Really Excellent Explanations with Nuclear Engineer and Precautions to Limit Health Risks from Nuclear Plant Radiation in Japan and Other CountriesBeyond the basic explanation reproduced below I have found an additional 3 very comprehensive explanations and discussions of what can be done to protect from the nuclear radiation and fallout if it happens in Japan and in other countries.
All electric power plants use something to turn turbines which turn generators and so make electricity. In the case of hyrdoelectric dams (eg. Hoover Dam) the water falling on a wheel turns the turbine which turns the generator and makes electricity. In the case of coal fired power plants, the heat from burning the coal,makes steam that turns the turbines that turns the generator that makes electricity. In the case of atomic (nuclear) power plants, the heat from a controlled atomic reaction (fission) makes steam which turns the turbine which turns the generator to make electricity.
Both the atomic bomb and atomic power plants use nuclear fission, a reaction, the so called "splitting of the atom". When the atoms are "split" energy is given off. But the fission in an atomic plant still is different than an atomic bomb. In the case of an atomic bomb the fission reaction goes into overdrive and huge amounts of energy are released immediately.. In the case of atomic power, the reactions are controlled and contained, the energy is given off in a regulated and controlled fashion.
Never the less, a veritable zoo of radioactive particles and radiation are associated with the nuclear reaction. If they get near you or in you they can do an assortment of nasty things such as messing up your dna..Usually these atomic creatures are all contained but the problem is when things go out of whack and some of these zoo creatures get out of their cages so to speak.
What has happened in Japan? Why is it dangerous?...The "nuclear fuel" is encased and manipulated by elaborate mechanisms to control the speed of the nuclear reaction and also to prevent any of the radioactive material from escaping into the environment around the plant. Among the control mechanisms is the use of water to cool things down.
Although the exact situation in Japan is rather opaque (at least to outsiders) it seems that the damage from the earthquake and tsunami has broken some of the protective containment mechanisms..Some of the broken plants have not been able to provide adequate water to cool the nuclear fuel and the tremendous heat of the atomic fission has literally melted at least partially the atomic fuel.
There are layers of protective barriers between the "zoo of particles and radiation" and the outside world. There is good news and bad news about radioactive particles. The good news is that the radioactivity decreases with time, the bad news is that the time can be anywhere from a few seconds to thousands of years. The time it takes for the material to decrease is measured by the half life. If a material has a half life of 1 month, that means in one month half the material will be gone, and after one more month half of the remainder will be gone,etc.
The zoo of particles and radiation is made up of things with short and long half lives as well. If the zoo creatures get out their danger depends on how close they get to you. Some are dangerous if they get near you, others are dangerous only if they get inside you.
Right now everyone is working to keep the zoo creatures from getting out and near to people.