Nuclear power starts out with the mining of fuel.
Uranium is relatively abundant, however most of it is found in ore where the
concentration of actual uranium is fairly low (ranging from 0.1% to 12%). Most of the mining is done in the
US,
Czechoslovakia,
Canada, and
Zaire with Canada producing about 35% of the world's uranium. Once the
ore has been collected, it is ground and through a series of
chemical reactions it is converted to
Uranium Hexafluoride. The mixture ends up being about 99.28%
U-238 and 0.71%
U-235. And here lies the problem, U-238 is not
fissionable and is useless in a reactor.
The mixture is put through a series of membranes that slow the movement of the heavier U-238 to increase the yeild to 3-5%. This process actually uses quite a bit of energy and is not very efficient. The final product is formed into pellets which are packed into rods filled with helium. The rods are sealed and placed in racks for use in nuclear reactors. On average, the rack for a Boiling Water Reactor contains 63 rods, and for a Pressurized Water Reactor it is about 264 rods. There are usually around 750 of these racks in a Boiling Water Reactor and 150 in a Pressurized Water Reactor. The fuel in a typical reactor lasts 4-6 years, after which it is typically stored until no longer radioactive (which is a fair bit of time, several hundred years at least). Outside of the US, the fuel may be reprocessed to capture usable fuel in the waste.
The reactor itself consists of a reactor vessel which contains the fuel rod assemblies. This vessel is surrounded by another container which is filled with water (usually). The water serves two purposes, to keep the reactor cool and to produce steam for power generation. This is then surrounded by a large concrete structure which absorbs any stray radiation. Now we get to the tricky part... Basically, U-235 is naturally unstable and will decay giving off neutrons. When this happens, the neutrons will normally fly off into space. In a reactor however, they are surrounded by other U-235 atoms which will absorb the neutron. Being unstable, when the neutron is absorbed they will split, releasing a great deal of heat and more free neutrons (typically 2-3 neutrons per split).
Now, for use in power generation, the core has to be slightly supercritical. A reaction is critical if on average, exactly one released neutron hits one other U-235 atom and causes it to split. It is subcritical if on average less than one neutron from each split hits another atom. Supercritical is of course when more than one released neutron causes another atom to split. For weapons use, you need a very supercritical reaction, but for power production you want it only slightly supercritical so that you can exert some control over the reaction without having a meltdown.
To control the reaction, the core uses control rods. These are rods which are made of materials that absorb neutrons. By raising and lowering them into the core, you can control how many neutrons are absorbed and thus the rate of the reaction. When the rods are fully inserted into the core, they absorb all free neutrons and thus the reactor is stopped. To start the reactor, the rods are slowly withdrawn and the reaction will begin naturally. The fuel rods also need to be rotated to keep the fuel from melting due to uneven heating (fuel near the outside of the rods absorb more heat).
Power is produced using the heat generated in the reaction, much in the same way as typicaly power production. The heat causes the surrounding water to reach very high temperatures. This hot water is then used, one way or another, to produce steam which is routed to a steam turbine (which I will need to do a node on in the near future). As the steam expands through the turbine blades, it causes them to rotate turning what amounts to a giant coil in a magnetic field, which thanks to the miracles of physics, produces electric current. Simple, eh?
So how about a few different types of reactors?
A boiling water reactor is a type of reactor that actually produces steam inside the reactor
core. Instead of having a high pressure system inside the core which keeps the water in liquid form, the system operates at a low enough pressure (typically around 70
atmospheres) to allow steam to form. The steam is vented through the top of the core where it goes through a few
filters to remove excess
water droplets before going on to the turbine. The excess steam is then cooled and pumped back into the reactor core. Becuase of this pumping, it also allows an extra bit of control over reactor power by varying how much water is pumped back into the reactor core. There are also additional pumps which can
pump in or remove water from the core in the event of a dangerous situation in the core.
A pressurized water reactor operates with the reactor cooling water in a
closed loop, seperate from the power generation system. The core operates at a pressure of about 160 atmospheres allowing the water to become
superheated. The water is pumped out of the reactor core and to a
heat exchanger (look, another node I need to do!), where it heats a secondary water source which turns to steam to power the turbine. This design is more
efficient than a boiling water reactor, and about 70% of reactors in the US are this type.
Now we get to the interesting one. A breeder reactor is basically a reactor that produces
fuel. Sounds good, eh? The basic idea is to use some of that
U-238 that is very very common and is typically just left laying around the earth or as part of waste. A breeder reactor is basically designed similar to a standard reactor, only the reactor vessel is surrounded by U-238. The reason this is done is because U-238 can easily be decayed into
Plutonium (Pu-239) by bombarding it with free neutrons.
Pu-239 happens to be a readily fissible material that can be used in nuclear reactors. So, by placing the U-238 around the core, you will gradually produce Pu-239 which is... more fuel!
The most common variation of this is a fast breeder reactor, which is a reactor that actually produces more fuel than it consumes. This can be done because U-238 is so abundant compared to U-235. Since on average U-235 releases 2.4 nuetrons when it splits, and only slightly more than 1 is used to continue the reaction, there are more neutrons creating Pu-239 than are used in the reaction. The breeding ratio is the ratio of the Pu-239 produced compared to the fuel used, and is generally around 1.4 in power production. Typically a breeder reactor uses a fuel with a much higher U-235 concentration, around 15-20%. On average it takes about 10 years for U-238 to degrade to Pu-239. This may seem like a long time but the design means that you can operate a reactor for 10 years on standard fuel and then another 14 years after that with the fuel produced. Thus you get 24 years of operation out of the same amount of fuel that a normal reactor would operate on for 10 years.
