Ok, I tried a google search but couldn’t find a site that explains in simple terms how nuclear power works, what’s the difference between the process in a nuclear power plant and a nuclear bomb and also the difference betwenn fusion and fission.
My basic understanding of the fission process is that the uranium (or plutonium - or other ***um) is very hot for some reason, heats the water, which, basicaly works like a steam engine, and provides the power…
So, anyone has a good link to provide or a simple explanation…
-I’m sorry if this thing has been done already but I’m posting from work and for some reason, I can’t use the search engine correct - even posting is really tough-
Ok. Lots of stuff here so let¡¦s take it one at a time.
Fission is what happens when the nucleus of heavy atoms split. Atoms are held together by incredibly strong but very short range forces; by breaking the atom apart you release energy (and a whack load of small pieces like neutrons). The pieces (neutrons) smash into other atoms and the process begins again. Thus the famous ¡§chain reaction¡¨.
Now by controlling the amount of neutrons that are available to continue the chain reaction you can stretch it out and moderate it. Basically most design use graphite or heavy water. What happens here is that the moderating material gets hot which allows for water to be run over it generating steam. Nuclear plants are big kettles with the fissionable material acting like the element (assuming you have an electric kettle¡K.never mind, it¡¦s a Canadian thing. ƒº)
Now if you have no moderating materials the chain reaction takes of and you wind up with a fission bomb, like the early atomic bombs.
Fusion on the other hand is where 2 small atoms (hydrogen, deuterium) get forced together and, again, generate heat. The trick is to keep these very hot gases close enough together and energetic enough that you actually get them to fuse. We can do it for small periods of time, but we have yet to learn how to get any net energy out of it (in a controlled fashion).
Uncontrolled, like a hydrogen bomb, the results are spectacular. They tend to have a fission bomb exterior with the fusion material in the centre. The fission reaction provides enough energy to trigger the fusion reaction. Think of it this way, fission bombs give kilo (thousand) tons of TNT, fusion bombs give mega tons.
Fissionable material is put into a “pile” with neutron absorbing moderators in the form of rods, carbon is common, which keep the pile at the radiation level of a self-sustaining chain-reaction. The rods are pushed into or withdrawn from the pile by automatic controls that keep the reaction rate at the desired level.
The pile gets hot because the energy released by the fission is absorbed by the pile raising the temperature of the pile. The heat is removed by circulating cooling water in the core. This cooling water turns to steam in the process and drives turbine powered electric generators.
The danger with such a fissionable core isn’t a nuclear explosion but rather a meltdown because of overheating. In order to get an explosion you have to bring subcritical masses of the fissionable stuff close together exceedingly quickly so that the exlosion occurs before they have a chance to melt down. In the early bombs they shot a piece of uranium into a sub-critical uranium mass with a “gun.”
One thing to notice is that there is a low point on the energy scale when thinking about nucleus size. I think it’s somewhere around Iron, 26 Protons (big WAG). What this means is that small nucleii will release energy if they are combined, and large nucleii will release energy if split. Fission is splitting, Fusion is combining.
Uranium is rather unstable and will fission all by itself (radioactivity), however, each time it does, it releases neutrons that can CAUSE another fission in a nearby Uranium nucleus. Thus, if you have enough of them together, you get a chain reaction (think boom) If you don’t have enough together, the chain reaction can’t really get a foothold, and it’s just radioactive and perhaps a bit warm. As has been mentioned above, the moderators control the speed of the reaction (and the resulting heat generated) by absorbing these neutrons.
Fusion is very hard to make happen because of the intense pressure needed to push the nucleii together. They won’t conveniently fuse the way Uranium fisses. Fusion bombs work by using a Fission bomb to generate the high temps and pressures needed to fuse the Hydrogen.
