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posted by janrinok on Tuesday August 11 2015, @03:37PM   Printer-friendly
from the power-for-the-people dept.

Nuclear fusion... ten and a few years away?

Advances in magnet technology have enabled researchers at MIT to propose a new design for a practical compact tokamak fusion reactor — and it's one that might be realized in as little as a decade, they say. The era of practical fusion power, which could offer a nearly inexhaustible energy resource, may be coming near.

Using these new commercially available superconductors, rare-earth barium copper oxide (REBCO) superconducting tapes, to produce high-magnetic field coils "just ripples through the whole design," says Dennis Whyte, a professor of Nuclear Science and Engineering and director of MIT's Plasma Science and Fusion Center. "It changes the whole thing."

The stronger magnetic field makes it possible to produce the required magnetic confinement of the superhot plasma — that is, the working material of a fusion reaction — but in a much smaller device than those previously envisioned. The reduction in size, in turn, makes the whole system less expensive and faster to build, and also allows for some ingenious new features in the power plant design. The proposed reactor, using a tokamak (donut-shaped) geometry that is widely studied, is described in a paper in the journal Fusion Engineering and Design, co-authored by Whyte, PhD candidate Brandon Sorbom, and 11 others at MIT. The paper started as a design class taught by Whyte and became a student-led project after the class ended.

[...] While most characteristics of a system tend to vary in proportion to changes in dimensions, the effect of changes in the magnetic field on fusion reactions is much more extreme: The achievable fusion power increases according to the fourth power of the increase in the magnetic field. Thus, doubling the field would produce a 16-fold increase in the fusion power. "Any increase in the magnetic field gives you a huge win," Sorbom says. While the new superconductors do not produce quite a doubling of the field strength, they are strong enough to increase fusion power by about a factor of 10 compared to standard superconducting technology, Sorbom says. This dramatic improvement leads to a cascade of potential improvements in reactor design.

They are calling it an affordable, robust, compact (ARC) reactor. Presentation [PDF].

ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets [abstract]


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  • (Score: 2) by Zinho on Tuesday August 11 2015, @04:34PM

    by Zinho (759) on Tuesday August 11 2015, @04:34PM (#221311)

    What they mean when they say 10 years... [xkcd.com]

    Is someone familiar enough with the design of these fusion reactors to explain how they intend to get the heat out of the reaction chamber? Eventually the point of this exercise is to boil water to spin a turbine. From what I've seen, we've so far had a bunch of difficulty even getting the reaction to be self-sustaining; extracting "waste heat" seems to be a "we'll cross that bridge when we get to it" sort of issue.

    So, Lentils, any insights?

    --
    "Space Exploration is not endless circles in low earth orbit." -Buzz Aldrin
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  • (Score: 3, Interesting) by Immerman on Tuesday August 11 2015, @05:35PM

    by Immerman (3985) on Tuesday August 11 2015, @05:35PM (#221335)

    My general understanding is that extracting heat is generally left as a "we'll cross that bridge when we get to it" sort of issue because it's (relatively) trivial to do. The technology for moving heat around has been pretty well refined over the past couple of centuries. There may be some material science challenges, but basically any cooling system suitable for a fission reactor should translate fairly well to a fusion reactor as well - the neutron flux per watt is only a few times higher after all.

    In addition, for many fusion devices the eventual goal is proton-boron fusion, in which case virtually all of the energy is released as high-speed helium-4 nuclei from which the energy can be electrostatically extracted, virtually eliminating heat from the energy-producing reaction, and relegating cooling systems to dissipating the waste heat produced by the control and containment systems.

    • (Score: 3, Interesting) by Zinho on Wednesday August 12 2015, @01:42PM

      by Zinho (759) on Wednesday August 12 2015, @01:42PM (#221673)

      Thanks for the reply. Part of the reason I was asking is that fusion reactors have some unique challenges that don't apply to fission plants. The fusion containment bottle is very "hands-off": the plasma (while very energetic) doesn't have a lot of mass, and if the plasma touches anything it cools off to the point that that the reaction stops. In contrast, the fuel rods in a typical fusion reactor don't much care if you snuggle a coolant loop up against them beyond how well the coolant absorbs/reflects/slows down any neutrons flying around.

      I was able to do some research, and it seems that the plan for how to collect energy from a tokamak is to run pipes of liquid Lithium through the containment vessel walls and use them to capture high-energy neutrons created by the fusion. Since the neutrons are at very high temperature, the neutron capture itself is the primary method of heat capture for the reactor. After the neutrons are captured the Lithium undergoes beta decay with Helium and Tritium as byproducts, which serves as a sustainable source of Tritium to feed the reactor. The waste helium you mention is retained temporarily in the containment bottle to keep the temperature up, but is eventually cycled out as a waste product (how they do this without shutting down the reactor is still a mystery to me).

      That neutron capture trick was not obvious to me, and kinda blows my mind. It's a great solution for the "no touchee" problem, and certainly gets the job done. As you said, this is a solved problem which explains why it doesn't get talked about much.

      --
      "Space Exploration is not endless circles in low earth orbit." -Buzz Aldrin
      • (Score: 2) by Immerman on Thursday August 13 2015, @05:17PM

        by Immerman (3985) on Thursday August 13 2015, @05:17PM (#222404)

        Okay, I understand your question better now, perhaps I can expand a bit on what you've learned (which was quite interesting, I hadn't known the details). In a fission reaction a fair fraction of the reaction energy remains trapped within the solid fuel since the large nucleus fragments can't escape and their kinetic energy is immediately thermalized, and thus your coolant must be in contact with the fuel rods to avoid having them melt down into an uncontrollable pool of fuel. Or alternately, in a liquid-fuel reactor, the fragments are much more free to move and the energy is thermalized over a larger area, reducing "hot spots". And obviously a meltdown is a non-issue when the fuel is already molten. Same basic idea though, you've just dissolved your fuel into the coolant to simplify things.

        By contrast, in a plasma-based fusion reaction nothing is restraining the fusion products except the magnetic containment field that's barely able to adequately contain the source plasma. Dump a bunch more kinetic energy into a product particle and most can escape containment without much trouble. To say nothing of free neutrons which tend to make up the majority of such particle radiation and are largely immune to magnetic containment due to their lack of charge (they do have a magnetic moment, but magnetically containing magnets is a very different mechanism than charged particles)

        In most* reactions though, fission or fusion, the majority of the energy tends to escape as non-particle radiation: high-energy photons in the X-ray and gamma ray spectrum that can pass through a significant amount of matter before being absorbed. That's what all the lead shielding around a nuclear reactor is there to stop. Particle radiation, even neutrons, is unlikely to penetrate even a thin piece of sheet metal. Most of the high energy photons though will easily pass through plasmas, fuel rods, magnetic containment, and the structural walls of the reaction vessel. They mostly get absorbed by the shielding and/or the coolant itself, and the heat is then put to work.

        *Fusion is a little different in that most reactions tends to create a lot more free neutrons per watt, and thus capturing them is more important to energy recovery. There are also some reactions, such as the proton-Boron I previously mentioned, where there is very little photon radiation as well, and thus virtually all of the energy is released as kinetic energy of the products. p-B being special in that all the products are identical He4 nuclei with a very narrow range of kinetic energies, and thus electrostatic energy conversion is extremely viable and most of that pesky inefficient heat can be avoided altogether.