Patent Number: 046577327
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention applies the principle of redundant design in order to seal a primary loop integrated into a prestressed concrete pressure vessel in a failure-safe manner. The prestressed concrete pressure vessel has two redundant seals, namely, a metal liner and a concrete body, which acts as an effective flow limitation. A precondition is that the radioactivity passing through the concrete body must be less than the value permitted for the installation in view of the environment. If this is not the case, the reactor must be shut down as a consequence of a "liner leak". If the condition is satisfied, the reactor may continue in operation, as the redundant seal of the concrete body is failure-safe. In the case of a leak of the inner cover of one of the closure devices, the escaping primary gas is discharged through a filter system and exhaust stack into the environment, while the fission products are retained in the filter system. In the space between the inner and outer covers, therefore, no appreciable excess pressure can be built up, so that the outer cover is failure-safe eve in relation to the sealing function. The outer cover, forming the fourth barrier, is thus comparable to the safety housing of the above-described nuclear reactor installation with a steel reactor pressure vessel. As radioactivity cannot escape into the environment from the primary loop through the prestressed concrete pressure vessel, or the closure devices of the passages, a tight safety housing is no longer necessary, and the tasks of containing the primary gas in a failure-safe manner and protecting the high temperature reactor against outside effects may be performed by the concrete pressure vessel alone. The additional construction effort required is relatively small, consisting essentially of the closure devices with the exhaust of the intermediate spaces. The invention thus makes possible the construction of a high temperature reactor in a highly economical manner, as no safety-associated reactor protection building is required. It is advantageous to locate the pipelines for the removal of primary gas leakages, to their connection with the exhaust stack, extensively in the concrete of the reinforced concrete pressure vessel, in order to contribute to the reduction of the risk potential of said pipelines, and for environmental reasons. The filter systems for the retention of fission products may also be installed within the walls of the prestressed concrete pressure vessel, thereby serving the same purpose. In order to provide securement against external catastrophes, such as an airplane collision, the prestressed concrete pressure vessel may be equipped with an appropriately designed protective installation. This may consist, for example, of a concrete building which simultaneously serves as the architectonic enclosure of the installations of the nuclear reactor. As this building is not charged with radioactivity from the primary loop, it may be structurally simple. A preferred embodiment of the invention can be seen with reference to the drawing. A cylindrical prestressed concrete pressure vessel 1 comprises a center cavity 2, which is clad with a metal liner 3. In the cavity 2, a high temperature reactor 4 is arranged, the core thereof comprising a pile of spherical fuel elements. The fuel elements consist of coated fissionable material particles embedded in a graphite matrix. The graphite matrix and the coating represent a first and a second barrier against the release of fission products. A cooling gas, e.g., helium, flows from top to bottom through the pile, as indicated by arrows in the figure. It is surrounded on all sides by a reflector 5, which is followed below by a hot gas collector space 6. The hot gas collector space is connected by the hot gas channel 7 with a plurality of steam generators 8 arranged about the high temperature reactor 4 in the cavity 2. Several circulating blowers 9 convey the cold cooling gas back into the reactor core. The reflector 5 is surrounded by a thermal shield 10, which also defines the guides for the return of the cold cooling gas. The prestressed concrete pressure vessel 1 comprises several large passages 11 in its roof area, each of which is closed off by a closure device 12. The circulating blowers 9 are arranged in part in said passages, thereby serving the purpose of installation and disassembly of the steam generators 8, and of other reactor components. Each closure device consists of two steel covers 13 and 14 arranged above each other, of which the inner cover 13 forms the seal for the primary gas. Between the two covers 13 and 14 of each closure device 12, an intermediate space 15 is present, and to which a pipeline 16 is connected. This pipeline, which is located for the most part in the concrete of the concrete pressure vessel 1, is connected through a filter system 17 to an exhaust stack 18. The filter system provides for the retention of fission products. To reduce the potential danger, in addition to the pipelines 16, the filter systems 17 are also arranged in the wall of the prestressed concrete pressure vessel 1. In place of the protective reactor building usually provided, and which forms the outermost barrier against the release of fission products, the instant invention provides for the prestressed concrete pressure vessel 1 as a barrier system. The liner thereof acts as a third barrier, and the concrete body, which provides for an extremely tight flow limitation, acts as the fourth and outermost barrier. The prestressed concrete pressure vessel thus has two redundant seals. In the area of the passages 11, the third and the fourth barriers comprise inner covers 13 and outer cover 14 of closure devices 12. In the case of leakage through the inner covers 13 into intermediate spaces 15, the outer covers 14, which are dimensioned for full design pressure, are not stressed appreciably due to the exhausting of the intermediate space through the pipelines 16, thus, they are failure safe as regards their sealing function. There is no need, consequently, for a tight protective reactor building.