Recently it was reported in Defense Daily that the White House Office of Management and Budget has moved to remove all research in fast-spectrum fission reactors from the DOE budget. This in turn prompted an outcry from the Secretary of Energy, Stephen Chu, who stated:
“Prohibiting research and development on fast reactors under the fuel cycle research and development budget line effectively selects the once-through fuel cycle as the only fuel cycle to be pursued in the United States”
With all due respect to the Secretary, there is another option. This is precisely the fork in the road that visionaries like Wigner and Fermi saw back in the late 1940s. If you pursue closed-fuel cycles based on uranium (and predominantly uranium-238) then Chu is right–the fast spectrum reactor is the only option for a closed-fuel cycle, since only in the fast spectrum can U-238 be productively consumed.
But…if you consider thorium as the basic fuel, as Wigner advocated, then you can have a closed fuel cycle WITHOUT a fast spectrum reactor, since thorium can be productively converted to energy in a “slow” neutron reactor (called a thermal-spectrum reactor) and thorium is the only option for this approach.
If the White House wants a transformational nuclear technology, that is not a fast reactor, closes the fuel cycle, increased safety and efficiency, opens the doors to hydrogen and chemical fuel production, is capable of using its waste heat for desalination, and is highly proliferation-resistant, then may I respectfully submit:
The Liquid-Fluoride Thorium Reactor!
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)
Several weeks ago, an engineer who has been involved in ocean-thermal-electric conversion (OTEC) technology wrote a piece in the Huffington Post quite favorable to thorium, based in large part on the recent article in WIRED magazine.
I enjoyed the article, and it caused me to reflect about my past enthusiasm about OTEC technology, and how at one point I thought it would play a large role in our energy future. It also caused me to reflect on how much my exposure to OTEC later influenced my concepts of LFTR and how it might be deployed and used.
For those who aren’t familiar with how OTEC works, let me give the briefest lesson in thermodynamics. Thermodynamics teaches us that wherever two “reservoirs” of energy exist, separated by a temperature difference, there is the potential to extract useful work. The efficiency at which that work can be extracted is directly proportional to that temperature difference.
I had just graduated from Utah State with a mechanical engineering degree when I picked up a copy of Marshall Savage’s book “The Millennial Project” and headed off to California for the summer to work on a rocket project before starting graduate school. “The Millennial Project” (TMP) is an incredibly ambitious text that argues for a step-by-step expansion of human presence into space, and it starts by saying we should build huge floating ocean colonies powered by OTEC.
OTEC utilizes those thermodynamic principles I previously mentioned by using the temperature difference between the cold waters of the deep ocean, where the temperature is only a degree or so above freezing, and the warm surface waters, where the Sun’s energy has brought the water temperature to about 80 degrees Fahrenheit. To use the expression we learned in thermodynamics for energy conversion efficiency, we first have to convert these temperatures from Fahrenheit to Kelvin. Let’s assume the deep cold water is 34 degrees Fahrenheit (274 Kelvin) and the warm surface water is 80 degrees Fahrenheit (300 Kelvin). The maximum efficiency that can be attained is (1 – (274/300)) = (1 – 0.9133) = 8.6%
So in the most perfect case, only 8.6% of the energy can be converted to work (or electricity) between these two sources. No problem, said the OTEC advocates, we have an unlimited amount of cold water and hot water! Let’s just pump more! So OTEC involved moving a LOT of cold water from the deep ocean to the surface. But the most exciting thing I learned in Savage’s book was that the deep ocean waters contained lots of nutrients, and so when they were brought to the surface they caused all kinds of ocean life growth to flourish.
This led to glorious depictions of floating ocean colonies, self-sufficient in energy, ecological oases of life and aquaculture, perhaps producing so much that they could be economically self-sufficient. It seemed so wonderful it was almost too much to believe.
I was motivated to act so I tried to read all that I could on OTEC and what state the technology was in. Of course, the real efficiencies that could be obtained by OTEC were substantially below that of the thermodynamic limit (which is pretty common in all real systems) but the geographic limitations of OTEC were much more severe than I thought.
This wasn’t exactly something you could park off the coasts of cities around the US and make power. You needed really warm water–tropical water–and so it was much better suited to the islands of the Pacific or the Caribbean. It seemed like an intriguing answer for them, but not very applicable to New York or Seattle. Ironically, the cold water was easy to find–cold ocean water can be found anywhere on Earth if you go deep enough, but the warm water was far less commmon.
