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PostPosted: Apr 29, 2014 5:19 pm 
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(Excuse me as a new forum member, if this topic has already been discussed here.)

On the Guardian's webpage (Guardian is a British national newspaper) there was an article about using an external accelerator to generate neutrons to drive a thorium reactor. The reactor would be subcritical, as the neutrons needed would be externally generated.

It said that for a reactor with 600MW output, only 20MW would be needed to drive the accelerator.

http://www.theguardian.com/science/blog/2012/feb/09/accelerator-nuclear-reactor

The only advantage I could see them claim was that the reaction could be stopped quickly by switching off the accelerator. However, I had believed that halting a reactor was not a difficult problem.

Does this idea make any kind of sense at all? It seems to me to be a costly way to make neutrons for an application where they would be produced automatically, if the reactor used a chain reaction in the normal way.

I'd be grateful for a brief comment on whether this is a promising way forward or just an idea that is going nowhere.

(More about it at http://thorea.wikia.com/wiki/Main_Page )


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PostPosted: Apr 29, 2014 5:36 pm 
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Martin A wrote:
(Excuse me as a new forum member, if this topic has already been discussed here.)
However, I had believed that halting a reactor was not a difficult problem.

Does this idea make any kind of sense at all? It seems to me to be a costly way to make neutrons for an application where they would be produced automatically, if the reactor used a chain reaction in the normal way.

I'd be grateful for a brief comment on whether this is a promising way forward or just an idea that is going nowhere.


It is a great idea if your specialty is accelerators and you need a new reason for the government to continue your funding.

You are perfectly right that stopping the reaction quickly on demand is something reactors absolutely have to do and have always done with one exception - Chernobyl where they had a few poor design choices and overrode the safety features and stress tested a failure mode.

So in my opinion (shared by many on this board) this is a road to nowhere.


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PostPosted: Apr 29, 2014 5:39 pm 
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Martin A wrote:
The only advantage I could see them claim was that the reaction could be stopped quickly by switching off the accelerator. However, I had believed that halting a reactor was not a difficult problem.


You are correct, turning off reactors is relatively easy. Essentially all thermal-spectrum reactors will turn themselves off without any operator intervention. What has been the persistent challenge has been the heating of the fuel rods from fission product decay after the reactor shuts off. This is the factor that threatens meltdowns, and accelerator-driven systems do nothing special to mitigate this problem.

Molten-salt reactors, on the other hand, have a very clever way to address decay heating through the drain of the core into a specially-cooled drain tank.


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PostPosted: Apr 29, 2014 9:14 pm 
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Lars has it spot on... high correlation of accelerator funded researchers with the idea of accelerator startup neutrons.

However, an interesting side note: an anti nuclear lay person I was talking with actually said he would be more confident of the safety of a nuclear reactor if it was initiated with accelerator neutrons -- as if a LFTR needed an additional on and off switch. Of course he had no idea about the actual thorium fuel cycle. It was as if an accelerator source was more acceptable because that "other stuff" comes from a dastardly reactor to start with, so it must be bad (never mind a LFTR would be getting rid of it.)


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PostPosted: Apr 29, 2014 9:28 pm 
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All of the accelerator-driven systems I've seen have to have a very fast-acting control rod system to "hold" the reactor subcritical (since it naturally "wants" to be critical, obviating any need for the accelerator). Somehow this Rube-Goldbergian arrangement doesn't seem to bother the accelerator people. Nature sends them a message and they ignore it.


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PostPosted: Apr 30, 2014 3:27 am 
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Somehow the accelerator driven reactors brings up an image of a huge windmill ventilating a coal furnace. Could be technically feasible but absolutely pointless.
I would rather have a reactor producing neutrons irradiating a sub-critical blanket of thorium, which eventually adds to the power of the reactor but is always available as a neutron sink.
A successor to Indian PFBR could be designed with MSR core and solid thorium blanket.


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PostPosted: Apr 30, 2014 8:53 am 
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Martin A wrote:
I'd be grateful for a brief comment on whether this is a promising way forward or just an idea that is going nowhere.


Keep in mind that what you will get are only opinions. Possibly backed by facts, but they are still opinions. I will give you mine, but you have to make your own.

The idea:
- The reactor is subcritical => you need extra neutrons to keep power constant
- The source of neutron is spallation: you accelerate protons and blast some nuclei (traditionally, heavy nuclei like Lead)
- The power needed by the accelerator comes from the reactor
- In case of trouble, the accelerator shuts down and thus the power produced in the core drops automatically.

