Energy From Thorium Discussion Forum

It is currently Feb 23, 2018 7:27 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 76 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6  Next
Author Message
PostPosted: Feb 21, 2011 9:44 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1058
U.K. Start-Up Aims to Cash in on Small Fusion Reactor


Quote:
A spherical tokamak could even form the core of a hybrid fusion-fission reactor, providing the neutrons to keep a fission reaction running. "Our expertise is in neutron sources and small tokamaks. We need to find the right partners and work with them to develop applications," Kingham says
.

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


Top
 Profile  
 
PostPosted: Feb 22, 2011 3:58 am 
Offline

Joined: Apr 19, 2008 1:06 am
Posts: 2239
I wish that in place of fusion, they would invest in Liquid Chloride fast reactors. They have enough plutonium from reprocessing to meet their electricity requirements for a long time.


Top
 Profile  
 
PostPosted: Feb 22, 2011 4:25 pm 
Offline

Joined: Dec 14, 2006 1:01 pm
Posts: 380
I agree; the Chloride reactors need development, and are more practical than a fusion system.


Top
 Profile  
 
PostPosted: Feb 22, 2011 5:04 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1058
IMHO, any fast spectrum reactor, chloride or otherwise, that burns nuclear waste should be non-critical especially if no U238 is provided to constrain control by Doppler shift.


Where should those external neutrons come from to compensate for the lack of delayed neutrons and provide control? The cost per neutron will be the lowest from a small fusion source.

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


Top
 Profile  
 
PostPosted: Feb 23, 2011 1:06 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
Thorium can also serve to provide a very prompt negative thermal coefficient due to doppler.


Top
 Profile  
 
PostPosted: Feb 23, 2011 1:36 am 
Offline

Joined: Nov 23, 2010 6:51 pm
Posts: 123
Pu isotopes still produce some delayed neutrons and a liquid chloride reactor should have lots of 238U. The only reason to build a reactor without fertile isotopes is to dispose of fissile isotopes in preparation for discontinuing the use of nuclear energy altogether. I don't think we're in a position to be doing that, nor will we be any time soon.


Top
 Profile  
 
PostPosted: Feb 23, 2011 2:07 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
A true two fluid reactor has no fertile in the core and hence does not get the fast control mechanism of doppler in 238U or 232Th. I would be concerned about the ability to control a true two fluid fast reactor.

A plutonium burner without u238 could make sense. It could be a machine that does three things:
1) burns around 1 tonne plutonium per year
2) produces around 1 GWe
3) generates 600kg or so of 233U per year (more if you remove fission products more frequently).

If the public has a primary concern about waste and plutonium then such a machine could be sellable to the public.


Top
 Profile  
 
PostPosted: Feb 23, 2011 2:40 am 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1058
There is no need for a two fluid breeder. Lars is right. Th-232 is just as good at doppler control and breeding as U238 is. Why produce plutonium when you can produce U233?

A chloride salt single fluid fusion/fission hybrid transuranic waste burner with thorium-232 breeding would produce 10e22 n/s and get another factor of 20 minimum increase through fission of transuranic waste using 14.2 MeV neutrons.

Protactinium would boil off above 400C as soon as it is transmuted to optimize U233 production and U232 could be extracted through on-the-fly salt reprocessing.

The production of U233 would be very high using this approach and contain a large percent of U232.

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


Top
 Profile  
 
PostPosted: Feb 23, 2011 8:15 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5060
If you add thorium and do at least breakeven breeding then I wouldn’t call that a plutonium burner. It is a U/Pu to U233/Th cycle transitioning machine. Waste burning is only one of the goals, the other is to get rid of coal, which it does by using thorium/U233 breeding. The cool thing about the LFTR is that it achieves numerous goals in just one reactor design.

I hope we can get commercial fusion going because 14 MeV neutrons are very useful. Unfortunately they also break apart any material known to man.


Top
 Profile  
 
PostPosted: Feb 23, 2011 2:31 pm 
Offline
User avatar

Joined: Dec 19, 2006 11:01 am
Posts: 396
Location: Knoxville, TN
IMHO both MSRE and ARE ran without any fertile in the core - the salt thermal expansion alone was good enough to keep them self regulating. Is there a problem going to higher total power? I though this scales.


Top
 Profile  
 
PostPosted: Feb 23, 2011 2:50 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
The thermal salt expansion is slower - you need to wait for the pressure wave to reach the exit pipes before there is actually less fuel in the core. In a thermal reactor (like MSRE) this is acceptable since the neutron generation time is longer. In a fast reactor the neutron generation time is much shorter and likely too short for the salt expansion to serve as the primary means of control.

The doppler effect is extremely fast - measured in femto-seconds - so it is plenty fast even in a fast spectrum.


