The Nuclear Regulatory Commission is ordering U.S. nuclear power plants to improve ventilation systems at 31 reactors with designs similar to those that melted down two years ago in Japan.
The order requires U.S. operators to upgrade vents to ensure they remain operable even during severe accidents, such as the March 2011 incident at Japan's Fukushima Dai-ichi nuclear plant.
The order stops short of requiring plants to install filtered vents, as some safety advocates and NRC's staff had urged. The filters are required in Japan and much of Europe, but U.S. utilities say they are unnecessary and expensive.
The commission said the order will improve safety and help prevent radioactive particles from escaping into the atmosphere after a Japan-style meltdown. The commission said it will study requiring filters.