In reply to Jones Beene's message of Sun, 12 Mar 2006 18:42:48
-0800:
Hi Jones,
[snip]
>Hi Robin,
>
>You said: According to
>http://atom.kaeri.re.kr/cgi-bin/nuclide?nuc=H-2&n=2
>the (n,2n) cross section for 14 MeV neutrons on D is 177 mb
>
>That sounds about right - but this is very low - and if memory
>serves this is actually about the same as the capture
>cross-section of oxygen for thermals - and you know how rare that
>is - plus since you get many more collisions at thermal energy -
>you will actually loose neutrons to the oxygen of heavy water
>faster than you can make extra neutrons via (n,2n)
The thermal neutrons are not really the problem, because they get
grabbed by the Li long before anything else gets them.
However what I did neglect to take into account are the competing
reactions with Oxygen for the high energy neutrons. I.e.
(n,p) Cross Section
* at 14 MeV = 43.70 mb
* Fission spectrum avg. = 20.26 micro barn
(n,d) Cross Section
* at 14 MeV = 15.40 mb
* Fission spectrum avg. = 3.888 micro barn
(n,alpha) Cross Section
* at 14 MeV = 109.0 mb
There is also
(n,na) Cross Section
* at 14 MeV = 36.61 mb
These add up to 204.7. mb. Still there are 2 deuterons for every
oxygen.
The elastic scattering cross section for O16 @ 14 MeV is ~ 900 mb.
So there's about a 19% chance that a fast neutron will be
destroyed by oxygen during a collision with oxygen. That cuts into
our supply of fast neutrons.
>
>> IOW we should get about 1.5 thermal neutrons out for every fast
>
>> neutron going in, and nearly all of the thermal neutrons are
>> going
>> to produce T. This should still be a breeder.
>
>Taking the ratio of the two - as you did is just a ballpark calc -
>and not the correct way to figure it -
Indeed, I miscalculated this anyway. Even using the same
assumptions, it should only have been about 39%, not 47%.
But that wasn't taking the oxygen reactions into account. The
margin keeps getting slimmer.
>but I will have to get the
>experimental data out of some hard copy files tomorrow - suffice
>it to say that the best TBR you can get with current technology
>using beryllium and highly enriched 6Li is about 1.3 in real
>tests -
That sounds about right.
>and the actual neutron (n,2n) ratio for any reasonable
>amount of heavy water is slightly underunity - for the reasons
>mentioned above.
This sounds like it might be a figure for pure heavy water, not a
Li6 solution. IOW in pure heavy water, at least some of the
thermal neutrons will eventually be absorbed uselessly.
>
>Millibarn cross sections allow for much unpredictability, in
>general, and it is tough to build a breeder on that kind of
>rarity - you could actually get a full neutron decay before a
>(n,2n) multiplication in "just" heavy water - with no lithium.
While technically true of course, I don't think this would make a
significant impact on statistics. Even at only thermal energies,
on average a neutron would undergo hundreds of trillions of
collisions before decaying (7E14). The chances of being swallowed
whole by anything at some point during those trillions of
collisions is so close to certainty, as to trivialize the
importance of decay.
With Li6 in the water, it is even more trivial.
>
>BTW - almost forgot - you CANNOT use dissolved lithium hydroxide
>in the blanket at all - as the T will preferentially displace a D
>in D2O and you will loose all of your precious tritium to
>tritiated heavy water - and it becomes such a small percentage
>that it is "effectively" lost.
This may be possible anyway, depending on the actual amount of
heavy water present. A very rough calculation yields a
concentration of about 1/1000 after a year of operation. It should
be possible to extract this, much as D is currently extracted from
normal water.
However I have no idea what percentage would be lost in the
process. Besides, about 5-10% of the T would also decay in that
year. (No one cares how much D is lost when producing heavy
water).
Bottom line, I still think this may be marginally possible, but it
would require more detailed analysis to be sure, and of course
real experimental evidence already gained doesn't hurt. ;)
[snip]
Regards,
Robin van Spaandonk
http://users.bigpond.net.au/rvanspaa/
Competition provides the motivation,
Cooperation provides the means.
