Questions About LFTR - Uranium 233 & Gamma Rays

[Nerdydude101]

So I have been reading about LFTR and I understand the concepts but there are a few things I have questions about, mainly it's possible use of uranium 233. Some articles I have read and diagrams I have seen make mentions if using uranium, however other articles and diagrams make no mention of uranium at all, so is it used in a lftr or maybe just in some lftr designs? If it is used then how is it used and how is it prevented from emitting gamma rays because from what I understand a lftr only emits alpha radiation. Thanks for any help you can give!

 

[Warpspeed13]

A LFTR converts fertile Th-232 into fissile U-233. This reactor will emit gamma because of U-232 poisoning of the U-233. U-232 is a very strong gamma emitter that is generated as a small fraction of that decay chain. The articles you read that claim that LFTR reactors are proliferation resistant, that resistance is because of the U-232. The U-232 is very hard to separate from the U-233 and handling it tends to kill people. And also a portion of that proliferation resistance is probably because U-233 is prone to spontaneously exploding when it is fashioned into a fission bomb.

 

[QuantumPion]

The U-232 is very hard to separate from the U-233 and handling it tends to kill people.

The half-life of the U-233 precursor, proactinium-233, is 27 days. You could chemically separate protactinium from irradiated thorium to produce pure U-233.

And also a portion of that proliferation resistance is probably because U-233 is prone to spontaneously exploding when it is fashioned into a fission bomb.

I've never heard this before. The SF rate of U-233 is lower than U-235 (and much lower than Pu-239) so why is this so?

 

[QuantumPion]

So I have been reading about LFTR and I understand the concepts but there are a few things I have questions about, mainly it's possible use of uranium 233. Some articles I have read and diagrams I have seen make mentions if using uranium, however other articles and diagrams make no mention of uranium at all, so is it used in a lftr or maybe just in some lftr designs? If it is used then how is it used and how is it prevented from emitting gamma rays because from what I understand a lftr only emits alpha radiation. Thanks for any help you can give!

Thorium is not fuel, only breeder material. LFTR's convert thorium into U-233 to produce energy. They require an initial loading of U-233, U-235, or Pu-239 in order to begin the breeding process. All nuclear reactor types and fuel cycles will produce gamma radiation from irradiated impurities and fission products.

 

[Nerdydude101]

So is it also not true that they are safer that a common nuclear reactor? I had heard that they were safe to be built in a higher density population area but if they met gamma rays then that wouldn't be true, also is this video inaccurate then?
https://m.youtube.com/watch?v=nYxlpeJEKmw

 

[Astronuc]

So is it also not true that they are safer that a common nuclear reactor? I had heard that they were safe to be built in a higher density population area but if they met gamma rays then that wouldn't be true, also is this video inaccurate then?
https://m.youtube.com/watch?v=nYxlpeJEKmw

The video is simplistic and misleading. It does not address the various engineering/technical challenges associated with a LFTR system.

It is the engineering that determines and assures safety.

Any fission system inherently produces fission products which are beta and gamma emitters. The fission products would have to be separated and processed, then ultimately deposited somewhere isolated from the environment. Each LFTR plant requires a processing system to extract/separate U-233 and recirculate it into the reactor system. At startup, a fissile inventory (U-235 or Pu-239) is required.

The plant efficiency will be determined by the peak temperature of the working fluid, and as temperature increases materials are challenged. It may be possible to use a Brayton cycle for power generation, otherwise, a steam Rankine cycle would be employed. Steam cycles introduce corrosion and erosion issues associated with the water interaction with the structural alloys.

 

[mheslep]

Agreed w one exception. If a molten salt reactor design must fall back to a Rankine cycle, steam will be obtained via heat exchangers. The reactor vessel itself and fission products will always be at a couple atm at most. An MSR is not a PWR.

 

[Warpspeed13]

The half-life of the U-233 precursor, proactinium-233, is 27 days. You could chemically separate protactinium from irradiated thorium to produce pure U-233.

Yes but that's not the problem the problem is separating the resulting U-233 and U-232. It requires the same equipment as separating U-235 and U-238. In addition it is far more radioactive than natural uranium.

I've never heard this before. The SF rate of U-233 is lower than U-235 (and much lower than Pu-239) so why is this so?

