I was going to post this as an ask hn question but since this article got a lot of eyeballs I thought I'd just post it here in the comments:

Ask HN: Is ITER built to last?

ITER is a demonstration model of a fusion power plant that is expected to yield enough energy to be comparable to utility scale power. That it may do, or may get close, or lead to a discovery that could make a derivative design mostly practical for these ends. ITER is an experiment so they don’t need to worry about utility hookups quite yet.

Why fusion energy? Fuel abundance is the selling point more than any other because water contains a great deal of fusion fuel in the small amount of deuterium scattered in it. Extracting deuterium is safer, cleaner, and cheaper than handling uranium or thorium ores and refining them into purified metals. Enrichment is very high technology and an industrial accident is extremely deadly. Deuterium as a fuel is inert and safe.

Tritium is like deuterium but unstable because a lone proton argues with two wives.

But we need it’s instability and reactiveness for ITER. Deuterium and Deuterium fusion is so hard to pull off we can’t imagine how to at this time (besides bombs) in amounts that are interesting. Unfortunately at this time Tritium can only be acquired in large quantities from fission but we had enough for experiments. ITER address this with tritium breeding blankets made of lithium. A 17 mev neutron slams through a very hot bottle of liquid lithium and hopefully bounces around long enough to thermalize to where it can be absorbed by the lithium contained in the bottle. Upon merger of lithium and the neutron, lithium will sometimes break down into Tritium which we need for fuel. So far so good.

Big deal, it converts neutrons to high test nuclear fuel. Whats the problem?

Fusion experiments (where fusion is being made in any significant amount) have usually been very short lived. So they haven’t really had to worry very much about what happens to those neutrons slamming everywhere at velocities impossible to imagine.

Thinking here, what kind of neutron flux are we dealing with? Beyond the napkin calculations I reason a few points to consider. First what kind of neutron flux is expected with current generations of fission plants? Well fission of uranium is a well known science and it turns out that most of the energy carried away from the process is in the kinetic energy found in the fragments. Only 30 Mev on average from a 200 Mev fission event is something in the form other than fast moving fragments. Per event it maybe similar to the energy fusion neutrons, I’ve not really dug deep into that. But overall, neutron kinetic energy is less than 15% of the total.

But unfortunately for us, fusion of Tritium and Deuterium's energy is basically all in the kinetic energy of the neutron. So comparatively speaking, fusion plants will have to deal with a neutron flux about 6.7 times higher per unit energy delivered.

Looking at fission reactors, ideally, neutrons don’t really get much farther than the fuel and the pressureized coolant and they are all submerged together. For safety reasons, concrete and lead can line the pressure vessel from the outside to prevent the from baddies getting out. And they are very bad, transmuting anything unlucky to brush up against a slow one.

And that brings me back to thinking about a practical fusion reactor. Sure it can stop most of those neutrons in a very very thick blanket of lithium but anything found before contents of that bottle is going to be blasted with a very very high amount of fast transmuting particles every minute, and every hour of the day. Eventually the structure and the magnets are going to be touched by some of these and cause problems. How long would it take? I can’t estimate that really. And maybe this is is all wrong and I’m not seeing this clearly. But I’ve not seen this discussion about the sheer amount and power of the neutron flux. Its hard to imagine anything in there lasting very long. Transmutations have effects on grain boundries and can lead to lattice dislocations. I am guessing that a significant mass fraction of the reactor may have to be scrapped and condemned within the first year of its tests.

Is there an answer to this? I don’t think handling neutrons this way is going to work out. Hotter reactors can burn pure deuterium but that also leads to significant neutron flux. Finding a material that is ideal as a shield that has the right magnetic permeability and other properties would be hard. Not to mention in general it is kind of out of this world trying pure isotope fusion.

It just gets so much harder beyond that with exotics like boron proton fusion that really is kind of an odd way of fissioning light nucli with protons.

ITER will probably do what the scientists expect and they might be happy at first. I hope they thought of the long term.

Could be I’m behind the times and someone already leap frogged ITER or solved it within the original plan. But I’ve not been up on the latest.