So I started thinking about how to “artificially” make the water warmer. First I was thinking about huge floating solar thermal generators, that would focus the Sun’s energy on a concentrator and use the cold ocean water to achieve conversion efficiencies on the order of 50%. Such systems sounded great if the skies were clear, but how often would that be the case at sea? Furthermore, the areas needed to generate power were vast, just as they are for terrestrial solar power, but the ability to use the deep cold ocean water to achieve high efficiencies and to create these ecological oases discussed in Savage’s book seemed so compelling.
Looking back, I’m amazed it took me so long to connect two utterly compatible positions to one another.
I had known about LFTR technology for five years before I finally put two pieces together in my mind–the ability of LFTR to make high temperatures from fission and the profound value of the deep cold ocean water–in late 2005. It came about because I has been working in Mississippi with a church group doing post-Katrina cleanup and I was trying to figure out how to deploy LFTR in a way that would be impervious to weather and hurricanes and earthquakes and so forth.
It then occurred to me that the logical place to deploy a LFTR would be in a submersible, where the environment around the submersible stays nearly constant even as storms rage overhead or earthquakes rumble underneath. I went on to discover that I was not the first person to propose submersible power reactors, but I was the first to propose that they be LFTRs and take advantages of the profound operational improvements that LFTR allowed (very long run times between reactor shutdowns, easy changing out of fuel, etc)
This vision of fleets of LFTRs, parked underwater around the world, providing electrical energy to coastal cities via underwater HVDC cables and using the deep cold ocean water to cool the reactor (not directly of course) has consumed me ever since. I have also realized since then that the rejected heat of LFTR is em
inently suitable to use for ocean desalination, making it possible that submersible LFTRs can be a huge source of fresh water for coastal communities.
This vision, which is still in development, owes its origins in large part to my initial interest in OTEC, and the directions of thought it led me. For that, I will always be grateful that I was exposed to that technology.
There is a fundamental fork in the road when it comes to plotting an energy future, a fundamental difference between two approaches, grounded in the very principles of nuclear physics. It was recognized at the dawn of the nuclear age by luminaries like Eugene Wigner and Enrico Fermi. This division went on to be implemented in national policies and national laboratories.
It is the basic difference between abundant thorium and abundant uranium, and how to use them.
Abundant thorium needs a fissile starter, but once started it can “burn” indefinitely in a thermal spectrum reactor. Thermal spectrum reactors are the only kind of reactors that can be built in their “most reactive” configuration, which is a very important safety feature. When a reactor is built in its “most reactive” configuration it means that any change to its geometry or its materials causes it to shut down by becoming less reactive.
Abundant uranium (consisting overwhelmingly of uranium-238) also needs a fissile start, but once started it can burn indefinitely only in a fast spectrum reactor. A fast spectrum reactor is one where every effort is made to keep neutrons at high energies and to prevent them from slowing down very much from their “birth” energies. This means very careful material choices must be made in the reactor to keep low atomic-weight materials (like hydrogen) out of the reactor. It also means that it is physically impossible to build a fast-spectrum reactor in its “most reactive” configuration. There are other configurations that are more reactive, and reactor designers must be careful to avoid these.
Burning abundant thorium means converting thorium into uranium-233 and burning it to make the neutrons to make more uranium-233.
Burning abundant uranium (U-238) means converting it to plutonium-239 and burning it to make the neutrons to make more plutonium-239.
In the thermal spectrum, uranium-233 makes enough neutrons in fission to keep the fire burning. Plutonium-239 doesn’t. In the fast spectrum, both fuels make enough neutrons to keep the fire burning, and the faster the spectrum, the more neutrons Pu-239 will produce in fission.
The original incentive to building a fast-spectrum reactor burning uranium-plutonium was to make not just enough plutonium to keep things burning, but lots of extra plutonium to start other reactors or for other purposes. There is an additional factor: fast-spectrum reactors take a lot more fissile startup material than thermal-spectrum reactors, because fissile material in fast reactors is less likely to cause fission in the first place.
Now, nearly all of the reactors in the world today do something else entirely. They burn up the very small fraction of uranium that is naturally fissile (U-235) in a thermal-spectrum reactor. They don’t worry about the fact that this is especially wasteful of uranium and could be considered unsustainable. Economics currently favors this approach and so they take it.
We in the thorium community, and most in the fast-reactor community, point out the value of using nuclear fuels like uranium and thorium sustainably, and in a way that will last a very long time. But we need to recognize that this is not a priority of the overall, U-235-burning nuclear community.
