"A Hard Look at U.S. Reactor Hardware After Fukushima"
"Over the objections of the nuclear industry, the staff of the Nuclear Regulatory Commission is planning to recommend the adoption of a new rule requiring American reactors similar to the ones at Fukushima Daiichi to install emergency vents with filters on them."
"The filtered vents would be required on two of the oldest reactor designs sold by General Electric. The idea is that their containments could be opened early in an accident to vent a puff of slightly radioactive gas and explosive hydrogen and thus prevent a buildup in pressure or explosions as an accident unfolds. The reactors did not have such vents originally, but most of the oldest models, equipped with Mark I containments, added vents in the early 1990’s.
After the Fukushima accident of March 2011, the commission ordered that vents be added to Mark II reactors as well but told its staff to quickly study whether filtered ones were necessary."
Source: NY Times, 11/05/2012