Shizuoka Prefecture, central Japan, while keeping offline the No. 3 reactor, which is currently undergoing maintenance. The amount of time they will be offline wasn't immediately clear.
This move to halt all operations at the plant comes after a request by Prime Minister Naoto Kan on Friday to shut the plant's three reactors until all planned safety measures against earthquakes and tsunami of similar magnitude to those seen on March 11 are implemented.
Separately, eight workers and a government inspector entered the reactor building of the quake-hit Fukushima Daiichi nuclear complex's No. 1 unit early Monday to survey conditions inside, another step toward bringing the complex's three damaged reactors under control.
It was the first time a government official took a firsthand look at the one of the reactors since they began leaking radiation after the devastating March 11 earthquake and tsunami.
"No noticeable damage or leakage was found in the area surveyed," said Hidehiko Nishiyama, spokesman for the Nuclear and Industrial Safety Agency, after the inspection. But he added the 30-minute survey wasn't sufficient to assess the condition of the vital pressure vessel, a thick steel cylinder that houses the nuclear fuel.
"There is a long way ahead to fully stabilizing all three reactors," another official said. "It is too early to rejoice."
The eight workers from plant operator Tokyo Electric Power Co. and one senior inspector from NISA entered the building with protective gear and face masks to check radiation levels. Workers plan to set up a heat exchanger by the end of the month as part of a plan to retrofit an air-based cooling system.
Later Monday, Chubu Electric Power Co. agreed to a government request to shut down its Hamaoka nuclear power plant in Shizuoka Prefecture until measures to protect against a tsunami are in place.
At the Fukushima plant, the inspection of the ground level of the reactor building was the first by humans since the complex was hit by the earthquake and tsunami. A hydrogen explosion the next day damaged the upper part of the building, and robots carried out an initial inspection last month.
During the 30-minute survey, the nine members of the group received cumulative radiation ranging from 2.7 millisieverts to 10.56 millisieverts, Mr. Nishiyama said.
Radiation levels in some places reached 600 to 700 millisieverts per hour, compared with an annual dosage limit of 250 millisieverts allowed for a male worker engaged in disaster relief work at the complex, he said.
In addition to the heat exchanger, workers plan to put up a new water gauge to track the operation to fill the whole steel containment vessel with million of liters of water and so submerge the fuel and the pressure vessel.
But the installation of such equipment is likely to be hampered by high radiation; the group found several "hot spots," especially around pipes suspected to be clogged with highly radioactive material.
Mr. Nishiyama said that for work to proceed without excessive radiation exposure, these spots would need to be shielded with lead sheets.
There are concerns the pressure vessel was damaged during the earthquake or as the nuclear fuel overheated in the days after.
The air-cooling system has become necessary for Reactors No. 1 to 3 because the regular water-based cooling system was disabled by the tsunami.
Correction
Monday was the first time a government official had seen one of the damaged Fukushima Daiichi reactors firsthand. An earlier version of this story incorrectly said it was the first time workers had entered the No. 1 reactor building since the March 11 earthquake and tsunami.
