The treatment of finite systems requires that boundary conditions be defined. In this simplified discussion we consider the case of a homogeneous medium surrounded by a vacuum. At the boundary of the medium, there is only an outgoing one-sided flux J+ while J_ = 0. This means that the current J = J+_ J_ > 0 and thus that, since grad(‘) > 0, ‘ decreases from the inner to the outer region. Extrapolating ‘ linearly in the vacuum region, where the diffusion equation is not valid, the extrapolated value should vanish at some distance dextra. By comparison with exact calculations one finds that ‘ is a good approximation of the true solution for dextra ~ 2D. This is usually very small compared with the multiplying medium size so that a simple, but sufficient, approximation of ‘, at least for qualitative discussions, is obtained by requiring it to vanish at the boundaries of the medium. To illustrate this, we solve the diffusion equation for a semi-infinite homogeneous reactor bounded by two parallel planes [55].