Nuclear weapons are largely machine-made, and assembled by people. But there are many technicians and scientists involved in the process.
The countries which currently make nuclear weapons, or at least have a nuclear arsenal, are the USA, Russia, France, Peoples Republic of China, UK, Israel, India, Pakistan and North Korea. Belarus, Kazakhstan, and Ukraine inherited weapons from the Soviet Union, but did not retain them. South Africa developed its own nuclear weapons, but uniquely gave them up.
There are - or were - active programmes to develop nuclear weapons in Iran, Libya, Syria, Brazil and possibly non-state players, such as Al Qaeda.
If you want the best chance of permanent employment, try the USA. Russia still has an active programme, but job security may be limited. They may also be suspicious as to why an American would want to work for them. If you are Jewish Israel may be an option.
If you want the money, try Iran or Al Qaeda. Though you may not live to spend it.
But whatever you do, you would have to complete a first class degree first, and pass stringent security checks.