According to their paper [0], the entire vaccum vessel is considered disposable and might survive only a few months. (It's also explicitly an experimental reactor meant to study neutron radiation damage, so they're hardly claiming it's a practical commercial design. But I think you're right, that the problem is basically unsolved).

    This allows the vacuum vessel to be replaced quickly,
    mitigating first wall survivability concerns, and permits a
    single device to test many vacuum vessel designs and
    divertor materials.
    
    [...] The replaceable vacuum vessel is made of corrosion-
    resistant Inconel 718, which maintains high strength and
    corrosion resistance at elevated temperatures.
    
    [...] Little research has been done regarding how Inconel
    718 responds to the irradiation environment of a fusion
    device [82]. However, studying the response of components to
    fusion neutron effects is part of the motivation for
    ARC. [...] It is unknown if Inconel 718 would behave
    similarly in a fusion neutron spectrum, but one expects the
    vacuum vessel would survive for at least 6-12 months (15-30
    DPA).

[0] http://arxiv.org/abs/1409.3540

Thanks for pointing that out. I don't think I conveyed myself well in my original post, this is a fantastic development but I feel like the article didn't emphasize that the development is really about moving towards the material science that we need. It felt like the article sensationalized the energy output per unit size and didn't focus enough on the material science challenges of the problem (as well as how much this will aide in their solution).

The authors clearly recognize this, I just wish the public understood it better.

The article was written by MIT's public-relations office, so interpret that as you like.

> "The challenge is the materials that compose the reactor become brittle and highly radioactive far faster than usable. Controlled fusion as an energy source isn't just challenging because fusion is difficult, it's also challenging because materials don't handle neutron bombardment well."

Isn't that problem solved with aneutronic fusion?

https://en.wikipedia.org/wiki/Aneutronic_fusion

Of course we should focus on getting fusion working (working in the sense of providing more energy than we put in), regardless of whether we use the aneutronic approach or not, but as the field develops I'm optimistic we'll find ways to have robust fusion reactors.

I can honestly say I'd never heard of that before you posted your link. I don't know enough to form a valid opinion here.

"I don't know enough to form a valid opinion here.".

Beautiful. This is pretty much the last thing I ever expected to read on the interweb.

Thank you for your honesty.

I'm not an expert in this field either, but I have been following the development of the Focus Fusion reactor being developed by Lawrenceville Plasma Physics, which is where I was introduced to the concept of aneutronic fusion. This video is a fairly decent introduction to their approach (note that the plan is to use the standard deuterium fuel first as it's cheaper and easier to work with, but the design can potentially support an aneutronic fuel such as pB11 in the future):

https://www.youtube.com/watch?v=O4w_dzSvVaM

The problem is, low neutron fusion reaction cycles require a very high pressure/temperature/plasma density to reach peak reactivity, substantially higher than D+T and D+D fusion.

If we do eventually progress to aneutronic fusion test reactors, we will first have to get neutron-emitting fusion reactions working to the level of substantial net power generation, even if a given test reactor only lasts a few months before neutron irradiation becomes unmanageable.

There are also reactions with fewer overall neutrons such as D-D fusion, which one of my professors was a large proponent of.

It seems like they address this issue as well:

"Another key advantage is that most of the solid blanket materials used to surround the fusion chamber in such reactors are replaced by a liquid material that can easily be circulated and replaced, eliminating the need for costly replacement procedures as the materials degrade over time. "It's an extremely harsh environment for [solid] materials," Whyte says, so replacing those materials with a liquid could be a major advantage."

It's been half a decade since I studied anything fusion related but IIRC regardless of using a liquid blanket you still have to have the machinery (powerful microwaves, particle injectors, etc) and the inner wall present.

Liquid breeder blankets are definitely the way to go but I don't think they are the full solution. (They are super neat because you can use them to make more fuel for fusion through neutron capture, so not only do you reduce radioactivity but you also reuse the energy in a useful fashion)

Embrittlement and neutron activation are already well understood and perfectly tractable problems, since they affect the fission power industry to the same degree. That comes down to the materials you use to construct your reactor.

There are thre main problems, only one is entirely engineering the other are on the border of engineering and physics. The most pressing problem is one of plasma containment lifetime. The longer you can contain a plasma the more feasible it is to operate a fusion reactor as a power source. Right now that time is measured in seconds. An equally important problem is plasma temperature, the higher the plasma temperature the higher the fusion rate, and the farther beyond break-even the fusion reactions will be. ITER is "designed" for containment times of up to a thousand seconds, about 15 minutes, though in reality it is likely to fall far short of that. And even that is a far cry from what would make fusion power production feasible engineering wise. As it happens, tokamaks are not necessarily particularly well suited to long containment times, but there hasn't been sufficient funding to thoroughly research all of the most promising designs (such as stellarators, gas-dynamic mirror fusion, spheromaks, etc.). It seems extraordinarily unlikely that the first and most easily constructed plasma confinement system would be the most capable one as well.

