Energy From Thorium Discussion Forum

It is currently Aug 18, 2018 1:30 pm

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 99 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6, 7  Next
Author Message
PostPosted: Feb 19, 2010 2:39 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1947
Location: Montreal
Lars wrote:
I think you will avoid the middle energies (1eV to 1MeV) much better in that reactor. In a fluorine based MSR I think you will jump pretty quickly into this middle range which leads to the main problem - excessive neutron capture by 238U.

Hmmm.... I would think that if fewer collisions get you to low neutron energies (as in inelastic collisions), then less time is spent in the middle energies (1eV to 1MeV), than when a long series of collisions is required to get there.
We can't both be right 8)

Remember that at energies > 600keV, F19 and Na23 have almost identical inelastic x-sections.
So the number of collisions to get out of the fast spectrum should also be about the same.
Its only once you get below 600keV that significant differences appear.....


Last edited by jaro on Feb 19, 2010 5:13 pm, edited 1 time in total.

Top
 Profile  
 
PostPosted: Feb 19, 2010 3:13 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
It looks like they are almost identical about 6 MeV (not 600keV).


Top
 Profile  
 
PostPosted: Feb 19, 2010 3:18 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1057
Lars wrote:
But the big difference is the presence of fluorine and its inelastic scattering cross-section.
(PS Still not sure I understand inelastic scattering properly so take this with a BIG grain of salt).

The chart shown is for a reactor with no fluorine - and hence no inelastic scattering. I think you will avoid the middle energies (1eV to 1MeV) much better in that reactor. In a fluorine based MSR I think you will jump pretty quickly into this middle range which leads to the main problem - excessive neutron capture by 238U.

Your reactor ideas may well work but I suspect you will need to avoid fluorides.


Fluorides are only a means to an end; they are just another tool in the design toolbox. The optimized performance of the reactor system is the goal. A moderator that can thermalize the fission neutron very quickly below 1eV is the means to that goal and that moderator is a combination of dueteirium and other low Z elements.

Kirk has said as follows:

Quote:
So there we have it--the perfect moderator is deuterium, and we simply have to find a substance that contains deuterium in a form that will be stable inside the reactor. No problem right?

No, there's the problem itself. It's really hard to find a substance that contains hydrogen and is chemically stable at the high temperatures of the fluoride reactor. The basic problem is that when those high-speed neutrons hit hydrogen or deuterium, they knock it clean out of the park, or at least out of whatever it's chemically bound to. Things would be okay if the hydrogen found its way back to where it came from, but typically the hydrogen finds hydrogen and becomes a gas, which comes right out of the moderator material.

Lithium hydride, beryllium hydride--they all have this problem. Hydrogen oxide (aka water) also isn't stable at the elevated temperatures of the reactor. For a time in the 1950s, hydroxides (and deuteroxides) of lithium and sodium were considered as high-temperature moderators, but they had extreme corrosion problems.


The design goal, then, is to make deuterium usable is a thermal reactor. The key is to use pressure. Not much is required, just a few BARs to keep deuterium bound to the low z elements of reaction friendly isotopes of lithium and beryllium and maybe boron.

The other method of deuterium stabilization is encapsulation in a pebble. A SiC or ZrC coating can keep a chunk of deuterium based moderator stable at high temperatures and pressures.

If a reactor system can be configured around these pebble and pressure techniques, the goal of an ultra high efficiency thermal spectrum reactor system is possible.

_________________
The old Zenith slogan: The quality goes in before the name goes on.


Top
 Profile  
 
PostPosted: Feb 19, 2010 4:09 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Axil, the heavy water is a fine moderator. Jaro and Lars are discussing whether a bi modal spectrum is feasible with fluorine in the fuel. U238 sometimes fissions if the neutron energy is high enough (more than 900 keV?) and that gives CANDU 10% bonus neutrons compared to thermal only spectrum. The problem is that fluorine will want to make the neutrons slow down rapidly upon collision. However no one seems to have looked in detail at a fluorine poor fuel such as the UF4 UF3 eutectic Jaro advocates, probably due to the conventional view that nickel alloys will be used. If those materials should be developed (carbon composite or otherwise) then the question is whether there is a small enough amount of fluorine atoms per volume in the UF4 UF3 to allow a big neutron bonus, because that would give a hybrid fast and thermal spectrum some attractive features.

Edit: fluorine not fluoride...


Top
 Profile  
 
PostPosted: Feb 19, 2010 4:28 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
It is the fluorine atoms that count. A 72% UF4 / 28% UF3 salt would be 78.8% fluorine atoms. I would not exactly call that a fluorine poor fuel.


