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PostPosted: Apr 12, 2016 9:05 pm 
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I was thinking about the accelerator driven subcritical reactors that have been proposed before. These reactors have a proton accelerator to have a subcritical mass of fissile material stay critical. The theory is that the reactor will shutdown if the proton flow is lost. What the people designing these systems know is that there is a possibility that even if the proton flow is gone the reactor may stay critical for an indefinite period, could be milliseconds, could be days. To account for this they have SCRAM moderator rods as part of the design.

Here's my idea, what if the reactor is allowed to stay critical after the proton flow is shut off? The accelerator is there only to get the reactor started. Once critical the moderator rods are used for control.

Why do this? It avoids the need for enriched seed fuel. Uranium centrifuges attract unwanted attention. This addresses many proliferation concerns.

I imagine a facility with multiple small reactors and a single proton accelerator. The accelerator is fired on the fuel in one reactor until it can stay critical on its own. That might take months, I know, but if the theory behind the accelerator driven reactors works then it can still be producing power for some of that time. Once one reactor is critical then the accelerator can move on to the next reactor. Once all reactors are critical then the accelerator can be shut off until needed again, perhaps even moved to another facility. Alternatively the accelerator can keep firing on a reactor core and used for bombarding radioactive waste that has been dissolved in the fuel salt. This could include rendering weapon grade material no longer weapon grade.

Good idea? Bad idea? Needs some work? Already been done?

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PostPosted: Apr 12, 2016 10:43 pm 
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Run the numbers. Super bad idea.


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PostPosted: Apr 13, 2016 4:10 am 
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Accelerator is more voluminous than a reactor. Whenever the idea has come in the past, I have suggested a reactor core as neutron source in lieu. With U233' it could be made as small as KAMINI experimental reactor running in India.
However, it is far better to start a thorium fire with Pu recovered from reprocessing, dozens or hundreds of tons available, or 20% LEU easily enriched. It was a big international effort to stop Iran going beyond!


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PostPosted: Apr 13, 2016 2:07 pm 
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FYI, there have been a couple of initiatives, regarding the Accelerator Ignited LFTR, which combine molten salts with sub-critical reactors:

GEM*STAR:

http://www.uxc.com/smr/Library%5CDesign ... EMSTAR.pdf

ADSMS (Texas A&M University):

http://accelconf.web.cern.ch/AccelConf/ ... 1_talk.pdf


An accelerator coupled to a reactor seems somewhat superfluous to me and is also costly.


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PostPosted: Apr 13, 2016 5:19 pm 
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camiel wrote:
FYI, there have been a couple of initiatives, regarding the Accelerator Ignited LFTR, which combine molten salts with sub-critical reactors:


That is not what I proposed. Those reactors maintain the reactor in a subcritical state, I propose driving the reactor to become critical as soon as possible, the proton beam be removed, and then the reactor continues to remain critical and produce power.

Perhaps I should have chosen a different term, such as "accelerator seeded reactor". The proton beam is only there to get the reactor started, not to keep it going. I am *NOT* proposing a return to the "energy multiplier" design that has been already shown to be impractical.

I realize that current proton accelerators are very inefficient, and therefore would take a large proportion of the power a reactor would produce to operate. What I propose is run the proton accelerator only so long as needed to seed the reactor, turn enough thorium into U-233, until it can breed enough uranium on its own.

If it takes months of running the accelerator to seed the reactor, and then it can run on it's own for years or decades then I'd think that the energy lost in that seeding time would be made up many times over until the reactor must be shut down because of neutron embrittlement or other reasons that would end the production life of the reactor. I assume the fuel from that reactor shutdown could then be used to seed another reactor, meaning the proton accelerator would not be needed again.

If the energy to produce the seed fuel by proton bombardment is less than that of using centrifuges then this would be a much easier case to make, especially for someone that does not already have access to centrifuges. If this takes more energy than enrichment by centrifuges then I suggest this may still prove viable if there is a desire to avoid the possible weapon production use of centrifuges.

One complaint of denatured molten salt reactors is that is does not do away with the need to have enrichment capability since this is still a "burner" design, it needs enriched fuel. Even with LFTR there is a need to have an enriched seed. With a proton accelerator to seed the reactor the enriched uranium is never outside of the reactor vessel. Any fuel removed from the reactor would be worthless for weapons unless enriched by centrifuges or another reactor designed to produce weapon grade material.

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PostPosted: Apr 13, 2016 5:42 pm 
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Run the numbers, you'll see how many decades it will take to start up the reactor this way. I wish it could work, I really do, but it doesn't.


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PostPosted: Apr 14, 2016 11:27 pm 
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Kirk Sorensen wrote:
Run the numbers, you'll see how many decades it will take to start up the reactor this way. I wish it could work, I really do, but it doesn't.

