An excellent paper was posted to the thorium-forum recently on the work being done at the Nuclear Research Institute at Rez in the Czech Republic. The paper was written by Jan Frybort and Radim Vocka and was entitled:
Regular readers of this blog know that I am a big fan of the two-fluid fluoride reactor. What is the two-fluid reactor? It’s the version of the fluoride reactor where the fuel salt (containing the fissile material, typically U-233) is kept separate from the blanket salt (containing the fertile material, typically thorium).
The one-fluid reactor, on the other hand, has the fissile and fertile materials mixed together in one salt.
Why do I like the two-fluid reactor so much? A couple of reasons. The first one is is that it offers some really exciting improvements in safety. It’s much more responsive the changes in temperature than a one-fluid reactor, because heating up the salt causes the fuel to expand out of the core more than the fertile material.
It’s a lot easier to process than the one-fluid reactor, because a whole class of fission products (the lanthanides) that behave a lot like thorium are kept separate from thorium.
And it’s a lot easier to scale up and down, because the blanket fluid acts like a neutron shield in each case.
So what’s the downside of the two-fluid reactor? There’s two big ones. The first is the problem of how to keep the two fluids separate in a way that doesn’t eat too many neutrons and will last for a good long while during regular operation without getting too messed up. The second is the tendency of the two-fluid reactor to have a problem if the blanket fluid changes–what’s called the “blanket void coefficient”.
I think both problems will ultimately be solvable, and I have some good ideas, but I don’t have the definitive answers yet.
But back to the paper from Rez…
In this paper, Dr. Frybort and Dr. Vocka look at the two-fluid reactor design that ORNL was working on back in 1967 with fresh eyes. They do this because they had been looking at one-fluid reactor designs and found them wanting in several categories, just as I have. They wanted a reactor with strong negative temperature coefficients, simple processing, and good performance, and the one-fluid reactor had serious issues in all categories. So they rewound the clock from 1974 to 1967 and looked at where the Oak Ridge Lab had gotten when they set the two-fluid reactor down.
Oak Ridge worked on two-fluid reactor design seriously for about two years, from early 1966 to late 1967. Their work is detailed in three semiannual progress reports (ORNL-4037, ORNL-4119, ORNL-4191) and one final report (ORNL-4528). All of these reports are available in the document repository linked from this site.
ORNL was specifically trying to define a “molten-salt breeder experiment” that they called the MSBE, that would have been a follow-on to the Molten-Salt Reactor Experiment (MSRE), that ran successfully from 1965 to 1969. In order to know what should go into an MSBE, they needed a pretty good idea of what the eventual “molten-salt breeder reactor” (MSBR) would look like. They needed to start with the end in mind, so they devoted themselves to trying to understand what a two-fluid MSBR looked like, and from there they would backtrack and try to define an experimental reactor that would prove its capabilities.
Defining an MSBE was a challenge. The MSRE was far less than that–it only had a single fluid and had no blanket where thorium would be converted into new fuel. So the first thing to do would be to define a new core geometry where both goals were being satisfied simultaneously. This is where ORNL ran into the first problem of the two-fluid reactor–the so-called “plumbing problem”.
They tried to fit blanket salt, fuel salt, and graphite elements in the core in a way that would work neutronically (it would achieve criticality) and in a way that would work from a fuel-cycle perspective (it would produce more fuel than it consumed). The second part required understanding the fuel and blanket processing system just as much as the reactor itself.
ORNL reactor designers experimented with a variety of core geometries. Most were hexagonal elements of graphite with recursive tubes of fuel flowing through them. The whole arrangement was then immersed in a bath of blanket salt. This arrangement was actually quite good at satisfying the first and second conditions. Very good in fact. It was also easy to process and had strongly negative temperature coefficients. All good. But it was not good holding up to the neutron flux. Data indicated that the graphite would shrink and then swell. As that happened, the relative volumes of fuel and blanket would change, and that would alter performance. Additionally, and this was never mentioned in any of the design documents, so well as I can discover–these designs suffered from the problem of the blanket void coefficient. If blanket salt drained out, core reactivity would go up–way up, and the reactor would respond by the core temperature increasing. Eventually it would reach the point where the freeze plug would melt and the core would drain out into the drain tank.
In the paper from Rez, the performance of this design was reevaluated using modern tools, and the authors verified that the two-fluid reactor had the advantages that ORNL scientists had anticipated. In part 4A, they found that the temperature coefficient was strongly negative and that the breeding factor was good. In part 5, they looked at changing the design to improve it, and found that by making the fuel channels bigger than the original design, they improved nearly all parameters. This is an important result, since it’s not often that you change a parameter and find improvements in nearly all outputs from that parameter.
