> in secret is impossible without the whole world knowing.

I'm curious about why this is

Outside of an actual test detonation, presumably this could all happen in a secure place?

For an example of how closely this is monitored see the Oklo fossil reactors[1]

The proportion of fissile isotopes being mined was off by a fraction of a percent, which caused the French government to launch an investigation. It turns out that millions of years ago the site had formed a natural fission reactor which depleted some of the fissile isotopes

[1]https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...

You need highly educated individuals, a massive amount of energy expenditure, a massive facility to house your centrifuges, and an active mine to dig up nuclear materials.

It isn't impossible to keep such a secret, but practically it would be incredibly difficult just through the energy requirements and mining scale which would be hard to hide without anybody asking what exactly are you mining and processing.

You need enough people to work on it that some information will leak, and the facilities needed to build nuclear power are pretty big (uranium refinement, etc.), big enough to be visible on satellite footage. Mostly the first point.

My guess would be that sales of the high-tech gear you need, like Uranium centrifuges, are strongly sales/export controlled. Probably someone would also notice if you start mining Uranium ore.

It requires very large, high powered centrifuges and tons of uranium. Requires an infrastructure project that is visible from space, even underground. And projects that large are difficult to keep secret anyway.

Espionage.