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Comment by maccam94 | Hacker News Reader

Commonwealth Fusion Systems plans to hit Q>10 net energy gain in their SPARC reactor in 2025, and then turn on a continuously operating ARC power plant around 2030. They plan to use a steel vacuum chamber which will need annual replacement. The rest of the ARC reactor will be shielded by a FLiBe molten salt blanket which will absorb neutrons and convert their energy into heat.

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A reminder again about this:

The ARC reactor described in the 2014 arxiv paper uses a lot of beryllium. To provide the 18 TW of primary energy used by the current world economy with these reactors would require 10 million tonnes of Be. The annual mine production of Be is just ~200 tonnes, and the global estimated (by USGS) Be resource (not reserve) is 100,000 tonnes.

The gross fusion power density of ARC from that paper is 0.5 MW/m^3, vs. 20 MW/m^3 for the fission power density of a PWR reactor vessel. ARC is going to be a much more complex and expensive way to boil water than a fission reactor.

My only counterargument would be that a lot of technology starts out requiring some rare or expensive element to function, but through improvement is later able to transition to some less exotic replacement.

In this case, lead would be more difficult but would still do the basic job of multiplying fusion neutrons. Last I saw, General Fusion was planning to use lead.

Lead (or lithium lead) would not work for ARC, because it is electrically conductive. It would not be possible to pump it in the strong magnetic field because of the back EMF generated. This is also why PbLi with vanadium structures was abandoned for DEMO: it proved impossible to make an insulating coating for the metal structures that would prevent the induced currents from flowing. Even slight cracks in the coating were found to be ruinous.

Ah, thanks, that makes sense.

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