[en] Reference is made to reactor containment vessels and the provision of improved thermal insulation on their inner surfaces, the arrangement described being particularly applicable to concrete pressure vessel of a gas cooled high temperature reactor or fast reactor, although it may also be applicable to a liquid cooled fast reactor. The internal surface of the containment vessel has a metal liner, having thermal insulation comprising a layer of ceramic bricks or tiles with one or more layers of compressible porous thermally insulating material interposed between the bricks and the metal liner. Retention means are provided by studs, secured on the metal liner and projecting through the interposed layer or layers, and by securing means by which the bricks are individually secured by the studs, the studs and securing means themselves being insulated from the vessel interior by proportions of the thermal insulation. The securing means exert forces on the bricks to hold the interposed layer or layers under compression between the bricks and the metal liner. There is provided adjacent to each stud a resilient device, that may consist of a helical spring, held in compression between the bricks and the metal liner. Full constructional details are given. (U.K.)