Sorry I was incorrect as to the requirements for premature destination. The main proliferation negator is the gamma emission. See the response on this article http://www.americanscientist.org/issues/pub/2010/5/a-thorium-future

 

[Warpspeed13]

So is it also not true that they are safer that a common nuclear reactor? I had heard that they were safe to be built in a higher density population area but if they met gamma rays then that wouldn't be true, also is this video inaccurate then?
https://m.youtube.com/watch?v=nYxlpeJEKmw

The gamma can be stopped by materials such as lead or depleted uranium. What makes them safe for high density population centers is the salt plug that holds the fuel in. If the fuel gets to hot the salt plug melts draining the fuel into a separate area separating it from the external neutron source.

 

[QuantumPion]

Yes but that's not the problem the problem is separating the resulting U-233 and U-232. It requires the same equipment as separating U-235 and U-238. In addition it is far more radioactive than natural uranium.

No, U-232 is generated by neutron absorption in the reactor. If you chemically separate the proactinium outside of the reactor and wait for it to decay to U-233 there will be no U-232 contamination.

 

[QuantumPion]

So is it also not true that they are safer that a common nuclear reactor? I had heard that they were safe to be built in a higher density population area but if they met gamma rays then that wouldn't be true, also is this video inaccurate then?
https://m.youtube.com/watch?v=nYxlpeJEKmw

It's difficult to compare the safeness of a hypothetical reactor design to real world power plants. While LFTR's have some potential advantages, there are also challenges, both known and unknown, which have to be analyzed. An LFTR would certainly still require a containment building.

 

[Warpspeed13]

No, U-232 is generated by neutron absorption in the reactor. If you chemically separate the proactinium outside of the reactor and wait for it to decay to U-233 there will be no U-232 contamination.

Ok true however you still encounter the same problems with getting the proactinium out as you do the U-233. The U-232 would irradiate everything in the vicinity until the proactinium was separated out. Also due to the short half life of proactinium I doubt there would be enough at anyone time to generate a usefull amount of U-233 outside the reactor. You would have to break open the reactor multiple times and separate out proactinium each time.

 

[QuantumPion]

Ok true however you still encounter the same problems with getting the proactinium out as you do the U-233. The U-232 would irradiate everything in the vicinity until the proactinium was separated out. Also due to the short half life of proactinium I doubt there would be enough at anyone time to generate a usefull amount of U-233 outside the reactor. You would have to break open the reactor multiple times and separate out proactinium each time.

The half life of proactinium-233 is 27 days which is plenty to reprocess. Chemically separating protactinium from the fuel is not very complicated. And since we are referring to a LFTR, it would be easy to get out since the fuel is already liquid and presumably reprocessing equipment is already part of the plant design. Additionally, the process can be made more efficient by having external breeding blankets.

 

[Warpspeed13]

Ok just focusing on the separation part of it though. You would need a remote handling facility due to the high radioactivity of the U-232. Anybody who had that kind of facility would have been better off building an enrichment facility for natural uranium. They would get a higher yield bomb without attracting unwanted attention by pilfering a LFTR for its proactinium.

 

[Nerdydude101]

But the point of a nuclear reactor is to generate energy, nt to create bombs, well I mean the government doesn't see it that way but if you exclude the bomb making would lftr be more efficient?

 

[Warpspeed13]

But the point of a nuclear reactor is to generate energy, nt to create bombs, well I mean the government doesn't see it that way but if you exclude the bomb making would lftr be more efficient?

That really depends on the design of the LFTR itself.

 

[QuantumPion]

Ok just focusing on the separation part of it though. You would need a remote handling facility due to the high radioactivity of the U-232. Anybody who had that kind of facility would have been better off building an enrichment facility for natural uranium. They would get a higher yield bomb without attracting unwanted attention by pilfering a LFTR for its proactinium.

Well no. By design, LFTR's use liquid fuel which is re-processed on site. This is completely different from enriching natural uranium, which requires extensive facilities and specialized equipment. It would not be hard to add capability for U-233 breeding and separation. The operation would be similar to that of plutonium production. The advantage of plutonium breeding vs. thorium is that plutonium breeding does not require high enriched fuel to seed the breeder, and the physics of Pu-based bombs is much more well tested and understood.

 

[QuantumPion]

But the point of a nuclear reactor is to generate energy, nt to create bombs, well I mean the government doesn't see it that way but if you exclude the bomb making would lftr be more efficient?

More efficient in what sense? Cost? Fuel efficiency? Thermodynamic efficiency?

 

[mheslep]

...The advantage of plutonium breeding vs. thorium is that plutonium breeding does not require high enriched fuel to seed the breeder, and the physics of Pu-based bombs is much more well tested and understood.

