"The Nuclear Regulatory Commission has allowed reactors to phase out some equipment that eliminates explosive hydrogen, the gas that blew up the outer containments of three reactors at the Fukushima Daiichi in Japan. The commission says it judged that at the American plants, the containments were strong enough that the equipment was not needed or other methods would do.
After the Three Mile Island accident in 1979, many reactors were required to install 'hydrogen recombiners,' which attach potentially explosive hydrogen atoms to oxygen to make water instead. At Three Mile Island, engineers learned that hot fuel could interact with steam to give off hydrogen. That caused the plant’s reactor to suffer a hydrogen explosion, although it did not seriously damage its containment. By contrast, the secondary containments at Fukushima Daiichi blew apart when hydrogen detonated inside them.
The change in commission policy was pointed out this week by a nuclear safety critic, Paul M. Blanch, who said that he had been involved in installing such equipment at Millstone 3, a nuclear reactor in Waterford, Conn."