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?

Boiling Water Reactor

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.

Pressurized Water Reactor

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.

Breeder Reactor

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.

Pebble Bed Modular Reactor

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.

Speaking of the types of reactor, there is one crucial difference between the graphite core and pressurized water reactors: passive safety. That is, what does it do uncontrolled, that is, if cooling fails? The answer: a pressurized water reactor stops, a graphite core reactor goes Chernobyl. Only Russians, North Koreans and Americans still use these potentially dangerous reactors.

Two things make up the basis of security of the reactor: the cooling system and the control rods. A nuclear reaction can produce power only with a moderator - without it, the reaction stops. The moderator is either graphite or pressurized water. In a pressurized water reactor, the same water acts also as the coolant. The control rods are used to slow or speed up the reaction, and using them the reaction can be stopped.

Now, if the cooling system and the control rods fail simultaneously, the reaction is too fast, and core will start to heat up. A graphite core will eventually catch fire, but the moderating water will start to boil. Steam is not a moderator, so the water reactor stops and doesn't produce heat. If the coolant drains away, the graphite core burns, but having no moderator, the water reactor stops.

No one could not blow up a pressurized water reactor with an explosion like in Chernobyl. Graphite core reactors are a risk.

Whatever happens, nuclear explosion does not happen in a power plant. The uranium is far too impure (3-5% 235U) to explode. It can overheat, but it never explodes like a nuclear bomb. If something explodes, it's because of the good old pressure and fire. This happened in Chernobyl. In any case, the radiating matter will not poison severely anything except the immediate neighbourhood of the reactor. The radiation released is usually only harmful, not lethal like from a bomb. However, the fallout of an exploded reactor has a longer half-life, making it dangerous for a longer time.

It must be acknowledged that in a nuclear fuel processing plant a failure can lead to an uncontrolled chain reaction, which radiates a miscellany of ionizing radiation, which can be harmful to the workers. Processing plants are also not as well protected as power plants.


A little reply or addition to DejaMorgana below: I've noticed that most people assign these three things together: nuclear blast, radioactive contamination and breaking the reactor shielding. These are very different things. Crashing a jet plane to a reactor dome could break the shielding, but the contamination would be very small, and certainly there wouldn't be any nuclear blasts. There will be no bomb-like blasts in reactors, because reactor-grade uranium is too impure. It is, however, poisonous, but even this is not excessively dangerous.

Remember that no matter if the contents of the reactor are radioactive, they are still poisons, not explosives. Poisons, no matter how poisonous, are dangerous to a certain extent only. The concept of the degree of severity applies to radioactive poisons as much as it applies to simple chemical poisons. It can be measured and controlled. Radioactivity is not magic. Radioactive contamination is not a supernatural evil force that permeates all barriers, but just another type of contamination!

Drinking toilet cleaner is crazy, and sprinkling it to food is equally insane. However, these are the only ways to make toilet cleaner lethal. It is possible to burn yourself with it, but that isn't likely to be lethal. There are definite ways to clean the poison. This applies to all poisons: poisons are not evil. They have their hazards, but these are predictable and well experimented. Radioactive ones are not an exception.

So, a jet plane crashing to a reactor dome is far from a worst case scenario. The likely toxicological effect is a small local contamination, which is immediately poisonous only to the people who clean the site. The likely consequence in the worst case scenario is the limitation of agriculture within a 20-km radius. You should be more concerned about why the jet would crash in the first place.

A Civilization advance.
Attempts to develop peaceful applications for the energy released by nuclear fission actually preceded its use in war. The eventual success of this endeavor turned out to be a mixed blessing. Engineers harnessed the immense energy produced by fission to create steam, which they then used to drive electric turbines, producing electricity. However, the radioactive materials necessary for sustained fission reactions are extremely lethal, disposal of the radioactive wastes generated by the process is difficult, and the risk of catastrophic meltdown can never be completely eliminated.
Prerequisites: Nuclear Fission and Electronics.
Allows for: Fusion Power.

Back to The Everything Civilopedia...

Nuclear fission power is one of the cleanest, safest, and most abundant sources of non-renewable power that can be used with today's technology. However, its use has been limited in the United States, only about one-fifth of all power plants in the US are nuclear. The vast majority of plants in the US are coal-fired. The reason for this is primarily political, not technological, because nuclear power has distinct advantages over coal.

Coal plants emit thick clouds of noxious smoke containing black soot, carbon dioxide, carbon monoxide, sulfur dioxide, and nitrogen oxides. All of these components have adverse effects on the environment. Coal mining is also dangerous business, with many people dying or getting injured each year because of mining-related accidents.

Nuclear plants, on the other hand, emit no polluting gases whatsoever. The only thing that comes out of those big cooling towers is steam. Because so little uranium has to be mined to make the same amount of power as coal, there is less chance for uranium mining accidents. When the resources of all possible fissionable material is added up, including thorium and the use of breeder reactor, there is actually more energy contained in nuclear fuel reserves than there is in coal reserves.