To attract their attention, we need to show how operating a thorium-burning, thermal-spectrum reactor like LFTR, or a uranium-burning, fast-spectrum reactor like the IFR, would make good, bottom-line sense to them and lead them to an improvement in profitably and economic performance over the U-235-burning light-water reactors of today.
So it is useful to compare how operations of a LFTR or an IFR would improve over a light-water reactor (LWR), and is there a potential for improvement?
The first item that LFTR or IFR supporters would probably like to hold up is the vastly increased fuel economy of these reactors. We can extract FAR more energy from thorium or uranium than a LWR can. But unfortunately, that is not a very big deal to the nuclear operators of the world. Even with recent cost increases in uranium, the cost of the fuel for an LWR is really pretty small versus the cost of operating the reactor. Even if uranium prices went way up, fuel costs are still pretty trivial compared to overall costs. So I don’t think this would make a lot of difference.
The next item that LFTR or IFR supporters would probably like to hold up is a vast reduction in the waste stream from these reactors. Because the fuel is burned up much more, there is far less waste. But again, at least in the United States, nuclear operators pay a tax of a tenth of a penny on each kilowatt-hour of electricity they produce using nuclear power. These taxes, which amount to nearly a billion dollars per year to the US Treasury, supposedly are set aside to pay for the eventual disposal of spent nuclear fuel in a way that is the US government’s responsibility, not the nuclear operators. So their response to less waste would probably be: “so what? We pay the tax and it’s the government’s problem.”
The real issue that LFTR or IFR supporters SHOULD be talking about is the cost of getting new nuclear plants built. Because nuclear plants are great once they’re built and paid for and operating. They make lots of money. But getting a new one build is a daunting concept for a utility. Capital costs, regulatory uncertainty, and an uncertain future market all combine to scare the pants off utilities who are thinking about new reactors. What is the story that LFTR or IFR have to offer?
LFTR has a great story to offer. LFTR operates at atmospheric pressure–there is no high pressure fluid (water) in the reactor that has to be held in at 3000 psi and under 9 inches of nuclear-grade steel. This in turn means the containment can be much smaller and closer fitting. Furthermore in an accident scenario the fuels in a LFTR go to the coolant, rather than trying to force the coolant to the fuel like we do in an LWR. Natural forces combine to lead to passive shutdown systems like the drain tank. All of these lead to a simplification of safety systems with even more safety rather than less. The fuel is chemically stable, the coolant is chemically stable, and there is no “driving force” trying to release radioactivity to the environment. Fluoride salts can carry LOTS of heat per unit volume, making them very efficient at moving the heat of the nuclear reaction to the coolants of the power conversion system.
IFR also operates at atmospheric pressure, but it uses an extremely chemically reactive coolant–liquid sodium metal. Because the coolant is so reactive, everything needs to be kept under a protective atmosphere of inert gas. Furthermore, liquid sodium doesn’t hold as much heat per unit volume as fluoride salt, so it takes a lot more liquid sodium to carry away the heat of the reactor in an emergency shutdown scenario. For this reason, IFRs are designed to sit in a huge pool of liquid sodium metal, so that there is enough to keep the solid rods of the reactor bathed in liquid sodium in all scenarios. This huge pool of liquid sodium makes the reactor physically very large for its power generation. It also means that there is more sodium to react in gross accident scenarios like a crack in the overall sodium vessel.
Both reactors can keep the overall reactivity of the reactor fairly constant over time, which is a big advantage over LWRs. LWRs “burn down” their fuel and their reactivity changes a lot from when the fuel is first loaded til it is removed. But LFTRs and IFRs generate replacement fuel about as fast as it is consumed, and overall reactivity can be kept fairly constant.
(next, can online reprocessing be economically advantageous for a utility? How?)
I love it when Stephen gives stupidity the treatment it deserves:
Thanks to monitoring the Twitter feed for “thorium” I find some really wonderful things on the net, like this–on their blog, “Global Artwork” did a size comparison of the amount of firewood, coal, oil, uranium, and thorium it would take to meet all of the world’s energy needs. The pictures speak for themselves!
Regular readers of this blog know that saving the uranium-233 is a very high priority of our small but enthusiastic thorium community. For those who are new, please read some or all of these posts:
EfT Forum: Save the Uranium-233!
Saving this precious substance, and saving the $300 million that will be WASTED trying to destroy it, would go a long way towards helping us have the thorium-powered future that we dream about.