Energy From Thorium Discussion Forum

I've seen it many times that MSRs can use the "spent" fuel rods from light water reactors as fuel. I'm just curious how this might work in practice. As I recall the fuel from these LWRs are fuel rod bundles made of zirconium tubes filled with uranium oxide pellets. The size, shape, and chemical composition is standardized and well known. Part of the problem with processing these fuel rod bundles is that they are highly radioactive even after years of cooling off. Seems to me that it would be highly preferable to have a method of processing these fuel rods in a manner that requires as little mechanical handling and human interaction as possible.

I was thinking that it might be possible to design a MSR so that it could accommodate a whole fuel rod bundle as part of the fueling process. I'm thinking that once the MSR is in need of fuel a hatch at the top of the reactor could be opened a fuel rod bundle lowered in and let the heat and neutrons eat up the bundle and dissolve it in the salt.

I realize this is not a trivial process as a lot of things need to be considered. The reactor would have to be large enough to swallow a whole fuel rod bundle and still have enough fuel salt to remain critical. The solid material won't all dissolve at once and the "chunks" that would inevitably break off would have to be kept away from pump impellers and such to prevent damage. The metals would have to be turned to salts so a fluoride gas would have to be bubbled into the reactor or some other means to keep the salt a salt.

I can see the presence of the oxygen from the uranium oxide might be a problem. I'm still not sure I understand all the chemistry that goes on inside a MSR but I'm guessing oxygen is bad. The other materials in a fuel rod seems like what would already be in the fuel salt. The uranium, transuranics, and fission products would be no different than what is in the fuel salt already. The zirconium in the fuel rods should just dissolve in the salt. I've read that ZrF4 has been used as a component of MSR fuel salts before so adding more zirconium should not be an issue.

Designing a MSR that can eat the fuel rod bundles whole I'm sure is an idea that might come in time. First we'd need to have to make a working MSR. Once we get to that point we can worry about making one that uses a less optimal fuel like spent fuel rods.

Other than feeding the fuel rods in whole how would spent fuel rods from LWRs get used as fuel in MSRs? What would the process look like to work around issues like radiation, proliferation, and environmental contamination?

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.

Sorry. You can't just insert the whole fuel rod. The neutronics won't allow that. The spent fuel from an LWR has already reduced the fissile content to around 1.5%. It requires a very thermal reactor to use fuel that is this low in fissile content.

The closest you could come would be very light processing of spent fuel rods from LWRs being used in a CANDU reactor (solid fuel) to extract a it more energy out of the fuel.

Yes, I realized as such shortly after I posted that. A reactor would need to quite large to both take the "hit" that such a large quantity of fission poison and still remain critical. Perhaps there could be a means to control how quickly the fuel rod bundle is dissolved in the reactor. I don't know, maybe as the rod bundle breaks apart the bits would sink to the bottom or float to the top until the chemistry is just right to allow the pieces to dissolve. The salt in between the jetsam and flotsam remains critical.

Perhaps the spent fuel rod could be dumped in at the same time as more fissile salts. As the reactor runs siphon off some U-233 and store it off in reserve tanks. If the Pa-233 is siphoned off as part of normal operation then this is nothing new that needs to be added. When the spent fuel rod bundle is put in the reactor then add the U-233 as needed to keep it critical.

Okay, probably a bad idea to add the spent fuel rod directly into the reactor. How about dissolving the fuel rod in the blanket salt of a two salt LFTR? This avoids the neutron economy issues of the core salt. What remains are the chemistry issues of the blanket salt. I'm sure that there would still be issues of the neutron economy in the blanket, I'm thinking and hoping they would be minimal.

I'm just thinking about how we could make these spent fuel rods into something very valuable. One thing I noticed in reading about coal energy is that the stuff that comes out of coal boilers is not "coal waste" or "coal ash" any more. What they call it now is "coal combustion products", as in it's not a waste anymore but it's a product that people want to pay good money for. They use it as aggregate in concrete mostly, sounds like they use a lot of it for that. For people to view the spent fuel rods as something other than waste we need to find a means to process it into something someone else would want to buy. Then we need to stop calling it "spent" or "waste" but call it a "product" so that people think it would be stupid to drop it in a hole.

The processes I described above involves a reactor designed to "eat" whole fuel rods. I thought of this because I put two ideas together, the continuous refueling of LFTR from bred fuel, and that "spent" fuel rods still contained valuable fuel for LFTR.

Getting to MSRs that can swallow a spent fuel rod whole is perhaps a leap too far. Perhaps a means of pyroprocessing spent fuel rods that does not require any neutrons as part of the process. Perhaps we could submerge a fuel rod bundle into a fluoride salt, something like ZrF4. We get the pot it's cooking in nice and hot, like 1000C. Then we bubble some fluoride gas through it, like HF. The zirconium cladding dissolves and becomes part of the solvent. The uranium is converted to UF6 gas and collected, which can be sold off to uranium refineries for making new fuel rods. The stuff that is left is fission products and transuranic elements which can be disposed of as radioactive waste like the entire fuel rod assembly would have been or can be processed further for medical and industrial uses. Perhaps the excess zirconium that is produced can be sold off as reactor grade zirconium for new fuel rods or other reactor components, I imagine that alone is valuable enough to make the process worthwhile.

The heat to drive this process can come from electric heaters or from a LFTR. If the heat is from a LFTR then one does not have to go far to dispose of the UF6 gas, it can be piped into the reactor instead of being sold off.

