Energy From Thorium Discussion Forum

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PostPosted: Dec 06, 2009 12:16 am 
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Lately on Nuclear Green, Charles has made several posts about the Indian AHWR program (here and here). It seemed to me that the EFT Forum needed a thread for this.

I should start by saying that the Heavy Water Reactor is apparently the best option for building a solid fueled thermal-spectrum breeder or near-breeder (in other words, an HWR can deliver inexhaustible Energy From Thorium :P ). And my personal belief is that because chemical processing is more economical in larger plant sizes, even when most large plants begin using on-site reprocessing (a la LFTR), smaller plants will likely stick with off-site reprocessing. Solid fuels may have a perceived safety advantage when it comes to used fuel transport for off-site reprocessing.

Attachment:
File comment: Make-up fuel vs. burnup for a PHWR near-breeder
IAEA-TECDOC-1155_PHWR_SSET.png
IAEA-TECDOC-1155_PHWR_SSET.png [ 53.13 KiB | Viewed 5164 times ]

IAEA-TECDOC-1155 (from 2000 ) showed this graph, which indicates that a HWR can break-even on the thorium-U233 cycle, but the fuel must be reprocessed much more often than desired (the burnup is 12 MWd/kg HM vs. 40-60 for LWR fuel), but less often than natural Uranium fuel (7.5 MWd/kg). However, if break-even is not required (i.e. make-up fuel is added during reprocessing), the resulting fuel burnup can be brought up to normal, and the make-up fuel use amounts to 10-20% of what a LWR w/ once-thru would use.

IAEA-TECDOC-1450 (from 2005) gives more info on the thorium fuel cycle, and on India's AHWR. At that time, the AHWR was envisioned as running as a near-breeder, with Pu make-up fuel from fast breeders.

Here is the link to a recent brochure on the AHWR. In this version, it’s fueled with Uranium enriched to 19.8% mixed with thorium, so the effective fissile concentration is 4.2%. This gives a burnup of 64 MWd/kg in once-thru mode. It appears that the thorium is in the same rods as the U238, so the bread U233 will be denatured. The U ore requirement is 13% less than for a LWR.

Here is the link to the Canadians: http://www.aecl.ca (the Canadians seem to have lost interest in thorium fuel as well as natural uranium, and their newest reactor, the ACR-1000 runs on enriched U).

So the questions are:
- Are we really justified in our refusal to license the HWR in the US?
- Is the proliferation resistance of the LFTR any better than the HWR? (I'm guessing not)
- Does a closed fuel cycle have any real benefit with today's low Uranium prices?
- Will India loose interest in thorium breeders now that they can import uranium?
- Will India really fuel their reactors with Pu and non-denatured U233?

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Nathan Wilson, MSEE


Last edited by Nathan2go on Dec 06, 2009 3:03 pm, edited 1 time in total.

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PostPosted: Dec 06, 2009 12:44 am 
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Nathan2go wrote:
So the questions are:

- Does a closed fuel cycle have any real benefit with today's low Uranium prices?
- Will India lose interest in thorium breeders now that they can import uranium?


A closed fuel cycle becomes important when we want to provide the world's energy needs using nuclear - roughly a 20x expansion from today. In this case, I'd expect uranium-235 to become noticeably more expensive. The second area a closed fuel cycle helps is with waste.

I doubt India will lose interest simply because for a little while they can import uranium. I think the last few decades have established in their minds the value of not being dependent on external supplies for their energy.


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PostPosted: Dec 06, 2009 2:17 am 
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Nathan2go wrote:
Here is the link to the Canadians: http://www.aecl.ca (the Canadians seem to have lost interest in thorium fuel as well as natural uranium, and their newest reactor, the ACR-1000 runs on enriched U)


It is doubtful if any ACR reactor will ever be built and the Canadian fleet is still run on NU. Canada has little interest in developing a thorium cycle because we have a vast supply of domestic uranium.


