Dear Fabrice Marty, in addition to the previous answer, try to calculate the integral product Psi* p Psi between a and b (a < r < b).

This is a math problem of an averaged QM momentum operator in a finite interval (a,b).

For most potentials, the averaged p with the Hydrogen wave function, or with a wave function in an external potential is zero, when the expected value has the physical meaning, that is, the interval of the integral goes from - infinity to infinity.

But if the wave function is not zero at the boundary, then there is QM tunnelling.

Please look at the following external link:

https://physics.stackexchange.com/questions/312911/average-momentum-in-quantum-mechanics-over-some-finite-interval-of-space