Patent Number: 047073276
Section: description

Referring now to the figure of the drawing in detail, there is seen a concrete structure 1 serving as a radiation shield, and a steel pressure vessel 2 in the concrete structure which is provided with a detachable cover 3 at its upper end. A core barrel 4 which is also metallic is disposed within the pressure vessel 2. The metallic core barrel 4 has a support or mounting in the form of a circular projection 5 which rests on a similarly circular flange 6 formed at the inside of the pressure vessel 2. The circular projection and flange divide the space between the core barrel 4 and the pressure vessel 2 into an upper space 8 and a lower space 7. The contact surfaces between the circular projection and flange are constructed with respect to their size and type of surface in such a manner as to form a gas-tight seal which is effective even for small pressure differences, between the lower space 7 and the upper space 8 between the pressure vessel and the core barrel. The core barrel 4 contains ceramic internal parts 9 formed of carbon blocks and/or graphite which surround a space for accommodating a fission zone 10 formed of a multiplicity of spherical fuel elements. Among other things, canals 11 are extended through the internal parts 9. Absorber rods can be moved through the canals 11 for controlling the fission zone, by means of conventional drives 12 disposed on the ceiling of the core barrel 4. The internal parts 9 also contain canals 13, through which cooling gas enters the space 7 from a non-illustrated heat sink through a hot-gas line 16. The cooling gas is conducted to an upper plenum 14, thereby cooling the internal parts 9, and is sent from the plenum 14 through the fission zone from top to bottom. The hot cooling gas flows together into a lower plenum 15 and is conducted to the heat sink through the hot-gas line 16 coaxially disposed in a nozzle 17 of the pressure vessel 2. The upper space 8 is likewise filled with helium which is preferably used as the cooling gas. However, the cooling gas in the space 8 is stagnant and can therefore be kept at a slight overpressure relative to the lower space, so as to prevent contamination of the gas by impurities which are unavoidably present in the cooling gas proper and can be further activated while passing through the fission zone. Since the core barrel 4 is gas-tight above the support 5, 6, the nuclear reactor cover 3 can be removed for repair and servicing purposes (such as to provide service on the absorber rod drives 12), without impairing the accessibility of the parts which will then be exposed due to radioactive contamination and without the danger of the air which than fills the upper space 8 from reaching the internal parts 9 and corroding them. The lower space 7 and the upper space 8 are in connection with each other through a first schematically-illustrated equalization line 18 which can be shut off by a valve 19. The line 18 provides the mutual matching of the respective pressures corresponding to the different operating conditions, required in normal reactor operation. However, the line 18 is not sufficient for equalizing the pressure differences suddenly occurring in the event of a major leak in the pressure vessel 2. For this purpose, a second equalization line 20 is provided. The line 20 begins near the lower end of the core barrel 4 and is brought through the gap between the core barrel 4 and the internal parts 9 to the upper end of the core barrel 4 where it is provided with a schematically-illustrated rupture disc protector 21. If required, the protector 21 quickly releases a flow cross section sufficient for pressure equalization and thus prevents a possible lifting of the core barrel 4 from its mounting 5, 6. The decay heat still developing even if the reactor is then shut down, sets the cooling gas contained in the interior of the core barrel 4 in a convective flow, which carries out the decay heat removal from the fission zone 10. The second equalization line 20 which is then open, carries the cold and therefore heavier air which has penetrated into the upper space 8, into the lower space 7, filling it. However, the air cannot enter into the interior of the core barrel 4 because this is prevented by the temperature stratification between the air and the hot gas. The foregoing is a description corresponding, in substance, to German Application No. P 33 45 457.4, dated Dec. 15, 1983, International priority of which is being claimed for the instant application, and which is hereby made part of this application. Any material discrepancies between the foregoing specification and the specification of the aforementioned corresponding German application are to be resolved in favor of the latter.