[pfdietz]

D + D -> 3He + n is actually tolerable, since the neutron is of low energy. In particular, it has a much lower cross section for (n,p) and (n,alpha) reactions in the reactor structure. Those are the really serious reactions that ruin the material by filling it with microscopic very high pressure gas bubbles.

What you want to avoid is D + T on the T generated from the other D + D reaction. In Helion's scheme, I believe the ions do not equilibrate during a pulse, so that tritium doesn't slow down before the plasma reexpands and is exhausted to the divertor. The DT fusion cross section peaks around a 60 keV center of mass energy, so keeping the T energetic can reduce its fusion rate somewhat.

This is another reason to have "field reactors" burning 3He generated elsewhere, with a high 3He/D ratio. The lower D concentration reduces DD reactions, but it also reduces DT reactions on any T that are produced.

There's an additional problem of purifying and reusing the D in the exhaust of the machines. You don't want to have to filter out all the tritium from this before every cycle. It's possible the field machines could have many go-arounds of this gas before sending the sufficiently contaminated D back to be filtered (or used in the dirty DD machines).