Switzerland has several operating nuclear reactors:

https://pris.iaea.org/PRIS/CountryStatistics/CountryDetails....

Switzerland also studied nuclear weapons production until 1988:

https://en.wikipedia.org/wiki/Switzerland_and_weapons_of_mas...

If the Swiss thought it was in the national interest to exit the Nuclear Non-Proliferation Treaty and crash-develop a nuclear deterrent, I think that they could achieve nuclear breakout quickly.