We consider a control constrained optimal control problem
governed by a semilinear
elliptic equation with nonlocal interface conditions.
These conditions occur during the
modeling of diffuse-gray conductive-radiative heat transfer.
After stating first-order necessary conditions, second-order
sufficient conditions are derived that account for strongly active sets.
These conditions ensure local optimality in an
Ls-neighborhood of a reference function
whereby the underlying analysis allows to use
weaker norms than $L^\infty$.