What is Contamination and Radiation?

Contamination is something that we have heard about a lot in the news lately.  However, as with most things in the news explanations have been sorely lacking.  So here is a brief primer on contamination and radiation.

Contamination is basically radioactive material in a place where it isn't suppossed to be.  It comes from a number of places such as byproducts of the fission process itself or from materials that have been "activated" by exposure to radiation.  That is, the energy from the fission process has actually rendered normally non-radioactive materials radioactive by adding energy to their atoms and making them unstable.  Contamination typically origniates in the reactor core, and is then carried around in the primary coolant.  Through various leaks in different systems and planned transfers to others the contaminated coolant makes its way to various areas of the plant.  The leaks of course present the biggest hazard for the spread of contamination.  Even when the water dries, it will leave the radioactive material behind to possibly be carried into the air or picked up on a boot or a tool.
Raidation is essentially the energy emitted from and unstable material.  It takes the form of gamma particles, beta particles (electrons), alpha particles (two protons and two neutrons, basically the nucleus of a helium atom), and neutrons.  The alpha particles are the most likely to cause damage as they have most mass and are the slowest moving and are therefore more likely to interact with the cells of your body, the can however, be stopped by a piece of paper.  Betas have less mass and are faster moving and so require more material to stop them, such as your clothes.  Gammas have little to no mass and move very fast and so will likely pass right through you without doing anything.  However, they are also the most plentiful form of radiation and so this is the kind to most wary of.  Basically, a handfull of pebbles won't hurt you much but a storm of them will.  Neutrons have mass but no electrical charge and so are unlikely to interact.  However, this also makes them harder to stop, like the gammas.  They also have a relatively large amount of mass making them dangerous when they do interact.  However, they are only present in appreciable quantities during the actual fission process. 
How dangerous a given piece of contamination is depends entirely on its makeup, that is, what sorts of fission products are present and how much?  Obviously, more material means more radiation which means more danger.  Also, if the contamination is largely something like nitrogen 16, it will not be dangerous for long as this has a half-life of minutes or hours.  Something like iodine has a half-life of days.  Potasium (like in your bannana) has a longer half-life.  The longer the half-life, the longer a bit of contamination will remain dangerous.

That's it for the primer.  Later this week, we'll start to get into exactly what the heck went on in Japan and how it will affect us here.