Energy From Thorium Discussion Forum

It is currently Dec 16, 2017 12:31 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 11 posts ] 
Author Message
PostPosted: Sep 22, 2010 11:52 am 
Offline

Joined: Dec 05, 2008 8:50 am
Posts: 337
It' s usually stated that the main drawback of fast reactors (including fluid fuel fast reactors) is the high fissile start up need, pratically in the form of costly (~ 100,000 $ per kg) plutonium or other LWR transuranics

But according to a doc in the repository section,
http://www.energyfromthorium.com/pdf/NA_MCFBR.pdf
a 1 GWe chloride fast reactor can be made critical with only 7,5 tonn of fissile and even much less, achieving still a positive and quite high breeding ratio.
Can you comment it ?


Top
 Profile  
 
PostPosted: Sep 22, 2010 3:29 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3070
Sounds about right. The fastish French design uses around 6 tonnes fissile but the spectrum isn't as fast. The fissile load requirements go up as the spectrum gets faster. 7,5 tonnes of fissile at $100,000 would be $750,000,000 - which is quite an expense. It isn't killer if we are talking about one chloride cleanup reactor per 20-50 fluoride reactors but if you are trying to create a chloride reactor that is targeted for bulk power generation then this cost is very problematic.


Top
 Profile  
 
PostPosted: Sep 22, 2010 6:36 pm 
Offline

Joined: Dec 03, 2008 5:23 pm
Posts: 137
Location: Oak Ridge, TN
The 7.5 tonnes/GWe is quite high. A thermal spectrum MSR has a fissile loading of 1.5 tonnes/GWe. An LWR has perhaps 3 tonnes of fissile/GWe.


Top
 Profile  
 
PostPosted: Sep 23, 2010 9:37 am 
Offline

Joined: Dec 05, 2008 8:50 am
Posts: 337
According to table I of the pdf above a molten chloride fast reactor can be even started up with less than 3 tonn of fissile, a level quite similar than LWRs


Top
 Profile  
 
PostPosted: Sep 23, 2010 9:53 am 
Offline

Joined: Dec 05, 2008 8:50 am
Posts: 337
Lars wrote:
Sounds about right. The fastish French design uses around 6 tonnes fissile but the spectrum isn't as fast. The fissile load requirements go up as the spectrum gets faster. 7,5 tonnes of fissile at $100,000 would be $750,000,000 - which is quite an expense. It isn't killer if we are talking about one chloride cleanup reactor per 20-50 fluoride reactors but if you are trying to create a chloride reactor that is targeted for bulk power generation then this cost is very problematic.


Actually, I think we' ll even need less than one fast chloride reactors every hundreds or even thousands LFTR of same capacity - assuming the only mission of fast chlorides is to burn transuranics waste from LFTR and the TRUs produced from LFTRs can be lowered to less than 100 grams per GWyear (a level easily achievable with current tech, as far I understand).

Obviously, different scenario if we want to use chloride fast reactors even to burn TRUs from current LWR fleet


Top
 Profile  
 
PostPosted: Sep 23, 2010 12:11 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3070
Getting down to 100milligrams is not easily achieved but I do think it is achievable.
This presumes feeding the actinides back into the LFTR on-site. This has a downside in that there is equipment on-site that can separate plutonium from fission products. Such equipment could be a proliferation risk and will cost something.

My preferred approach is to simply let the plutonium go into the removed salt-seeker fission products stream - roughly 20 kg Pu/year. This gets stored on-site until it is cool enough to move. Then move this to a central processing site with your secured plutonium extraction equipment and chloride reactor. A fast reactor with a feed of 238Pu will generate a considerable surplus of neutrons which could be used for generating startup fissile if desired or possibly for transmutation. If we don't feed the reactor anything but 238Pu then we would need a ratio around 50:1.


Top
 Profile  
 
PostPosted: Sep 23, 2010 1:56 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
I'm not yet convinced that using 'pyro' methods to exctract SNF TRU will cost the $100/gram that PUREX high purity Pu239 costs.

If, at scale, it turns out to be only $10/gram, its a whole different ball game.


Top
 Profile  
 
PostPosted: Sep 23, 2010 3:48 pm 
Offline

Joined: Dec 05, 2008 8:50 am
Posts: 337
Lars wrote:
Getting down to 100milligrams is not easily achieved but I do think it is achievable.
This presumes feeding the actinides back into the LFTR on-site....


100 milligrams per MWyear ?
Just curios, does it change anything if LFTR themself are started up with LWR transuranics spent fuel ?

If you reduce transuranics production to 20 kg/GWyear, then you need at least a couple of big Cl fast reactors, for a fleet of LFTR of at least 70-80 GWe - while only some tens or hundreds MW of Cl fast reactors for that LFTR power installed, instead; considering the costs and proliferation concerns involved it' s a factor to take seriously in consideration, I guess


Top
 Profile  
 
PostPosted: Sep 23, 2010 5:15 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3070
Got up too early this morning. 100 grams/GWe-year.

Over the long haul there is no significant change. There is a somewhat higher leakage rate initially - though I suspect we can simply send the waste back through the actinide extraction system a second time and get the leakage down to the identical level.


20 kg / GWe-yr is the quantity of TRUs produced assuming we remove them fast enough that only a small portion gets burned up. Since plutonium seems like such a bogeyman and it would reduce costs at the majority of sites I'm thinking along the lines of lots of LFTR iso-breeders w/o actinide feedback and a central site to both extract the actinides and host a machine that consumes them.

Assuming a GWe machine can eat between 400 and 800 kg actinides / GWe-yr one would need 2-5 such machines for every 100 LFTRs.


Top
 Profile  
 
PostPosted: Sep 24, 2010 3:01 pm 
Offline

Joined: Dec 05, 2008 8:50 am
Posts: 337
By the way, if you envisage to install a so big nuclear capacity of fast chlorides (say, a few % of the LFTR power installed with a TRU production of "only" 20 kg/GWyear), what do you think to do with extra U-233 produced in the thorium blanket (if you envisage to have one of this) - considering that LFTR are (or should be) still self-sufficient breeders and don't need any external fissile

For example, for a power installed of, say, 100 GWe of LFTR you need about 2 or 3 GWe of fast chlorides burning every year 90 GWyear/year*20 kg/GWyear ~ 1,8 tonn of Pu (or TRU) from LFTR and producing with a breeding ratio in the range of 1-1,4 an extra U-233 in the Th blanket in the range of ~ 2-2,5 tonn/year. It seems that this U-233 surplus is pratically useless or am I wrong ?


Top
 Profile  
 
PostPosted: Sep 24, 2010 3:24 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3070
For quite a while (>100 years I'd guess) we will be expanding nuclear power so any extra fissile generated could be used for start up.

Once we have completed that task then the neutrons will be surplus. It is plausible to use them to transmute the iodine-129. Iodine is bioconcentrating so it poses more danger than one would imagine from its lifetime. It has the nice property that on absorbing a neutron it decays to Xe so we can remove it promptly once it has transmuted. Personally I would not expect it is worth the money and trouble to isolate the iodine and treat it this way rather than putting it in geological storage but not my call.

Otherwise, the neutrons are truly surplus. A neutron stream can do interesting things so I'd imagine someone will find a use for them. If just want to throw the extra neutrons away, we can let them transmute ordinary iodine into xenon.

Funny to consider what to do with too many neutrons :)


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


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