Comparing LFTRs and IFRs to LWRs (part 2)

In a previous post I began comparing how a utility that operated nuclear reactors might look at a liquid-fluoride thorium reactor (LFTR) or an integral fast reactor (IFR) relative to their existing light-water reactors, with a specific look at whether or not operating these advanced reactors would confer an economic advantage for them.

Despite how much LFTR and IFR advocates like to talk about fuel efficiency and reduction in nuclear waste, I concluded that neither would be particularly compelling to a utility with today’s fuel costs and regulatory environment.

Which leads us into fuel reprocessing, because both LFTR and IFR plan to incorporate fuel reprocessing within the confines of the reactor building, and for the LFTR, perhaps even within the reactor cell. How would integral reprocessing be viewed by a utility?

Initially, most likely with great hostility. In today’s LWR-powered nuclear world, fuel reprocessing, if it is done at all, is something that is done entirely separately from the reactor. It involves transporting highly-radioactive spent fuel many miles, or in the case of the Japanese, across the world. LFTR and IFR can make that trip much simpler right away by making the transport of fuel a very local affair. The flip-side to this is that LFTR and IFR had better offer a scheme for reprocessing that is FAR simpler than the PUREX-type aqueous reprocessing technology used for LWR fuel, because that process is most definitely not one that could be scaled down and co-located with the plant.

To achieve vastly simplified reprocessing, both LFTR and IFR employ fuel forms very different than the solid-uranium-oxide fuel commonly used in LWRs, and this choice is made very consciously to try to make the reprocessing end of things much simpler. In the case of LFTR, the fuel (and the blanket) consist of thorium and uranium fluoride salts dissolved in a carrier salt of lithium and beryllium fluoride salts. This fuel form is totally stable chemically and in hard radiation fields. Best of all, it can be reprocessed in its existing form. It does not need to be changed chemically to some other form to reprocess.

The IFR uses metallic uranium/plutonium fuel. Using solid metal fuels (rather than solid oxide fuels like an LWR) has some definite performance and reprocessing advantages. It makes the fuel much more thermally conductive and it makes changing the fuel into another chemical form far more chemically favorable. Metals “want” to oxidize, either with oxygen or nitrogen or fluorine or chlorine. In the case of IFR, the metal fuel (when it comes time to reprocess) is removed from the reactor and reacted with chlorine to form a chloride salt analogous to the fluoride salts used in LFTR.

In chloride salt form, reprocessing is executed on the IFR fuel, separating fission products from actinides like uranium, plutonium, americium, etc., and then the chloride salt is electrolytically separated (using a flow of electrical current) back into metallic form. The metal is cast into a new fuel rod and reintroduced into the reactor for another round of burnup.

The goal in LFTR or IFR processing is the same–to keep burning fuel and to separate out the “ash” of fission, the fission products. The chemical form of the fuel while it’s being reprocessed is also similar: LFTR uses fluoride salts and IFR uses chloride salts. But there is an important difference. LFTR’s fuel is already in the right form to reprocess. That means that reprocessing can take place continuously, while the reactor is operating, whereas IFR has to be shut down in order for the solid fuel to be removed and cooled for a period. Then IFR’s fuel is chemically changed into a chloride salt, processed, and then it has to be changed back again.

It is this shutting down and starting up of a reactor that would probably be the part where you got the utility’s attention in a big way. They make money when reactors are running and making heat and turning generators. They don’t make money when reactors are shut down. They’re probably not going to be too keen to begin with about an integral reprocessing system, but one that involves frequent shutdowns and startups is going to be especially unattractive to them. LFTR has the potential to actually improve on LWR availability, because even LWRs have to shut down every 18 months or so for refueling and fuel reshuffling. A LFTR could potentially go much longer between scheduled shutdowns, probably limited by the turbomachinery rather than by the fuel. But an IFR is going to have to shut down frequently for fuel reprocessing and reshuffling. And utilities are NOT going to like that.

While IFRs could run longer on a given fuel load, they will do so at the detriment of their breeding performance and their fuel quality. LFTRs don’t give up performance in the fuel cycle to run longer, because in one sense, they are reprocessing all the time.

So if a utility is looking at the bottom line of how many hours of the year the reactor is turning the electrical generator, they’re going to see a big advantage for LFTR over the IFR.

(next time: other economically beneficial products from LFTR vs. IFR)



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