Implications:
- You want a multiplication factor (keff) as high as possible but still < 1 (ideally 0.9999...), so as to get a nice bonus from fission (the accelerator is here to compensate for the "1-keff", if you want).
- The bonus from fission translates directly in economies for the power needed by the accelerator.
- Since you are subcritical you do not really care about reactivity coefficients: they tend to become less favorable with increasing Minor Actinides content, which is a technical limitation for solid fuel, critical fast reactors when you want to burn the said actinides.
- The other thing you do not really have to care about is the build-up of fission products that absorbs neutrons and that basically decide when you have to refuel in a critical reactor.

Negative implications:
- Ideally you would like your keff neatly constant in time but in real life it is not. It may go up (when breeding, for example) or down (by burning fissile material) and managing this is quite a challenge. Giving it a substential margin penalizes you.
- As it was mentionned, you have to guarantee that your keff will stay between reasonable values even in accidental scenarios, a fact that is overlook by many proponents. Just the hot-to-cold reactivity may surpass some crazy margins people think are achievable (I remember seeing an assumed keff of 0.997 once). Sometimes it is even legally defined that you can not be considered as a subcritical facility if your keff is not below a certain value (often 0.95). That means you may have to comply with such legal requirements.
- Current accelerators are not reliable enough to provide 24/7 beam power. They trip every one in a while. For the reactor, that means going from full power to 0 in an instant - repeatedly. That will shorten the lifetime of the reactor - at best. Plus, it's impractical for power production.
- The subcriticality does not prevent accidents involving loss of cooling (like Fukushima, for example)
- The accelerator will cost you a bucket. If we want nuclear technology to survive, we need to be cost-competitive. This goes in the wrong direction.
- In any case most systems assume reprocessing of the fuel, even implicitly. There is anyway a fluence limit on the cladding and fuel of solid-fuel systems, so you will have to do something with the fuel anyway. It's unlikely you can burn all Minor Actinides in one pass without exceeding the said limits, even with Inert Matrix Fuel.

Conclusion:
- Unpractical for large-scale power production
- Some benefits for niche applications (e.g. waste burning), but that's all.
- I see little improvement compared to other systems with similar goals (solid fuel fast reactors and MSRs)

Outlook:
Some folks propose to combine molten salt liquid fuel and accelerators, like McIntyre and his group at Texas A&M. I find this idea at least more promising than traditional solid-fuel ADSs (Rubbia's "Energy Amplifier" included).


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PostPosted: Apr 30, 2014 10:55 am 
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am something of a heretic on this site on a number of issue, so no-one will be surprised when I say this is one of them.

I think it might well be a great idea to have a few ADSs purely to destroy the last of the MAs. LFTRs would do a grand job for the bulk of the MAs but eventually, even they will be overloaded, IIUTC.

Another thing to note is that proton acceleration may get cheap enough to allow Very Small Modular Reasctor ADSs using a small block of NU (or FP stripped SNF) as the core. When that happens, there may actually be multi-residence systems for condos or apartments, units for hospitals and other low power industries. So ADS for large power plants? No, not really, certainly not needed. ADS for final burning, could be beneficial. ADS for VSMR? Hmmm.

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PostPosted: Apr 30, 2014 1:16 pm 
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KitemanSA wrote:
am something of a heretic on this site on a number of issue, so no-one will be surprised when I say this is one of them.

I think it might well be a great idea to have a few ADSs purely to destroy the last of the MAs. LFTRs would do a grand job for the bulk of the MAs but eventually, even they will be overloaded, IIUTC.

Another thing to note is that proton acceleration may get cheap enough to allow Very Small Modular Reasctor ADSs using a small block of NU (or FP stripped SNF) as the core. When that happens, there may actually be multi-residence systems for condos or apartments, units for hospitals and other low power industries. So ADS for large power plants? No, not really, certainly not needed. ADS for final burning, could be beneficial. ADS for VSMR? Hmmm.


With an MSR you can burn off the MAs down to the level that we can separate the MAs from the fission products. BUT you will always have some MAs in the fission products so you still need to do geological storage of the fission products eventually. Just how much effort we want to expend to reduce the MAs further will be for society to prioritize. A fast spectrum MSR can burn off more of the MAs in each pass so it makes the sense in the long run to have fast spectrum MSRs to do the final cleanup passes.

An ADS could supply a fraction of 1% of the neutrons needed to burn off MAs. In a thermal spectrum the MAs will need more neutrons than it generates. This can be supplied by external fissile makeup (235U additions). In a fast spectrum many MAs isotopes will fission - it likely even supplies enough neutrons to not require additional neutrons. So I don't see a place for ADS even for burning off MAs if we have a fast spectrum MSR. If for some reason, a fast MSR is not allowed AND burning off MAs is really important AND you can get a reliable, continuous, affordable ADS at significant power levels then maybe it makes sense. Not the place I'd invest.


