Abstract: A joint for interfacing a steel head closure and a steel-lined prestressed concrete reactor vessel (PCRV). The steel head closure is bolted to a thick steel annular plate and a thermal barrier is arranged in a volume bounded by the head ring, the liner and the concrete of the PCRV wall. The thermal barrier is made of zirconia sand or other material having a low thermal conductivity, a high bulk modulus equal to that of concrete, and a low shear modulus. Differential thermal expansion of the vessel head closure and PCRV is accommodated by a flexible connection of the PCRV liner to the inner periphery of the head ring. In the alternative, the steel head closure is interfaced to the top of the PCRV via a sliding joint or a thermal stress relief ring. In accordance with a further alternative, thermal stresses in the steel vessel head closure are mitigated by reducing the temperature of the head closure by installing thermal insulation on all internal surfaces thereof.