Crossposted at Politicook.net
This diary was inspired by NNadir's diary earlier today on Dailykos.com and some of the comments associated with it. Whilst fusion promises a tremendous of essentially nonpolluting energy on the face of it, the technical, practical, and logistical problems associated with it are insurmountable with current technology. Here is primer on how fusion works and the associated problems.
Most of us know that sun applies fusion to make helium out of hydrogen. However, the mechanism for that is very complex and requires a couple of steps that require more energy than they release, net energy sinks, that make that approach unfeasible in terrestrial applications.
The net equation for solar energy generation is given by:
4H = 1He + energy.
We can not approach the pressures in the core of the sun, where this reaction occurs, and so can not fuse four hydrogen nuclei in one helium one, because that would require either a four-body interaction (the probability for which is extremely low, even in the core of the sun) or the complex cycle that the sun uses (I am sure that Wiki has an entry for "Carbon cycle", but that is way too complex to cover here). We cheat and use deuterium and/or tritium.
Ordinary hydrogen has a nucleus that contains a single proton (charge +1, atomic weight 1, it is also called protium). Deuterium is a rare form of natural hydrogen with a proton and a neutron as its nucleus (charge +1, atomic weight 2, also called "heavy hydrogen", and the water made from it, D2O, is called "heavy water"), and tritium is an unstable, artificially produced isotope of hydrogen with a proton and two neutrons as its nucleus (charge +1, atomic weight 3). The nuclei are respectively referred to as protons, deuterons, and tritons.
Deuterium is good for us because helium has a nucleus composed of two protons and two neutrons, which are very nicely provided by two deuterons. Both mass and charge are conserved, so deuterium really good. Because of kinetic energy and momentum considerations, tritium is even better reacting with deuterium, and if I have space I will go into that. We will see.
There are a couple of fundamental issues that have to be covered to understand any sort of nuclear energy, be it fission or fusion. The most important, basic one is the Einstein mass/energy equivalence, as expressed by the famous equation E = mc^2. This simply means that mass and energy are two faces of the same coin, but it is costly in terms of energy to turn over the mass face, and very lucrative to turn over the energy face, by a factor of 1.6 x 10^17 meters squared/seconds squared. That is a lot! In other words, a little mass becomes a tremendous amount of energy, and a whole lot of energy is required to make a very little mass.
Another concept that is important is activation energy. This is the energy required to get a process started, regardless whether the process releases energy or requires even more to continue. A good, everyday example is lighting a fireplace. It takes more heat to get it going than comes out until the kindling is going well. Then the logs provide enough heat to keep themselves going (unless you are using elm) and provide the excess heat to warm you.
The final concepts are the two fundamental forces involved. In the case of either fission or fusion, the strong nuclear force is involved. This is the force that binds nuclei, and is essential for any nucleus with more than one positive charge (that would be hydrogen in all three forms). Without the strong interaction, the positive protons in nuclei with more protons than one would fly apart. The strong force is, well, strong, but very short ranged. It follows an inverse r-^6 relation (read as "an inverse radius [separation distance] to the sixth power), which means that nucleons (protons and neutrons) interact only when very close together. This force, as far as we know, is always attractive.
The other one, which is not important for fission, is the electrostatic force. This is the familiar force that causes that annoying static cling and frizzy hair on dry days for those of us lucky enough to have enough for an interaction. This force has two manifestations, either attractive (for positive charges interacting with negative ones), or repulsive (two like charges interacting with each other). The most important thing is that the electrostatic force follows an inverse r-^2 relation (read "an inverse r-squared relation"), meaning that it has a much longer reach than that of the strong force. Since, in fusion, the nuclei are always positively charged, for purposes of fusion this force is always repulsive.
