You should specify for the 2D grid :

However, the user is let free to create his own boundary condition (in
the case where the 1D grid is only outside the 2D grid for instance).

The boundaries of the 1D grid are open.

The OuterSourceMass option applies to the 1D grid only. It should not
be used, as the 1D grid is supposed to cover the whole disk, with open
outer boundary.