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PostPosted: Dec 21, 2011 3:00 pm 
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Using fluorinator bottoms certainly sounds attractive, with only one processing step required to get the fissile, that must be very simple and economical. Too bad we get the lanthanides along. We can't have cake and eat it.


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PostPosted: Dec 21, 2011 4:25 pm 
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That is a lot of fission products to add to the reactor. Offhand maybe 400kg fission products to get 250kg of plutonium of which only about half is fissile. I'm not sure that is a net positive for our reactivity. Also, better check out the thermal load of such a product. After you remove 98% of the used fuel composition the heat is 50x more intense.


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PostPosted: Dec 21, 2011 4:50 pm 
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We discussed that before, but didn't got any solid numbers on the reactivity penalty. Most of the nonvolatile fission products aren't too bad in neutron capture, compared to highly reactive plutonium (still 60-70% fissile). Some of the lanthanides being an important exception.

Perhaps it is good enough for a converter reactor. One problem might arise with trifluoride solubility, as the lanthanides and plutonium are fed in as trifluorides. Not a big problem for a CR of 0.99 but it will be for CR of 0.9


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PostPosted: Dec 21, 2011 5:14 pm 
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Yes Lars, it certainly isn't a great fuel but for some purposes it could be fine. And again, while LWR spent fuel is not ideal, the ratio of Pu to fission products is more than twice as high in CANDU spent fuel.

David LeBlanc


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PostPosted: Jan 03, 2012 5:15 am 
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Sorry guys this is a bit of a time warp, out of order WRT the current post.
Cyril R wrote:
There is a public perception issue because people don't know there are multiple isotopes of plutonium and some of these can't be used to make a weapon because they fission prematurely. In fact most people don't know what an isotope is, or what a spontenous fission is.

David wrote:
It is true that by IAEA rules, only Pu with more than 80% Pu238 is considered safe guarded and subject to less stringent rules. It doesn't matter if we agree or disagree, that just happens to be the current regulations. As for other mixes like spent fuel Pu, for every web entry that claims it impossible to make a weapon there is a counter one saying it possible. I think the only thing we can say with certainty is that for mixes with significant isotopes other than weapons grade +93% Pu239 it would be very difficult and take a very high level of technical savvy to make any explosion beyond a "fizzle" of just a small fraction of a kiloton (like at least one of North Korea's tests)

Cyril R wrote:
The lowest quality plutonium ever to be used in a weapon was 85% Pu239. It didn't work very well but they used enough plutonium to get 20 kiloton yield. It was spent fuel from low burnup English gas cooled reactors. In regulatory terms, this is considered fuel grade plutonium, the intermediate category between weapons grade and reactor grade. Below 85% Pu239 the yield drops off very rapidly.

I found an interesting discussion on the issue.

http://www.fas.org/nuke/intro/nuke/O_9705.htm

First up, that is an excellent link, it helps provide some context to those forms of Pu that pose the greatest proliferation risk and those forms more proliferation resistant.

I've done some reading recently on Pu and I cannot claim any expertise on the topic, but I'm happy to share some of what I have learned.

- Aside from 80% Pu238 there does not appear to be any accepted isotopic mix that cannot be weaponised (we can of course discuss the effectiveness or lack thereof);
- Fizzle is highly likely as the quantity of Pu240 increases beyond 7%, this is due to the device taking itself apart prematurely through predetonation as the assembly is coming together;
- The yield of a Pu based device where Pu240 is present is a statistical result (assuming a well built device), as the Pu240 content increases the probability of getting full yield diminishes and the probability of getting a fizzle yield increases, but there is a statistical range of expected outcomes (i.e. one device, multiple potential yield values within a range);
- I don't care if it just a fizzle yield destroying just one building (and spreading a significant amount of radioactive material) instead of a much bigger area, the prospect of a possible detonation of a Pu based device in an urban environment is completely unacceptable and not to trifled with or dealt with by encouragements that people are ignorant and need more education;
- As much as we do need more education on the risks associated with Pu in a rational well considered way and how to best manage those risks, I think that the chances of winning the public over to the point of view that shipping and using separated Pu as top-up fuel is a good idea are slim to none IMO;
- I accept that scientifically and rationally there are far more effective pathways for inflicting harm than to divert reactor grade Pu, but not everyone is scientific or rational;
- I was hoping to find some silver bullet that would make Pu safe (in relative terms), I did not find one, although isotopic mixes containing significant quantities of Pu240 and Pu238 are a great start, keeping Pu only in irradiated form with fission products all seem like good ways of making the material unattractive for diversion;
- I am intrigued by Lars suggestion of mixing in some thorium with Pu, I'd like understand that or similar options better than I do now.