The main purely engineering hurdle is cost effectiveness. Which is about being able to construct a plasma containment and heating system and all of the other components of a fusion reactor (some straightforward, some not) within a reasonable cost, not multiple billions of dollars for a single 1 GW plant. There breakthroughs in superconducting wire and so forth can have an enormous impact on the economic viability of fusion power, so it's a hugely important step.

One thing mentioned in the article was another advance in this design, where some of the materials that deal with the most intense neutron bombardment are replaced by a liquid that can be pumped out and circulated. Presumably there's still a containment vessel for this liquid that is a maintenance pain, but they do seem to be working on the problem.

The liquid blanket technology is nothing new or unusual and, from what I recall, seems to be generally agreed as the way to go. You are exactly correct about the containment vessel, and there is also the raw machinery needed to maintain the containment field and the reaction. I think the lifespan for these things is on an upper bound of 3 years, and I can't remember if that is at power plant or experimentation scale (experimental reactors run way less frequently and create less radiation than a power plant would).

It's an interesting problem to me because fusion is largely solved and well understood from the physics perspective (which is what most people consider the challenging esoteric part), the remaining challenge is a much more 'mundane' task of developing economic methods to create the needed materials.

> "It's an interesting problem to me because fusion is largely solved and well understood from the physics perspective (which is what most people consider the challenging esoteric part), the remaining challenge is a much more 'mundane' task of developing economic methods to create the needed materials."

On one level, yes, it's a well understood problem. I mean when you get to the point when Make can teach you how to build your own fusion device I think it's safe to say we have a fairly decent handle on what's involved:

http://makezine.com/projects/make-36-boards/nuclear-fusor/

However, the challenge of producing a nuclear fusion device that generates more energy than it consumes is still a technical challenge. Perhaps it'll just be a question of better materials, but from a layman's perspective the plasma confinement/control aspect does not appear to be a completely solved problem.

Not a nuclear physicist, so this is my armchair layman's "why don't they just..." analysis.

But doesn't nuclear fission work off of neutron bombardment? Why not build a hybrid reactor where the "shield" undergoes fission while the core is generating fusion and perhaps they can produce a useful amount of power, perhaps with the fission "shielding" powering the magnets or some such?

China's actually working on a reactor like that: http://www.scmp.com/tech/science-research/article/1840219/ch...

Advantages: the fusion side doesn't have to quite hit breakeven, and on the fission side you can handily break apart any transuranic atom, even more effectively than fast reactors. It'll destroy any transuranic waste (which is most of our nuclear waste, and almost all the long-term waste), or directly fission thorium or unenriched uranium.

On the other hand, fission people don't like the complexity and fusion people don't like introducing all that hassle with fission products and decay heat. But it may well find a niche for nuclear waste elimination at least, and produce a lot of energy along the way.

What would be the advantage of a hybrid like that over a straight-up fission reactor?

If done correctly you fusion fission everything to pure iron?

checks the box for fusion?

more seriously, it could split the energy production to some ratio of fusion/fission and produce less hard to handle waste. Since the fusion reaction provides the neutron source, it may not be as necessary to provide as fissionable material which may not be able to meltdown?

I dunno, I have no knowledge or experience at all, this is my "if I was a sci-fi writer this is a thing I would put into a story"

The light elements that you prefer for fusion, have a lower absortion rate for neutrons than the heavier elements, which you don't prefer to absorb neutrons. In other words, to get usable fusion, you need so much neutrons that a lot of material in/out of the reactor will get too radioactive.

This strikes me as the general problem with nuclear power: it works so well -- in a sense -- that it tears apart the fabric of matter itself, making its components, fuels, wastes, etc. absolute hell to handle. It's tough to use matter to contain matter-decomposition machines.

There's also the problem of getting rid of the ash. Conventional tokamaks have divertors at the base (and sometimes the top) to get the spent plasma out of the chamber. These plates inevitably come into contact with the plasma, and we just don't have the materials to withstand that yet. We're not unimaginably far off, but it's unsolved. To add to the fun, some of the more promising materials have the rather entertaining property of being explosive under standard conditions...

Regarding brittleness - TRIZ suggested to build a reactor vessel that is replaceable in parts - or to use a shield substance that takes the damage instead. Not an machine engineer though.