In part 7 of the paper, they mention the second key issue with a two-fluid reactor–the problem of the blanket void coefficient. Since the original ORNL design had the problem, and since they modeled only parametric variations on that original design, it’s no surprise that the problem still shows up. It must be fixed, probably through a new design approach to the two-fluid reactor. I have some ideas, most all of them based around physical situations where a loss of blanket fluid leads to a loss of moderation. I anticipate that this could be done by floating moderator elements (graphite) in the blanket salt, so that as the level of the blanket salt falls, the moderation decreases more than the absorption decreases from the loss of blanket. These ideas definitely need more modeling, but I think they are essential
The scientists from Rez have done us all a great service by “blowing the dust” off the old ORNL design, figuring out how it works with modern tools, and figuring out a few ways to make it better. There is a lot more work to be done, but recent work like this is often the catalyst to getting more scientists and engineers working on a problem that has the potential to have such an important result.
Thank you Dr. Frybort and Dr. Vocka!
I have appealed many times on this blog to our public leaders to save the precious resource that is uranium-233. I have explained in an earlier blog post why the uranium-233, if used in a liquid-fluoride thorium reactor (LFTR) represents “unlimited” energy, because it can “catalyze” the consumption of abundant natural thorium and the inventory of U-233 remains even at the end of the reactor lifetime.
But why save this particular 1000 kg of U-233? If we’re going to start hundreds of LFTRs, there isn’t enough U-233 to start all of them, and we’ll have to use other sources of fissile material. So why worry about this batch of U-233?
Reason #1: Destroying the U-233 is a waste of money.
The Department of Energy is estimating that the cost of “downblending” U-233 with depleted uranium (predominantly U-238) followed by “disposal” at the Waste Isolation Pilot Plant (WIPP) in New Mexico will cost approximately $380 million dollars. This is the estimate as of December 2006, and it came from an article in the Knoxville News-Sentinel that is no longer available, but the text can be viewed on the thorium-forum. $380 million is a LOT of money, money that’s being used to destroy U-233 rather than develop the LFTR technology that turns it into unlimited energy.
Reason #2: LFTR runs better when it is started with U-233 than any other fissile fuel.
The LiF-BeF2 dissolves uranium better than plutonium, and if you start LFTR on U-233 you start it the way that it will be running in the long run. It takes a lot of neutron absorptions for U-233 to turn into something other than uranium. You have absorb one (U-234), two (U-235), three (U-236), and finally four (U-237, decaying to Np-237) neutrons before you form your first transuranic nuclide. By contrast, if you were to start LFTR on highly-enriched uranium (which would be a very attractive way to dispose of HEU that was produced for nuclear weapons) you only have to absorb two neutrons to get to Np-237. Plus, HEU has a certain amount of U-238 in it, and U-238 is only one neutron absorption away from forming a transuranic isotope (Pu-239). Obviously starting on plutonium, you’ve already put a lot of transuranics in the reactor to begin with.
This may sound a bit confusing, so I’ve made an image to try to graphically depict what happens in the reactor when you start on different fuels. U-233 fissions more favorably, its products have more opportunities to fission, and it has further to go before it forms a transuranic.
Reason #3: U-233 allows us to prove “thorium burning” in a prototype reactor.
One of the basic underlying assumptions of the LFTR is that we can “burn” thorium, in other words, we produce enough U-233 to replace that consumed. By starting a prototype reactor on U-233, we can actually show this in a real-world case early on, rather than trying to extrapolate backwards from a HEU or plutonium startup. It is going to be important to demonstrate that LFTR can really burn thorium, and a U-233 start is the only conclusive way to demonstrate this.
Reason #4: The aged U-233 contains valuable decay products that could save lives.
The U-233 in the DOE’s inventory has had 30+ years to decay. That means that some special isotopes like actinium-225 and bismuth-213 have had time to form in the stored U-233. These actinides, which can ONLY be formed through U-233 decay (not fission) have shown that they could be very promising isotopes for certain types of radiotherapies that could save thousands of lives every year. Even the Inspector General of the DOE has appealed to save the U-233 to save lives.
To summarize, saving the U-233 saves money, lets us demonstrate LFTR’s ability to burn thorium, and could save lives. Please appeal to your congressional leadership to save this precious resource.
Yes it is true; the use of wind power is a constant reminder and an insult to all the millions of people that suffered and died in the world wars. And the reason for this is steel.
