Comparing LFTRs and IFRs to LWRs (part 1)

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?)

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