In summary I would say that as an opinion, I think that that reactor grade Pu could be very useful for starting MSR cores and I support that. I don't see it having much of a future as top-up fuel unless well mixed with other irradiated material. While LEU remains very reasonably priced I don't see much upside in advocating for separated Pu as a safe everyday fuel and I think that we need to choose our battles carefully.

I'm reasonably confident I'll get decent sort of a serve for this post, but if I've gotten any of the above wrong, I'm sure that others will correct me and hopefully we can all learn something along the way. I hope that people find this post useful in some way.


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PostPosted: Jan 03, 2012 2:35 pm 
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Just to add another quick bit of info. I know some of the logic expressed for using Pu startup and top ups on a simpler graphite moderated converter MSR is that one could stick with an otherwise pure Th-U233 system and obtain a higher conversion ratio than if Low Enriched Uranium is used as startup and top up (because the U238 that tags along is a poorer fertile than Thorium).

For example without processing you can get a Conversion ratio of 0.8 easily with LEU as top up (i.e. the DMSR 30 year) up but you could probably do 0.9 with the same 30 year batch time if you just have Pu and Thorium (and of course the U233 produced insitu). Thus as a rule of thumb you might need only half the makeup fissile Pu for a U238 free concept than the amount of U235 top up you'd need with the traditional DMSR design. Without even getting into the proliferation issue there is another thing to remember in this comparison. That is that even with cheaper fluoride volatility processes that each gram of fissile Pu is much more expensive than each gram of fissile U235 (in LEU). Fissile Pu costs at least 100$/gram, more likely 200$/gram for PUREX at least. U235 is only 40$/gram at current prices (again in LEU) so the presumed fuel cycle savings can disappear completely.

Yes, yes, I know the refrain is "someone else" will pay to remove the Pu from spent LWR fuel but the huge cost can not be just ignored. The Japanese plant soon to open (or should I say, if it opens) will produce only about 7 tonnes of LWR Pu a year at a capital cost of 20 Billion$ (La Hague was probably about the same cost to build but double the output, still extremely expensive option though and about a billion a year to run).

David L.


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PostPosted: Jan 03, 2012 3:02 pm 
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I'm not sure that the added security is such a problem. I'm basing this on the considering MOX. With mox you have very clean Pu (much cleaner in terms of low radioactivity level than we would use) mixed with uranium. There is a difference between shipping mox and shipping just the Pu - though i think this is more from politics than real risks.

I'm thinking if you ship your Pu mixed with thorium you have a similar deal to MOX. Actually much better since it is much harder to separate the thorium from the plutonium than to separate uranium from plutonium. Further, it would be reasonable for us to include some radioactive fission products in the mix so it would be more proliferation proof than the existing MOX procedures. Hence, I am thinking it would be fair game to think about a global system that takes used LWR fuel and in a secure facility removes the Pu (perhaps together with some fission products) and mixes the result with some thorium. The PuTh mix is then sent to production power generation reactors.


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PostPosted: Jan 03, 2012 7:08 pm 
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Lars, I think that those are some important suggestions, if Pu is to be used and shipped, then as you suggest mix it with a bunch of other things that make it quite undesirable. From my frame of reference, I see irradiated fuel salt as a potential carrier that should make the heavy metal contained within difficult to access and be quite unattractive for diversion.

Just a quick sidebar on MOx, I think that I read somewhere that one of the purposes of the PRISM reactors in the UK's MOx plans is to irradiate to finished MOx fuel assemblies for a time prior to sending them to storage as low burn-up irradiated MOx fuel, which presumably breeds even more Pu239 from the U238 MOx matrix. Oh my gosh, what a convoluted, complex, ineffective and super expensive way to deal with Pu and other TRU.

