It would not need be manhattan project size, it’s a precision engineering problem not a fundamental physics problem mixed in with an industrial scale production problem.
There have been plenty of very expensive defense programs that were kept quite secret for a very long time.
I’ve known and worked with people with top secret clearances, they just didn’t talk about their work, it’s not that hard.
And everyone of these scientists is voluntarily giving up the opportunity to move a basic working fusion reactor into the private sphere? They could make billions and become the next GE or Tesla.
If just one of them had a startup bone in his body, we would see a small fusion reactor startup making consistent progress that seems amazing. It's not happening.
A scientist or engineer who left a classified project to do the same thing in a startup would very quickly find themselves in prison. Once you gain access to classified information, it is very very easy to convict you of very many crimes.
It's a situation that is apparently very difficult to understand for many people. You work on a classified program and you understand the situation. Nobody goes off and decides to turn their classified knowledge into a business.
Unless your assange, snowden, goldamns sachs ceo turned regulator or retired military general gone into the private sphere or fbi agent turned security consultant or blackwater pmc agent.
I get the value of the myth, people instinctively acheive what they believe is doable. It's just a myth though.
And until a working fusion reactor is build, it is indeed a fundamental physics problem, with all the energy involved, which is not at all completely understood since we are in quantum mechanic level down there.
There have been plenty of very expensive defense programs that were kept quite secret for a very long time.
I’ve known and worked with people with top secret clearances, they just didn’t talk about their work, it’s not that hard.