Chubu Electric Power Co. acted at a special board meeting after Prime Minister to install more emergency backup generators and other equipment and improve the water tightness of the reactor buildings.

The Hamaoka plant lacks a concrete sea barrier now. Sand hills between the ocean and the plant are up to 50 feet high, deemed enough to defend against a tsunami around 26 feet high, officials said.

The company said the reactors would be shut down after preparations are made, but it did not specify a target date.

The government earlier estimated the improvements could take two years.

The operator of the Fukushima nuclear plant, Tokyo Electric Power Co., has said the tsunami that wrecked critical power and cooling systems there was at least 46 feet high.

The March 11 quake and tsunami left more than 25,000 people dead or missing on the northeast coast and triggered the worst nuclear crisis since Chernobyl in 1986.

At the Fukushima plant Monday, workers and nuclear inspectors briefly entered the No. 1 reactor building to monitor radiation levels, as part of preparations to install a cooling system in the badly damaged unit that had been off-limits for weeks due to high radiation levels.

Workers first entered the building last week to connect ventilation and air filtration equipment to try to reduce radiation in the air. The utility hopes to start allowing workers into the building for full-scale work soon.