That was my understanding too. What other "nuclear energy" related use would there be for a hidden Geiger counter?

Nuclear power != Nuclear weapons, or preventing the spread of them.

I have heard Russia decided to build more dangerous reactor types like the one in Chernobyl because they are cheaper + they could use the same radioactive material for nuclear weapons.

Nuclear power is a prerequisite to nuclear weapons though.

/edit: if you don’t want your country to be an international pariah

The first atomic bombs were dropped a few years before the first nuclear power plant came online.

Yes, but there weren’t pesky inspectors sneaking around with custom iPods then either