Shielding: how it works.
Nuclear fission spews forth lots of radiation. Fast neutrons, thermal neutrons, gamma rays, beta particles (electrons), positrons (anti-electrons), alpha particles (helium nucleus), fission products, and other subatomic particles and energy. Some of these things can hurt you under the right circumstances. Most radiation can be greatly reduced with the appropriate shielding.
Shielding is measured in tenth-thickness. A tenth thickness is the amount of shielding you need to reduce the level of radiation to 10%. Example: two tenth-thicknesses would reduce radiation coming from a source to 1% (a tenth of a tenth).
Fission products: these are usually poisonous heavy metals, that may be radioactive or stable, and may be ionized. These are really only a problem in a nuclear bomb, as these contaminate the whole area as fallout. Luckily they cannot penetrate your skin so you can wash them off with soap and water. Don't eat them and you should be fine. Right now the workers at the Japanese plants need to wear anti-contamination suits to keep any of these fission products from hurting them. The general public need not worry about these as they won't enter the atmosphere in any significant amount beyond the first level of containment. Alpha particles are pretty much the same thing, and only cause problems if ingested. They can be blocked by a piece of paper.
Neutrons: These come in Fast, and the slow moving variety called Thermal. Fast neutrons generally pass through the body without causing any harm, as long as they come in small quantities. When they slow down is when they can hurt the body by ionizing cells as they pass around the body. Luckily they are effectively shielded by water, but it takes quite a bit. The tenth thickness for water is 12" (if I recall correctly). This means that almost all neutrons never leave the reactor compartment, as it is a huge pressurized water system.
Electrons and Positrons: these tiny particles could burn you since they can slightly penetrate your skin. Luckily they are so reactive that they are caught in other reactions before they get very far. Absolutely no risk unless you ingested a beta-emitting cookie.
Gamma Radiation (photons): Ok this is the bad one. Gamma radiation is naturally comes from the visible and invisible light coming from the sun. Most of the really bad gamma rays are blocked by our magnetic belt and our atmosphere, but some still makes it to earth to give you a sunburn. Every wonder why the X-ray technician hides in a little room while giving you an x-ray? These pass right through your body, and depending on the gamma's energy levels it could cause a few ionizations to damage cells along the way. Luckily metal makes a very effective shield against gamma's. Lead has a 2" tenth-thickness, and steel has a 4" tenth thickness. An theres is literally tons of steel surrounding the reactor core. This is why it is safe for radiation workers. Unfortunately, when there is a partial meltdown, you end up with some nasty radioactive gases like Nitrogen that emit gammas. The good thing is that they have an extremely short half-life. This means that they will detect high radiation levels around the plant when pressure needs to be vented, but significantly less within a short distance, and barely above background levels once you are outside the immediate vicinity.
Edited by damric - 3/16/11 at 5:43pm