Internal Boundary Conditions For Diffusion Equation?.

Hello,

I'm modeling simple diffusion in a 2D geometry; however, the geometry contains some internal boundaries that I would, at times, like to be able to represent as walls. Consequently, I need to specify a zero flux across these internal boundaries. While it is easy to specify a flux discontinuity, specifying a zero flux on both sides of the internal boundary is not obvious.

I suspect there is a simple answer, but I have yet to be successful in finding it.

thanks iosman