Abstract: A clay layer of low permeability and of a thickness of about 2 meters, depending somewhat upon the permeability, immediately covers and laterally surrounds the external concrete wall and roof structure of the nuclear reactor building, this layer extending at least down to a ground water draining or leading ground layer. Above it is a layer of gravel, sand or porous stone of relatively high permeability, typically somewhat less than a meter thick, and on top thereof an earth fill layer of less permeability than the intermediate layer is provided, which is typically 8 meters thick. The clay layer, which could also be a loam layer, prevents the emergence of radioactive materials in the event of cracking of the concrete structure by an accidental malfunction and absorbs aerosols and water-soluble fission products. The gravel layer converts the convective mass flow of the emerging materials into a diffusion flow and prevents the spreading of cracks in the covering layers.