Every patent is contained in the grammar of English. Finding a meaningful pattern that others haven't yet, that seems to be what a patent needs. They found some prime numbers, very useful, and someone else found the nuclear reactor, also useful.

The nuclear reactor's design follows directly from the current state of engineering (valve?steam?superconductor?etc) and some properties that follow directly from physics (uranium and water -> good).

So it's wacky to patent a number, but not that wrong. Don't forget finding primes is actually a pretty expensive exercise.