Earth Covered in-the-Ground Nuclear Reactor Facility

This invention concerns a nuclear reactor facility of the underground type of construction in which at least a part of the building of the nuclear facility, particularly the building portion containing the reactor, is embedded in undistrubed ground and an outer concrete wall and roof structure projecting above the top edge of the surrounding natural landscape is covered with earth fill, spread thereover.

The underground type of structure for nuclear reactor facilities is sought to be used for safety reasons. The possible gain in safety margin is to be weighed against the additional costs for the emplacement of a nuclear reactor facility of that type. It is known to locate reactor facilities in rock caverns. This has been done heretofore, however, only for nuclear reactor facilities of relatively low power (cf. "Review on Underground Siting of Nuclear Power Plants", UEC-AEC 740 107, 1974).

Rock caverns for nuclear reactor facilities of higher power are generally not available in most landscape regions. Constructions embedding the facility in the ground are therefore recommended. It is disclosed in a publication of W. Kroger et al., "Unterirdische Bauweise von Kernkraftwerken" in the 1975 Annual Report of the Kernforschungsanlage Julich GmbH, that in the case of a nuclear reactor facility, half or entirely sunk into the ground, safety of the external concrete walls of the building of the facility was increased by overlaying them with up to 10 meters of earth and, for the protection against the strongest external effects, up to 20 meters of earth.

Additional precautions are, however, to be considered in the design, particularly in the design of the portion of the building containing the reactor, if in the case of so-called hypothetical disturbance cases, that go beyond the greatest accident to be postulated, sufficient products penetrate into the inner chamber of the reactor building, and a crack formation in the outer concrete wall structure from the disturbance accident must be taken into account, such that radioactive fission products can get into the environment of the reactor facility.

It is an object of the invention to provide such precautions in the underground type of construction for nuclear reactor facilities, that in spite of the formation of cracks that might occur in one of the external concrete wall or roof members of the underground building of the nuclear reactor facility in the case of a hypothetical malfunction resulting from the giving-off of fission products, no contamination of the environment of the nuclear reactor facility will take place.

Briefly, the external concrete wall and roof structure projecting above the natural landscape is surrounded and covered by a first layer of clay and/or loam extending at least down to a ground-water-diverting ground layer a second layer of greater permeability consisting of gravel, sand, porous stones or the like is provided above the first layer, and a top layer of earth fill that is at least 3 meters thick and has a permeability less than that of the second layer is then spread over the latter. In the case of a malfunction accident, the emergence of radioactive materials from the outer concrete structure is strongly reduced and aerosols and water-soluble fission products are held back by the clay and/or loam layer surrounding the concrete structure because of the low permeability, between 0.001 and 0.1 darcy (1 darcy 0.987.times.10.sup.-8 cm.sup.2) of the clay and/or loam layer. Most advantageously, this layer has a thickness from about 1.5 to a maximum of 3 meters. It extends down to the ground water draining ground layer and thus prevents the contamination of ground water. In the layer next upward from the clay and/or loam layer that consists of gravel, sand or porous stones such as slags or porous building material as well as crushed stone, or can also be provided, for example, of glass fiber material, and has a layer permeability between 1 and about 100 darcy and a height from about 0.5 to a maximum of 1 meter, the convective mass flow of the emerging materials is converted into a diffusion flow, so that the radioactive materials that manage to penetrate through the clay and/or loam layer into the overlying earth fill layer, that has a permeability between 0.01 and 1 darcy, reach the overlying layer only by diffusion at the surface. As a result of the long diffusion times thereby resulting, the radioactive materials relevant for radioactive contamination are largely decomposed already within the earth fill covering.

In order to provide protection also against contamination of the deeper-lying ground layers with radioactive fission products in the case of crack formation in the external concrete walls, particularly if at the location of the nuclear facility it is necessary to contend with ground water velocity greater than 10 meters per day, or if a ground layer is present that has a small adsorption capacity for water-soluble fission products, a further development of the invention is provided whereby the clay and/or loam layer surrounds the outer concrete structure down to the floor plate of the reactor-containing building portion. It is particularly effective for the clay and/or loam layer to fill an intermediate space between outer concrete wall structures and reinforced excavation cavity walls.