Just build a MSR/MOSART, separate the U and the Zr cladding from the TRU by fluoride or chloride volatility, put the U into storage, recycle or landfill the Zr. Burn the TRU and ensure that your FP stream has a low enough actinide level to permit 500 year storage. Existing SNF is safe enough as it is, so just park it in that irradiated self-protecting form in dry cask storage until the Pu and other TRU can be separated and immediately placed into a working MSR core for incineration, no need to hold stockpiles of separated fissile. Much simpler, cheaper and better. It might take 300 years and quite a number of MSR's, but you will get there in the end for a lot less $ than these MOx schemes. (I'll put my soapbox away now)


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PostPosted: Jan 03, 2012 8:59 pm 
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Lindsay wrote:
Just a quick sidebar on MOx, I think that I read somewhere that one of the purposes of the PRISM reactors in the UK's MOx plans is to irradiate to finished MOx fuel assemblies for a time prior to sending them to storage as low burn-up irradiated MOx fuel, which presumably breeds even more Pu239 from the U238 MOx matrix.

This doesn't sound right. I don't think you can ship radioactive MOX to LWRs for burnup there. I do not believe the LWR's are set up to handle hot fuel.
Quote:
Just build a MSR/MOSART, separate the U and the Zr cladding from the TRU by fluoride or chloride volatility, put the U into storage, recycle or landfill the Zr.
So far so good.
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Burn the TRU and ensure that your FP stream has a low enough actinide level to permit 500 year storage.

Here it is more tricky. Unless the spectrum is very fast I don't think you can simply burn the TRU+fp and keep building up the fission products. Eventually you need to separate the TRUs from the fission products. If you add both TRU and fission products from LWRs then eventually comes quicker. I think the equipment and skills that can separate Pu from fission products is a proliferation risk and likely needs to be secured. It may be that we have to create centralized, secured processing centers to separate the Pu from the fission products. We do not need (or actually want) Pu that is as clean as needed for LWRs so this opens the possibility of a less expensive process (like pyroprocessing) that is also more proliferation resistant. Just maybe the pryoprocessing will be inexpensive enough and proliferation resistant to allow it be deployed with each reactor park.
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Existing SNF is safe enough as it is, so just park it in that irradiated self-protecting form in dry cask storage until the Pu and other TRU can be separated and immediately placed into a working MSR core for incineration, no need to hold stockpiles of separated fissile. Much simpler, cheaper and better. It might take 300 years and quite a number of MSR's, but you will get there in the end for a lot less $ than these MOx schemes.

Agreed.


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PostPosted: Jan 04, 2012 3:47 am 
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PRISM uses metal fuel, U-Pu with some Zr for stability. If it is built we can also use it with a Th-Pu metallic fuel. Zr is not required due to Th excellent chemical and physical characteristics. Th has a lower reactivity swing than U238 so the PRISM can run in deeper burn mode (more economical). The Pu is effectively destroyed without making any more TRUs, U233 so created can be used for LFTRs, and it comes with more U232 for proliferation protection, because fast neutrons do some n,2n that makes U232.

Lars makes some really good points, keeping Th with the Pu helps with proliferation issues. A pyroprocess that always keeps the actinides together (including fertile) would be quite nice to have. I've been thinking about a process that uses everything in the metal form (as the fuel), boils the fission products away, leaving actinides metals.


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PostPosted: Jan 04, 2012 5:42 am 
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Lars wrote:
Lindsay wrote:
Just a quick sidebar on MOx, I think that I read somewhere that one of the purposes of the PRISM reactors in the UK's MOx plans is to irradiate to finished MOx fuel assemblies for a time prior to sending them to storage as low burn-up irradiated MOx fuel, which presumably breeds even more Pu239 from the U238 MOx matrix.

This doesn't sound right. I don't think you can ship radioactive MOX to LWRs for burnup there. I do not believe the LWR's are set up to handle hot fuel.
I may have gotten this wrong, I will try to find out more and come back with references if I can.
Lars wrote:
Here it is more tricky. Unless the spectrum is very fast I don't think you can simply burn the TRU+fp and keep building up the fission products. Eventually you need to separate the TRUs from the fission products. If you add both TRU and fission products from LWRs then eventually comes quicker. I think the equipment and skills that can separate Pu from fission products is a proliferation risk and likely needs to be secured.
Agreed I didn't mean to imply that the FP's should stay in-core for an extended period. I was deliberately vague on how that processing might be done, I think that FP removal from the reactor will be required, but possibly as a two step process. The first stage is a rough cut to get most of the FP's out of the reactor, but the rare earth FP waste stream may carry more Pu and actinides that we want to bury. Therefore it may be expedient to have second stage processing at a secured facility to strip the remaining small quantities of Pu and TRU's out of that FP stream with a view to targeting a 500 year storage period for FP's not the 10,000 - 100,000 years associated with once through LWR fuelling.