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PostPosted: Dec 06, 2009 11:15 pm 
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AHWR is stated to be producing sufficient U233 for the next cycle. Pu shall have to come from fast breeders. That gives a conversion ratio of nearly 60%. PHWR with thorium fuel may have given a better conversion ratio due to better neutron efficiency. To get a near breeder you may have to use the tricks of Shippingport breeder like avoidance of neutron eating control rods and multiple seed and blankets.
In the Dittmar's articles in the oil drum, the typical breeding ratio of so-called fast breeders has been given as 1.05(PFBR). You have to maintain higher neutron energies to get better breeding ratio. A metallic fuel fast breeder may also produce a fissile break-even performance in Th-U233 cycle.


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PostPosted: Jan 07, 2010 11:37 am 
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Nathan2go wrote:
........the Heavy Water Reactor is apparently the best option for building a solid fueled thermal-spectrum breeder or near-breeder (in other words, an HWR can deliver inexhaustible Energy From Thorium :P ).

A report titled "An Overview of the Potential of the CANDU Reactor as a Thermal Breeder" (J.B. Slater, AECL-5679, Feb. 1977) concludes that a break-even equilibrium thorium fuel cycle is relatively easily atained in a CANDU reactor.
However, the fuel assumed in the report is HEU (61% U233, 6% U235), which would not even be considered these days, for political reasons (i.e. non-proliferation).

With more advanced modifications (but still solid fuel, no on-line FP extraction), the report finds that a breeding ratio up to about 1.11 can be obtained.

It would be interesting to adjust the Neutron Balance sheet in TABLE 2 of the report, in light of a molten-salt-fueled CANDU with on-line FP removal (zap item 7 absorbtions), carbon fuel channels (item 9), and of course reduced Th232 inventory and absorbtion (item 1) to allow politically-acceptable NU or SEU use, instead of LEU/HEU.

Its unfortunate that AECL-5679 only discusses "the potential of the CANDU reactor as a thermal breeder" in the context of the "pure" thorium fuel cycle: I was more interested in how close to break-even breeding a CANDU with a mix of NU/Th or SEU/Th could get.
The other adjustment I would like to look at is increased fuel channel diameter (fewer fuel channels & calandria tubes), for both reduced structural absorbtion and for a boost in fast fission factor (no water in the fuel channels, and fluorine being significantly LESS moderating than oxygen in UO2 -- mass 19 instead of 16). The latter should be especially interesting in an equilibrium core, where Pu241 gives a big neutron production boost for the U-Pu portion of the mixed fuel cycle (which is what the Radkowski concept is based on).
There is clearly a research domain in this field that hasn't been investigated in detail to date.....


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PostPosted: Jan 07, 2010 8:08 pm 
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jaro wrote:
Nathan2go wrote:
........the Heavy Water Reactor is apparently the best option for building a solid fueled thermal-spectrum breeder or near-breeder (in other words, an HWR can deliver inexhaustible Energy From Thorium :P ).

A report titled "An Overview of the Potential of the CANDU Reactor as a Thermal Breeder" (J.B. Slater, AECL-5679, Feb. 1977) concludes that a break-even equilibrium thorium fuel cycle is relatively easily atained in a CANDU reactor.
However, the fuel assumed in the report is HEU (61% U233, 6% U235), which would not even be considered these days, for political reasons (i.e. non-proliferation).
With more advanced modifications (but still solid fuel, no on-line FP extraction), the report finds that a breeding ratio up to about 1.11 can be obtained.

Even a highly neutron efficient thermal design like PHWR (or CANDU) has poor prospects as a breeder. Molten salts are not as neutron efficient and a breeder or self-sufficient thermal LFTR sounds unlikely. I wonder how much improvement can be brought about by constant removal of Xe. I think fast spectrum is more desirable if we want to burn all the U238 or run on thorium fuel.
We must think of alternative designs without carbon moderator and less moderating salts than FLiBe.
AHWR design is not a breeder.


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PostPosted: Jan 07, 2010 8:27 pm 
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Solid fuel reactors do have a significant neutron loss to Xe.
LFTR does not and can indeed breed using the Th/u233 cycle.

