This is a bit out of my area of expertise. I've never had a need to take another 120 hours of specialty training to work at Hanford but some of my cohort have.
The Nuclear Regulatory Commission, the United States Environmental Protection Agency, and other U.S. and international agencies, require that licensees limit radiation exposure to individual members of the public to 1 mSv (100 mrem) per year, and limit occupational radiation exposure to adults working with radioactive material to 50 mSv (5 rem) per year, and 100 mSv (10 rem) in 5 years.
What the hell does all this mean you might be asking? As simply as I can say it works like this. We are being exposed to radiation on a regular basis from what's called background radiation. That can be naturally occurring radiation from radon gases, X-rays from the doctor or dentist and other sources like the burning of coal just to name a few. Fortunately most of these doses are very small in the fractions of an Sv. Radiation used to be measured in rads but that didn't take into account what was being exposed and what possible damage it might be doing to a human body. An Sv is a Sievert is named after Rolf Sievert, a Swedish medical physicist famous for work on radiation dosage measurement and research into the biological effects of radiation. It takes into account what part of the body or combination of body parts are effected. Were the exposures from ingestion, inhalation, or skin contact? Not something you can put your finger on with ease but easily measure with a photo sensitive badge like the type atomic workers are required to wear.
So should we be concerned about the releases in Japan? That will depend on many factors but judging from the distance from here to Japan it's likely that most of the radiation will dissipate before it reaches our shores. Rain would wash it into the ocean which would dilute it further. Remember the Russians were noted for dumping their nuclear waste in the oceans off their coast for years. Not a good thing I know but a fact none the less.
On the matter of nuclear power plants I've always been against them. Not that I'm a purist in terms of environmental issues but even from a economic sense they make no sense. The amount of energy and materials required to build, operate and then dismantle a plant is far more than the amount of energy they can produce in 25 years (their lifespan). There are several such plants that are ticking time bombs operating well beyond their useful lives. Then there's the issue of the waste because no body wants it in their backyard.
An Update on the Fukushima Daiichi plant: Seems the 40 year old plant had some safety violations in the past. 29 violations to be exact. And the president and several of his board members were forced to resign because of this. A few things to consider are that the plant is operating beyond its' life span as I pointed out earlier and that safety inspections were falsified. A nice invitation for disaster. But as we all know money and profits come first even in a global market place.
Update 2: Here's a video from Russia Today of the explosion hope it works.