I am generally refraining from comment on the Japanese situation, because I don't have much useful to say, and the media is full enough already of newspaper articles about retweets of amateur analysis of sensationalistic news reports based on the utterances of people who don't know exactly what's happening and might not want to tell the truth if they did. But a couple of things.
Thing 1: There are some engineering PhDs
(well, at least one) declaring that "the plant is safe now and will stay safe".
As another engineering PhD, I would be extremely reluctant to make such confident assurances outside my field of expertise, and I think the guy cited above - who is mech eng, NOT nuclear
- is unwise to do so. (In fairness, I'll also note that he doesn't appear to have intended his post for widespread distribution, or perhaps he'd have been more circumspect.
Honestly, even if he WAS a nuclear engineer, I'd have reservations. Nuclear power plants are built to tremendously high engineering standards, but they're complex systems and Sod's Law is a powerful thing.
Thing 2: Since none of us know what's going on inside the reactors at the moment, we're all talking about the one thing we can quantify: radiation measurements. That's well and good, but if you're doing so, please make sure you're using the units correctly.
(Sv) is a measurement of how much harmful radiation a person has received*. If you catch about 1 sievert or more in a short period, you're probably in trouble. A millisievert (mSv) is one-thousandth of a sievert, and a microsievert is one-millionth of a sievert.
Sieverts per hour
(minute, second, year) is a measure of how fast that exposure is accumulating. If the radiation level is 100mSv/hr and you stay there for an hour, you're going to pick up 0.1Sv. If it's 400mSv/hr and you stay there for 15 minutes, you're also going to pick up 0.1Sv.
What this means is that if a radiation level is just reported in "sieverts", with no "per $TIME_UNIT" attached, you're missing information. It is quite likely that it's meant to be "per hour", but who knows?
Think of it like this:
Arnold: I make $5000 a week.
Barbara [writes newspaper article]: Arnold makes $5000.
Carla: I make $25,000 a year.
David [writes newspaper article]: Carla makes $25,000.
Edgar [reads both newspaper articles]: Hey, Carla makes five times as much as Arnold!
If your sources don't understand the distinction between total
exposure and rate
of exposure, they're probably not useful sources, and trying to draw conclusions from them is a bad idea.
*There are many other radiation-related units out there - rads, curies, becquerels, grays, rems, etc etc, with different meanings and purposes. Wikipedia should have a good discussion if you're curious.