Dr. Peterson thinks AHTR can be a breeder in the future with appropriate processing.

A 238U/239Pu cycle does not breed in the thermal spectrum.


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PostPosted: Jan 07, 2010 8:51 pm 
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jagdish wrote:
I wonder how much improvement can be brought about by constant removal of Xe.

Xe is responsible for a large fraction of neutron absorptions by FPs.

In AECL-5679, the table of "Neutron Balance for CANDU Core at Equilibrium" lists the following figures, normalized to 1 neutron absorption in U233:

Quote:
Absorptions...............................................................Current CANDU Design

1. Th-232..........................................................................1.03
2. Pa-233..........................................................................0.02
3. U-233...........................................................................1.00
4. U-234...........................................................................0.08
5. U-235...........................................................................0.10
6. U-236 + higher actinides.....................................................0.01
7. Fission Products.............................................................0.11
8. Coolant, Moderator...........................................................0.04
9. Fuel Sheathing and Structural Materials....................................0.07
10. Leakage, Control and Power Shaping.....................................0.09
-------------------------------------------------------------------------------------------
11. Total..........................................................................2.55

Production

12. U-233...........................................................................2.29
13. Th-232..........................................................................0.03
14. U-235...........................................................................0.21
15. Other...........................................................................0.02
--------------------------------------------------------------------------------------------
16. Total............................................................................2.55

Conversion Ratio = (U233 + U235 production) / (U233 + U235 destruction)
= (1.03 - 0.02 + 0.08) / (1.00 + 0.10)
= ~1.0

To reach a BR of up to 1.115, the report credits mostly four items: 1) higher D2O purity, 2) improved nuclear materials, 3) improved power shaping and 4) reduction in fuel rating.
Eliminating FPs from the fuel - as in a molten salt version of CANDU - would conserve more neutrons than all four of the items above combined.


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PostPosted: Jan 07, 2010 10:45 pm 
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jaro wrote:
... AECL-5679... concludes that a break-even equilibrium thorium fuel cycle is relatively easily atained in a CANDU reactor.
However, the fuel assumed in the report is HEU (61% U233, 6% U235), which would not even be considered these days, for political reasons (i.e. non-proliferation).
...

Hmm, so a LFTR on pure-thorium cycle would likely be in the same boat.

jaro wrote:
...
It would be interesting ... to allow politically-acceptable NU or SEU use, instead of LEU/HEU. ...

Huh? I would think a reactor that combined on-line processing with NU fuel would be a serious WG-Pu proliferation concern (if the reactor were diverted or cloned). On-line refueling of CANDUs already seem worse to me than LWRs.

jaro wrote:
... I was more interested in how close to break-even breeding a CANDU with a mix of NU/Th or SEU/Th could get...

Isn't that what the Indian AHWR-thorium cycle represents? But it's only slightly better than a LWR: 40% energy from thorium vs. 30% energy from bred-Pu.

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PostPosted: Jan 07, 2010 11:35 pm 
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Nathan2go wrote:
Hmm, so a LFTR on pure-thorium cycle would likely be in the same boat.

You bet.

Nathan2go wrote:
Huh? I would think a reactor that combined on-line processing with NU fuel would be a serious WG-Pu proliferation concern (if the reactor were diverted or cloned). On-line refueling of CANDUs already seem worse to me than LWRs.

This is only an issue for a fresh core.
After less than a year of operation, the Pu isotopics become the same as an LWR -- definitely NOT "WG-Pu", and of course quite useless for weapons use.
Besides which, the on-line processing equipment provided with the reactor is only designed for FP removal. All the TRUs are intended to stay in the core and burn up.
As for existing CANDUs, the SNF Pu isotopics are similar to those of LWRs, and the commercial plants marketed world-wide have no means for clandestine transfer of SNF bundles outside the plant.

Nathan2go wrote:
Isn't that what the Indian AHWR-thorium cycle represents? But it's only slightly better than a LWR: 40% energy from thorium vs. 30% energy from bred-Pu.

