As far as I know, I'm the only forum member here with any experience in nuclear power. While Chernobyl is what leaps to peoples minds, what is happening in Fukushima Dai-ichi is more like what happened at Three Mile Island, at this point, although it's likely to get worse unless they can restore cooling in some method.
This is especially critical now that fuel rods are exposed
http://www.msnbc.msn.com/id/42066534/ns/world_news-asia-pacific/?gt1=43001 (for clarification, radiation does not "spew" out from a reactor or any other source)
Detectors showed 11,900 microsieverts of radiation three hours after the blast, up from just 73 microsieverts beforehand, Kinjo said. He said there was no immediate health risk because the higher measurement was less radiation that a person receives from an X-ray. He said experts would worry about health risks if levels exceed 100,000 microsieverts.
The radiation that is being released is 1/10 of the levels that cause concern, and while any exposure to radiation entails risk, at this point I think officials are being cautious out of the fear associated with nuclear energy.
For comparision, 1,000 microsieverts = 1 REM. Here is what nuclear energy workers in the US are allowed to receive on an annual basis
(
http://www.jlab.org/div_dept/train/rad_guide/dose.html) While the amount being leaked may seem higher than what a US worker is allowed to receive, radiation drops off dramatically over distance. In the military we used TDS to minimize our exposure. Minimize your Time in the radiation field, maximize your Distance from the source, and increase Shielding.
What is a concern in this type of situation is contamination. Contamination is small radioactive particles that can be carried on wind or water currents. A good way to visualize this is that radiation is the smell, contamination is the pile of poo creating the odor.