Yes, I see where I went wrong with my assumptions. Even the accelerator driven reactors started with fuel that had some fissile material in it. If starting from only thorium as fuel the accelerator would have to transfer enough mass to turn the non-fissile thorium into a critical mass of fissile U-233. I read through the articles that camiel linked to and one of them claims to be able to run from natural uranium.

camiel wrote:


If that is true then I assume that a LFTR could be started with natural uranium and the accelerator and once critical then the accelerator could be removed and make-up fuel could be thorium. Since other reactor designs have been fueled with natural uranium and shown to be practical then it should not be too much of a leap to think that a molten salt reactor with natural uranium fuel and with the help of an accelerator could become critical.

Again I realize that the energy needed to start a reactor in this manner is quite likely to exceed that of running centrifuges to achieve the same amount of fissile material, and possibly by a wide margin. I'm working with the assumption that centrifuges are undesirable or inaccessible because of treaty obligations or other legal restriction. The CANDU reactors have shown that solid fuel reactors are practical with unenriched fuel, I'd like to see that same thing for molten salt reactors. If a MSR cannot start on natural uranium then, IMHO, the next best thing is an accelerator started one.

Unless there are MSRs that can be started with natural uranium alone. Is that feasible?

A solution proposed in another thread was to use a CANDU to produce fuel for starting MSRs. This would likely take years, or even decades like using an accelerator to start a LFTR, the difference being that the CANDU would produce power in that time instead of consume it.

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PostPosted: Apr 15, 2016 8:31 am 
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Kurt Sellner wrote:
A solution proposed in another thread was to use a CANDU to produce fuel for starting MSRs. This would likely take years, or even decades like using an accelerator to start a LFTR, the difference being that the CANDU would produce power in that time instead of consume it.


With proper fuelling, the CANDU can be a U233 producer for reactors just as some reactor designs were specialised for producing power and plutonium for weapons. Th-Pu(RG) or Ih-20%LUE fuel could give a high burn up and 2000FPD of reactor life. A fresh batch of thorium could be irradiated every 500 day or so. A metallic thorium could be electro-refined to separate the U233.


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PostPosted: Apr 15, 2016 11:52 am 
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jagdish wrote:
Kurt Sellner wrote:
A solution proposed in another thread was to use a CANDU to produce fuel for starting MSRs. This would likely take years, or even decades like using an accelerator to start a LFTR, the difference being that the CANDU would produce power in that time instead of consume it.


With proper fuelling, the CANDU can be a U233 producer for reactors just as some reactor designs were specialised for producing power and plutonium for weapons. Th-Pu(RG) or Ih-20%LUE fuel could give a high burn up and 2000FPD of reactor life. A fresh batch of thorium could be irradiated every 500 day or so. A metallic thorium could be electro-refined to separate the U233.


Use a CANDU to produce fuel for itself. Start out with a fertile seeded thorium fuel. Adjust the CANDU so that those short fuel bundles can be added at one end while being extracted at the other. Put each bundle in for long enough to breed a bunch of Pa233 and then extract and shelve for ~60 to 90 days and then repace it. Around and around it goes, breeding, relaxing, burning, ...

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PostPosted: Apr 15, 2016 12:02 pm 
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Well, itself and fat reactors. There is so much of used and depleted uranium about!


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PostPosted: Apr 16, 2016 10:13 am 
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A high intensity proton beam acceleration with a continuous wave output of roughly 1mA producing 1GeV protons, which seems reasonable to me, will accelerate a mole of protons every 96.5 million seconds.
That is pretty much 3 years per mole.

If we assume every proton impacts a 238U nucleus in the 'primary' target and undergoes ludicrous spallation-fission with the typical 20 daughter fast neutrons produced that gives us a mole of neutrons every 54 days.
If we assume no downstream fissions (which is probably unrealistic but lets go with it as a conservative estimate) and all of the neutrons are absorbed in a solid target compposed entirely of something with atomic mass ~238 then we would expect to produce roughly 4.3g of fissile material per day.
If they all fission and double the number of neutrons that improves but still looking at grammes per day.

Which is enough for it to be a worry for a bomb programme (as linacs get much cheaper) but not really for civil use.

EDIT:

Correction, this study seems to suggest a surprisingly efficient LINAC with a 120mA beam current at up to 1GeV is feasible, with ~57% efficiency at high currents. And this is CERN so they tend to know what they are talking about for accelerators.
120mA beam current with the x20 multiplication assumption tends to lead to a mole of accelerated protons every 223hrs, which leads to a mole of daughter neutrons every 9.3 hours, which leads to roughly ~600g of fissile material produced per day.

Assuming you use uranium for the primary target and thorium for the primary target you might bea ble to produce enough fissiles to power the accelerator.
After all 600g/day is 219kg per year, which diluted to 4.2% for an ESBWR (assuming 233U is the same as 235U, which I know it isn't) with 0% DU is ~5.2t of fuel, which has an electricity production of ~2TWh (e) for an average power of ~230MWe, whereas the accelerator would require something like 175MWe [drive power, not beam power].

