>You omitted the "... but there's a much bigger one from your whole country having large amounts of cheap, clean power"
Not to go tinfoil hat here but... if we had a relatively small fusion reactor, lobbyists would do all they could to make sure they never became operational for connection to the grid.
- Fossil fuel lobbyists
- Power company lobbyists
- Fossil fuel worker union lobbyists
- Ship, train and semi industry/union lobbyists (that transport the insane amounts of coal and oil-related products)
You'd also possibly see the unions that cover anyone working with power lines as you'd be able to set up a bunch of independent grids and stop servicing/repairing, and start removing, high transmission lines that blanket the country which also could get various metal worker unions lobbying against it as production of these massive amounts of cable (if done domestically) would drastically be reduced as why bother crossing vast stretches of land when you can just drop a reactor in a rural area and just serve the region.
When/if we develop fusion reactors that are economically viable, you will remove thousands to hundreds of thousands of jobs in a relatively short period in the United States alone. Voters aren't too found of losing their jobs and the lobbyists of their industries have lots of money to burn fighting such changes (this is why you'll never see the IRS thrown out in exchange for something like a federal income tax only, it would cut 75,000 federal jobs over night and their union + the voting power of them and ever tax preparer, CPA etc would fight hard to prevent it).
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AS far as secrets, there are very few people that know where the United States keeps any of it's various strategic stock piles. Those remain extremely closely guarded secrets where a very small number of people know at any given time where that stuff is stored/warehoused, how much, what variety etc. That's not conspiracy, that's just cold hard fact, and that's stuff like vaccines/medicine/fuel etc.
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A hypothetical stretgic use:
The strategic value of keeping such a reactor/technology secret. If you can power your known underground bases/alleged DUMBs (deep underground military base), without needing any energy input from the surface you'd want to keep that very hush hush because it gives you far longer backup power capabilities than generators and fossil fuels stored onsite could provide.
Having such a technology also allows you to actually create DUMBs. You can make an electrically powdered tunnel boring machine, go to something like a known cave or mine site, point it in a direction and go for mile after mile if you felt so inclined. If you're doing this in an area with known caverns/an old mine you never even need to bring what you bore out up to the surface, you just fill caverns/mine shafts. Such tunnels and bases have been alleged to exist since the cold war days and they may or may not exist, but if you had a compact fusion reactor technology that was classified from the start, the longer you keep it a secret the longer you have to exploit it for such hypothetical situations as this.
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Edit: Previous HN comment about him/his work
https://news.ycombinator.com/item?id=19229721
Looks like he's previously worked on systems for 'Hybrid Aerospace-Undersea Craft' for something to operate both underwater and as an aircraft a compact fusion reactor would probably be what took it from being a novelty (like the various goofy boat-cars and car-planes) to an effective vehicle.
That's an interesting idea on its own. You could travel fast distances, presumably quite rapidly, through the air and drop below radar, slow down to enter the water and submerge, then proceed to whatever. Or you could act as a sub, if you're detected launch out of the water and accelerate away in the seconds of confusion you create when you suddenly go from a water target to an air target.
Similarly, it could simply be an awesome power source for a rather large seaplane. Rapid troop/cargo transport, even better if you could submerge for travelling in deep water then pop up in coastal water and seaplane in the rest of the way.
loses the rest of his afternoon to daydreaming about a compact fusion reactor.