Energy From Thorium Discussion Forum

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 Post subject: Re: Thorium production
PostPosted: Jun 08, 2009 9:34 am 
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I mean, given the fact that many countries don't have enough TRU to start a large fleet of MSR breeders and considering the superiority (at least for many of you) of moderator free, epithermal spectrum MSR version (because it avoids irradiated graphite/heavy water (***) waste at the end of the life and allow the use of both neutron economy poorer, cleaner and more economic salts, achieving at the same time break even breeding), it could be wise as intermediate solution to use low enriched uranium in DMSR while waiting for a total switch to thorium breeders, building up meanwhile as much TRUs as possible for future "pure" thorium breeders. Has it any sense or the TRU production in DMSRs is anyway too low ?



Alex,

Yes a DMSR will probably have to be graphite moderated because of the large resonance cross sections of U238. As far as TRU production, because the cross sections for fission and absorption are so high in a well moderated spectrum, there is actually quite little TRUs that stay in the salt. In Oak Ridge's DMSR there was less than a tonne of Pu at the end of 30 years and it would be of poor quality for starting other types of molten salt reactors because the fissile fraction is so low. I am not sure how much Am or Cm would be present but likely just a few hundred kg. Thus the DMSR is not a good way to make fissile for other reactors but to me that is part of its overall proliferation advantage. The DMSR does finish its life with a good amount of denatured uranium though which is great for starting yet another DMSR (i.e. keeping the same plant going after changing the graphite as well). A reminder that ORNL named two designs DMSR, one had processing for fission products and tries to break even, the other that I prefer is much simpler and leaves the soluble fission products in the salt until you shut down or swap out the entire salt load (with one time only processing, likely in a central facility).

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When you all say that DMSR didn' t include a processing plant, you mean only the fuel (uranium + thorium + actinides), not the fission or the activation products, otherwise I don' t undserstand how safety is assured, in case of loss of coolant/fuel accident, if the FPs are not extracted from the reactor (maybe the TRUs are not so dangerous...)


Alex,

The simplest DMSR would have no processing of the fuel whatsoever on site (except He bubbling and some chemistry control to deal with any oxide contamination). I've mentioned that at the end of the salt's lifetime we'd likely do a one time only processing (30 years in ORNL's study, we might change that though). That processing would likely either be by equipment brought to the site and run for awhile (removing the uranium content is easy, the rest is harder) OR we'd send a plant a fresh load of salt containing denatured uranium to start and trade for their spent salt off to a central facility perhaps even in another country.

Yes, that does mean that by the end of the plant's lifetime we are carrying more fission products in the salt than other reactors. These are really only the stable soluble fission products though which would basically stay in the salt in almost any imagined plant accident. As well, in a major accident it is more the short lived fission products, say a few day half life, that are the big worry because they are so radioactive. In term's of those FPs, there isn't much difference in the amount between a MSR that processes out fission products on a 10 day cycle or 30 year cycle.

I am guessing that something like a 15 year period between salt swaps proves the most attractive. In this case we'd have roughly the same fission product load compared to an operating PWR. A PWR runs fuel for only 5 years but 100% of the fission products stay in the fuel and they also run at a lower thermal efficiency so they produce more fission products per year.

David L.


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 Post subject: Re: Thorium production
PostPosted: Jun 08, 2009 9:42 am 
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Here is a summary of what I understand about the fuel processing (corrections welcomed). About 20% of the fission products are removed by He bubbling, another 40% by noble metal extraction. Most of the last 40% are removed by the liquid metal exchange system. If you want to get only clean salt then you need to use the vacuum distilling method.


Lars,

Those numbers seem about what I thought as well. Do you have any reference though for the 20/40/40 split? I've been on the lookout for the exact values for quite some time. I hate to put anything into a publication unless I'm sure of the numbers and able to reference where they came from. I suppose we could figure it out from scratch by looking at the fission yield of each fission product and then putting them into each category (Xe+Kr, noble+seminoble metals, everything else).

David


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 Post subject: Re: Thorium production
PostPosted: Jun 08, 2009 12:34 pm 
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David,
I used ORNL's report on fission products to group things that come out as noble metals, those that are "salt seeking" and those that come out with the He sparge.
I then took a nuclide chart and for each mass starting at the excess neutron end went down the decay path until it came to something fairly stable.
I then weighted each by the yield for that mass number and added them together.

This was just a first pass. Doing it more precisely, I should factor in the extraction rates (I think there were a couple of mass numbers where the decay rates and extraction rates are similar enough that we would get some extracted into say noble metals and some would decay to be salt seeking. The other factor would be nuclear cross sections where some of the fission products capture a neutron before they get extracted. I would guess this will primarily apply to the nuclides extracted by slow processes like liquid metal or vacuum distilling.


