Energy From Thorium Discussion Forum

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PostPosted: Apr 25, 2014 4:55 pm 
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[My first posting as a newcomer to this forum.]

I have read that a reactor based on thorium would produce less long-life radioactive waste than a conventional reactor using uranium.

If such information exists, I'd like to see a table giving estimates of the number of becquerels of radioactivity remaining after 10, 100, 1000, 10,000, 100,000, 1,000,000 years for each MWh produced in:
(a) a uranium reactor
(b) a thorium reactor.

Is this, or something similar, available anywhere?

Thanks for any information
Martin


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PostPosted: Apr 25, 2014 6:44 pm 
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Welcome Martin,

The comparison is normally made between a molten salt, thorium reactor such as a Liquid Fluoride Thorium Reactor (LFTR) that recycles the transuranics until they are gone and a once through light water reactor. Existing reactors are mostly LWRs. Heavy water reactors (CANDU) are also around - they have a somewhat similar radio-toxicity to LWRs.

The radioactive elements in spent fuel can divided into two classes: fission products and transuranics.

Fission products are the two pieces that result when an atom is split. Most fission products are intensely radio-active for a short time so that within 300 years they have decayed. There are a few (7) long-lived fission products - but their radioactivity is so low that they pose no danger. So we can live with fission products by properly isolating them from the environment for 300 years. This is a reasonable thing to do.

Transuranics (TRUs) are elements heavier than uranium. These come into existence when an uranium atom absorbs a neutron rather than being split by it. These can have half lives in the tens of thousands of years range so proving we can keep them isolated from the environment is a challenge. Yet they have sufficiently high decay energy to be a radioactivity risk. Current reactors generate around 300kg of TRUs per gigawatt-year.

TRUs will fission if placed back into a reactor and when they fission they are completely destroyed. This can be done in LWRs but it only modestly reduces the total TRUs we have. Fast solid fuel reactors will do a better job at reducing the TRUs. But a molten salt fueled reactor will do the best job (personal bias showing through - long argument as to why). In a molten salt reactor it is feasible to reduce the TRU content to the capability of chemical isolation. Minimizing the TRUs flowing into the waste stream hasn't been an explicit goal so far but reducing the TRU content 100 fold is generally accepted as feasible and some results from ORNL suggest we could even get to a 10,000 fold reduction - if we as a society deem this worth the effort.

I would recommend the wiki article on LFTR and the American Scientist article (wiki ref 6) as good starting points.

Figure 6 in "Liquid Fluoride Thorium Reactors". American Scientist 98 (4): 304–313 has a figure showing the toxicity versus time. Note that this is sieverts not becquerels. Becquerels aren't particularly meaningful as they are simply the number of atoms that decay per second and say nothing about the energy released in the decay or the likely harm to humans from that decay.


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PostPosted: Apr 25, 2014 7:03 pm 
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Martin A wrote:
[My first posting as a newcomer to this forum.]

I have read that a reactor based on thorium would produce less long-life radioactive waste than a conventional reactor using uranium.

If such information exists, I'd like to see a table giving estimates of the number of becquerels of radioactivity remaining after 10, 100, 1000, 10,000, 100,000, 1,000,000 years for each MWh produced in:
(a) a uranium reactor
(b) a thorium reactor.

Is this, or something similar, available anywhere?

Thanks for any information
Martin

Consider enrolling in an introductory nuclear sciences course.
I just completed this one (the class is still open, you can still join and finish the course if you cram 40 30 minute lessons in 2 weeks):
https://class.coursera.org/nuclearscience-002/
It was very useful to me.

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PostPosted: Apr 25, 2014 8:11 pm 
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CANDUs can be manipulated to burn a far larger fraction of their produced transuranics in the once through cycle.
This is because spent CANDU fuel has a very low enrichment of fissile materials (both residual 235U and various plutonium isotopes).

But I digress - it is also unfair to use the once through LWR cycle as your frame from comparison - future recycling after an extended cooling cycle would produce a result comparable to LFTRs with proper engineering.
(Recycling using current mature technologies gets cheaper as the activity of the feed material decreases due to reduced degradation of your extractant materials by the gamma flux released by the fuel).