Perceived drawbacks of nuclear power include radiation emission, nuclear waste, and meltdowns. Many would be surprised that a coal power plant emits more radiation than a nuclear one! Nuclear plants have concrete shielding more than a meter thick. In a test of the reactor's shielding and strength, testers crashed a jet plane into a nuclear reactor. The plane disintegrated, with barely a dent left in the concrete. Terrorists would be wasting their time messing with nuclear plants. Nuclear waste is stored in shielded containers, so there is no radiation threat in that either. In fact, some nuclear waste can be recycled into nuclear batteries, a lightweight source of portable power.

As for meltdowns, there were two: Chernobylin the USSR and Three Mile Island in the US. The former was terribly designed with few safety measures, while the latter was well designed. The meltdown of the well-designed Three Mile Island plant resulted in no casualties. Chernobyl's meltdown did result in 4000 deaths, but as I said before, Chernobyl had all the safety measures of car with no seatbelts and its brakes cut. When nuclear plants are properly designed, meltdowns do not pose a serious risk.

Thanks to Brontosaurus for the number of deaths caused by Chernobyl

Although there are quite a few fuzzy areas in anglopwr’s writeup above, I’m just going to respond to one very misleading snippet:

“Nuclear plants have concrete shielding more than a meter thick. In a test of the reactor’s shielding and strength, testers crashed a jet plane into a nuclear reactor. The plane disintegrated, with barely a dent left in the concrete.”

Not really. Nobody ever crashed a plane into a nuclear reactor, and NRC officials have actually gone on record to state that nuclear reactors were never designed with such events in mind. Nuclear plants are designed to withstand tornados, earthquakes, and similar catastrophic events, but NOT suicide attacks by hijacked jet liners.

But there was a test similar to what is described above. In 1988, an unmanned F-4 Phantom, ballasted with water and mounted on rails, was “flown” into a concrete wall at 480 MPH. As reported, the plane crumpled, and penetrated only about 2 inches of concrete. A very impressive test - except it wasn’t meant to be a test of nuclear reactor safety. The wall the F-4 crashed into was not a simulation of a nuclear plant’s wall. It was a 12-foot-thick wall mounted on an air cushion. The test was designed to study impact forces by measuring how far the impact would push the wall. Breaking through the concrete was the last thing any of the involved scientists wanted to achieve. Furthermore, the F-4 was ballasted with water to give it the same weight as a plane fully loaded with fuel, and its final weight was 42,000 pounds. Needless to say, crashing a 412,000 pound 767 loaded with fuel into a fixed wall would have slightly different results.

Big words, you say. Nice fearmongering, DM. But can you back it up with any facts? Well, actually I can. Because according to a 1982 study by the Argonne National Laboratory in Illinois - a study which was conducted by request of the DOE and the NRC - the explosion from a 707 crashing into a containment dome at 466 MPH would probably overwhelm the reactor’s shielding. Note - that’s a 707, which weighs 336,000 pounds. In 1982 those were big jets. But we’ve “advanced” considerably since then. The 767s that were flown into the World Trade Center weighed 80,000 pounds more than that and carried a lot more fuel.

Other studies, again conducted for the NRC at the Lawrence Livermore National Laboratory, found that a 125,000 pound jet had a 32 % chance of piercing a containment building’s six-foot base and an 84 % chance of breaking through the dome.

Ramming a plane into the containment dome isn’t the best way to attack a nuclear plant, either. There are other possibilities with much greater chances of success. I’m not going to discuss them, but the statement that “terrorists would be wasting their time messing with nuclear plants” is a gross misrepresentation. I’m not saying every nuclear reactor in the world should immediately be dismantled to prevent such an occurrence, but to claim that they are invulnerable is a disputed opinion, to say the least.

(I'm not quite sure how vuo's "reply or addition to my w/u" is really replying to me, since I never once mentioned nuclear blasts or radioctive contamination. I'm very well aware that breaching a containment dome will not produce a nuclear blast, and I didn't say it would. This is not a writeup for or against nuclear power. It is a correction to a false statement regarding how vulnerable nuclear plants are to being rammed by jet liners, and that is all it is.)


SOURCES:

  • LYMAN, EDWIN, ‘Statement on the Science Article “Nuclear Power Plants and Their Fuel as Terrorist Targets,”’ Nuclear Control Institute, 2002. Available online at
    http://www.nci.org/
  • IVRY, B. AND NUSSBAUM, A., ‘Indian Point casts nuclear shadow over North Jersey’, North Jersey News, 2002. Available online (cached) at
    http://www.nci.org/02/04f/11-04.htm
  • SERVATIUS, TARA, ‘Fuzzy Science’, Creative Loafing, 2002. Available online at
    http://charlotte.creativeloafing.com/newsstand/2002-10-02/metrobeat.html

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