There was a small release of radioactive steam at Three Mile Island in 1979, and there have also been a few releases at Fukushima Daiichi. These produce radiation at about the level of one dental X-ray in the immediate vicinity and quickly dissipate.
If a meltdown does occur in Japan, it will be a disaster for the Tokyo Electric Power Company but not for the general public. Whatever steam releases occur will have a negligible impact. Researchers have spent 30 years trying to find health effects from the steam releases at Three Mile Island and have come up with nothing. With all the death, devastation and disease now threatening tens of thousands in Japan, it is trivializing and almost obscene to spend so much time worrying about damage to a nuclear reactor.