I don't believe the seed need be HEU, but only enriched sufficiently to sustain a reaction for some time.

 

[Nerdydude101]

It's ability to convert the fuel into energy, we convert almost none of uranium into energy, but from what I've read we can convert a large amount if the thorium into energy but from what people on this post have said I'm starting to doubt everything I've read haha.

 

[Warpspeed13]

Well no. By design, LFTR's use liquid fuel which is re-processed on site. This is completely different from enriching natural uranium, which requires extensive facilities and specialized equipment. It would not be hard to add capability for U-233 breeding and separation. The operation would be similar to that of plutonium production. The advantage of plutonium breeding vs. thorium is that plutonium breeding does not require high enriched fuel to seed the breeder, and the physics of Pu-based bombs is much more well tested and understood.

Ok we'll then were getting into a nation state level endeavor. I've been approaching this in regard to what prevents an individual or small group from getting the U-233.

 

[QuantumPion]

I don't believe the seed need be HEU, but only enriched sufficiently to sustain a reaction for some time.

Yes, but during the Manhattan project they had no source of fissile material to start with, so it was easier to go the plutonium route and there was little incentive to later switch over to thorium.

 

[QuantumPion]

It's ability to convert the fuel into energy, we convert almost none of uranium into energy, but from what I've read we can convert a large amount if the thorium into energy but from what people on this post have said I'm starting to doubt everything I've read haha.

Well that part is true. Any breeder reactor will be far more fuel efficient than a standard fuel cycle, and any reactor design that includes on-site reprocessing will have reduced or even eliminated spent fuel disposal issues.

 

[mheslep]

The point was that there's no proliferation risk due to the seed required for a LFTR startup

 

[Astronuc]

It's ability to convert the fuel into energy, we convert almost none of uranium into energy, but from what I've read we can convert a large amount if the thorium into energy but from what people on this post have said I'm starting to doubt everything I've read haha.

As QuantumPion indicated it is important to clearly define what type of efficiency one is considering, e.g., thermodynamic efficiency, or fuel utilization, i.e., burnup (energy per mass of fuel).

In a conventional LWR, about 4.5% to 5.5% of the U is converted into energy. It could be greater if certain design, reliability and safety requirements could be met (but that's a different topic). One of the constraints on LWR fuel is the accumulation of fission products and transuranics (TU) and the impact on fuel reliability and safety (e.g., solid and gaseous swelling of the fuel, and rod internal pressure, and how that affects the requirements of fuel/core coolability and reactivity control (i.e., ability to reliably shutdown reactor in response to various AOOs and postulated accidents)).

In an LWR, fuel may be used for two or three cycles, with each cycle being typically 18 to 24 months (in the US and parts of Europe). Each batch of fuel removed is on the order of 34% to 50% of the core. The oldest fuel is removed, and fresh fuel is added. The oldest fuel contains the fission products and TU accumulated from the two or three cycles of operation.

The benefit of the LFTR is that fission products are removed and the Th-based fuel cycle does not accumulate as much TU as does the U-based fuel cycle. However, the LFTR system does require a reprocessing system that will accumulate the U-233, recycle the Th-232, and accumulate the fission products that then have to be fabricated into a safe form (typically encapsulated ceramic). The waste forms then have to be cooled.

Thermodynamic efficiency is also factor. LWRs have a range of thermodynamic efficiencies from ~32% to ~38%, while some gas-cooled reactors have expected efficiencies of ~42%. That also improves the MWh/MTHM. While it might be possible to have comparable thermodynamic efficiency with an LFTR, one must realize that the reprocessing plant will consume some amount of the electrical energy produced - and each LFTR needs it's dedicated reprocessing plant (and waste storage).

It may be desirable to have modular LFTR units with a common reprocessing plant, and then one has to decide if the LFTR units are moderate in size (e.g., 200-300 MWe) or larger (1000 - 1500 MWe). The power ratings will determine the size of core, which then determines enrichments.

 

[Astronuc]

The point was that there's no proliferation risk due to the seed required for a LFTR startup

Not as long as the fissile material (U-235 or Pu-239/241) is diluted in Th-232 or mixed fluoride salt.

 

[Warpspeed13]

Yes, but during the Manhattan project they had no source of fissile material to start with, so it was easier to go the plutonium route and there was little incentive to later switch over to thorium.

