Neutrons come out of a fission event at high speed, about 20% the speed of light. These are called fast neutrons. In almost all nuclear reactors we've built, we deliberately slow these neutrons down with a moderator material (which must have light atoms in it: things like graphite, water, beryllium). It's much easier to get a pile of nuclear fuel to chain react when you slow neutrons down, largely because Uranium-235's liklihood of absorbing a slow neutron is much higher than its liklihood of absorbing a fast neutron.
(Aside: slow neutrons are often called "thermal" neutrons because they're in thermal equilibrium with the atoms around them. At room temperature they're going 2200 m/s, which corresponds to an energy of about 0.0253 electron-volts, slowed down from 2 million electron-volts when they emerged from fission)
A fast-neutron reactor ("fast reactor") is a reactor where the neutrons that emerge from a fission event are kept moving fast, simply by not putting a moderator into the design. These reactors use heavy nuclei or very low density material as coolants (sodium metal, lead-bismuth eutectic, helium, etc), so as to not slow down any neutrons. It takes much more fissile fuel to get a pile of nuclear fuel chain reacting in this configuration, but once you get it going, you get some very nice benefits.
Once started, fast neutron chain reactions have a vast surplus of neutrons going around. This is for two main reasons: (1) The number of neutrons emitted per fission increases dramatically as the neutron speed increases above a threshold around 1 MeV, and (2) parasitic absorption of fast neutrons is low for all nuclides. With so many extra neutrons around, you can afford to put extra "stuff" in your reactor, such as spent nuclear fuel (nuclear waste) or extra fertile material (uranium-238 or thorium-232).
You can make these reactors into breeder reactors (which convert U-238 or Th-232 into fissile fuel in such a way that could power the entire of humanity 10x over for a few million years, at least, using known resources), or you can make these reactors into burner reactors, whose job it is to transmute used fuel from other reactors into shorter-lived material (similar to what OP's article is about, but much more practical).
A definitive guide to understanding transmutation of spent fuel in fast-neutron reactors is put out by the UN's International Atomic Energy Commission [TRS-435]:
A handful of fast-neutron reactors have been built, starting with tiny single critical mass assemblies in the 1940s, to the first true demo of breeding more fuel than you consume in EBR-1 in 1951 to the EBR-2's safety demo weeks before Chernobyl showing that the reactor could passively shut down and cool itself without any human intervention or external power or control rods inserting, to India's PFBR fast reactor that's been under construction for the past 18 years.
Only the Russians have successfully operated fast reactors commercially, via the BN-350 and BN-600. In general, fast neutron reactors are considered more complex and expensive to build and operate than traditional water-cooled thermal neutron reactors.
We once thought uranium was very scarce, and so we put a lot of money into fast breeder reactors. Then it turned out that there's lots of uranium and fast reactors are largely on hold internationally. France just announced the cancellation of its national fast reactor program (ASTRID). Russia delayed their next BN fast reactor, saying their VVER (slow neutron) reactors are bout 4x cheaper. China and India are pushing forward. The US shut down its last fast reactors (EBR-II and FFTF) in the early 1990s. Many nuclear startup companies are now exploring options to get back into fast reactors.
The US DOE is putting forth a major project to build a new fast reactor very similar to the FFTF, but in Idaho instead of Washington and with metal fuel rods instead of oxide ceramic ones. The project is called the Versatile Test Reactor (VTR) and is actively discussed in current nuclear news.
No. Weaponization of nuclear fission technology was completed in 1944. Plutonium production plants of similar design to those used in the Manhattan Project were used throughout the Cold War (at Hanford and Savannah River). There are myths about various nuclear technologies being cancelled because they "couldn't be weaponized" (usually talking about molten salt reactors), but these myths are quite wrong.
Fast reactor tech hasn't been popular because it's more expensive than regular old water-cooled fission reactor tech. To keep neutrons going fast you have to use exotic coolants like sodium metal. Since you don't want to mix a chemical hazard with a radiological one, you insert an additional intermediate heat transfer loop into the system. Hot radioactive sodium transfers heat to hot non-radioactive sodium which transfers heat to water which boils and turns a turbine to crank a generator to push electrons around to provide low-carbon service to human quality of life.
Can you please not call sodium "exotic"? Doing so adds to an irrational fear of relatively benign technology.
