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Comment by erikgrinaker | Hacker News Reader

Much of the (valid) criticism in this article relates to the deuterium-tritium fuel cycle. This is the easiest reaction to accomplish on Earth, so most experimental reactors are designed with this fuel in mind, and we're certainly having a hard enough time making even this work.

However, I've always considered D-T fusion an intermediate step on the path to aneutronic fusion, such as Helium-3 or proton-Boron reactions. These avoid most of the radiation issues, as well as the tritium-breeding problem (although Helium-3 sourcing presents its own challenge). Since the fusion products are electrically charged the reactor could possibly also generate electricity directly, without a steam turbine and the associated energy loss. Unfortunately, it requires temperatures that are an order of magnitude higher than D-T (well beyond a billion degrees Kelvin), so we'll need to learn to walk before we can run.

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The nice thing about fusion neutrons is you get to control the isotopes, you have no control over fission waste isotopes.

Some fission isotopes are really icky to deal with, as everyone has heard...

On the other hand if you don't like dealing with cobalt-60 waste at your fusion plant, simply stop using cobalt alloys in your reactor vessel.

It turns out to be "not that big of a deal" to design a fusion plant where neutron activation isn't important. The quotes are because nothing is easy in fusion but as a problem its pretty low on the list.

This is true. Back at Fiat Lux when we designed our D-D reactor, we intended it to sit inside a pool of water and borax. Since we didn't need to regenerate tritium, just absorbing the neutrons with boron was the cheapest solution. As far as I know, Borax is the cheapest effective neutron shielding known. We would have liked to have built our vacuum chamber out of purely Al (since Al-28 has a two-minute half-life), but we went with steel for cost reasons.

Unfortunately, we never made enough neutrons to activate anything worthwhile. Nevertheless, it is certainly possible to work around neutrons through design decisions.

Sure, activation in itself isn't necessarily a big problem, but I believe that embrittlement of the blanket and other plasma-facing surfaces due to the high neutron fluxes is one of the major engineering challenges for ITER and similar tokamak designs.

Billion Kelvin operating temperatures make me super skeptical about the practicality of that solution in my lifetime. It's just so many orders of magnitude beyond where our materials science is.

It's very high, but not as bad as it sounds. There is less than 1 gram of fuel in the reactor at any given time, so it's extremely diffuse, and mostly contained by magnetic fields. Much of the energy can also be radiated away by facilitating ion-electron recombination before the plasma reaches any materials.

The current design for ITER, operating at 150 million Kelvin, focuses the exhaust plasma onto a dedicated divertor surface with an estimated peak heat flux of 20 MW/m2. Experiments show that tungsten handles this fairly well, although with some cracks appearing after many cycles, but this is an active area of research.

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