Abstract: In a reactor vessel for pebble beds at high and varying temperatures having a side wall and a base formed of a multiplicity of stacked blocks of heat-resistant material and held together by an outer cylindrical or polygonal ring and supported on a foundation, the base and the side wall, respectively, being formed of a plurality of sectors having substantially vertical, radial parting lines therebetween, the sectors being supported with slight friction on the foundation and being braced against the outer ring, the sectors having boundary surfaces with a pebble bed, support surfaces on the foundation and abutment surfaces against the outer ring, the respective surfaces of the boundary surfaces, the support surfaces and the abutment surfaces having a convex mutual inclination whereby each of the sectors is held together in itself by external forces and is forced by its own weight and the weight of the pebble bed into a definite position.