Top
 Profile  
 
PostPosted: Feb 19, 2010 5:08 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Lars wrote:
It is the fluorine atoms that count. A 72% UF4 / 28% UF3 salt would be 78.8% fluorine atoms. I would not exactly call that a fluorine poor fuel.


It certainly is compared to a majority FLiBe carrier unless you feel like using HEU. The French work has data on a LiF carrier but they assumed a lot of it (>70% LiF). It will certainly be better fur UF4 UF3 even with NU. Maybe it's still too much fluorine. I hope not because that fast fission bonus is quite nice to have. Using more enriched uranium reduces the effect, but it also reduces the fast fission bonus from U238 in the first place, with a lower U238/U235 ratio...


Top
 Profile  
 
PostPosted: Feb 19, 2010 5:15 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1947
Location: Montreal
Lars wrote:
It looks like they are almost identical about 6 MeV (not 600keV).

Ooooops..... another senior moment..... I meant to say Na23 of course -- corrected it in the previous post. Sorry about that -- and thanks for pointing out my errors.


Top
 Profile  
 
PostPosted: Feb 19, 2010 5:20 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1057
Cyril R wrote:
Axil, the heavy water is a fine moderator. Jaro and Lars are discussing whether a bi modal spectrum is feasible with fluorine in the fuel. U238 sometimes fissions if the neutron energy is high enough (more than 900 keV?) and that gives CANDU 10% bonus neutrons compared to thermal only spectrum. The problem is that fluorine will want to make the neutrons slow down rapidly upon collision. However no one seems to have looked in detail at a fluorine poor fuel such as the UF4 UF3 eutectic Jaro advocates, probably due to the conventional view that nickel alloys will be used. If those materials should be developed (carbon composite or otherwise) then the question is whether there is a small enough amount of fluorine atoms per volume in the UF4 UF3 to allow a big neutron bonus, because that would give a hybrid fast and thermal spectrum some attractive features.

Edit: fluorine not fluoride...




Quote:
Axil, the heavy water is a fine moderator


Yes, the heavy water is a fine moderator. But there are other compounds that are better because they contain more deuterium by weight than heavy water. Heavy water cannot be used in a high temperature environment that increases the thermal efficiency of the reactor. Other deuteride compounds can withstand temperatures as high as 1000C.

If the reactor operates at these high temperatures, doppler broadening becomes a factor.

When a reactor gets hotter, the accelerated motion of the atoms in the fuel increases the probability of neutron capture by U-238 atoms.

When the uranium is heated, its nuclei move more rapidly in random directions, and therefore see and generate a wider range of relative neutron speeds.
U-238, which forms the bulk of the uranium in the reactor, has very distinct energies at which it absorbs neutrons, so that it will be thousands of times more likely to absorb a 6.67eV neutron than at 8eV neutron. As the random motion of the U-238 atoms increases though, the more likely it is that atom will be moving away from the 8eV neutron at the right speed that the neutron speed, in the U238 frame of reference, will be 6.67eV. This increases the number of neutrons absorbed by U238 atoms by reducing the number of neutrons available to cause the more useful U233 breeding function and reducing the breeding capability of the reactor.

The hotter the reactor gets, the more U238 steals neutrons from the U233 formation process. A weak moderator like fluoride only amplifies the problem because the neutrons stay in the greatly Doppler expanded U238 absorption zone much longer thereby further increasing the probability that u238 will steal neutrons from U233 breeding.

_________________
The old Zenith slogan: The quality goes in before the name goes on.


Top
 Profile  
 
PostPosted: Feb 19, 2010 5:35 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1057
Lars wrote:
It is the fluorine atoms that count. A 72% UF4 / 28% UF3 salt would be 78.8% fluorine atoms. I would not exactly call that a fluorine poor fuel.



I suggest a natural uranium based uranium deuteride nano-powder suspended in a lithium deuteride liquid.

_________________
The old Zenith slogan: The quality goes in before the name goes on.


Top
 Profile  
 
PostPosted: Feb 19, 2010 6:12 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
jaro wrote:
I meant to say Na23 of course -- corrected it in the previous post.

Yes they do look similar.
But in a fast reactor don't they use metalic fuel so the neutrons get to hit 238u many times before it gets to the sodium. In our case the fluorine atoms are mixed into the fuel so it will be worse for us. BUT it also makes me wonder about my understanding of inelastic scattering. I may need to spring for a more advanced book. Any recommendations for a follow on to "Introduction to Nuclear Engineering" by Lamarsh?


Top
 Profile  
 
PostPosted: Feb 19, 2010 6:47 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1947
Location: Montreal
Thanks Cyril.
Cyril R wrote:
U238 sometimes fissions if the neutron energy is high enough (more than 900 keV?) and that gives CANDU 10% bonus neutrons compared to thermal only spectrum.