Top
 Profile  
 
PostPosted: Feb 23, 2011 3:20 pm 
Offline
User avatar

Joined: May 24, 2009 4:42 am
Posts: 826
Location: Calgary, Alberta
Lars wrote:
The thermal salt expansion is slower - you need to wait for the pressure wave to reach the exit pipes before there is actually less fuel in the core. In a thermal reactor (like MSRE) this is acceptable since the neutron generation time is longer. In a fast reactor the neutron generation time is much shorter and likely too short for the salt expansion to serve as the primary means of control.

The doppler effect is extremely fast - measured in femto-seconds - so it is plenty fast even in a fast spectrum.

The effect of salt expansion will occur at the speed of sound. The speed of sound in dense fluids is extremely fast, I can't tell you right now how fast, but if we know the neutron generation time, we should be able to make some meaningful comparisons, one to the other.

Another point in this area would be how quickly can you insert excess reactivity, remembering the the core is already in the optimal geometry for criticality, there are no reactivity adders for fuel slumping for example, so I wonder what credible excess reactivity events are possiblle, how big are they and how quickly can they be inserted. The French work showed that when inserting 1000 pcm (roughly equivalent to dumping 99 kg of U233 into a DMSR by my estimate) , that unless you could insert that in 0.01s or less you could not even get the effect of 1000 pcm as the combined self regulating effects clipped the actual reactivity response observed to some number less that 1000 pcm if the insertion time was greater than 0.01s. An insertion time of 0.1s resulted in approximately 400 pcm of observed reactivity after accounting for the self regulating effects during that time period.

Accepting of course that those numbers are for a core with substantial quantities of fertile material present, but those are the only ones I have access to.


Top
 Profile  
 
PostPosted: Feb 23, 2011 4:21 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
Neutron generation time varies depending on the neutron spectrum. I don't have the data collected in a place I can refer to but I could provide my recollections from various articles. It is certain that there are errors here but it should give the right overall trend.

Reactor Generation Time
CANDU 55 seconds
LWR a few seconds
MSBR ?? many mSeconds?
French hundreds of uSeconds?
Fast metal oxide
Fast metal
Chloride MSR ? useconds

In particular, the chloride MSR has one of the fastest spectrum around. If you had a chloride reactor you would need to check that the spectrum extended low enough that the doppler effect kicks in (I think this is very likely but still needs to be checked). If you had a chloride reactor with no fertile in the fuel salt then I'd be very concerned about the control and careful analysis would be needed (and I'm guessing would show that we should not do such a thing).

Yes, the French work on stability of the reactor simply chose some high PCM insertion and some quick insertion rates to find the performance limit of their reactor. The connection to plausible causes for such reactivity insertion was scant. I think there is enormous margin between the insertions they hypothesized and any physical means of adding reactivity. The biggest cause was a sudden stop of the pumps (which would make it so that all delayed neutrons are retained in the fuel salt rather than losing 1/3 of them to salt outside the core). But I have a hard time picturing the mechanism to cause a flow of 12 tonnes/sec of fuel salt flowing at 1-3 meters/sec to be stopped in 10mSec.


Top
 Profile  
 
PostPosted: Feb 23, 2011 7:36 pm 
Offline
User avatar

Joined: May 24, 2009 4:42 am
Posts: 826
Location: Calgary, Alberta
Yes I'm struggling to see that too, that's a lot of mass and monemtum, that combined with some natural circulation, it is hard to imagine all of that stopping in 0.01s as you mention.

Our mutual friend the REBUS 3700 concept design has a mean effective neutron life time of 2.24 x 10-2 s (Ref Bokov 2005) which is a lot slower than I would have thought for a true fast spectrum design, albeit with plenty of fertile material similar to TMSR.

Now to find the speed of sound in molten salt...


Top
 Profile  
 
PostPosted: Feb 23, 2011 8:22 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1954
Location: Montreal
According to LA-13638,
Quote:
Analysis of data from KEWB and CRAC has led to relatively simple computer codes that follow the early transient behaviour well and rely on thermal expansion and the formation of radiolytic gas for the shutdown mechanisms.

In the KEWB systems, two quenching mechanisms seem to be dominant over a wide range of excursions.
The first of these is the rise in neutron temperature and thermal expansion as the core temperature rises, resulting in a prompt temperature coefficient equal to –2 ¢/°C at 30°C. This effect is sufficient to account for the observed yield of excursions starting near prompt criticality, but is inadequate for more violent transient experiments. The second quenching mechanism is bubble formation. The available evidence supports the contention that during the spike, void space, consisting of many very small bubbles (microbubbles) with internal pressures of from 10 to 1000 atmospheres, is created by the fission process. The bubbles later coalesce and leave the system, giving the observed gas production coefficient of about 4.4 l / MJ.
Growth of these microbubbles seems to involve the repeated interaction between fission fragments and existing microbubbles from earlier fissions. Thus, a quenching mechanism proportional to the square of the energy release can be invoked. This model is successful in describing the solution transients, notwithstanding imprecise knowledge of the manner in which the bubbles form and grow.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 76 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6  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