Comment by fastball | Hacker News Reader

Comment by fastball | Hacker News Reader

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Working on fusion doesn't mean we're not working on better fission. We should do both. I think both fusion/fission are great but your comment is misleading at best.

- Fusion produces much less radioactive waste byproducts then fission. It's not zero, but it's a significant difference.

- The problems with nuclear weapons proliferation are much easier to handle with fusion. For starters, you're not transporting enriched uranium fuel around. Also you don't end up with fun transuranics like plutonium which can be readily used to make weapons, unlike any key byproducts of fusion. Hell, the main byproduct of our fusion reactors is gonna be Helium, which we actually need more of because it is a very useful element that experiences shortages due to not being contained in the atmosphere and not being super prevalent in the earth's crust.

- "Extremely fussy" is a selling point. Extremely fussy means that if something goes wrong you don't have a runaway chain reaction that makes everything go boom. It's impossible to design a fusion reactor that can melt down. Meanwhile, melting down is the default mode for fission reactors and needs to be carefully designed around.

- Fusion is the only viable energy source for long-term space travel/colonization.

- The point about net energy surplus is kinda nonsensical. Of course we're not there yet, that's why it's a problem we're actively working on and not something we've already solved. Your point is literally "we shouldn't develop this technology because we haven't developed this technology yet".

Also not sure how we can simultaneously need to "come up" with a good fast breeder reactor design while at the same time it's apparently "tried and true technology".

> - "Extremely fussy" is a selling point. Extremely fussy means that if something goes wrong you don't have a runaway chain reaction that makes everything go boom.

Sorry, I should've elaborated a bit on this point. I'm talking about the expensive containment system which will be subject to extreme conditions and have a short lifetime[1].

> Under reactor-relevant conditions, the following are the most serious damaging mechanisms: thermally induced defects such as cracking and melting of the plasma-facing material (PFM); thermal fatigue damage of the joints between the PFM and the heat sink; hydrogen-induced blistering; helium-generated formation of nanosized clusters; and neutron-induced degradation of the wall armor via reduction of the thermal conductivity, embrittlement, transmutation, and activation.

> Further serious lifetime-limiting PWI processes are caused by material irradiation with hydrogen isotope ions (D+ and T+) and impurities that—depending on their impact energy—will sputter the wall material. The eroded species will be deposited elsewhere, for example, on unshielded parts of the vacuum vessel, on blanket modules, or on less severely exposed divertor targets (outside the separatrix strike zone). Implantation of hydrogen isotopes into the surface of the PFM will result in severe embrittlement of the wall. This also has a strong impact on its cracking resistance, in particular during short transient thermal loads (i.e., ELMs). Helium will also be implanted into the surface of the wall armor or buried in redeposited surface layers. Implanted helium tends to migrate (depending on the prevailing temperature) and to form tiny bubbles that again can interact with implanted hydrogen. In several fusion-relevant PFMs (e.g., tungsten) helium can initiate rather substantial changes in surface morphology, such as the growth of tiny tendrils or “fuzz” on the surface of the PFM.12 These layers can easily reach several micrometers in thickness. These effects need to be considered as a potential source for the release of dust particles and contamination of the burning fusion plasma.

So I'm not talking about the fail-safe nature but rather the extreme cost and technical difficulty of containing the reaction for the amount of time that would be needed for fusion to be a viable commercial energy source.

[1] https://aip.scitation.org/doi/10.1063/1.5090100

I think one of the advantages of the General Fusion approach is that both what the heat gets transferred to and what bears most of the neutron flux (and hence gets irradiated) is the liquid metal, which is presumably easily replaced (and could presumably even be done incrementally while the reactor is live, since it's going to be flowing through a heat exchanger anyway)?

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