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PostPosted: Jan 04, 2012 9:02 am 
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Quote:
The first stage is a rough cut to get most of the FP's out of the reactor, but the rare earth FP waste stream may carry more Pu and actinides that we want to bury. Therefore it may be expedient to have second stage processing at a secured facility to strip the remaining small quantities of Pu and TRU's out of that FP stream with a view to targeting a 500 year storage period for FP's not the 10,000 - 100,000 years associated with once through LWR fuelling.


This is also what Lars favors, get out what you can reasonably at the reactor online processor, then use central facility processing for the trifluoride semi waste gunk.

It may actually be preferable to leave the high activity wastes at the reactor site, not ship Pu and fission products. In stead use a team of dedicated processors that come by every once in a while, servicing a large fleet of LFTRs. These take their equipment with them. Possibly this could be 'sensitive' equipment in terms of proliferation, it could be a fully safeguarded team.


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PostPosted: Jan 04, 2012 11:19 am 
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One thing that caught me off-guard was the heat content of our spent fuel. In a cycle where you only feed thorium you could simplify the on-site processing by letting the Pu flow with the salt-seeker fission products. This would amount to 15-20 kg/GWe-yr. When considering transporting this stuff to a central processing site we have to worry about how hot it is. I'm thinking we will have considerably higher power density than LWR spent fuel of similar aging so it may require extra thought.


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PostPosted: Jan 04, 2012 12:24 pm 
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That's true. ORNL said the limiting factor for the vacuum still operation in batch mode, was the decay heat removal, and they used a high efficiency (sodium pipes) active heat removal system.

I'm thinking it is best to let the hot stuff stay in a Hastelloy N container sitting in the hot cell. It could sit there hot and help keep the hot cell at high temperature. In the buffer salt pool type reactor design, double walled canisters of hot stuff could sit on the bottom of the buffer salt pool to help keep it hot. During normal operation, a buffer salt heat exchanger keeps the buffer salt from getting too hot, and use the heat in the power cycle. It saves on containment cost and on heating cost during extended shutdowns, while allowing full passive containment cooling.


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PostPosted: Jan 04, 2012 6:45 pm 
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Lindsay wrote:
Lars wrote:
Lindsay wrote:
Just a quick sidebar on MOx, I think that I read somewhere that one of the purposes of the PRISM reactors in the UK's MOx plans is to irradiate to finished MOx fuel assemblies for a time prior to sending them to storage as low burn-up irradiated MOx fuel, which presumably breeds even more Pu239 from the U238 MOx matrix.

This doesn't sound right. I don't think you can ship radioactive MOX to LWRs for burnup there. I do not believe the LWR's are set up to handle hot fuel.
I may have gotten this wrong, I will try to find out more and come back with references if I can.
My bad folks, I could not find the item that I originally read or thought I read. The reference to irradiating material in PRISM was in relation to burning down Pu to a lower grade within PRISM, NOT what I had suggested, sorry about that. Good catch Lars, thank you.

http://www.world-nuclear.org/info/inf98.html
Quote:
An early application of PRISM is proposed to secure and utilise the UK's stockpile of some 100 tonnes of reactor-grade plutonium. Two PRISM units would irradiate fuel made from this plutonium (20% Pu, with DU and zirconium) for 45-90 days, bringing it to 'spent fuel standard' of radioactivity, after which is would be stored in air-cooled silos. The whole stockpile could be irradiated thus in five years, with some by-product electricity (but frequent interruptions for fuel changing) and the plant would then proceed to re-use it solely for 600 MWe of electricity generation, with one third of the fuel being changed every two years. The cost of the plant would be comparable to a large conventional reactor, according to GE-H, which is starting to develop a supply chain in the UK to support the proposal. No reprocessing plant (Advanced Recycling Centre) is envisaged initially, but this could be added later.
What a silly expensive wasteful scheme though. It has some logic if they are sitting on a lot of weapons or fuel grade Pu, but wouldn't it make more sense to simply mix low and high burnup Pu together to degrade the isotopic quality of the resulting mix? Sounds like it should be a lot cheaper than building a couple of PRISM reactors to achieve a similar level of isotopic degradation. Also if the fuel is loaded with DU they'll be producing Pu239 at the time as destroying it which sounds like an extremely inefficient approach.


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