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PostPosted: Mar 08, 2010 10:52 am 
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Won't your fission will always be dominated by odd-mass nuclides - even if you spectrum is fast enough to get a boost from the even-mass nuclides?


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PostPosted: Mar 08, 2010 1:35 pm 
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jaro wrote:
The main problem I see with increased levels of transuranics in the core, especially the even-mass ones, is the same for all reactors: delayed neutrons cease to be effective in reactor control, because their energy at birth is less than half that of prompt neutrons from fission, so they are unable to contribute to the fission chain (even-numbered TRUs being able to fission only with fast neutrons).
This means we can't use these "lost" delayed neutrons to top-off the multiplication factor (so to speak), to avoid being critical on prompt neutrons alone -- an uncontrollable situation, as you know.


Thank you again, Jaro. I've seen several references to TRUs making reactors less controllable, but never why. There are a few issues you've brought up here.

1. even-numbered TRUs having essentially no fission cross section down in the low energy part of the spectrum. This shouldn't contribute to a control issue, right? It is as issue for neutron losses, I understand.

2. birth energy of delayed neutrons is less than half of the energy of prompt neutrons. A quick look at ENDF data seemed to show this is equally true of U-235, Pu-239, and Pu-241. I can see how, with a lot of U-238 around, the lower birth energy means that the delayed neutrons are more likely to get a resonance absorption and less likely to make a fission. So, it's clear that having a lot of U-238 makes for a control problem, but I don't see why the transuranics contribute to the problem more than U-233, U-235 or Pu-239. Incidentally, I didn't see any data for U-233's normal fission neutron energy spectrum.

Is the resonance absorption problem in Th-232 quite as bad as the resonance absorption in U-238? From looking at the ENDF plots, it looks like the relevant region is smaller, but there are so many peaks that it's very hard to tell.


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PostPosted: Mar 08, 2010 5:56 pm 
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Lars wrote:
Won't your fission always be dominated by odd-mass nuclides - even if your spectrum is fast enough to get a boost from the even-mass nuclides?

In most reactors it will.

But the delayed neutron fraction is small as it is : if you reduce it further -- such as by losing delayed neutrons outside the core, or by increasing the fuel fraction that is insensitive to delayed neutrons -- then controllability suffers to some extent.
Of course its not a black-and-white issue, but rather one of degree.....

Note that fuel made from LWR SNF with high burnup -- today around the 50GWd/tonne mark, and climbing -- can have quite a high proportion of even-mass nuclides (plus Am241 & Np237, which behave similarly).


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PostPosted: Mar 08, 2010 6:15 pm 
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Is the fraction of fissions generated in even mass nuclides (+np237 + am241) significant (say > 20%) in any reactor?


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PostPosted: Mar 08, 2010 6:30 pm 
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iain wrote:
1. even-numbered TRUs having essentially no fission cross section down in the low energy part of the spectrum. This shouldn't contribute to a control issue, right? It is an issue for neutron losses, I understand.

Personally, I find it easier to understand by looking at extreme cases:

For example, you could have a critical reactor with pure Pu240, or pure Pu238, or pure Pu242, or pure Np237, or pure Am241, or a combination of these, because their fast fission x-section is large enough, and extends a bit below 1MeV.
But because these nuclides don't react (fission) with any neutrons much below that -- notably all the delayed neutrons -- such a reactor would be totally uncontrollable (though it might make a good bomb, in some cases).

If you fuel a reactor with only half the load comprising these TRUs, then the delayed neutrons will only have half the effect they would if all the fuel were made of stuff like U233/235 and Pu239/241.