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 Post subject: Re: Thorium production
PostPosted: Jun 08, 2009 12:58 pm 
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There is also a variety of volatile FP fluorides that may come out in the He sparge....

Some FPs may also combine chemically to form volatile compounds, such as the iodides and bromides, that may also come out in the He sparge....


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 Post subject: Re: Thorium production
PostPosted: Jun 08, 2009 4:54 pm 
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FROM ORNL-4865
Table 5.1. Fission product data for inventory calculations (page 17) provides fission yields for around 40% of the fission products.

From page 27
salt-seeking elements (Wb, Sr, Y, Zr, Cs, Ba, La, Ce, and rare earthsj

Noble-metal fission product elements (Nb, Mo, Tc,Ru, Rh, Pd, Ag [Cd: In, Sn?], Sb, Te, and I) do not form fluorides which are stable in salt at the redox potential of the fuel salt. Niobium is borderline

Iodine, exemplified by 3 1 1 , is indicated to be in the form of iodide ion at the redox potential of fuel salt, with little I2 being stripped as gas in the pump bowl


Page 97 table 10.2 has more fission product yield information.


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 Post subject: Re: Thorium production
PostPosted: Jun 09, 2009 4:56 pm 
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David wrote:
.... A reminder that ORNL named two designs DMSR, one had processing for fission products and tries to break even, the other that I prefer is much simpler and leaves the soluble fission products in the salt until you shut down or swap out the entire salt load (with one time only processing, likely in a central facility)



Frankly, this is totally new to me. Where can I read more about ?

Quote:
When you all say that DMSR didn' t include a processing plant, you mean only the fuel (uranium + thorium + actinides), not the fission or the activation products, otherwise I don' t undserstand how safety is assured, in case of loss of coolant/fuel accident, if the FPs are not extracted from the reactor (maybe the TRUs are not so dangerous...)
....
Yes, that does mean that by the end of the plant's lifetime we are carrying more fission products in the salt than other reactors. These are really only the stable soluble fission products though which would basically stay in the salt in almost any imagined plant accident. As well, in a major accident it is more the short lived fission products, say a few day half life, that are the big worry because they are so radioactive. In term's of those FPs, there isn't much difference in the amount between a MSR that processes out fission products on a 10 day cycle or 30 year cycle


Ok, now I understand that


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 Post subject: Re: Thorium production
PostPosted: Jun 09, 2009 5:53 pm 
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The simplest DMSR would have no processing of the fuel whatsoever on site (except He bubbling and some chemistry control to deal with any oxide contamination). I've mentioned that at the end of the salt's lifetime we'd likely do a one time only processing (30 years in ORNL's study, we might change that though). That processing would likely either be by equipment brought to the site and run for awhile (removing the uranium content is easy, the rest is harder) OR we'd send a plant a fresh load of salt containing denatured uranium to start and trade for their spent salt off to a central facility perhaps even in another country.


I would expect a filter to collect most of the noble metals in a 2.4 hour period. There is some discussion about how much of the noble metals will come out with He sparging - though I'm inclined to not expect much from He sparge as far as noble metals go. I would prefer to know where the noble metals go by designing a specific filter to extract them. Letting them settle in the heat exchanger seems like a formula for clogged heat exchanger tubes. This is an area where we would do well with a salt loop, non-radioactive to see what helps extract the noble metals best. The noble metals look like 40% of the fission products by mass (not sure by neutron stealing).


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 Post subject: Re: Thorium production
PostPosted: Jun 09, 2009 6:00 pm 
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Kirk,
You have a fission product Java program showing decay. It must know the yield per FP and the lifetimes. If we add extraction rates per nuclide and extraction port (He sparge, noble metal filter, fluorination, liquid bismuth exchange 1 or 2, vacuum distilling are the ones I can think of) then we should be able to show what fission products go where and the thermal load at each location. In the process, I believe we also end up with some isotropic separation (due to the differences in decay chains).

Would it be possible to post the data of half-life for each nuclide?


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 Post subject: Re: Thorium production
PostPosted: Jun 10, 2009 5:53 pm 
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Alex,

ORNL TM 7207 is all about the "30 year once through" DMSR without fission product processing. ORNL TM 6413 is the DMSR that processes for fission products to break even on breeding but still stays denatured. They are both in the repository.

David L.


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 Post subject: Re: Thorium production
PostPosted: Jun 11, 2009 3:07 am 
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David wrote:
Alex,

ORNL TM 7207 is all about the "30 year once through" DMSR without fission product processing. ORNL TM 6413 is the DMSR that processes for fission products to break even on breeding but still stays denatured. They are both in the repository.

David L.


ok, thanks...very interesting


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