Patent Number: 050842340
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, the essential protecting components preventing escape of radioactive radiation and substances are schematically shown as provided in a boiling water reactor. The fuel elements 10, being each composed of a plurality of fuel rods, are arranged in a reactor pressure vessel 12 being filled with water to about two thirds of capacity. The reactor vessel 12 consists of a special steel and has a wall thickness of about 20 cm. For the controlling of the reactor core, i.e. for the controlling of the nuclear fission, the reactor vessel 12 has arranged therein a plurality of so-called control rods 14 which are provided between the fuel elements 10 and, by drive means 16 outside of reactor vessel 12, are displaceable in lengthwise direction. The fuel elements 10 have a cladding 18. Further, circulation pumps 20 for circulating the water are provided in the reactor vessel 12. A feedwater line 22 and a steam discharge conduit 24 are connected to the reactor vessel 12. The water coming from the condenser flows, via conduit 22, into the reactor vessel 12, is heated therein due to the energy released during nuclear fission and, at the same time, is evaporated. Through conduit 24, the water issues from the reactor vessel 12 and reaches the turbines. The reactor vessel 12 is surrounded by a concrete shell 26, having a thickness of about two meters, which is also called a biological shield. The reactor vessel and the concrete shell thereof are accommodated in a steelmade safety container having a wall thickness of about 3 cm. The safety container 28 has its outside provided with a sealing skin having a wall thickness of about 4 mm. The safety container 28 is arranged within the reactor building 30. The reactor building consists of steel-reinforced concrete and primarily serves for protection against external influences. The absorption casing of the invention is arranged on the respective inner side of the safety container 28 and of the reactor building 30. The absorption casing consists of a plurality of layers. Hereunder, the construction and the series of the layers are described with respect to an absorption casing which is arranged on the inner side of the reactor building 30 (FIG. 2). The inner surface of the wall of the reactor building 30 has a layer 32 of titanium applied thereto. This titanium layer 32 is provided for absorbing the radioactive radiation of plutonium and, therefore, is necessary only with fast breeding reactors wherein plutonium is obtained upon nuclear fission. Nonetheless, the titanium layer 32 has been included in FIG. 2 for reasons of completeness. On the titanium layer, there is provided a layer 34 of an elastic material having a thick layer 36 of lead arranged thereon. The lead layer 36 serves for absorbing the gamma radiation. As seen from the reactor core, a layer 38 of cadmium, boron, hafnium or beryllium for the absorption of neutron radiation is arranged before the lead layer 36. The interspace between the layer 38 and the lead layer 36 is filled by a layer 40 of elastic material. At a distance to the layer 38, there is arranged a layer 42 of aluminium for absorbing the alpha and beta radiation. Finally, on the inner side of the aluminium layer 42 being averted from layer 38, a layer 44 of a zirconium alloy is provided. The zirconium-alloy layer 44 prevents escape of gaseous fission products and, forming the innermost layer on the inside of the absorption casing, has the smallest distance to the nuclear reactor. The interspace between the zirconium-alloy layer 44 and the aluminium layer 42 as well as the interspace between the aluminium layer 42 and the layer 38 are filled with elastic material 46 and 48, respectively. The individual layers 42-44 are assembled from individual plate members. All of the plates of the same layer are bolted to each other; the plates of different layer are also bolted to each other. The elastic material of the layers 34, 40, 46 and 48 compensates the mechanical stresses in the absorption casing which are caused by the differences in expansion and shrinking of the layers upon heating and cooling, respectively. The absorption casing of the invention can also consist of self-supporting layers. In this case, the zirconium-alloy layer 44, the aluminium layer 42, the layer 38 of cadmium, boron, hafnium or beryllium, the lead layer 36 and, if provided, the titanium layer 32 will be arranged at mutual distances; the interspaces in this case can remain substantially free of material so that the layers of elastic material can be omitted. Instead of these layers 34, 40, 46 and 48 of elastic material, deformable spacers can be used which are fastened to the layers between which they are arranged. As already mentioned, the innermost layer 44 is made from a zirconium alloy. As a material for the layer 44, Zircaloy is particularly suited (Zircaloy is a registered trademark). However, also every other zirconium alloy offering reliable protection against gaseous fission products can be used as a material for the layer 44. The thickness of the individual layers for absorbing alpha, beta, gamma and neutron radiation and for sealing the absorption casing against gaseous fission products is chosen in dependence of the intensity of radiation. As to the thickness of the individual layers relative to each other, it is to be noted that the zirconium-alloy layer 44, the aluminium layer 42, the layer 38 of cadmium, boron, hafnium or beryllium, and the titanium layer 32 have substantially the same thickness while the lead layer 36 is substantially of triple thickness in comparison to each of the before-mentioned layers. The relation of the individual layers with respect to their thickness is graphically rendered in FIG. 2; in this graphic representation, the steel-reinforced concrete wall of the reactor building 30, having a thickness of about 1.50 m, can be shown in part only.