It appears that 40% is about the best you can do with a solid-fueled HWR using LEU material (as noted above, the Th-HEU CANDU could be a breeder, but no nation could put it on the market, due to political non-proliferation issues.....)


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PostPosted: Jan 07, 2010 11:41 pm 
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Lars wrote:
A 238U/239Pu cycle does not breed in the thermal spectrum.

Correct.
That's why a bi-modal thermal/fast design is required, combined with on-line FP removal.
Its called the HW-MSR or CANDU-MSR.


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PostPosted: Jan 08, 2010 8:52 am 
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Just a quick point. No matter how fast you process to remove fission products you will never come close to removing all neutron losses to fission products because the of the very high cross sections of many rare earths. As well you have to balance in the losses to the carrier salt which for example in the MSBR were higher than to fission products.

Likely you can do very nicely with a HW-MSR but I don`t think it will be as good as you think. In regards to the fast fission bonus you often mention, I don`t think that will be nearly as large as you think either since fluorine inelastic collisions almost instantly bring most neutrons down from the very high energies where you can get a fast fission bonus out of U238. If you look at neutron spectrum curves from the French work there is a big missing piece of the spectrum at high energy, it is due to the fluorine effect. Another good example of this is that even with the huge amounts of fissile material they propose they still get a quite poor fission to capture ratio out of their plutonium (barely higher than the MSBR).

David L.

P.S. Can you post the AECL paper on thorium from 1977, it would be a very interesting read.


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PostPosted: Jan 08, 2010 9:45 am 
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David wrote:
No matter how fast you process to remove fission products you will never come close to removing all neutron losses to fission products because the of the very high cross sections of many rare earths.

True, but I think that keeping the concentration very low makes those large x-sections irrelevant.
Rare earth FP extraction has been discussed before -- and should be considerably easier in a carrier salt-free system.

David wrote:
As well you have to balance in the losses to the carrier salt which for example in the MSBR were higher than to fission products.

As you know, my preference is for a carrier salt-free system, using a UF4-UF3 (-ThF4) eutectic.

David wrote:
P.S. Can you post the AECL paper on thorium from 1977, it would be a very interesting read.

Certainly.


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PostPosted: Jan 08, 2010 11:01 am 
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The challenge comes from the ThF4, fission product mix not the LiF or BeF.
You can simply discard (or save for a future day) the thorium and leave it with the extracted fission products but then you would want to process rather slowly. We have 40 tonnes or so of thorium in a 1Gwe reactor so removing fission products every few months and discarding 40 tonnes of thorium is rather impractical.


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PostPosted: Jan 08, 2010 11:39 am 
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Lars wrote:
The challenge comes from the ThF4, fission product mix not the LiF or BeF.
You can simply discard (or save for a future day) the thorium and leave it with the extracted fission products but then you would want to process rather slowly.

Thanks for the reminder Lars.
I suppose it depends how much Th there would be in a reactor started on NU/Th or SEU/Th.
Due to the very low fissile content, the Th fraction would also be limited -- perhaps less than a third of the total load.
If break-even conversion could be achieved in the bi-modal thermal/fast flux without ANY Th, then I wouldn't bother putting it in at all.... just keep adding DU as the FPs are extracted....

Also, the extracted fission products CAN NOT be stored in pure form, since they are too hot.
A matrix material is needed in which the FPs are diluted.
If you don't like to use Th for that purpose, then something else needs to be used instead.
As Th is "dirt cheap", I'm not sure any other candidates are better....

Lars wrote:
We have 40 tonnes or so of thorium in a 1Gwe reactor so removing fission products every few months and discarding 40 tonnes of thorium is rather impractical.

Not sure what reactor type you are refering to, but a channel-type HW-MSR of ~0.6GWe capacity could work on just ~3 cu.m of fuel salt (including external inventory). Only a fraction of that would be Th. So we're looking at about 6 tonnes of Th, not 40.

Perhaps the best option would be to have rapid fuel processing for Xe removal (plus any other easy neutron absorbers), with the rare earths removal processing on a slower schedule..... (its a different process anyway, so seems practical)


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