I'm sure I'm missing some vital steps in the chemistry here. I'm assuming that HF mixed with the UO2 from the fuel rod would result in UF6 and... something left over. I'm sure water might result but it's not likely to survive long at such temperatures and in the presence of such volatile chemicals.

I'm just tossing ideas out there. I'm no nuclear engineer. There is obviously some very intelligent and educated people here and I'm glad I am allowed to participate.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.

There is some good background info at: http://en.wikipedia.org/wiki/Nuclear_reprocessing#PYRO-A_and_-B_for_IFR-- check also the paragraph(s) above it. The IFR (Integral Fast Reactor) project did a bit of laboratory bench scale work on reprocessing spent fuel rods in order to eliminate the long-lived TRUs in a fast reactor. I had been quite the fan of the IFR project when I learned of it a dozen years ago, but always feared all that liquid sodium. I switched to MSR's and LFTR once I learned of them a few years after that, and understood them better.
I think the long term answer for LWR spent fuel rods will turn out to be MSRs using molten chlorides so that the fast spectrum will transmute the TRUs, but there is a large amount of basic research needed in comparison to the fluoride MSR systems.

The existing pyroprocessing technology developed for sodium fast breeder reactors uses molten salts as the solvent and electrolyte. The uranium and actinides can be preferentially removed from the salt once the fuel rods are dissolved. This is practically the same as dumping fuel rods into the reactor; instead you dump lightly processed salt. You end up skipping fuel fabrication. You may even be able to use the same online processing equipment the MSR is using.

The best idea on the subject I have read so far is given by Kirk's blogs in forbes:=
http://www.forbes.com/sites/kirksorense ... -digester/
This can fit in nicely in place of fast breeder in the IFR scheme of things. Chloride salts will have synergy with chloride volatility in pyroprocessing.

Kurt,
David LeBlanc gave me an answer to a similar question earlier. http://www.energyfromthorium.com/forum/viewtopic.php?f=2&t=3775

As many on this forum are familiar, direct consumption has many many challenges, but various forms of pyro processing provide the opportunity to convert SNF to a salt form or a salt or metallic form. Then we can repurpose most of that material as MSR fuel. I recommend that you take a look at flame fluorination, it converts SNF to a fluoride base from which basic separations of the different materials can take place.

However none of that increases the fissile content of the U which will be very low.

The other way to go about this would be with a fast breeder reactor. Then the fission products are less of a poison and the uranium (if fed slowly enough) would provide the fertile. However, there is an awful lot of SNF. For every GWe-yr you created with LWRs you would need to create 20 GWe-yrs with the fast reactor. Personally, I'm nervous about fast reactors and not like to see so many fast reactors. I would rather have lots of thermal reactors and possibly a few fast reactors to clean up the residual TRUs. This road will not burn up the SNF/U but will take care of the TRUs. I see no reason to be concerned about the SNF/U.

Used fuel rods have to be cooked (generally referred as reprocessed) before they can be digested. And it requires a breeder to convert more of fertile U238 or thorium to fissile matter.
Fast reactors, with higher neutron energy do it better. MSR, with liquid fuel may be at an advantage as part of fission product poisons (Kr, Xe) can be removed during operation. Fast MSR may be the right used fuel digester.

Definitely one very practical solution, if we can separate FP's, U and TRU from one another, the U can be stored as future fuel or diluent for U233 bred from thorium, the FP's disposed of, and the TRU's burned in fast MSR's. I remain optimistic about the potential for fast spectrum MSR's, but there's a lot of work to be done.

The work on fast spectrum solid fuel reactors is going on in Russia, China and India. MSR's, both thermal and fast spectrum are way behind.

Several thoughts on this.
The TRUs can be used as a starter charge for a Liquid Fluoride Thorium Reactor and by using the U as a diluent to achieve sustainment level rather than breeder level, you create what I like to call the 2.2+ fluid LFTR.

Once you are at the state with processing the unspent fuel for LFTRs, the FPs can also be segregated. At that point it seems wise to extract any FPs that are economical to do so, Cs and Sr seem a good starting point. Cs for gamma irradiation of stuff (food, etc.) and Sr for RTGs or SRMs.

_________________
DRJ : Engineer - NAVSEA : (Retired)

For thermal LFTRs U-238 is a loss. It is a necessary evil for denaturing but we build up excess U238 over time so we add as little of it as necessary. Thermal LFTRs will never consume the LWR spent fuel uranium.

It is possible to get more power out of the LWR spent fuel uranium by running it through a CANDU. But I do not understand a policy goal to consume the SNF/U - it is only modestly different than depleted uranium.

Separating uranium from the rest of the spent fuel is very practical using fluorination. This is something ORNL did for MSRE to switch from using U235 to using U233. They did it with lab scale equipment in 12 days (if I recall correctly).

Separating Cs from the spent fuel should be fairly reasonable using vacuum distillation - though I'm less confident that the benefits are worth the trouble unless it is a natural byproduct of doing vacuum distillation for other reasons.

Separating the plutonium from the fission products is high value but not trivial. There are several ideas floating around (intense fluorination, liquid metal exchange, oxidation, electro processing) but all still have various challenges.

I have no idea how to separate the strontium from the other fission products. It is hard to say it will be financially attractive (though I am not hopeful).

Sr-90 is an alpha emitter with a 1.3 ba xs.
Leave it with the TRU.
It wont get in the way.