Now, because the goal is to create fuel as well as burn it, the design has to be a bit different. No water is used because it slows down the neutrons and reduces the amount of Pu-239 produced. Typically in place of water, sodium is used for cooling. This poses some design problems as sodium will react violently with water. The sodium is piped to a heat exchanger which heats water to produce steam for power generation, and all the components must be carefully designed to prevent the introduction of any water to the sodium path. Sodium has excellent heat transfer characteristics however, and with careful design can be safely used. However, the sodium does become radioactive which leads to some extra handling and storage problems with used sodium.
The other major problem with this type of reactor is that due to the higher concentration of U-235, it is actually capable of producing a small scale nuclear explosion should a core meltdown occur. In the US, this type of reactor has not been allowed since the 1970's, mainly due to concern over the fact that it produces weapons grade fuel. In addition, the supply of Uranium has exceeded expectations, the demand for power has been lower than expected, and breeder reactors have proved to be rather expensive to build and maintain. Thus, it has not been a popular design and proably won't be barring any major uranium shortages. To my knowledge there are no currently operating breeder reactors in the world, although many countries are researching them with plans for future usage.
see that node
Open Pool Research Reactor
I'm adding this here because I have some intimate familiarity with this type and they are somewhat common at universities and corporations. Basically this is a small reactor used for research and other purposes and does not generate any power. They generally range between 10 kW and 2 Mw. For the most part, they are used for things like irradiation, gamma spectroscopy, and a variety of research purposes such as radiation damange testing and detector testing. So let me go ahead and describe the one we have at Ohio State University.
The reactor building sits west of campus in a small unassuming building. You go in the door, and bam, there is the reactor. It is a 500 kW open pool water cooled reactor. The reactor was initially installed in 1961 operating at 10 kW and using enriched uranium at 93% U-235. Around 1980 is was upgraded to operate on 19.5% U-235 (about the max you can get without having weapons grade fuel) and the capacity was increased to 500 kW. The reactor core sits in a containment vessel about 20' high, with the core itself being around 4' high. It utilizes 7 (I think) fuel rods that hold about 3.5 kg U-235, and 4 control rods (3 of which are filled with boron, the other is just water filled to act as a fine tuning mechanism). A metal walk runs around the top of the reactor where you can look down through 15' of water to the reactor core. The containment vessel is made of high density concrete and is flared out at the bottom to provide adequate radiation absorption. Because of it's small size, you can safely stand at the top with nothing but 15 feet of water between you and the reactor core while it is operating. Neat. It almost looks like a narrow swimming pool. The vessel holds a total of about 5700 gallons of water that is filtered daily and changed about every 5 years.
The control room is small and not very high tech, most of the equipment dating back to the 1960's. To start the reactor, you first lower some PuBe down next to the reactor (this gives off about 10^7 nuetrons per second and helps to get things started faster). You then pull the control rods out 30 cm and keep an eye on the reactor power. The control mechanisms are nothing fancy, you simply press a button for which control rod you want to move, and then push a switch up to raise them and down to lower them. An analog gauge tells you the height. Contrary to what you might think, there doesn't really need to be a lot of accuracy here. All the control rods do is allow more nuetrons to hit fuel, so you don't have to raise them all at the same time or even get them at exactly the right distance . The rods themselves are lifted by electromagnets that grab the metal rods, every now and then they may fall off... This doesn't cause any big problems, it basically just shuts the reactor down, however in a commercial power plant the lost revenue due to this could be huge.
Ok, so you flip the switches and raise all the control rods to 30 cm. There are nuetron detectors next to the reactor core that measure how many nuetrons are being released. From this, the computer is calibrated to determine how much power is being produced. At 30 cm, you keep an eye on things for a few minutes to make sure everything is operating properly. At this point the reaction is subcritical and it would shut down on it's own if you removed the external nuetron source. Once you are happy that everything is going ok, you raise the rods to about 40 cm and again make sure everythin is ok. At this point, you remove the external neutron source and pull the rods up to about 44 centimeters. On our reactor, this is where the reaction gets supercritical. You leave it here until you get up to the power you want to operate at (or, raise the control rods higher if you want to get there faster). Once you have reached your desired power output, you then slowly begin to lower the rods until the power output stabilizes. At this point you basically have to fine tune it using the power output gauge to get it critical. Once you are there, you are all set.
To stop the reaction, you just lower the control rods all the way down. In an emergency situation, you can release the power on the electromagnet which causes the rods to fall down to the base in about .5 seconds. Otherwise you just slowly lower them back down. You'll still get some power output for a little while, but within a few minutes it will be off. And that's all there is to it.
To see a picture of this in action, go to http://www-nrl.eng.ohio-state.edu/. On the right, you'll see a square box that kinda looks like a safe. This is the reactor core where the fuel is kept. The four tubes going into the center of this box are the housings for the control rods (which are inside those tubes). Right below the core is the primary nuetron detector, with 4 others on the right of the core. The tube to the left of the core is where the external neutron source is lowered. The other tubes you see are all for inserting materials near to the core for research. The blue light you see is caused by particles coming from the core at very very high speeds. When these particles strike the water, the water emits the extra energy as visisble light. It's kinda neat to look at. Keep in mind this picture is taken from the observation point at the top of the water pool, you can stand right there while the reactor is operating.