I received a question from one of my readers this morning regarding the damaged Fukushima Daichi nuclear reactor having another meltdown and a radiation cloud reaching the West Coast of the United States. There is so much misinformation on the internet about Fukushima that I'm not surprised that people are concerned. If I wasn't a nuclear engineer that knew the facts, I would be concerned too.
The article the reader sent to me warns that steam emanating from the damaged reactor building is evidence that the nuclear material has reached criticality and is melting down and releasing radioactive material into the atmosphere. However, the steam is likely rainwater evaporating according to this report directly from TEPCO (Tokyo Electric Power Company). No change in radiation dose has been observed since the steam started appearing. No worries, there will be no "radiation cloud" reaching California "within 3-5 days" that would harm your health.
Check out Hiroshima Syndrome, a blog written by one of my nuclear engineering colleagues and a good source for debunking Fukushima conspiracy theories on the internet.