User talk:Kenton

Hello,

I am a new user.

Kenton

Fusion
Hello Kenton,

I am not quite confident on fusion but "no waste at all" sounds a bit too good to be true. This page says that a deuterium-deuterium results either in a Helium-3 atom and a neutron or a tritium and a proton. Now, I understand (from another source) that neutrons produced in a deuterium-deuterium reaction have a lower energy then ones in a deuterium-tritium reaction and this would result in less radioactive waste but would it be entirely waste-free?

If you know this better than me and can explain this or point me to some sources, I could accept your version.

Welcome to Wikipedia! Andris 10:41, Aug 26, 2004 (UTC)

Hi!, and Welcome! Im afraid I reverted only on the basis of dimly recolected A-level physics... and so you may well be right with respect to the difference between the various versions of the fusion reaction. Dont get put off editing... if your convinced, put it back! (though you might also like to add a note to Talk:Cold Fusion, as the change looks a bit counter-intutative!)Iainscott 11:10, 26 Aug 2004 (UTC)

Hello Kenton, you asked me on my personal page to comment on this question. Unfortunately, I don't know that much about fusion, but I agree with the page mentioned above my Andris that in a deuterium-deuterium reaction, there is also the possibility that neutrons can be produced. However, I don't know how big the production cross section for that process is compared to the cross section for helium production, and I also don't know which energy these neutrons will have (well, calculating that last thing is fairly easy - when I have time, I'll do it). But I would suspect that the cross section is large enough so that indeed radioactive waste is produced (as a first guess, I would even say that the cross section for this reaction is greater than the one for helium production, since the phase space of the end state is greater when two particles are produced instead of only one). Bjoern 18:40, 26 Aug 2004 (UTC)

From these web pages on hydrogen isotopes and helium isotopes, I get the masses of the atoms (in atomic mass units; notice that I can use the atom masses here instead of the nuclei masses, since the electron masses cancel out): H2 2.0141, H3 3.016, He3 3.016 (the small mass difference between a neutron and a proton apparently here is cancelled due to the different binding energy (?)), He4 4.0026. This corresponds to energies of (in MeV): H2 1876, H3 2809, He3 2809, He4 3728. The energy corresponding to the mass of a neutron is 940 MeV. Hence the energy released in a H2+H2 ---> He3+n reaction is 2*1876 MeV - 2809 MeV - 940 MeV = 3 MeV. This energy will be distributed to the He3 atom and the neutron, according to their masses. In the cms system, the neutron will get an energy of 2.3MeV, the He3 the remaining 0.7 MeV (calculating non-relativistically here due to the rather small energy). In contrast, in a H2+H3 ---> He4+n reaction, the released energy is 1876 MeV + 2809 MeV - 3728 MeV - 940 MeV = 17 MeV. In the cms system, the neutron gets 13.6 MeV, the He4 the remaining 3.4 MeV. So in this reaction, the neutron has an energy which is greater by a factor of approx. 5! IIRC, slow neutrons will tend to make nuclear reactions more than fast neutrons (in nuclear reactors, the neutrons have to be "moderated", i.e. slowed down, for the reaction to work), so my first guess is that the neutrons produced in a H2+H2 reaction would produce more nuclear waste by interacting with the surrounding shielding! Additionally, as you already said, one has also to consider the production of radioactive tritium. Unfortunately I still do not know the cross sections for all these processes; I will tell you when I find out. My argument about the phase space above was probably misguided, since when He4 is produced, it has excess energy which is radiated of as a photon - so in both cases, the phase space of the end state has two particles (although in one case, there are two nonrelativistic particles, whereas in the second, there is one nonrelativistic particle and a photon). Bjoern 10:24, 27 Aug 2004 (UTC)

I found it! These reactions, complete with their branching ratios, are described in the "Conventional Fusion FAQ" (which is regularly posted e.g. to sci.physics). The branching ratios for H2+H2 ---> He3+n and H2+H2 ---> H3 + p are both around 50%, the branching ratio for H2+H2 ---> He4 is only about 0.0001%. And, as I mentioned above, the neutrons coming from the reaction H2+H2 ---> He3+n probably will produce at least as much radioactive waste as the neutrons produced in the standard produced reaction with H2+H3 (well, the details will depend on the shielding of the experiment, especially on the means used to absorb the neutrons (and the protons from the other reaction)). So, apparently we can forget this (nice) idea, sorry. Bjoern 13:53, 27 Aug 2004 (UTC)