A nuclear reactor installation includes a pressurized-water reactor pressure vessel positioned in a reinforced-concrete structure forming a wall surrounding the pressure vessel and forming an annular space around the pressure vessel, this space containing a layer of discrete elements such as gravel, ceramic bodies, steel balls, etc. This layer directly contacts the concrete wall, possibly through a steel sheet or plate skin and between it and the pressure vessel, a thermally-insulating and pressure-resistant encasement is built from separable segments cast from suitable concrete, a steel sheet or plate skin preferably being interposed between the layer of discrete elements and this encasement, the discrete elements being compacted so that radial force transmitted from the pressure vessel via the encasement is transmitted through the discrete elements to the concrete wall formed by the reinforced-concrete structure. The encasement, layer and skin interposed between them provide via their total thickness, for zero-travel restraint of the pressure vessel in its radial direction, via the concrete structure's wall, when these parts and the pressure vessel are thermally expanded during operation of the reactor. When the reactor is shut down and it and the various parts are cold, the layer of discrete particles and the skin between them and the encasement, are proportioned so that with thermal contraction the skin is free from the encasement, and the latter, being made of separable segments, may be removed segment-by-segment.