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PostPosted: Apr 26, 2014 4:28 am 
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Many thanks for the helpful and informative replies.

My knowledge of nuclear power is pretty much limited to "The Elements of Nuclear Power" (3rd Ed) by D J Bennet and J R Thompson, which I read attentively about twenty years ago. (However, it's a book that goes into quite some detail.)

I am interested in forming my own opinion on the use of thorium reactors. I have come across statements (sorry I can't give references) about the total waste generated which vary from one extreme (the waste is negligible) to the other extreme (much the same as with conventional reacotrs). And all expressed qualitatively, without numbers. Hence my question, aimed at getting an understanding in quantitative terms, even if it's only an approximate overall picture.

Becquerels - yes I can see that it might not be the appropriate unit to measure the total waste.

I know that there are all sorts of proposals for burning up transuranides, rather than use the once-through cycle currently used. I've no idea how likely that is to be put into practice and I know that there are arguments against doing it. With my own outlook, which tends towards pessimism, I'll be convinced when I see it in operation. So for the time being, I'm interested in comparing thorium reactors with LWRs as they are currently operated.

I've printed off the Am Sci article and I'll be reading it with interest over the weekend.

I'm still hoping to find a side-by-side comparison of the figures of total waste (in sieverts, if that is an appropriate measure - I'd need to convince myself on that) of the total radwaste remaining (after 10, 100, ... years) per MWh from:

[1] LWR's operated as they are at present.
[2] Thorium reactors in the most likely format to be constructed and operated.

Many thanks again for taking the time to reply to my question.


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PostPosted: Apr 26, 2014 5:38 am 
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You can easily find this kind of information by Googling images. Here is one of many:

Image

As you can see this considers ingestive radiotoxicity. Which misses the main point about nuclear energy, that wastes are small, isolated, and immobilized. So they don't get eaten. Even if wastes are poorly buried and get into the water table, only a fraction dissolves, then only a fraction makes it through the filtering effect of the soil, then only a fraction of the remainder gets taken in by plants, then only a fraction of that ends up in humans, etc.

Might as well discuss the ingestive toxicity of paint. Certainly, paint is toxic, but paint is not for eating. Everyone knows that. I got enough household chemicals around to kill several people, but that's just being silly.

TRU oxides are ceramic materials. They aren't going anywhere. They aren't going to be eaten.


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PostPosted: Apr 26, 2014 8:48 am 
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Many times, when folks say the Th waste stream is the same as current LWR waste streams, it actually is in reference to Th used in solid form in a LWR. The key with LFTR is that it is in a molten salt which gives it many advantages over solid fuel.


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PostPosted: Apr 26, 2014 10:41 am 
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Martin A wrote:
[2] Thorium reactors in the most likely format to be constructed and operated.

Many thanks again for taking the time to reply to my question.


I know of a solid fuel thorium reactors design in India. It should generate less plutonium in a once through that LWRs simply because most of the 238U has been replaced with 232Th. It uses 20% LEU rather than 4% so it will have around 1/5th the 238U and as a crude guess will produce around 1/5th the plutonium compared to an existing LWR in a once through operation.

The first molten salt reactors likely will use minimal reprocessing. All molten salt fueled designs I know of extract the offgases and almost all extract the noble metals. Together these represent around half of the fission products. In addition, since there is no fuel cladding the fuel lifetime isn't limited by integrity of the cladding. So with limited reprocessing the two designs I'm familar with target a fuel life around 30 years. The longer the lifetime the less plutonium per GWe-year.

In a solid fuel reactor, the fuel needs to be very well understood since it will be in place for 6 years or so and if there is a hot spot anywhere in the fuel it will cause the cladding to fail there. When recycling fuel the isotropic mix is dependent on where the fuel was in the reactor, the power history, how the fuel got shuffled around, and the properties of the fuel rods around it. Practically, it gets to be difficult to recycle the fuel more than once or twice due to these uncertainties.

