Japan's regulator says 10 of the country's nuclear power plants and other facilities have yet to complete work to prevent massive inflows of rainwater into buildings in the event of torrential rain.
The Nuclear Regulation Authority has called on operators to finish the work within a year.
The NRA urged them to take the steps after rainwater got into the Shika nuclear plant in central Japan and short-circuited a distribution switchboard last September.
At a meeting on Wednesday, NRA officials said 10 plants and facilities have yet to finish waterproofing areas of buildings where pipes enter from the outside.
New regulations established after the 2011 Fukushima Daiichi accident require operators to protect plants' power sources and reactor-cooling systems from inflows of rainwater and tsunami.
Some of the 10 plants and facilities are equipped with a drainage system for rainwater. But the regulator is urging the additional measures for greater safety.
Work has yet to be completed at: the Onagawa plant in Miyagi Prefecture, the Fukushima Daini plant in Fukushima Prefecture, the Kashiwazaki-Kariwa plant in Niigata Prefecture, the Hamaoka plant in Shizuoka Prefecture, the Shika plant in Ishikawa Prefecture, the Tsuruga plant in Fukui Prefecture and the Shimane plant in Shimane Prefecture.
Others are nuclear fuel reprocessing plants in Aomori and Ibaraki prefectures, and the Monju fast-breeder reactor in Fukui.
Officials say such measures are already in place at the restarted plants in Kagoshima and Ehime prefectures.