The invention is further described by way of illustrative example, with reference to the annexed drawings, in which:

FIG. 1 is a diagrammatic cross-section of an underground nuclear reactor facility;

FIG. 2 is a basic diagram of the covering of the external concrete wall and roof structure of the facility, and

FIG. 3 is a partial cross-sectional view of the external concrete wall and roof structure of the nuclear reactor facility in the vicinity of a side of the excavation for the building.

As can be seen from the drawing, the reactor facility is completed embedded in undisturbed ground 1. In the reactor building 2, there are housed a nuclear reactor 3 with a primary cooling loop within a safety container 4. The outer concrete wall and roof structure 5 of the reactor building 2 that projects above the top edge 6 of the natural landscape contour is covered by an earth layer 7. Alongside the reactor building 2, still other building portions belonging to the nuclear reactor facility are located that are covered by earth fill along with the reactor building portion 2, FIG. 1 showing an input passage 8 and a sluice 9,10 among the ancillary building portions of the installation. The fill covering 7 extends all the way to the machinery building 11 in which the equipment for energy conversion is located.

As is shown in FIG. 2, the protective covering 7 of the external concrete wall and roof structure 5 consists, going from the inside to the outside, first of a clay layer 12 immediately surrounding the concrete wall and roof structure 5. The clay layer 12 is continued down to the ground-water-draining ground layer 13 and is 2 meters thick in the illustrated example. The height of the clay layer--as also the height of the other layers of the covering--is determined primarily in accordance with the permeability of the particular material utilized. In the illustrated example, a clay having a permeability of the order of magnitude of 10.sup.-3 darcy is illustrated. The clay layer 12 is covered by a layer 14 that is 0.7 meters thick and consists of gravel having a permeability of the order of 10 darcy. Finally, an earth fill layer 15, 8 meters thick, lies over the gravel layer 14 and consists of earth fill having a permeability of the order of magnitude of 10.sup.-1 darcy.

The clay layer 12 hinders the emergence of radioactive materials and absorbs aerosols and water-soluble fission products in the event of an assumed hypothetical malfunction accident accompanied by crack formation 16 in the outer concrete wall and roof structure 5. In the gravel layer 14, the convective mass flow of the emerging materials is interrupted and converted into a diffusion flow. The layer 14 also impedes the spread of cracks in the earth fill and thereby also an undesired breaking through of a crack all the way to the surface of the fill layer 15. In the earth layer 15, the radioactive materials that still occasionally penetrate the layers are transported further only as the result of diffusion. Convection flow and diffusion flow in the covering are indicated in FIG. 2 by various arrows.

The covering 7 shown in the illustrated example not only holds back the radioactive fission products emerging from the reactor facility in the case of hypothetical malfunction accidents accompanied by additional undermining crack formation in the outer concrete wall structure, but is also capable of absorbing other shock loads, for example in the case of impact from a falling aircraft. In order to provide also for protection against the strongest external effects, the earth fill layer 15 can be covered further by a shield paving of concrete.

FIG. 3 shows a partial cross-sectional view of the outer concrete wall and roof structure 5 in a form of construction provided for unfavorable ground qualities at the building site of the reactor installation. The clay layer 12 surrounds the outer concrete wall structure 5 down beyond the ground-water-draining ground layer 13 all the way to the level of the bottom plate 17 of the reactor building 2. The clay layer 12 fills an interval between the outer concrete wall structure 5 and its insulating layer 18 and a reinforced excavation retaining wall 19. The clay layer is 2 meters thick. The clay layer reaching all the way to the bottom concrete floor 17 is of particular importance where a ground water velocity of more than 10 meters per day is present and/or ground layers are present that have a low adsorption capability for water-soluble fission products.

Although the invention has been described with reference to a particular illustrative embodiment, it will be recognized that variations and modifications are possible within the inventive concept.