Fuel made from high-burnup LWR SNF can have a nuclide mix not very far off this half-and-half ratio (plus of course lots of U238).
One mitigating factor is in fact the U238, because it yields an unusually high fraction of delayed neutrons per fission.
As we all know, U238 is desirable for other reasons as well (ie. nonproliferation politics).

iain wrote:
2. birth energy of delayed neutrons is less than half of the energy of prompt neutrons. A quick look at ENDF data seemed to show this is equally true of U-235, Pu-239, and Pu-241. I can see how, with a lot of U-238 around, the lower birth energy means that the delayed neutrons are more likely to get a resonance absorption and less likely to make a fission. So, it's clear that having a lot of U-238 makes for a control problem, but I don't see why the transuranics contribute to the problem more than U-233, U-235 or Pu-239. Incidentally, I didn't see any data for U-233's normal fission neutron energy spectrum.

No.
The thing is that U238 fission x-section is too low to sustain criticality on fast neutrons, whereas the even-mass TRUs are comparable to the odd-mass ones in the fast spectrum.
As noted above, U238 is in fact desirable, for its anomalously high delayed neutron yield per fission (even though it itself doesn't react with them -- its only a "donor").

iain wrote:
Is the resonance absorption problem in Th-232 quite as bad as the resonance absorption in U-238? From looking at the ENDF plots, it looks like the relevant region is smaller, but there are so many peaks that it's very hard to tell.

I haven’t checked, but the point is that Th232 barely fissions at all, and won’t provide any delayed neutrons, like U238.


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PostPosted: Mar 08, 2010 6:47 pm 
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Lars wrote:
Is the fraction of fissions generated in even mass nuclides (+np237 + am241) significant (say > 20%) in any reactor?

As I understand it, only one-third of LWR core fuel is changed out each year (or year-and-a-half, depending on the plant's schedule).
That one-third that's nearing its end of life in the reactor will have a composition such that a significant fraction of fissions will be generated in those even mass nuclides (+np237 + am241).
But its only one third of the core load.
Currently, MOX is made from Pu only -- no Np237 or Am241 -- so a bit more than one-third of a core load can probably be tolerated.
Its the reactor physicist's job to figure out the tolerable limits on TRU loading, and the corresponding fueling strategies (....I'm not one of them)


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PostPosted: Mar 08, 2010 6:58 pm 
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I can see this as something worth checking in a very fast reactor that doesn't use 238U.
So perhaps it is an issue with some of the solid fuel concepts for burning TRUs in a thorium/TRU mix.

In a LFTR (or MSR-HW) most of the fissions happen in a slower spectrum where delayed neutrons will also cause fission.
I suspect the effect you describe will have an impact but a minor one (roughly we will lost a fraction of the delayed neutrons coming back into the core as (fast fissions ) / (total fissions).


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PostPosted: Mar 08, 2010 8:09 pm 
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Lars wrote:
In a LFTR (or MSR-HW) most of the fissions happen in a slower spectrum where delayed neutrons will also cause fission.

Definitely, the spectrum is key!

In fact, the importance of spectrum can hardly be over-emphasized.

For example, most people probably don't realise how vastly different LWR spectrum is from CANDU.

LWR spectrum is really more like that of an epithermal machine, with quite a large fast-fission contribution.

By contrast, CANDU spectrum has a great big hump at the far left end, near 0.03eV (which BTW might be a good pun on Canadian politics :lol: ).

Similarly, I would expect most of the LFTR concepts discussed here to be fairly epithermal, while HW-MSR would be more CANDU-like (over-moderated).

Since the over-moderated reactor is more dependent on thermal fission than epithermal reactor types, they can handle higher TRU loading.
As a matter of fact, CANDU can indeed safely handle a full-core MOX load, whereas LWRs can not.
This was one of the big CANDU selling points to the American GNEP program, for TRU disposal.

Note also that the Radkowski concept is basically an extension of the LWR spectrum into the fast reactor domain, by having the central portion of each fuel assembly loaded with extra-densely packed metallic fuel pins.
As the LWR spectrum is already relatively fast, it is claimed that the Radkowski scheme can be applied in existing LWRs without modification.


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PostPosted: Mar 08, 2010 8:24 pm 
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One important point in the spectrum - you need to have a significant portion of your neutrons captures in the resonance range to benefit from the very fast negative temperature coefficient generated by Doppler.


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