So far we have talked about natural reactors, fast neutron reactors, plutonium production reactors, and thermal power plant reactors.
There is one cool reactor that I would be remiss in discussing, and that is the TRIGA reactor. I worked at a facility where two of these were located, and was licensed to operate them both. One was the very first TRIGA reactor ever built, rated at 250 KW (thermal), and the other was a MK IV model, rated at 1.5 MW (thermal). These reactors are swimming-pool reactors, and so they don't generate steam or electrical power using steam turbines.
In fact TRIGA stands for (T)raining, (R)esearch, (I)sotope production, (GA) General Atomic, the manufacturer.
In the late 1950's there was a desire to promote "Atoms for Peace". This was Eisenhower's attempt to invoke the power of the atom for peaceful purposes. The world was understandably horrified by the images of Hiroshima and Nagasaki, as well as the possibility that the budding cold war might turn into a hot war at some point.
The invention of TRIGA reactors went a long way toward fulfilling that vision. Unlike other reactors, TRIGA reactors have a solid moderator that is cast into the fuel itself. Therefore there is a homogenous blend of solid moderator and fuel. The moderator is Zirconium Hydride, and as you would expect, the hydrogen atoms do the moderating.
Because about 60% of the moderator is solid and homogenous with the fuel, this reactor has what is known as a "prompt negative temperature coefficient of reactivity". In other words, the very instant a runaway nuclear reaction begins and starts causing fuel temperature to increase, the solid moderator temperature also instantly increases, which in turn reduces the available thermal neutrons. This provides a VERY rapid damping of the runaway nuclear reaction.
If you recall in an earlier post, it is important (in all reactors except TRIGA) to never allow the reactor to be critical on prompt neutrons alone, because each generation of neutrons only last 10^-14 seconds. There is no way to control a reaction that proceeds so quickly, so the small fraction of delayed neutrons are what allow us to control reactors. Chernobyl, SL-1, and the Borax Experiment were each prompt criticality events that ended badly.
TRIGA reactors though, can easily tolerate a prompt critical event. Doing this is called "Pulsing" the reactor. Any reactor can be pulsed, but only a TRIGA can do it more than once ;) In fact, the record reactivity insertion into any reactor was TRIGA, at 5.22 times the value needed to be prompt critical. Because the moderator heats up as rapidly as the fuel, it shuts the reactor down just as soon as heat is generated, in a few thousandths of a second, without operator intervention.
TRIGA reactors ended up being sold around the world. Being low-power, they weren't practical for making weapons, and the solid UZrH moderator was incredibly difficult to extract from the fissionable fuel, so using the fuel for making weapons was not possible. Even so, currently manufactured TRIGA fuel has been reduced from 20% U-235 down to 7% to prevent proliferation.
Here is a video of a TRIGA reactor being pulsed to 2.5 x prompt criticality. Any other reactor would vaporize the fuel and create a steam explosion, blowing water upwards out of the tank!
Below is an image of a TRIGA reactor at the bottom of the pool, while not in operation. As you can see, it's quite simple. The fuel elements rest on a bottom grid plate, and are kept vertical by the upper grid plate. The fuel can be grabbed by a long-handled pole with a ball-type coupler at the end, similar to how modern hydraulic couplings work.
The rods sticking down into the core are just aluminum shafts that connect the drive motors to the control rods (which are partially out of the core).
The inner ring around the core is a lazy susan. Samples can be dropped into a number of holders in this dry ring. When the reactor is in operation, the ring rotates to ensure each sample is exposed to equal amounts of neutron flux. This is useful for performing neutron activation analysis on several samples at once.
The outer ring is a graphite reflector/moderator, which reduces the amount of fuel needed. The cans outside the reflector are neutron detectors, for determining what power level the reactor is at. The little lanyard at the bottom is attached to a neutron source (usually Americium/Beryllium). This makes sure there are enough neutrons available to start the reactor up. Also its a daily test to pull it and stick it next to each neutron detector and make sure they work OK before you start the reactor up.