The No Flux node is added by default and is the default boundary condition on exterior boundaries. It should be used on boundaries across which there is no mass flux. The condition is applied as:

By default, the No Flux node prescribes a vanishing diffusive flux at the boundary. This is appropriate when the relative convective velocity at the boundary is zero. To include the convective flux, select Include in the Convection section.

The No Flux node is not available in 0D.