Things took a very bad turn last night in Japan. Unit #2 did breach containment. Levels of radiation that are dangerous local to the plant have been released.
Unit #4, which had been in cold shutdown, suffered what is believed to be a hydrogen explosion. I am reading and hearing reports that the spent fuel pool became uncovered at some point and caught fire here as well - trying to confirm. That would be extremely bad news. There are other reports saying that the fuel did not catch fire. Here's hoping that is the case.
We have gone deep into the wilderness of worst case scenario for this type of plant. I must admit I never would have thought that something like this could have happened in Japan. We are now talking about events that I have only studied in theoretical problems when I was in school.
Radiation Exposure Terms
Here is a quick reference for you regarding the language and terms used to describe radiation dose.
Japan and most of the rest of the world uses a unit called a "sievert" to measure dose to the body. In the U.S. we use the term "rem."
1 sievert (Sv) = 100 rem (R)
100 millisieverts (mSv) = 10 rem (R) = 10,000 millirem (mR)
Here is a link to a quick calculator to convert the values between rem and sievert. This might be necessary as reports in the U.S. media may cite units in rem and in international media they will cite dose in terms of sieverts.
NOTE: The doses that have been provided so far have all be in terms of dose per hour. This is important. The rate of absorption drives how long you can work safely in a specified radiation field. Always try and confirm the rate of the dose being provided in a media account.
Effects of Radiation
In the U.S. a radiation worker is allowed to take 5 rem per year. Normal members of the public are limited to 100 millirem (mR) per year. These figures are dose allowed above normal background radiation.
For perspective, in a report out of the NYT this morning, we see that radiation levels of 400 mSv/hr were recorded at the plant. That is 40 rem/hr. That can produce measurable and negative effects to health.
At the end of this crisis, the death or sickness toll from the radioactive release will probably be minimal compared to the loss of life from the earthquake and tsunami, but the fear factor involved in radiation will drive a disproportionate response.
The long-term effects should be minimal - with the plant sealed off, the reactors entombed in concrete and lingering radiation low if not normal background.
This disaster is going to have a lot of long term effects around the world. Remember that energy use drives economies. I'll hold off on speculating at the moment. There are other things to worry about.