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Comment by dbcooper | Hacker News Reader

I remember that article. An older friend had worked on fluidised-bed reactors at Tsinghua in the 1990s, and I sent it to him.

Using helium indicates a problem with kinematic viscosity of cooling gases? Nitrogen would be non-reactive enough, I assume?

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Helium has excellent heat transfer properties (low Prandtl number) and does not undergo nuclear reactions when subjected to a neutron flux.

Nitrogen will undergo an (n-p) reaction to produce carbon-14 which has a half-life of 5700 years.

Yup. That's why regular PWRs take care not to nitrogen to pressurize or flush the primary coolant loop after maintenance.

And why reactors that use nitride fuels use fuel made with the isotope nitrogen-15.

The front runner steel for use in fusion reactors, EUROFER-97, contains a necessary small amount of nitrogen. This is enough under some nations' rules to render it into intermediate level radioactive waste after use, due to the carbon-14 content.

Oh yeah. I have a friend who was working on researching fusion-safe steels. Solving it fully is going to be a real engineering challenge.

Apparently, even a small natural niobium contamination would make it a low-grade waste.

Yes, niobium is the other problem. I think steel makers are used to adding alloying elements, but not set up for excluding them to ppm levels. I saw someone bemoaning that the steel alone for DEMO would cost 3 billion euros (or dollars?), and I've wondered if this is the reason.

https://scipub.euro-fusion.org/wp-content/uploads/eurofusion...

"[...] in the first layer, nearest the plasma, the rate of production of 94Nb – via neutron capture (n,γ) reactions on the stable 93Nb of niobium – is so high that Eurofer in this region is predicted to exceed the France-LLW limit within the first year of operation, and consequently would not be disposable as LLW under French regulations for more than 1000 years."

It's not just the steel. Beryllium typically contains about 100 ppm U, and an estimate of the cost of purifying it enough to avoid excessive fission products was another billion.

It will likely be necessary to start with steel made before atmospheric nuclear tests began.

There are many uses for such steel already.

https://en.m.wikipedia.org/wiki/Low-background_steel

I don't think that would make any difference. The problem isn't traces of radioisotopes in the initial steel, the problem is vastly larger amounts of radioisotopes created in the steel under intense neutron bombardment due to absorption in certain stable impurity elements.

And when I say "intense", I mean it: the integrated neutron flux the first wall of a DT fusion reactor would be exposed to over its operational lifespan is comparable to, or even greater than, that experienced by components of a nuclear weapon in the brief interval of a nuclear explosion.

Nitrogen is a big neutron absorber in gas-cooled reactors. It's actually used as a secondary shutdown mechanism in the UK's AGR reactors. So if it was to leak out (lose pressure) you'd see an increase in power at the same time as a loss of cooling... Not a great idea!

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