Ok, I’ll try. First fusion. If you were to weigh a hydrogen atom and a helium atom, you will find the latter is slightly less massive than four of the former (actually, I should have said to determine their masses). On the other hand, it looks like the constituents of a helium looks a lot like four hydrogens. (Strictly speaking, the hydrogen has 1 proton, 1 electron, while the helium has 2 protons, 2 electrons and 2 neutrons, but since a free neutron will spontaneously decay into one electron and one proton, this is a reasonable point of view.) So if you could somehow induce 4 hydrogens to combine to make one helium atom, there is some missing mass. And that appears as energy in accordance with E = mc^2. The actual process is not quite that; for example one possibility is to combine one tritium (a hydrogen atom that has two extra neutrons) and one ordinary hydrogens, but the principle is the same. The mass of the product is just a bit less than that of the constituents and the difference appears as energy. That’s fusion. And in stars it continues all the way up to iron and is the main source of energy for stars (the other source being gravitational potential energy–the energy of the infalling material, almost all hydrogen and helium).
What happens past iron? Well to make any element past iron requires energy since the mass of the constituents is now less than the mass of the product. And these transferric elements are in fact made only during supernovas of massive stars in the one or two seconds that there is enormous energy around that the star is trying to shed. Thus supernovas are the source of all the gold and lead and … and uranium. Thus if you can split those elements into smaller constituents energy will be released. You are just recapturing the energy from some long ago and far away supernova. And this is what happens both in a nuclear reactor and an atom bomb. Uranium is naturally radioactive. When it decays it gives off two or three neutrons. If you build a big enough pile of uranium, enough of these neutrons will hit other uranium atoms and cause them to split–or fission–releasing more neutrons. If this happens in a controlled way, you have a reactor that is generating heat that can be used to generate electricity. In an uncontrolled way you get a nuclear bomb. There are various kinds of uranium (they differ in the number of neutrons) and U-235, the one with 143 neutrons, is especially unstable and easy to fission. But that kind is a very small percentage of uranium as mined, so one of the things you have to do to build a bomb is to enrich it. This was done during the war at enormous cost. Gaseous UF_6 (uranium hexafluoride) was spun in large gas centrifuges and the lighter U-235 concentrated somewhat in the center, while the heavier U-238 moved to the outside. Repeat until you have the concentration you need. There are, of course, many many details omitted.
By the way, fusion is usually done two different ways, by fusing
deuterium gas into 1 helium atom or fusing Lithium Deuteride into
an unstable beryllium atom which then splits into two helium atoms.
Now, a question arose: what’s up wit plutonium? I thought that nuclear bombs were made with plutonium (not uranium)? Can you use plutonium for nuclear power too?
Or is it mostly the same but uranium is more easier to find so it’s used more often?
Plutonium can be used, but historically, in the US, it hasn’t been, because of security concerns. Just recently though, Duke Power of North Carolina has gotten aproval to use Mixed Oxide Fuel (Which contains oxides of both Uranium and Plutonium) in some of it’s reactors. This is partly because it also provides a way for the US Department of Energy to rid itself of a couple of tons of surplus Plutonium left over from Cold War nuclear weapons production.
There are several types of reactors - the Boiling Water Reactor, the Pressurized Water Reactor, and the Breeder Reactor. A good explanation on how each type of reactor works (how it makes power) can be found here.
FWIW, the US Navy uses PWR exclusively on its nuclear vessels.
No, this is not true. Coolant and moderator serve two distinct purposes.
In a pressurized-water reactor, plain water serves as the moderator, and its moderation feature has nothing to do with preventing an atomic explosion.
In your typical PWR, a fuel atom splits and forms lots of random bits, and throws off a handful of neutrons. It turns out that if the neutrons are going too swiftly, they cannot interact as well with the fuel. What this means is that the slower the neutrons travel, the better chance they have of striking another fuel atom and causing another fission.
They call these slower neutrons “thermal” neutrons, because they are traveling at a speed that is consistent with the surrounding temperature.
It is the job of the moderator to slow down neutrons, thereby increasing the reaction rate. If the moderator weren’t there (and something else were used to cool the reactor), the reaction would cease.
