Coupling for joining conduits in a dielectric heating reactor comprising: a connector for forming a sealed joint between conduits, and an expandable electromagnetic shield arranged to surround joined conduits such that electromagnetic shielding of the joined conduits is maintained in response to dimensional changes due to temperature and/or pressure.Furthermore, an electromagnetic heating reactor comprising: an electromagnetic enclosure having a pressurisable interior and a reactant supply conduit at least partially enclosed within the pressurisable interior, wherein the pressurisable interior is arranged to at least partially counterbalance pressure within the reactant supply conduit and further wherein the interior of the reactant supply conduit is sealed against the pressurisable interior.