Re: Fusion Power

From: Iain Strachan (
Date: Fri Jun 06 2003 - 15:08:27 EDT

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    Hi, Lawrence,

    Yes, I rather hastily bashed off a response before checking the facts.

    A web-search indicated that you can get Tritium from both Li^6 and Li^7,
    though the Li^6 (which is around 7.6% of naturally occurring Lithium) is the
    main reaction.

    A detailed description of Tritium breeding technology can be found at

    On asking a colleague who had worked for some time on Tritium breeding
    technology, he indicated (and this is confirmed on the above link) that in
    order to get the breeding ratio above 1 (so as to breed rather than
    consume), you need to have an intermediate reaction that multiplies
    neutrons, and this is accomplished using Beryllium (but I don't know what
    the reaction is).

    The indications are that by using Lithium to breed Tritium, there is an
    extremely large potential source of energy; a rough calculation showed that
    even if all the earth's power were generated by Fusion, the amount of
    Lithium that could be mined would provide 5000 years worth of energy, and if
    one also extracted Lithium from sea water, this could be extended to several
    million years. But according to my colleague, the D-T reaction is only an
    intermediate technology; one would prefer to get a D-D reaction, but the
    temperatures required are much hotter. Ideally, a P-P cascade of reactions
    would ultimately be used. (I was not aware of this possibility until

    The timescales for even a D-T reactor are very long. The next planned
    experiment (ITER) will take 12.5 years to build and 12.5 years to run. This
    will not be a reactor, but a physics experiment to get the parameters right
    for a reactor & also (I believe) to perfect the Tritium fuel cycle.
    Following on from ITER, the plan is to produce a demonstration fusion
    reactor (DEMO), which again will take 12.5 years to build and 12.5 years to
    run. So the timescale for us being able to start building fusion reactors
    en masse looks like around 50 years. I obtained this info from a physicist
    I know who works on JET.

    > I'm happy to get info from people who have worked in the fusion
    > business.
    > In your response below, you assume that Li^7 is stable, and is
    > the fuel for more tritiuim. But, as George Murphy and the
    > original Hutchinson article pointed out, the stable isotope is
    > Li^6. If it were Li^7, you might indeed get more neutrons in the
    > output, and have a Litium chain reaction going to produce
    > tritium. But the reaction
    > Li^6 + N -> Li^7 -> He^4 + H^3 is the one George
    > indicates.
    > Li^7 is extremely unstable, with a half-life of maybe 10^-12
    > seconds. *

    I'm not sure this is correct? On the following web-page:

    the naturally occurring isotopes of Lithium are listed. Li^6 accounts for
    7.59% of naturally occurring Lithium, and Li^7 for the remaining 92.41%

    > So what we need then is an estimate of the efficiency of this
    > process for regenerating tritium, taking into account the losses
    > of neutrons in the reactor vessel, and the cross section of Li^6
    > for the above reaction. It must be much less than 100%, leaving
    > the need for some outside source for most of the tritium, such as
    > fission reactors.
    > Thanks, Iain.

    As I indicated above, one uses a Beryllium intermediate reaction to multiply
    the neutrons to get the breeding ratio above 100%.

    Hope this is a bit more enlightening than my first hasty response!

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