"The Nuclear Industry and Venting, Round 2"
"The debate over making post-Fukushima Daichi improvements to American reactors is getting down into the details, and one focus is pressure relief vents."
"The idea behind venting a nuclear plant is that if a reactor overheats, chemical reactions will produce steam and gases that could overpressurize the containment building. The containment is a major line of defense against the release of radioactive materials, and rather than let it burst like an overfilled balloon, the idea goes, it would be more sensible to let the reactor dump a little bit of slightly radioactive gas into the environment.
Boiling water reactors like the kind used at Fukushima and at many sites around the United States have smaller containment vessels and are therefore more prone to overpressurization, and they use more metals that can give off gas if exposed to extreme heat."Source: Green/NYT, 07/06/2012