User:Dr Moebius/sandbox

The D-He3 reaction and its potential attractiveness for peaceful nuclear fusion power
Among nuclear fusion reactions, the ones of greatest potential interest for energy production are the following:

D + D --› T + proton + 4.04 MeV, D + D --› He-3 + neutron + 3.27 MeV, D + T --› He-4 + neutron + 17.6 MeV, T + T --› He-4 + 2 neutrons + 11.27 MeV, D + He-3 --› He-4 + proton + 18.34 MeV, Li-6 + neutron --› T + He-4 + 4.8 MeV.

As shown in figure 1, the D-T reaction has the best combination of high cross section and (relatively) low temperature at which it reaches its high reactivity. This had made the D-T reaction the preferred one in military applications and in (future) peaceful energy production alike.

D + He-3 —› He-4 (3.67 MeV) + p (14.67 MeV),

where D is a deuterium nucleus, He is a helium nucleus—with isotopes containing only one neutron and the regular one with two neutrons—and p is a proton. The kinetic energies of the helium nucleus and proton escaping the reaction are expressed in million of electron-volts [definitions to be inserted here, or link to public Wikipedia articles on energies at the atomic and nuclear scales]

18.34 MeV of energy are produced in total in this reaction, therefore, with an efficiency slightly higher than is the case in the D-T reaction on which ITER [link to to be inserted to the relevant web pages of the European Commission Directorate General for Research and Innovation and to the “Fusion for Energy, “F4E”] will be based: about 5 GeV of rest mass, when fusing deuterium (one proton, one neutron in the nucleus) with helium-3 (two protons, one neutron in the nucleus), give rise to 18.34 MeV of energy, a fraction of 3.67 E-3 of the reacting mass is converted into energy, about 4% more efficiently than is the case in D-T fusion. But, in the case of D - He-3 fusion, all of the energy produced in the reaction comes out as electrically charged products. This does not so much matter to weapons designers, for whom the high energy neutrons from the D-T reaction are best used to fast-fission the tamper of the second stage, but this is potentially very much relevant to the design of future fusion reactors for electrical power production. Indeed, both the reacting deuterium and helium-3 are non-radioactive, nor are the fusion products, helium-4 and a proton; there are (almost, see below) no neutrons, so little or no activation of the fusion reactor vessel; last but not least, the alpha particles (the He-4 nuclei) keep the plasma burning, while the high energy protons could drive a high-conversion efficient direct cycle reactor, dispensing it from using blankets (also used as tritium-breeders in a D-T reactor) and thermal turbines to convert high temperature steam into electricity. It is however somewhat incorrect to term the D - He-3 reaction as fully aneutronic, a totally clean reaction which will prevent any activation of the reactor structure, resulting from neutron bombardment. Indeed, along with D - He-3 reactions proper, some D-D reactions will occur nevertheless in the burning plasma, even though D - He-3 reactions will dominate over D-D reactions; this would be so, because the cross section for D - He-3 fusion is about ten times that of the two possible D-D reactions combined, as these cross sections peak at comparable temperatures. These D-D reactions will produce both neutrons (with an energy of 2.45 MeV) and—from an activation point of view, worse—tritium. Of course, this tritium will react in turn with the ambient deuterium, via the "normal" D-T reaction, producing the very energetic 14.1 MeV neutrons one sought to dispose of in the first place. If, on the one hand, the quantity of tritium being generated in this two-step process will be limited, the peak reactivity of the D-T reaction (its cross section), on the other hand, is also about ten times that of the D - He-3 reaction. Furthermore, since the D-T cross section peaks at lower temperatures than the D - He-3 cross section, densities profiles will need to be carefully optimized, lest to have a relatively higher D-T reaction rate in the outer regions of the plasma, where temperatures are lower and closer to the vessel inner walls. In the end, it turns out that a "simple" D - He-3 reactor will not be truly aneutronic, but it should be acknowledged that will still produce a neutron radiation field intensity about 10% that of a D-T reactor of the same fusion power. One should, however, not infer unduly from these arguments than advanced fuels, like D - He-3, hold no promise for future energy-producing fusion reactors. Reducing the flux of 14.1 MeV by 90% would be an extraordinary engineering feat and, we recall, this result has been derived for equal fusion power outputs; if one takes into account that a D - He-3 reactor will not need a thermal to electrical converter system, resulting in potentially higher efficiencies [more engineering-minded readers might likely add edits and links here, especially to the efficiency achieved in very high temperature cycles], the advantage in lower neutron activation at equal electrical output would be proportionally even higher. Of further interest to this fusion reaction, schemes were proposed a number of years ago to use polarized deuterium nuclei in the D - He-3 mixture, so as to suppress the D-D reaction responsible for the tritium production. The idea of polarized nuclei did not receive much continuing attention, despite its stimulating character, as it remains technically quite a formidable one: how to maintain aligned spins in a nuclear burning mixture with temperatures in the range of 100 to 400 keV? Purely aneutronic character aside, if the D - He-3 reaction is however so attractive, why then nuclear fusion is normally foreseen, at least for first generation fusion power stations, as being based on the D-T reaction? For one thing, the D-T is the easiest to achieve, since its cross section peaks at about 100 keV, whereas the D - He-3 reaction cross section peaks at about 400 keV. But, of course, the chief reason is that there is hardly any He-3 available on Earth, aside from traces in the atmosphere arising from cosmic rays and from volcanism, as some He-3 is produced in the course of radioactive processes in the Earth interior. Tritium, on the contrary, can be produced rather more easily in a number of ways: for peaceful purposes, as by-product of Candu [insert link here to relevant web pages for Candu reactors] and to the Ontario power reactors, and in future fusion reactors by breeding it out of lithium in the reactor blanket [a possible edit here would be “One might surmise that this is conceptually similar, but considerably more benign, to the way a dry two-stage thermonuclear weapon produces most of the tritium it needs “on-the-go”, out of its lithium-deuteride mixture, during the thermonuclear burn of the warhead second stage”]. It has been surmised by some authors that He-3, on the other hand, might perhaps one day be mined on the surface of the Moon to fuel a more advanced nuclear fusion-based world economy; this would be certainly an intriguing, albeit perhaps not yet entirely practical, possibility.