So you are actually surprisingly close to breakeven, even diluting the 233U with pure 238U rather than NU or RepU. Using ~0.7% enriched U (either NU or RepU) would give us 6.2t of fuel per year, raising average reactor power to ~300MWe, giving double the beam input power.

Sure its still rubbish compared to more conventional technologies, but honestly I am amazed it actually gets near breakeven at all.


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PostPosted: Apr 16, 2016 4:16 pm 
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jagdish wrote:
With proper fuelling, the CANDU can be a U233 producer for reactors ...
Th-Pu(RG) or Ih-20%LUE fuel could give a high burn up...
A metallic thorium could be electro-refined to separate the U233.

If someone has access to enriched U-235 then why bother with the CANDU to produce fuel for the MSR? The LEU can be used to seed the MSR, no?

I'm working on three assumptions:
- MSRs are good
- Centrifuges for enrichment are bad
- MSRs cannot be started from unenriched fuel

We assume MSRs are good because they are safe, run on thorium and natural uranium, produce valuable fission products, etc. We assume centrifuges are bad because they can be used to produce weapon grade material. Enrichment by other means, such as neutron bombardment in a CANDU, is considered "safe" because it makes the production of weapon grade material difficult or impossible. My last assumption, that MSRs cannot be started from unenriched fuel, is something that perhaps with some advancement in technology may prove false but for now, as far as I know, no one has shown it to be even theoretically possible.

I proposed using a proton accelerator to seed a MSR since I thought that might be a means to avoid any nuclear weapon proliferation issues as well as avoid the costs of continuously running the accelerator in the "energy multiplier" designs proposed elsewhere.

We are seeing a number of nations consider using nuclear power for their energy needs but this often comes with concerns from outside and within. Other nations might be concerned that a neighboring nation is using a civilian nuclear power industry to hide military efforts to build nuclear weapons. This concern of weaponizing fissile material may lead to difficulty to create or import enriched fuel.

Using an accelerator to seed a MSR might be expensive but it avoids a lot of political problems. Using a CANDU seems reasonable if it does not require enriched fuel of its own to produce seed fuel for a MSR. Even if it takes years for a CANDU to produce seed fuel for MSRs it is at least producing power during this time. Also, once this nation or other entity has seed fuel for the MSR I assume the MSR could process or consume the spent fuel from the CANDU and avoid the problems of waste production associated with solid fuel reactors.

Basically I'm looking for a means to get nuclear power while avoiding the complaints of those against nuclear weapon proliferation specifically and just nuclear energy generally. I believe that LFTR and other MSRs are the way to do this but without a means to start the process without the enrichment concerns then we have political problems.

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PostPosted: Apr 16, 2016 4:37 pm 
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KitemanSA wrote:
Use a CANDU to produce fuel for itself. Start out with a fertile seeded thorium fuel. Adjust the CANDU so that those short fuel bundles can be added at one end while being extracted at the other. Put each bundle in for long enough to breed a bunch of Pa233 and then extract and shelve for ~60 to 90 days and then repace it. Around and around it goes, breeding, relaxing, burning, ...


I envision something similar. Get CANDU going with natural uranium, and once it's got enough neutrons to spare to breed new fuel then feed in some thorium filled fuel rods to breed U-233. Once enough U-233 is stockpiled then start up a MSR. Now you have a CANDU and MSR to use to breed fuel for another MSR. This process becomes a chain reaction of it's own, each new reactor brought online shortens the amount of time needed to get fuel to start the next one. When a CANDU fuel rod reaches its end of life it can be brought to one of the MSRs for processing.

Here's a question that someone here might be able to answer, does the fuel in those fuel rods need to be a ceramic? What I envision is that instead of oxide pellets in those zircalloy tubes there could be salt pellets. That way when the fuel is "spent" the reprocessing can consist of cutting off the end of the tubes and dumping the salt into a MSR. I realize it is not quite that simple because the tubes are filled with radioactive gasses that shouldn't be released to the atmosphere, but the process should be not much more than what I describe.

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PostPosted: Apr 16, 2016 7:01 pm 
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A CANDU running on natural uranium does not have a lot of neutrons to spare. We can not put a lot of thorium in the reactor without poisoning it, and there are not many neutrons escaping from the reactor. The quantities of 233U produced will be relatively small (but there is still the plutonium which is produced in much higher quantity by the CANDU).

A country can buy a starting charge of enriched uranium (for the LFTRs or fast reactors) from states which already have a legal enrichment technology. There is no need to spread the enrichment technology.

A faster way to produce 233U (if we absolutely need to start on 233U) is to use a fast reactor and put thorium in the blankets like the Indians want to do (assuming you have access to enriched uranium or enough plutonium to start the fast reactors).


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PostPosted: Apr 17, 2016 5:47 am 
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AHWR is designed to use thorium in a heavy water reactor. But not built so far. Horizontal tube version could also be built. A blanket could be added to maximise the production of U233.


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