Looking to the Past, Planning for the Future
Since giving my talk at TEAC2 several weeks ago on my proposed plan for our nuclear future, I’ve been spending a lot more time thinking about this issue, the plan, and how to describe what I would propose to do.
Sometimes it helps me to sort out thoughts by drawing a picture, but in this case, as I sketched out my plan, I found that I needed to sketch out descriptions of how nuclear operations have been conducted in the past, how they are conducted currently, and what the conventional view of our nuclear future is.
First of all, let’s consider the period right after World War 2. That may seem like a long time ago, but decisions were made then that still have ramifications to this day. In that time period, stretching from the late 40s until well into the 1960s, the overwhelming concern of the US Atomic Energy Commission (USAEC) was to produce weapons-grade plutonium and weapons-grade uranium for use in nuclear devices. Producing electrical energy from nuclear power was way down the list of priorities.
Here’s a sketch of how things worked back then. They would mine uranium and process the ore at a mill. Then some of the ore would be converted from natural uranium dioxide (the form it’s in in the earth) to uranium hexafluoride (UF6), which is a gas. When I say “natural” in these descriptions, I generally mean its isotopic consistency, in other words, that the proportions of U-235 and U-238 are the same as those found in nature (0.7% U-235 and 99.3% U-238 respectively).
So the NUF6 (natural uranium hexafluoride) would be enriched in a huge enrichment plant. Some of the first of these were in Oak Ridge, Tennessee, but later facilities were at Portsmouth, Ohio and Paducah, Kentucky. At these enrichment facilities, the composition of the uranium was changed at incredible expense. Most of the uranium ended up “depleted”, which means it has less U-235 than when it started. Some ended up “highly-enriched”, to the point where the uranium was nearly all U-235.
Another path was taken to make plutonium. This time, natural uranium was loaded into special heavy-water reactors at places like Savannah River that would lightly irradiate the uranium (I sometimes call it “toasted” uranium) in order to convert some of the abundant U-238 into plutonium-239. This plutonium was then separated chemically at an aqueous reprocessing facility and used for weapons construction.
Then we can take a look at how the nuclear approach looks today. We still mine uranium and enrich it, but now the enrichment level isn’t as high as is needed for weapons. Our reactors (at least in the US) don’t use “highly-enriched” uranium, they use low-enrichment uranium (LEU). It’s still produced in those big enrichment plants at great expense though. So we make LEUO2 fuel (low enrichment uranium oxide) and stick in our light-water reactors. As the fuel “burns”, plutonium is produced, but it doesn’t have the same composition as the weapons-grade stuff. It’s called “reactor-grade” plutonium.
We also have a lot of highly-enriched uranium that’s coming from the decommissioning of nuclear weapons. In a bizarre waste of energy, this HEU is being “downblended” with depleted uranium to also make nuclear fuel for light-water reactors. The energy investment originally required to make HEU was titanic, so downblending it is not my favorite idea for what we should be doing, but it’s what we’re doing nonetheless.
As you can see from the graph, our weapons-grade plutonium isn’t currently being used, and our small U233 inventory is sitting there too. We have a huge inventory of reactor-grade plutonium in our spent nuclear fuel, but the current plan (and it’s still law until they change the law) is to send it to Yucca Mountain.
Well, that doesn’t make any sense because Yucca’s been cancelled, right? So we turn to our next slide about the “conventional” view of our nuclear future. In this approach, we take the weapons-grade plutonium we have from decommissioned weapons and we make “MOX” fuel out of it. MOX stands for mixed-oxides, and it means that you have a fuel that is formed from plutonium and depleted uranium, and you mean to burn it up in a light-water reactor. Like LEUO2 fuel, you can only get part of the energy out of the MOX fuel that’s in there, so running it through the reactor doesn’t release all of the energy of the plutonium. But it does degrade it, probably enough that it’s no longer weapons-grade but now reactor-grade.
Our small U233 inventory, as I have bemoaned so many times, is slated for destruction in this scenario by mixing it with depleted uranium and burying it somewhere, probably in the Waste Isolation Pilot Plant (WIPP) facility in New Mexico.
It’s also possible that we might use aqueous reprocessing to recover reactor-grade plutonium from our light-water reactor fuel and make more MOX out of that, although the prospects for doing in this in the US aren’t so likely. France is doing this right now.
In this scenario, we still need a Yucca-Mountain type facility. We may reduce the need somewhat, but it’s still there.
Finally, we reach the scenario I propose, which is an attempt to completely destroy our weapons-grade plutonium and HEU while at the same time kick-starting our thorium-powered future into high gear. In my proposed scenario, we use the weapons-grade and reactor-grade plutonium in a liquid-chloride reactor. The liquid-chloride reactor can completely burn this stuff up, and if we jacket the reactor with thorium, we can make new U-233 to start all the LFTRs that we need to build to get energy independent.
We also stop downblending of HEU but use instead to start special versions of LFTR that are intended just to make more U-233. So these special LFTRs will live on a diet of HEU and produce U-233 in their blankets to start other LFTRs.
We take all of the spent, exposed uranium oxide fuel (XUO2) that our light-water reactors are producing and we fluorinate it. Most of it will come out as UF6 that we could send back to the enrichment plant if we want. The transuranic waste (TRU) which is mostly plutonium is sent to the chloride reactors to be burned up.
To make all of this happen we’re going to need a lot of fluorine to make all this fluoride salt. The average LFTR fuel is over 50% fluorine by weight. Here’s a chance to kill two birds with one stone. Most of the uranium we’ve ever mined is sitting outside of the enrichment plant in barrels as depleted uranium hexafluoride (DUF6). Each of those uranium atoms is locking up six fluorine atoms. Furthermore, disposing of depleted uranium in this chemical form (UF6) is a really bad idea. UF6 is chemically unstable, because the uranium is just barely holding on to those last two fluorine atoms. UF4 is much more stable, and UO2 is more stable than that (in nature). So we need to convert all of that DUF6 to DUO2 and recover lots and lots of fluorine. The recovered fluorine is then used to fluorinate spent uranium oxide fuel from light-water reactors, and to form new fuel and blanket salts for all the LFTRs we will build. Once DUO2 has been produced from DUF6, it could be disposed on in the same mines from whence it originally came. It will never be as radioactive as natural uranium (and its decay products).
In the scenario I lay out, the U-233 is precious, and we stop all efforts to destroy it. Rather, we want to make much more of it. We want LFTRs burning up HEU to make U-233. We want chloride reactors burning up plutonium to make U-233. Uranium-233 is the constraining factor in our thorium expansion because we have so little and we need so much.
The thorium itself will be easy to come by, provided that we begin to mine for rare-earth elements again, and stop relying wholly on Chinese imports. Thorium is always found with rare-earth elements, and will be available in large quantities for essentially no cost. They may actually even pay us to take it!
The goal of this plan is to address all of the heritage HEU and plutonium both from our weapons-program and from light-water reactor operation, as well as to get us off of fossil fuels and running on thorium. Another goal of this plan is to eliminate the need for a Yucca-Mountain style repository.
I think it can work! What do you think?
(here’s the original slides if you’re interested…)