Finally, the other key concept is the packing fraction. This is what makes both fission and fusion to work. Without going into the rigorous mathematics, it simply means that a helium nucleus (two protons and two neutrons) has a slightly lesser mass than the two protons and two neutrons that two deuterons supply. This difference is mass is exactly accounted for by E = mc^2. This conversion of mass to energy is the entire source of the energy supplied by fusion. Likewise, when uranium is fissioned into two smaller nuclei, those nuclei have a combined mass slightly smaller than the uranium nucleus had. The mass differences are less when fissioning large nuclei into small ones than when fusing little bitty ones to larger ones, but the mass to energy equivalence remains as defined by the Einstein equation.
Now, let us pull all of this wankish stuff together. Please come with me, because the results will make the rather, by necessity, disjointed facts presented above, come together and make sense. Science is not hard. Memorizing a bunch of facts is hard, at least for me. But once I see an integrated system, the parts of the system begin to make sense.
For fission, such as uranium-235 or plutonium-239, the reaction just happens if enough material (the critical mass) is brought together. No other energy is required. (In fission bombs there is a high explosive, but that just serves to put the smaller, noncritical masses together very fast and make a critical one. The explosives serve only as a means to move things together fast.) This is because the activation energy for nuclear fission is just about zero. Why is this? Because fission is induced by neutrons emitted by a heavy nucleus that spontaneously emits one, and that neutron is absorbed by a nearby nucleus, causing it to fission and release a couple of more. Those neutrons infiltrate other nearby nuclei, causing them to fission. This is the classic nuclear chain reaction. It happens so fast when critical mass is reached that an uncontrolled explosion occurs. It is possible to control it by reducing the number of neutrons available for induction of fission. That is what control rods do in a power reactor. They are composed of substances that absorb neutrons and thus reduce the rate of fission in the mass of the fuel.
Fission has a zero energy barrier (another name for activation energy) because neutrons have no charge, and so are not affected by the electrostatic repulsion of the very positively charged nucleus. In fact, slow neutrons (thermal neutrons) are more apt to be captured in many cases than fast ones, since no electrostatic repulsion is involved.
Thus, a fission reactor can be operated from complete shutdown (control rods inserted to absorb enough neutrons as not to allow any chain reaction), to meltdown, depending on the inherit design (it is possible to design one not to melt down, but it is inefficient), when no control rods or other means of cooling are available. In other words, you can tune a fission reactor to do what you want, if you are technically competent.
Fusion is a different animal. To attain fusion, two or more (two in terrestrial cases) positively charged nuclei must be moved together such that the strong force is dominant in comparison with the electrostatic repulsion. That is hard to do, because of the r^6 and r^2 relations described a while ago. The sun can do it because gravity helps a lot, but even the core of the earth does not have that kind of gravity. We have to rely on heat.
In a thermonuclear device (the hydrogen bomb), a fission device is utilized to provide the heat, and enough pressure, to get deuterium and tritium to fuse. Dammit, I wish that it would not have worked, but it did. Schade. This results in the primer, the fission part, emitting neutrons that interact with the surrounding shell of lithium deuteride, converting the lithium into tritium, that reacts after a few nanoseconds with the deuterium, forming helium (and lots of noxious materials as well, and radiation from the gamma to the radio, see my previous posts on the electromagnetic spectrum, available under my profile).
So, how do we do this for power production? At present, we do not. Whilst I am optimistic for future developments, we have no economical way of triggering the reaction, no materials capable of containing the heat required to contain the starting materials, no materials capable of moving that heat to do useful work, and no materials capable of absorbing the extreme radiation emitted in the gamma to prevent injury.
I may sound like a pessimist insofar as fusion is concerned, but I am really not at all. I just have real concerns about the technology that is available at present not only to control it, but to gain useful energy from it. It is a dead end? Not in any way. Can we exploit it next year? Not in any way. Is it the future? Oh, hell, yes.
I will stick around for a while to answer question, respond to comments, and so forth. Ye who know me know also that I do not post and run unless something unexpected happens. Warmest regards, Doc