Molten salt reactors rapidly mix the fuel and have no cladding to fail. This allows us to use fuel of somewhat uncertain isotropic mix. MSRs also allow easy fuel removal/addition over time so that changes in reactivity of the fuel are easily compensated for. These are the reasons I am much more hopeful that an MSR can recycle the TRUs better than a solid fuel reactor.

Finally, MSRs do not require accurately machined fuel rods so that fuel fabrication is simpler and can be accomplished even if the fuel is radioactive. That also makes recycling the TRUs more viable.

It is still likely the recycling the TRUs will cost more than enriching uranium. Many nuclear engineers feel that TRUs do not represent a significant health hazard - as Cyril mentions - you shouldn't eat it. In the US (and through its influence much of the nuclear power in the world) recycling is not done for proliferation reasons.

WILD SPECULATION WARNING
Whether we recycle or not is likely a question to be answered by society rather than nuclear engineers. If society deems it worthwhile we can recycle TRUs in an MSR. In the US, Europe, Canada, and Japan I expect the answer will be that we will have to have the proven capability to do recycling to greatly reduce the TRUs before these societies are going to allow a big expansion of nuclear. In the rest of the world I don't expect this will be a requirement for many decades.


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PostPosted: Apr 27, 2014 7:24 am 
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Martin A wrote:
I am interested in forming my own opinion on the use of thorium reactors. I have come across statements (sorry I can't give references) about the total waste generated which vary from one extreme (the waste is negligible) to the other extreme (much the same as with conventional reacotrs). And all expressed qualitatively, without numbers. Hence my question, aimed at getting an understanding in quantitative terms, even if it's only an approximate overall picture.
I also think it may be partly due to the way different people define wastes. For example, I define Spent Nuclear Fuel as a resource tainted by a bit of waste (the fission products that are not worth removing for sale). With my definition, all nuclear power produces the same amount of waste, tho silly people do want to needlessly waste perfectly good resources.

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PostPosted: Apr 28, 2014 12:43 pm 
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There are significant differences between the once-through LWR, once-through MSR, and closed LMFBR fuel cycles. These differences are especially prominent if you take the view that Allison MacFarlane (current NRC chairwoman) has taken...she is skeptical of lower waste volume claims from closed LMFBR fuel cycles because of the low-level waste created by repeatedly reprocessing the fuel. I don't necessarily agree with her, but that bugaboo does not apply at all for an MSR.

The game changer for an MSR in "waste terms" is the ability to keep the TRU and solid fission products in the salt for a very long time. That longer exposure to high neutron flux gives you a much more attractive waste profile, period.


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PostPosted: Apr 29, 2014 8:47 am 
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Many thanks again for the replied to my original query. I found pretty much what I was hoping to find as Figure 6 of the paper that I was recommended to read http://www.energyfromthorium.com/pdf/AmSci_LFTR.pdf .

The paper makes clear what I should have realised anyway:

[1] The fission products that need to be disposed of are pretty much the same for LWRs and MSRs, both in quantity and properties. (Because the fissile elements are U-235 and U-233 respectively with about the same energy and spectrum of fission products for each fission.) (Much the same if the LWR also uses Pu-239 from reprocessing.)

[2] The transuranic elements are produced in much greater quantities in LWRs because of the U-238 which is present, which serves no useful purpose but which is transmuted into transuranics by the neutron flux present.

I'm assuming what I state here is ok but if I've missed a major point, I'd like to be corrected.

Thank you again.
Martin


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PostPosted: Apr 29, 2014 9:31 am 
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Well the 238U does serve a useful purpose in a LWR.

Significant quantities of it are fissioned to produce energy - especially as burnups increase still further.
This effect is esspecially apparent with PHWRs.

For instance the Romanian SEU CANDU study shows 1.2% enriched uranium getting burnups of 21GWd/t - which is roughly 2.15% of the fuel being fissioned.

So 45% of the energy produced comes from the fission of 238U or its derivatives even in a once through cycle where 235U extinction is assumed.