There is something else -- possibly more important:

When we talk about an "equilibrium core", we need to remember that there will be a fair bit of Pu -- including 239, 240 & 241.
Not only do these have higher fast fission x-sections than U238 (and even U235), but they also have a high "eta" (n emitted per n abs).
The Radkowski reactor patent in particular depends on high "eta" -- especially from Pu241.

Cyril R wrote:
The problem is that fluorine will want to make the neutrons slow down rapidly upon collision...

As shown in the graphs on the previous page, there is little difference between F19 and Na23 -- the latter being the typical fast reactor coolant.

Fluoride fuel has also been compared to chloride fuel -- in some of the French publications.

In this graph, note that in the highest energy end (marked “fast”), the fluoride trace closely follows the chloride trace -- the slight difference being solely due to the large mass difference: 35.5 vs. 19, which means a bit less energy decrement per collision with chlorine than with fluorine, thus maintaining a bit more neutrons at a given energy.

Its only below about 600keV (marked “intermediate”) that the fluoride trace suddenly drops a lot -- due to the difference in inelastic x-sections at those energies.

However, below about 1MeV the big neutron bonus from high eta values no longer applies, so we're not losing anything there.

Moreover, we want to get below the intermediate energy range as fast as possible, to avoid resonance absorbtion in U238.

For that purpose, F19 is ideal, since inelastic collisions get us there in fewer steps than a long series of elastic collisions.
The proof of that is in the French graph: At energies below intermediate, the fluoride trace is above the chloride trace, indicating that more neutrons made the successful transition in the former case than in the latter case : The dip in the intermediate part of the fluoride trace merely means that few neutrons stick around there for long before moving over to the left....
Also, the higher F trace on the left side indicates that neutrons were NOT lost to resonance absorbtion – otherwise the trace would remain below the one for chloride fuel...(the chloride stuff does also absorb more at lower energies).

Image


Top
 Profile  
 
PostPosted: Feb 19, 2010 6:55 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
You make a good argument - perhaps my very limited understanding of inelastic collisions is not right.
I'll wait with further comments until I gain a better understanding of inelastic collisions - which may be a month or two if I have to first find and order a book.


Top
 Profile  
 
PostPosted: Feb 19, 2010 7:07 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1947
Location: Montreal
Lars wrote:
in a fast reactor don't they use metalic fuel so the neutrons get to hit 238u many times before it gets to the sodium. In our case the fluorine atoms are mixed into the fuel so it will be worse for us.

This would be true if the fuel rods were fat & relatively well separated (ie. "heterogeneous" lattice), and if the fuel were metalic.

In fact, fast reactor fuel pins are VERY thin -- a few millimeters OD -- due to the high enrichment & consequent high flux & volumetric power, that must be transfered to the sodium.
Thus they are virtually "homogeneous" types of core -- equivalent to a solution or alloy, from a neutronics point of view.

Most LMFRs built to-date used oxide fuel.
There were a few exeptions, like EBR II (intended to be succeeded by IFR, also with metal fuel).


Top
 Profile  
 
PostPosted: Feb 20, 2010 10:26 am 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1947
Location: Montreal
Incidentally, in the last graph above, the materials are fluorides and chlorides of Thorium-232, not Uranium-238.

That's just because I haven't found the version for U238.

As it turns out though, Th232 is a pretty good surrogate for U238 -- at least in the "intermediate" energy range discussed above -- implying that the effect seen should indeed be due to the difference between fluorides & chlorides, and applicable to both fertile species.


Image


Top
 Profile  
 
PostPosted: Feb 20, 2010 3:16 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1947
Location: Montreal
Lars wrote:
BUT it also makes me wonder about my understanding of inelastic scattering. I may need to spring for a more advanced book. Any recommendations for a follow on to "Introduction to Nuclear Engineering" by Lamarsh?

Why would you need another book ??

Lamarsh, p.42 wrote:
Inelastic scattering. This process is identical to elastic scattering except that the nucleus is left in an excited state. Because energy is retained by the nucleus, this is clearly an endothermic reaction. Inelastic scattering is denoted by the symbol (n, n'). The excited nucleus decays, as explained in Section 2.7, by the emission of gamma-rays. In this case, since these gamma-rays originate in inelastic scattering, they are called inelastic gamma-rays.


Lamarsh, p.449 wrote:
Shields designed for the purpose of attenuating very fast neutrons.... slow down the neutrons by inelastic scattering. This process is surprisingly effective.
....Suppose, for example, that a 14-MeV neutron is inelastically scattered by iron (A = 56). Then from Eq. (10.62), E' = 3 MeV. This represents an average energy loss of 11 MeV, which is substantially more than the neutron would have lost in an elastic collision even with hydrogen.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 99 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6, 7  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group