The NRC issued the order, which requires all reactors with "Mark I" and "Mark II" containment buildings to install or improve "hardened" vents so they can handle the pressures, temperatures, hydrogen concentrations and radiation levels from a damaged reactor, the agency said in a statement Thursday.
"Strengthened vents will help these plants continue to protect the public and the environment even if emergency systems can't immediately stop an accident," NRC Chairman Allison M. Macfarlane said. "By safely releasing built-up pressure and hydrogen, the plants will preserve the buildings that contain radioactive material."
Dave Lochbaum, director of the nuclear safety project at the Union of Concerned Scientists, a non-profit group that tracks reactor regulatory policy, said the order was progress, the Minneapolis Star Tribune reported.
"It is a step forward," Lochbaum said.
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