Steel was used to kill, maim and terrorize countless millions of people from 1914 to 1919 and 1939 to 1945. It was used in rifles, in tanks, in artillery shells and hand grenades. All of it culminating with the steel birds Enola Gay and Bockscar dropping atomic bombs on the cities of Hiroshima and Nagasaki. Steel and war are forever linked because you simply cannot wage war without steel.
The connection between war and wind power is steel. Practically every wind turbine in the world uses steel. Steel is everywhere in them: in the tower that holds up the turbine; in the gearbox; in the bolts that hold it together, just to mention a few examples. This of course means that wind power always connected with the use of weaponry and war.
Wind power is an insulting tribute to the memory of those who died in the world wars. Turning away from wind power and, in turn, weapons and war should be a true lasting legacy and memorial of those victims.
This is too funny…but is it any less crazy than this?
As evidence that nuclear weapons and nuclear power are connected, and should be opposed jointly, she cited Iran — which has claimed to be processing uranium for peaceful purposes but which U.S. leaders contend is preparing to build bombs.
Remember, any country that claims to want to smelt steel for windmills could be secretly diverting some of that steel to military purposes! For that reason, I propose that we ban the proliferation of steel-making capabilities. If they want steel, they can ship iron ore to an approved steel-making country and we will ship them the finished product of their choice…
Why is uranium-233 so precious? Because in a liquid-fluoride thorium reactor, U-233 represents essentially unlimited energy. How can that be so? Because in a LFTR, U-233 “catalyses” the consumption of thorium, which is natural and abundant. Every kilogram of U-233 represents roughly a megawatt of power in a LFTR–forever.
This might sound like some kind of “perpetual motion” machine, but it’s very much grounded in nuclear reality. U-233 is what thorium turns into when exposed to neutrons. U-233 is fissile, thorium is not. But thorium can capture the neutrons from fissioned U-233 and then replace the U-233 consumed.
So a LFTR, started on U-233, will burn through its original “start charge” fairly quickly, but will continue to form new U-233 at the same rate it’s consumed. So after 1, 10, or 100 years, the same amount of U-233 is there as was there when the reactor got started.
Here’s some images describing the inventory of U-233 that the DOE currently has at Oak Ridge National Lab.
One kilo of U-233 in a LFTR for 10 years: ten megawatt*years (87,600,000 kilowatt*hours)
One kilo of U-233 in a LFTR for 100 years: 100 megawatt*years (876,000,000 kilowatt*hours)
The longer you use U-233, the more it’s worth. Let’s say electricity sells for a nickel per kW*hr.
One kilo, one year: half a million dollars.
One kilo, ten years: 5 million dollars.
One kilo, one hundred years: 50 million dollars.
But the DOE is determined to destroy this precious resource (and we have about 1000 kg of U-233) by mixing it with U-238 and making it worthless for future use. What’s worse, they’re spending hundreds of millions of dollars to make this precious resource into waste!
Why are we sabotaging our future by destroying U-233?
Call your congressman (especially if you live in Tennessee) and beg them to intervene!
As I previously mentioned, I was involved in a proposal to the Advanced Research Projects Agency-Energy (ARPA-E) on an investigation in using liquid-halide reactors (both chloride and fluoride) along with salt-based reprocessing techniques to investigate how to address our issues surrounding spent nuclear fuel–often erroneously called “nuclear waste”.
In a nutshell, we proposed to fluorinate the uranium oxide spent fuel, turning the constituents of the fuel from oxides to fluorides.
Uranium, comprising more than 95% of the fuel, would be removed by further fluorination to uranium hexafluoride (UF6). UF6 is the input material to uranium enrichment processes. Thus, 95% of spent nuclear fuel could be sent to an enrichment plant in this process for additional use. Its isotopic composition would be fairly similar to natural uranium, with a little more U-235 and a lot more U-236.
The fission products would be separated for decay, since they decay fairly quickly to stable nuclides. The ones that were already stable could be partitioned and sold.
The transuranic nuclides like neptunium, plutonium, americium, and curium are the things that give you most of the headaches from a long-term disposal or fuel management approach. This is where most of the effort in the proposal would have gone–into a strategy for burning these up. My favored approach is to put them in a fast-spectrum liquid-chloride reactor and burn them up. If the chloride reactor was surrounded by a thorium blanket we could make new uranium-233 fuel for LFTRs, and the LFTRs wouldn’t produce these transuranics in the first place.
So, here’s the proposal. Mind you, it isn’t the “full” proposal, but rather an 8-page “teaser” that was designed to attract ARPA-E’s interest and keep us from having to go to the trouble of making a full proposal if they weren’t interested. For that, I appreciate their strategy of letting us know early if they didn’t think we would be making an interesting proposal to them.