Molten sodium is easy to pump, non-corrosive to many metals including steel, and a good heat transfer medium. The chemical industry uses it routinely. In a nuclear reactor, it would also chemically sequester a particularly annoying fission product (I-131) in case of a fuel element leaking. Unpressurized, it is good up to 800C before it boils.
By contrast, water at 300C will corrode most steels. At higher temperature, it will corrode zircalloy, forming hydrogen. And it really wants to be a gas, hence the giant containment buildings around LWR.
What did you say? "But sodium explodes in contact with water?" That's easy to solve, just keep the water out. A sodium cooled reactor should be coupled to a supercritical CO2 turbine instead of a steam turbine. That removes the problem of leaking and exploding heat exchangers.
(Footnote: I like molten salts better than sodium. But sodium is still better that water.)
Sodium coolant is exotic from an operational point of view. Exotic doesn't mean dangerous. Sodium systems from a probabilistic risk statement are usually 100x safer than pressurized systems because of passive decay heat removal capabilities. Exotic means "different from normal." So far, all operating sodium-cooled plants have suffered from operational and economic challenges due to sodium's characteristics. This is solvable with experience, but it's not easy.
Have you ever studied a detailed procedure for heating up a large sodium valve from the solid sodium phase to the liquid? If not I highly recommend doing so. It's truly remarkable. Molten salt systems are the same level of complexity, but much more radioactive in the primary system. Remote maintenance of this stuff is totally doable, but it sure as hell is exotic. Rickover's characterization of sodium systems stands to this day.
Traditional sodium systems with void-swelling resistant structural materials may not go to high enough temperatures to strongly justify the tech development needed for a sCO2 balance of plant in the short term. Sodium-water steam generators with leak detection have worked well-enough to build a few more of them while sCO2 turbomachinery gets figured out at the 100s of MW scale.
To the contrary. You can use these things to breed very dangerous material for bombs. Hence most industrial countries (say Germany) are nudged by the superpowers to not implement such devices. If Iran would build one, for instance, it would immediately be bombed by Israel or become a nuclear superpower in a few weeks of operation.
You can breed weaponizable fissile material in fast reactors. But by far the easiest way to make bomb material in a reactor is what they did in the Manhattan Project, using graphite moderated thermal neutron production reactors (not fast reactors). And it's even easier still to just build gas centrifuges and enrich uranium without needing a reactor at all.
Becoming a nuclear superpower weeks after operation of a fast-neutron reactor is a bit of an overstatement.
A reactor with a fast, as opposed to thermal, neutron spectrum. Among other things, implies that it must be cooled by something else than plain water, which is is a neutron moderator (that is, slows down the neutrons to thermal velocities). Often the coolant of choice is a metal like sodium or lead.
Fast reactors enables breeding, that is producing more fissile material from the fuel than is consumed. They are also better at burning up various isotopes that thermal reactors have trouble with.
Yeah, no. We should not exchange large quantities of relatively slowly decaying radioactive materials (hence the long half-life) hidden somewhere deep underground with a very dangerous device somewhere close to population.
The long half-life [actually the mixture of lengths of half-lives] is really the problem. You've got stuff that's super-hot [fast decay] and so very dangerous mixed with much slower-decaying [so not as dangerous] stuff that needs to be isolated for a super long time. If you run it through a fast reactor/reprocessor, you can use most of the energy and be left with mostly just medium-duration stuff, that will burn out in a reasonable timescale for underground storage, say 1000 years.
We have many dangerous devices extremely close to the population already. Cars kill more people in a year than all nuclear reactors have killed in the whole time of their existence. Should we bury the cars underground instead?
A very dangerous device. I suppose you have heard about that irresponsible experiment conducted in 1986 where they simply turned off all cooling to a reactor? Remember what happened?
No, not Chernobyl. I'm talking about EBR-II, a fast reactor. Nothing happened. That's how dangerous the device is!
You mean a reactor that is cooled with Sodium, a material that, according to Wikipedia, spontaneously explodes when it comes into contact with a rare substance called water?
Not spontaneously, only when contacting water. But you see, there is no water in such a reactor. And it can't get in, because if water can get in, other stuff can get out, and stuff getting out of a reactor is a disaster anyway, so the thing is a closed system. Even though, since sodium cooled reactors are not pressurized, stuff wouldn't necessarily come out of a small leak.
On the other hand, water at 300C and 200bar will explode spontaneously without coming into contact with anything. A steam explosion is no fun, according to Wikipedia.