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PostPosted: Apr 29, 2014 12:12 pm 
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Martin A wrote:
Many thanks again for the replied to my original query. I found pretty much what I was hoping to find as Figure 6 of the paper that I was recommended to read http://www.energyfromthorium.com/pdf/AmSci_LFTR.pdf .

The paper makes clear what I should have realised anyway:

[1] The fission products that need to be disposed of are pretty much the same for LWRs and MSRs, both in quantity and properties. (Because the fissile elements are U-235 and U-233 respectively with about the same energy and spectrum of fission products for each fission.) (Much the same if the LWR also uses Pu-239 from reprocessing.)

[2] The transuranic elements are produced in much greater quantities in LWRs because of the U-238 which is present, which serves no useful purpose but which is transmuted into transuranics by the neutron flux present.

I'm assuming what I state here is ok but if I've missed a major point, I'd like to be corrected.

Thank you again.
Martin


U-238 has a useful purpose, just like Th-232. They're both fertile. After one neutron absorption they become fissile Pu-239 and U-233 respectively. If further neutrons are absorbed without fission, you start producing transuranics. Eventually even those transuranics will fission, it just takes time and more neutrons.

In a LWR it's difficult to give those transuranics enough time to fission because of the buildup of fission products in the solid fuel and the resulting poor neutron economy; reprocessing the fuel solves that problem but its more expensive than simply using new fuel.

In a MSR you don't have the same neutron economy constraints because:
1) gaseous fission products (e.g. Xe-135) bubble out of the salt into the off-gas system
2) some other fission products can be removed through online chemical reprocessing, if you choose to do that


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PostPosted: Sep 02, 2016 6:14 pm 
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jaro wrote:
alexterrell wrote:
I make the claim that nuclear waste is radioactive for Centuries, not millenia, on the grounds that Pu and other transuranics are not waste, but fuel. What amount of Transuranics can it be allowed to contain before we can dump it into a cheap borehole?

One reasonable "standard" might be natural analogues.

For example, Protactinium-231 in the U-235 decay chain, has properties similar to Plutonium-239 -- notably half-lives close to 30,000 years, and alpha decay.
Natural abundance of Pa-231 is comparable to Radium-226.
Discussion moved from:

Re: Congratulations to David LeBlanc (ORNL)

Congratulations, Dr. LeBlanc.
Since Kirk Sorensen got the ball rolling with his LFTR concept under the name Flibe there have been additional startups in the MSR category. China started their initiative promising to patent their own flavor of MSR even though it was teaming up efforts with American schools. Transatomic has had the most publicity of the startups mostly because of the young fresh faces behind it. Leslie Dewan was featured on the Fareed Zakaria show. Not long ago Terrestrial Energy came on the scene in December 2012 with David LeBlanc’s KISS approach introducing the IMSR. In order to avoid the obstacles of regulation of thorium they propose using uranium as the main fuel source. Next came ThorconPower (pdf) announced their plan which to many people’s surprise was planning their prototype without thorium. Thomas Jam Pedersen gave a talk at the TEAC7 on June 2015 introducing another MSR company called Copenhagen Atomics
Back to the radioactivity remaining after 10^N years question and related points.

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PostPosted: Sep 08, 2016 3:16 pm 
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E Ireland wrote:
CANDUs can be manipulated to burn a far larger fraction of their produced transuranics in the once through cycle.
This is because spent CANDU fuel has a very low enrichment of fissile materials (both residual 235U and various plutonium isotopes).

But I digress - it is also unfair to use the once through LWR cycle as your frame from comparison - future recycling after an extended cooling cycle would produce a result comparable to LFTRs with proper engineering.
Ummm, no, it can't. The U/Pu fuel cycle CANNOT burn as much of the U238 as a Th/U cycle can burn thorium, no mtter how the thermal cycle is engineered. The Pu fission just does not produce enough neutrons to burn up the fuel all the way. Pu thermal fission only produces ~1.9 neutrons while a mininmum of 2 is needed to burn fully even in a neutronincally perfect reactor.

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