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 Post subject: A Reactivity Question
PostPosted: Feb 28, 2010 2:16 am 
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For a core that contains similar amounts of U235 and U233. If one wants to maintain the same reactivity in a thermal design, if U233 increases, what reduction in U235 would be required to maintain the same overall reactivity?

Thanks


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PostPosted: Feb 28, 2010 9:47 am 
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Not sure if this is much help at 1am local time...

At least for HEU purposes, U233 seems to be counted as 5/3 as reactive as U235 ((5 * U233 conc + 3 * U235 conc)/3 <= 20 for LEU), so, I dunno, 5 parts u235 equals 3 parts u233 for reactivity?

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PostPosted: Mar 01, 2010 12:36 pm 
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They are likely quite close but would completely depend on the neutron spectrum. U233 gives you about 10% more neutrons per absorption but the cross section for fission is a little larger for U235 (pure thermal spectrum anyhow). Something that would have to be modeled for each design but likely quite close in terms of reactivity (my guess is U233 has the edge).

David L.


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PostPosted: Mar 01, 2010 1:06 pm 
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Fission-to-capture ratios are plotted here.....
Only the last five items are plotted (note checked boxes).
The subscripts refer to the different species above, starting with "0" and "1" for (n, fission) and (n, gamma) for Pu239

Note that the U233 trace only drops down below some of the others at about the 2eV mark....


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PostPosted: Mar 02, 2010 4:58 am 
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Thanks, so if I am reading this right, the magenta trace is U233 and the cyan trace as U235, so in this slow thermal range the U233 looks to be significantly more reactive that U235. Nice to know.


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PostPosted: Mar 02, 2010 6:32 am 
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Yes, you're reading it right.

One needs to also take into account the neutrons per fission, which is typically about 1.2 x higher for the plutoniums than for the uraniums, so the difference we see here is less pronounced in a plot of "eta" than in this fission-per-capture plot.


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PostPosted: Mar 02, 2010 8:39 am 
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Here is \eta for U an Pu isotopes (Fig 2.2 at page 35, interestingly pg 37 in my print edition):
http://books.google.com/books?id=y1UgcgVSXSkC&pg=PA35


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PostPosted: Mar 02, 2010 9:13 am 
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Thanks Ondrej,

I think this figure from a textbook illustrates a problem that is often seen in these references: The traces look rather "sketchy" compared to actual lab database plots, and of course you don't have any control over what species of nuclides is plotted -- in this case, U238 is left out of the fast end of the spectrum, where it makes a significant contribution.
Other nuclides of interest are Pu238 and Am241.

Would you happen to know of a nuclear reaction database that allows computations combining MF1 and MF3 data ? .....BNL's NNDC is really awkward in this respect.


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PostPosted: Mar 02, 2010 9:28 am 
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Hi Jaro,

I dont know about a better web accessible database than the NNDC one. However I know some people at NNDC who work on it, so if you have a suggestion for improvement I can forward it to them.


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PostPosted: Mar 02, 2010 10:20 am 
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Great idea !

Right now, their ENDF manual states that there is no way to combine MF1 and MF3 data in computations (ex. neutron production as function of energy (MF1), and fission/absorbtion x-sections as function of energy (MF3)).

Maybe you could ask them for a hint on how to do this patch with the least headaches....


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PostPosted: Mar 02, 2010 11:20 am 
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It's weird. You can get fission X-sec (y0), capture X-sec (y1) and Nu-bar (y2) all plotted, then add y0/(y0+y1) to get the fission probability as y3, THEN form y2*y3 to get eta. But if you try y2*y0/(y0+y1) directly, it doesn't work.


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PostPosted: Mar 02, 2010 11:49 am 
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The other reason 233U is better than 235U is what happens after a capture.

10% of the time 233U will capture the neutron instead of fissioning. When it does so, it creates 234U which is in essence a fertile isotope. One more neutron capture and you get fissile 235U.

20% of the time 235U will capture forming 236U. But 237U -> 237Np doesn't fission unless the neutron is going really fast. 238Pu is similar. Only at 239Pu do you get back to a fissile element.

So a capture in 235U costs you almost twice as many neutrons as a capture in 233U (on top of being twice as likely).


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PostPosted: Mar 02, 2010 3:21 pm 
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Thank you all for that feedback. I'm fascinated by that, the U233 does look to have quite superior values over U235 in the thermal region.

So for the DMSR discussion thread http://www.energyfromthorium.com/forum/viewtopic.php?f=29&t=2227 where some of the challenges include the need for cleaning the fuel salt of FP's to improve neutron economy and the eventual accumulation of significant quantities of U238. If one had the option of adding small quantities of U233 laced with U232, that could be a very effective addition for boosting reactivity while avoiding the addition of any more U238 to the pot. A small amount of material might go a long way in those circumstances.


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PostPosted: Mar 03, 2010 1:06 am 
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Lindsay wrote:
Thank you all for that feedback. I'm fascinated by that, the U233 does look to have quite superior values over U235 in the thermal region.

So for the DMSR discussion thread http://www.energyfromthorium.com/forum/viewtopic.php?f=29&t=2227 where some of the challenges include the need for cleaning the fuel salt of FP's to improve neutron economy and the eventual accumulation of significant quantities of U238. If one had the option of adding small quantities of U233 laced with U232, that could be a very effective addition for boosting reactivity while avoiding the addition of any more U238 to the pot. A small amount of material might go a long way in those circumstances.



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If one had the option of adding small quantities of U233 laced with U232,


This can only come with the acceptance of the pure thorium fuel cycle as a viable route to nuclear power. Until that time, thorium and its operative agent, U233 will be relegated to the role of a fuel extender and anti proliferation agent for the uranium fuel cycle.

No regulatory framework exists for what you suggest. U233 must always be denatured with an abundance of U238. Yes, an unfortunate happenstance.

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PostPosted: Mar 03, 2010 4:23 am 
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Axil wrote:
This can only come with the acceptance of the pure thorium fuel cycle as a viable route to nuclear power. Until that time, thorium and its operative agent, U233 will be relegated to the role of a fuel extender and anti proliferation agent for the uranium fuel cycle.

No regulatory framework exists for what you suggest. U233 must always be denatured with an abundance of U238. Yes, an unfortunate happenstance.

Thanks Axil, I might be missing something here (or just dim),a pure thorium fuel cycle produces U233, and from your second statement that U233 must always be denatured with U238. So doesn't that mean that a pure Th/U fuel cycle cannot occur within the existing regulatory framework on the basis of the existing denaturing requirements?

That could be hard to fix. Unfortunate, yes and some other words too.

In terms of dosing the fuel salt with U233 at a secure facility to boost the reactivity, the amount than can be added and stay within the (1.67U233 + U235)/TotalU =< 20% limit appears to be really small, so I don't think that would work. So I'm 0/2 at moment.


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