Optimized reinforcement of nuclear power plant structures for aircraft impact forces

Abstract

Reactor buildings of nuclear power plants and, to some extent, also other buildings of the plant, according to the present safety requirements, have to be able to withstand aircraft impact forces. The building has to withstand this loading only once since afterwards it will be out of use. Accordingly, other criteria for design and the necessary safety measures are valid than in the case of service loads. Large deformations and the development of large cracks due to such loadings are insignificant from a construction point of view for reinforced concrete structures, i.e. the stresses can build up to the ultimate load carrying capacity. From the nuclear safety point of view, however, some restrictions are possible in this regard, e.g. to obstruct the penetration of fuel through the cracks. Basically all mild steels, with large ductility and without brittle fracture under sudden load increases, are suitable for this purpose. High stresses in the structure would, however, require uneconomical concentrations of mild steel. It is for this reason that the use of high strength steels, e.g. St 110/135, has been introduced in Germany for this kind of loading. Since the permissible deformations of reinforced concrete structures due to aircraft impact are large, a new kind of reinforcement is at hand. Through the use of wire strands or cables of high strength steel it is possible to reach a condition of cracks and large deformations due to ultimate loads in zones of point loading. The reinforcement takes on a distinctly curved shape and is able to carry the normal loads and shears through a suspension-structure action. The deformability of the structure for the analysed limit load state can be further increased through a bond-free net. This measure allows a more uniform stretching of the cables or strands over a larger zone. By making use of the higher allowable deformability of the structure and this type of reinforcement, savings in longitudinal reinforcement are possible. Reinforcement for shear is not relevant with this model and serves only a structural purpose. The structural thickness can be decreased as it depends only on the protection from penetration and is independent of the requirements for bending and shear.