Advanced Fuel Cycles

Some day the inhabitants of this planet will look back at the clumsy magnetic bottle, the D-T tokamak, which is described in the previous chapters. The tokamak will seem like an old IBM Selectric typewriter with font balls compared to Microsoft Word on a 2-GHz notebook computer. The deuterium-tritium reaction is a terrible fusion reaction, but we have to start with it because it is easy to ignite. It generates power in neutrons, which make everything radioactive so you cannot go near the reactor. The neutrons are hard to capture and also damage the whole structure of the machine. And you have to breed the tritium and keep it out of the environment. There are much cleaner fusion fuels that we can use in next-generation magnetic bottles.

These future magnetic bottles will hold denser, hotter plasmas for a longer time. Then we can use reactions that do not produce the intense flux of energetic neutrons that plagues D-T reactors. Here is a list of the main possibilities.

D + D ^ T + p (half the time)

D + D ^ He3 + n (half the time)

D + He3 ^a + p p + B11 — 3a p + Li6 — He3 +a He + Li —12a + p p + Li7 — 2a He3 + He3 — a + 2p D + Li —— 2a

‘Numbers in superscripts indicate Notes and square brackets [] indicate References at the end of this chapter.

F. F. Chen, An Indispensable Truth: How Fusion Power Can Save the Planet,

DOI 10.1007/978-1-4419-7820-2_10, © Springer Science+Business Media, LLC 2011

In this list, D stands for a deuteron, T for a triton, p for a proton, and a for an alpha particle (He4 nucleus). He3 is a rare isotope of helium with only one neutron instead of two. Figure 10.1 compares some of these reactions with D-T. What is shown are their reactivities, which show how fast fusion occurs for each fuel mix at each ion temperature.

The special role of D-T is immediately apparent. Not only does it fuse much faster than anything else, but the peak occurs at a much lower temperature. The 50-keV temperature of the peak can already be achieved. We next describe the advanced fuels in groups.

The first group involves only deuterium, which is plentiful in water. It can fuse with itself two ways, either producing T and p, or He3 and n. When it goes the first way, the proton is harmless, but the T will quickly react with D in a DT reaction and produce a 14-MeV neutron. When D+D goes the second way, it produces harmless He3 and a weaker neutron. So the DD reaction is not completely clean; there are neutrons, but much fewer of the dangerous ones. Forty percent of the energy comes out as charged particles (p, T, He3, and a), which keep the plasma hot and can give up their energy electrically instead of through a thermal cycle. The neutron damage to materials is greatly reduced. These two DD reactions will occur simultaneously, but their reactivities are very low even when summed. However, there is a gain of a factor of 2 because both reactants are the same. That is, each deuteron can react with all the other ions instead of with only the half that are tritons, as in a DT reactor. However, this still leaves the DD reaction with a much lower rate than DT.

The reactions in the second group have the next highest reactivities and are the most promising ones. D-He3 has sizeable reactivity at low temperature and produces no neutrons. Unfortunately, you cannot keep deuterium from fusing with itself, so there are DD reactions going on at the same time. But the energy in neutrons is reduced by a factor of 20 relative to DT, and this is an almost clean reaction. The problem is that He3 does not occur naturally. It can, however, be mined on the moon. It is estimated that there are a billion tons of He3 just under the surface of the moon, enough to supply the world for 1,000 years if it could be brought down here [3]. Mining machines have been designed which could dig 1 km2 of the moon’s soil, down to 3 m depth, to get 33 kg of He3 a year [4]. If the moon is ever colo­nized, this would be the fuel used. Finding deuterium there may not be as easy, and the much harder He3-He3 reaction (Fig. 10.1) would have to be used. Burning D-He3 on earth will have to wait until space shuttles can reach the moon. Nonetheless, the simplicity of the engineering is so attractive that a D-He3 reactor has been designed [5].

The p-B11 reaction is the most attractive one at present. The reactants are not radioactive, and only helium is produced. Without neutrons, all the shielding and blankets of DT reactors are unnecessary. Fusion power plants can dispense with the tritium recovery and processing plant, as well as with remote handling equipment. Only hydrogen and boron are used. Boron is plentiful on earth, and B11 is its main isotope. We commonly use 20 Mule Team Borax, a cheap cleanser. All the energy comes out as fast alpha particles. Since these are charged particles, there may be a possibility of direct conversion of the energy into electricity without going through boilers and turbines. This can be done by leading the alphas into a channel where they can be slowed down with electric fields, thus producing electricity directly, or by capturing the synchrotron radiation emitted by the alphas spiraling in a magnetic field. However, boron is not a light element; it has a charge of 5 (Z = 5) when it is fully ionized. When electrons collide with ions, they produce X-rays at a rate that increases with Z2. Though it is not hard to shield against these X-rays, they repre­sent energy that is lost to the plasma, and it is harder to raise the plasma tempera­ture. Special methods being developed to overcome this is described in a later section.

All the other reactions on our list have very low reaction rates, as exemplified by p-Li6 and He3-He3 in Fig. 10.1. Reactants with atomic number Z above 2 have two other problems besides low reactivity. First is the synchrotron radiation loss mentioned above. Second, there are competing reactions when there is a large number of proton and neutrons, and they can combine in different ways. For instance, p-Li7 looks like a great reaction, producing two alphas. However, p + Li7 ^ Be7 + n (a neutron) is also possible [6], and this happens 80% of the time. The two reactions in the third group above form a chain reaction in which the He3 generated by p-Li6 can react with Li6 to regenerate the proton, and only alphas are the result. However, the reaction rate is low, and there are competing reactions.

Speaking of chain reactions, it was Hans Bethe who invented the famous carbon cycle that allows hydrogen fusion to occur in the sun at a comparatively low tempera­ture. Carbon is used as a catalyst that regenerates itself. Other chain reactions for the sun have been devised since then. No one so far has found a chain reaction for advanced fuels that will allow them to burn at lower plasma temperatures on earth. However, there has never been a large-scale effort to find such a chain reaction.

The last reaction listed above, D + Li6 ^ 2a, looks very attractive, but there are five competing reactions that produce nasty products. It has an interesting story. Lithium is the lightest solid element. Lithium hydride (LiH) is a glassy, solid material. It is one of the hydrides mentioned in Chap. 3 for carrying hydrogen in hydrogen cars. If we replace natural Li with Li6 and H with D, we get lithium-6- deuteride, a similar solid that is easy to transport and store. There is no informa­tion on this reaction on the Web because apparently it is useful for making hydrogen bombs, being easy to carry and producing a large amount of energy, 22 MeV. In a bomb, the reaction is set off by neutrons, and the nasty by-products would be just fine for the purpose. Mention of tests involving this reaction can be found in public histories of atomic bomb development in the USA. In a fusion reactor, however, a deuterium-lithium plasma would be hard to ignite, and the neutrons and gamma rays emitted would be hard to manage. The reaction rate [7] is about 28% of that of He3-He3, the lowest curve in Fig. 10.1. Furthermore, the competing reaction D + Li6 ^ Be7 + n occurs 3.5 times more often, producing neutrons. This is the reason many clean-looking reactions are not actually viable for a reactor.