Actually they went with reactors that bread plutonium because that also produced materials for bombs. Some of the scientists on the Manhattan project such as Eugene Wigner and Alvin Weinberg actually advocated that the thorium fuel cycle be used.

 

[mheslep]

Not as long as the fissile material (U-235 or Pu-239/241) is diluted in Th-232 or mixed fluoride salt.

Or before. There would be no need to ship HEU to the reactor to start it, thus no need to produce HEU.

 

[Astronuc]

Or before. There would be no need to ship HEU to the reactor to start it, thus no need to produce HEU.

On the other hand, if one has a mixture of U-235 and Th-232, then somewhere in the process, one has HEU, since low LEU is < 5% U-235 and 95% U-238.

If there is little or no U-238 in the Th-based fuel, then somewhere one has to make HEU (predominantly U-235).

 

[mheslep]

On the other hand, if one has a mixture of U-235 and Th-232, then somewhere in the process, one has HEU, since low LEU is < 5% U-235 and 95% U-238.

If there is little or no U-238 in the Th-based fuel, then somewhere one has to make HEU (predominantly U-235).

I'm unaware of a reason why some U238 can not be introduced into a LFTR as part of a starter seed of LEU.

 

[QuantumPion]

Actually they went with reactors that bread plutonium because that also produced materials for bombs. Some of the scientists on the Manhattan project such as Eugene Wigner and Alvin Weinberg actually advocated that the thorium fuel cycle be used.

My point was that they could have produced U-233 from thorium for bombs, except that they didn't have a source of HEU or Pu to seed a thorium reactor to start with. So they went with Pu breeding, because that can be done with natural uranium. And once they had worked out how to do Pu breeding, even though they now had fissile material to potentially switch over to U-233 breeding, there was little reason to do so. If Oak Ridge had been able to produce enriched U-235 before they worked out how to breed Pu, they very well may have gone the thorium breeding route instead.

 

[nikkkom]

any reactor design that includes on-site reprocessing will have reduced or even eliminated spent fuel disposal issues.

But added issues with having a small reprocessing plant at *every* reactor. Reprocessing plant is not an easy thing to build and maintain - ask Brits or US.

And you still have to dispose of fission products. They aren't in the fuel anymore (if your reprocessing plant works well), but they still exist.

 

[mheslep]

But added issues with having a small reprocessing plant at *every* reactor. Reprocessing plant is not an easy thing to build and maintain - ask Brits or US.

True, though a major concern with reprocessing spent uranium fuel is that it is unavoidably also a plutonium factory. This is not the case with a thorium cycle reactor, which produces negligible plutonium.

And you still have to dispose of fission products. They aren't in the fuel anymore (if your reprocessing plant works well), but they still exist.

Also true, but again with a thorium reactor the fission products are not mixed in with long half-life uranium actinides made via neutron capture. The radiotoxicity of the fission products alone decays in a ~hundred years to a point requiring ten thousand years for the same level in uranium actinides.

 

[nikkkom]

True, though a major concern with reprocessing spent uranium fuel is that it is unavoidably also a plutonium factory. This is not the case with a thorium cycle reactor, which produces negligible plutonium.

I'm not talking about security or proliferation concerns. The technical challenges of building reprocessing plant are bad enough per se.

British plant has repeated bad leaks.
Japanese plant has problems with vitrification equipment. Years and years behind schedule. Not operational yet.
Not-yet-completed US plant is horribly expensive, took more than a decade to build, and is not even a real, full-cycle reprocessing plant - it can't reprocess real reactor fuel, it will only vitrify existing Hanford waste.

Now imagine this saga repeating at every reactor site.

 

[mheslep]

I'm not talking about security or proliferation concerns. The technical challenges of building reprocessing plant are bad enough per se.

The two issues are not separable in spent uranium processing; that is, one would have hard time pointing to a significant piece of a spent uranium reprocessing plant design and say it is not influenced by the need to account for and secure every mg of plutonium or other uranium actinides in the spent fuel. Such plants also must accommodate the routine transportation of spent and reclaimed fuel, another challenge not required of the closed loop recycling likely to be used in an MSR.

...
Not-yet-completed US plant is horribly expensive, took more than a decade to build, and is not even a real, full-cycle reprocessing plant - it can't reprocess real reactor fuel, it will only vitrify existing Hanford waste.

Now imagine this saga repeating at every reactor site.

I grant processing is a challenging chemical problem, but the cost and schedule problems you cite here are largely driven by the security, proliferation, and long-term waste issues inherent in spent uranium.