Abstract
The generalized Semenov/Frank-Kamenetskii problems which govern the steady couplings between radiation/chemical heat-release and conduction are shown to be deducible from a variational principle whatever the shape of the reacting body, at least when the latter has a convex outer surface and radiation follows an Eddington-like law specialized to grey properties. A few examples are given to illustrate how this can be exploited in practical situations.