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PostPosted: May 01, 2014 1:44 am 
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Effectively all energy sources are an "energy amplifier". Every energy source starts with some sort of input energy. That energy return on energy input (EROEI) varies by how and where we get it of course. What this accelerator driven reactor design does is take an energy source known to have an EROEI of about 100 to 1 to something that has an EROEI of 10 to 1. That translates to dollars out vs. dollars in.

I imagined a reactor much like this "thorium energy amplifier" myself. I imagined a reactor much like a two fluid MSR. There'd be a blanket molten salt to soak up the neutrons from a center core. With my idea the center core would not contain an enriched U-233 fissioning but instead a Farnsworth fusor. The center fusor would fuse deuterium to produce neutrons to breed thorium into fissile uranium. I quickly realized issues with this design.

If started with no seed fissile material the fusor would have to run for a month or more before the Pa-233 decays into U-233. Then once there is enough U-233 in the blanket where its at least critical with the center fusor operating then it'd be easy enough for the reactor to produce enough neutrons on its own if it weren't for the big hole in the middle. It might be possible to make the blanket thick enough that it could remain critical on its own without the fusor running, then it is just an oversized single salt reactor. It would also mean having to run the fusor even longer at the start since there is much more material that would need to be irradiated to get critical.

Allowing the blanket salt to get critical would be trivial, just don't remove so much U-233 from the core that it goes subcritical. With the blanket salt critical then no power needs to be put into the fusor, there's enough neutrons now and the fusor takes a lot of power to run. So what do you use the fusor core for now? Perhaps fill it with a moderating gas. Vary the moderating effect by varying the pressure and/or composition of the gas.

Maybe fill the now idle fusor core with U-233 molten salt and make it a fission reactor core in a two salt reactor. This gives us much more power out from the same reactor. It has all the safety of the fusion-fission design since the neutrons flowing from the center can be stopped by roughly the same way, drain out the fuel.

In my mind the people that propose accelerator driven MSRs are relying on the fear of fission power, want money for their pet project in accelerators, have a poor understanding of MSRs, or some combination of the above. They are a solution looking for a problem.

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PostPosted: May 01, 2014 2:45 am 
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An ADS being promoted as a burner for MA's also known as transuranics, is a steam hammer to kill a fly. Any fast spectrum reactor, solid or liquid fueled, could do the job as an additional benefit. Let the major actinide, the Pu-239, as a major part of recovered transuranics, burn in the reactor as fuel and incinerate the minor actinides with it.
As for in situ conversion and burning of thorium, an ADS is a bigger tool than an enrichment plant added as fuel processing system for a LWR. Just put in an additional reactor core to generate the neutrons. You could combine a fast core to generate more neutrons and moderated area for better absorption. The engineering would be less voluminous.


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PostPosted: May 02, 2014 6:30 am 
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Many thanks for all the interesting and helpful replies.

"Somehow the accelerator driven reactors brings up an image of a huge windmill ventilating a coal furnace. Could be technically feasible but absolutely pointless."

Yes, that very well puts into words my impression.


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PostPosted: May 02, 2014 7:31 am 
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Well I am pretty sure almost all the transuranics have critical masses (even 242Pu does) which sort of implies that we can burn them to extinction in a fast spectrum reactor after an IMF burn-down in a conventional reactor.


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PostPosted: May 03, 2014 6:10 am 
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I realized I was overthinking the issue with comparing ADSR with LFTR. The safety of both systems comes from the effect that part of the power from the reactor is used to keep the reaction going. In the ADSR the power is used to drive a very complicated particle accelerator. In LFTR the power is used to cool the freeze plug. In both cases if power is lost the reactor shuts down.

Now, what happens if power is not lost but the reactor gets too hot? In ADSR, if I understand the proposed design correctly, the safety systems are identical to LFTR except for the proton accelerator part. ADSR relies on the negative feedback from the thermal expansion of the fuel. There is the added safety of requiring a constant flow of protons to keep it going but this is just an overly complicated method compared to the freeze plug. Seems to me that the ADSR would require something similar to a freeze plug anyway since it looks like it is possible to drive the ADSR supercritical by producing too many protons from the accelerator.

More simply, LFTR is ADSR without the proton accelerator.

Looks to me like LFTR gives all the benefits of ADSR without the cost and complexity of the proton accelerator.

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PostPosted: May 03, 2014 7:32 am 
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An LFTR would have serious issues attempting to burn very heavy actinides in very large amounts though.

Since you have to stay away from being prompt critical and delayed neutrons would be almost useless for fissioning the transuranics - so you risk strangling the reactor if you are not careful.

An Accelerator driven system has a supply of make up neutrons so it can burn purely transuranics without risking prompt criticality if you want.


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