Against my better judgment...

Splitting the Atom - A Primer in Oversimplification

 

The whole idea is to start a chain reaction. What happens if you bombard a fissile isotope with neutrons of sufficient quanitity and velocity? One is likely to be absorbed into the fissile isotope's nucleus. Consider U-235, (ok, 235U), a common reactor isotope. When it absorbs a neutron you end up with U-236 which is highly unstable. It fissions, "breaking down" into other lower level products and releases quite a bit of kinetic energy (heat); it also releases 2-3 free neutrons. These 2 or 3 neutrons are absorbed into other nearby nuclei, which in turn fission and release their own free neutrons. What you've got is a self-sustaining chain reaction a.k.a. critical mass.

 

This is great for a bomb. Ever hear the phrase "weapons grade" plutonium? It basically means it's highly refined stuff, much more so than the fuel in nuclear reactors, since the idea is lots of power density and an extremely fast release of that energy. If you can bring two masses of fissile material together (creating a supercritical mass) faster than the time it takes for spontaneous fission, you've got yourself a great bomb. PU239 is ideal for its spontaneous fission probability, but bringing the masses together like with Uranium would be too slow. In this alternative the idea is to "crush" a sphere of PU239 using traditional explosives, compressing the material into a density (and quickly enough) to go critical. Seperating the TNT from the nuclear payload pretty much guarantees the weapon is "safe".

 

But what about harnessing nuclear energy for electricity? In this case we want a controlled reaction. How to do that? If we absorb free neutrons, we effectively put limits on the chain reaction taking place. This is the function of control rods in a reactor core. Expose more of the rods and you absorb more neutrons.

 

In a traditional elec plant coal or gas is burnt releasing heat, the heat turns water into steam, the steam drives turbines, then the steam precipitates back to water through cooling towers (or pumped into cooling ponds). Nuclear plants operate the same way only the fissioning atoms are generating the heat in the reactor. In fact the towers one associates with a nuke plant are not reactors at all, they're cooling towers for the coolant. The one caveat is that the water circulating through the reactor becomes radioactive and so cannot be pumped through cooling towers or ponds directly, rather, the cooling system is a closed loop and transfers heat to a secondary cooling system through heat exchangers, much the same way a car transfers heat in liquid-form to air through a radiator (heat exchanger).

 

How safe are they? Nuclear plants have a lot of backup systems, including containment vessels around the reactor, emergency cooling systems if pumps should fail, systems that initiate a SCRAM when parameters go out of limits, and so forth. That said, we can't forget 1. Murphy's Law,  2. Corporate Bottom-Lines and 3. People can be pretty stupid.