I have read a little about these since seeing them mentioned in a thread a few days ago. For the uninitiated, these are tokamak fusion reactors which consume more power than they produce but happen, like most fusion reactors, to produce a lot of neutrons. These neutrons could be used to burn spent reactor fuel from conventional fission reactors or burn sub-critical piles of fissionable products like Th-232. My question is what kind of processing is necessary to prepare waste for burning? How different would a fresh vs. spent fuel reactor be? Could you mix the fuels? What are the byproducts? Is any of this stuff known?
This is a complex question to answer, because not all waste or other potential fuel sources are comparable. Most fuel today is designed for a “once through” fuel cycle because reprocessing just isn’t economical. This means it is often contained or packaged in such a way that it is difficult to extract it for processing; whether it could be used in such a form as fuel in a hybrid fission-fusion reactor depends on the type of packaging (i.e. how much neutron absorption occurs and what kind of solid state transformations occur) and the configuration of the reactor. Using natural fertile fuels (fuels that are not themselves fissile but are capable of being transmuted into fissile elements) may turn out to be more economical, although being able to process nuclear fuel into an inert final product may be both politically and ecologically desirable enough to overcome additional processing cost.
The largest advantage of this over normal fission reactors is being able to largely dispense with the costly processing and enrichment processes to generate fuel grade uranium, and the resultant handling of caustic, toxic radioactive waste that is produced during processsing. Additionally, such a reactor would be natural sub-critical; in other words, if you shut down the non-sustainable fusion source, fissile production decays and fission stops, or at least drops to a low level. While this doesn’t make the reactor completely fail-safe, the possibility of a runaway reaction that results in a catastrophic escape into the environment substantially lower. And the resulting waste products will be substantially less radioactive. (There may still be some residual radioactivity from handling fixtures and such, but it may be low level waste that doesn’t require special handling or containment.
Although no one has built a production-level hybrid fission-fusion reactor, the basic theory and concepts are sound and not much of a stretch from existing reactor technology. The ability to simulate the reactions involved and generate a design is well within existing nuclear engineering practice, and is less complex, computationally, than thermonuclear weapon design.
Although you mention tokamaks, the neutron sources I’ve seen for these applications have been either inertial confinement fusion generation (laser pulse ignition) or electrostatic fusors like the Farnsworth device. While the power output of such devices is significantly lower then unity, the objective is the production of fast neutrons that are absorbed by atoms that then decay releasing thermal neutrons. This obviates the need for directly converting the normal fast neutrons produced in the lower energy fusion reactions (which are a large portion of energy released) into usable thermal neutrons that can then be converted into heat energy to power a thermodynamic cycle, one of the hurdles to making fusion power generation feasible once sustainable fusion reactions are achieved. Hybrid reactors don’t require a continuous neutron source and could therefore make use of existing non-continuous fusion reactors.
This would certainly be a good bridging technology between existing fission reactors and true fusion capability (which, at current development rates, is a minimum of forty or fifty years from commercial viability), which could provide for projected energy demands indefinitely, especially if progressively supplemented by solar and wind power.
I mentioned tokamaks because the first article referenced a scheme to burn high-level waste by placing it in a ring around the reactor. The illustration accompanying the article and depicted it as room-sized, but I don’t know if that is accurate. The proposal probably came from tokamak researchers at my alma mater (UT Austin), so it was partially a pride thing :). Are they poor sources of fast neutrons? Also, why do you want fast neutrons? I thought slow neutrons were better for inducing fission.
I don’t really understand why once-through fuel would need much special processing in order to be inserted into the reactor. Is it just a matter of efficiency?
Tokamaks are (in theory at least) very efficient once you get a stable self-sustaining plasma and inherently generate a large neutron flux, but take a lot of energy to get the plasma hot enough to fuse, and end up losing a lot of energy to Bremsstrahlung loses unless the plasma is very tightly and complexly controlled.
For fissile fuels–element isotopes that are inherently unstable and will absorb thermal (slow moving) neutrons–you want thermal neutron sources, either inherently or by the use of a moderator like graphite or heavy water; this can make the reactor easier to control and more efficient. However, for non-fissile but fissionable fuels (those would not ordinarily maintain a chain reaction but will fission due to external neutron bombardment) and fertile materials (those that do not spontaneously decay but will absorb neutrons and convert to fissionable material) fast neutrons are often desirable. Since the energy from D-T fusion results in fast neutrons, any attempt to thermalize them wastes most of the energy that you’d rather put into making fissionable material that generates a shower of thermal neutrons suitable for heating water, helium, or some other working fluid to power a thermodynamic cycle.
This depends on how much processing and separation you need to do in order to make efficient use of the material. The material probably cannot just be inserted as discarded fuel rods; it would have to be removed and processed into some form for effective use, and many forms of current or proposed packaging of fuel for a once-through cycle may make this problematic. For instance, the fuel pellets for a modular pebble bed reactor (PBMR) are contained within a shell of pyrolytic graphite, which acts as a moderator and protects them but may interfere with being sprayed with large amounts of fast neutrons and can cause phase transitions in the moderator that were unintended in the original design. (See “Wigner defects” and “Windscale fire” for an example of what can happen as a result of unintended interactions with neutrons.)
Still, hybrid reactors offer the promise of power generation that is both potentially safer and less complex than pure fission reactors, may make much fuller use of fuel and render it inert, while also significantly reducing the amount of waste produced in the fuel cycle. And it can probably be developed to commercial viability in 5-10 years rather than 40-60 years for true, self-sustaining nuclear fusion.
I am somewhat familiar with PBMR fuel from a layman’s standpoint, but to my knowledge, all the PBMR reactors that exist are research/pilot reactors and probably don’t account for that much high-level waste. I have only seen pictures of CANDU fuel bundles and stored spent fuel from LWRs. Is it possible to add some of this type of spent fuel to a bunch of thorium 232 to burn it? IOW, could you combine power production and LWR waste processing in one reactor or would the functions be different enough that you would want a separate reactor for each? Also, how would you extract the power from the reactor? Could you use high temperature inert gas like you could with a PBMR? Are there any serious projects to develop this technology? Are the challenges more on the engineering side or the science side (or the regulatory side)?
I also have to say that I have been geeking out about this since you mentioned it.