Water does this job quite nicely: How do you slow down a speeding neutron? If it strikes something big, it will simply ricochet off, like a ping-pong ball striking a bowling ball. If it strikes something tiny, it will plow through, like a bowling ball through a pile of marbles. It turns out that the best way to slow down a neutron is for it to strike slower objects of identical size. The proton in the nucleus of a hydrogen atom fills the bill well. When a neutron strikes a few protons, it is similar to a cue ball striking a few billiard balls; it gives up much of its velocy in each collision. Water has plenty of hydrogen atoms, so it makes an excellent moderator. Since water has such a high heat capacity, it also excells as a coolant. Pressurized water reactor design is thus quite simple and stable.
Oh, and to add something for the OP:
The bulk of the energy produced by a nuclear reactor comes from the kinetic energy of the fission fragments: the random junk that is produced when a uranium atom splits. In addition, these fragments are truly random; when I was in high school, I saw charts showing fission sequences and thought that uranium always splits into the same pieces. It doesn’t.
This is incorrect. The moderator does not absorb neutrons. See my post above.
This is also incorrect. The control rods and moderator are distinct components.
The moderator exists to slow down neutrons so that they can cause fissions. The control rods do absorb neutrons, and so they can be used to shut down the reaction.
Here’s a subtle point: The control rods do not control power output (at least in a pressurized water reactor). The rods are raised once to an appropriate level to start up the reactor and rarely moved until the reactor is shut down or the power distribution within the fuel changes (because of fuel being exhausted).
So what happens when someone needs a whole bunch of power from the reactor suddenly? The additional steam being suddenly drawn out of the steam generator brings the average temperature of the coolant down. The coolant (water) becomes more dense. Since the coolant is also the moderator (as described in my prior post), the hydrogen atoms are closer together and more likely to slow down neutrons. This increases the population of thermal neutrons, and thereby increases the reaction rate, with no human intervention. Likewise, when the reactor is generating copious amounts of juice and suddenly nobody needs the power, the steam demand drops, the temperature of the coolant builds up, the density of the coolant decreases, the hydrogen atoms are further apart, few neutrons are slowed, less thermal neutrons are available, and thus the reaction rate drops. Again, totally automatically.
PWR design is nice because of this: they are inherently stable.
It’s there; you just don’t see in the basic equations.
First, we can basically ignore the electrons, as they don’t take part in fusion.
A lone proton (as in a [sup]1[/sup]H, or protium nucleus) has a mass of 1.008 daltons (amu). Four protons, completely unsurprisingly, have a mass of 4.032 daltons. A [sup]4[/sup]He nucleus, however, has a mass of only 4.029 daltons, despite free neutrons being about 0.5% heavier than free protons. Those 3 millidaltons are the “missing mass”, which in p-p fusion are released as gamma photons, positrons, and neutrinos (a neutron is one particle; trust me on this for now).
Incidentally, the basic reaction p + p -> [sup]2[/sup]H + e[sup]+[/sup] + v is a weak-force-mediated reaction (most fusion reactions are strong-force-mediated reactions), which runs at a rate of about 40 orders of magnitude slower than strong-force reactions. What this means, in effect, is that you need to pile up a mass of hydrogen the size of a star to get it going at all.
It’s binding energy. The resulting arrangement of the particles is a lower energy state than the original arrangement. A rough and grossly inaccurate analogy would be to imagine 2 magnets sitting on a tabletop, separated. Bring them closer and they will suddenly snap together releasing energy. You had 2 magnets (particles) before and 2 magnets (particles) after, yet energy has been released (i.e. if you had a coil of wire around them, electricity would be generated) and they are in a lower energy state than before. You will have to add energy to the system to return them to their previous arrangement.
You guys are doing a terrific job on this, and I cannot add much of anything to the basic discussion going on. I’m just posting to mention that long-time SDMB member Ultress works for our local utility in a capacity where she is a spokesperson for their nuclear power plant, in hopes she will do a vanity search and stop by this thread to go into more details on how fission plants work in practice.
One more thing, the “water” in contact with the fissionable material is in a closed loop system which interacts with a second water system. The second loop is what turns to steam and drives the stream turbines, then it is cooled back into condensate for the reheating. The water which cools the second loop (the third loop, being an open one) is what goes into the cooling towers you see.
