Patent Number: 052290670
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of International Application Serial No. PCT/DE90/00880, filed Nov. 15, 1990. The present invention relates to a liquid metal cooled nuclear reactor for a nuclear power plant on the modular principle in a reactor cavern fitted with cooling surfaces, having a reactor tank which contains a reactor core, one or more heat exchangers and primary pumps. Such a nuclear reactor is known from Published European Application No. 0 308 691 A1. According to the modular principle, which has already been proposed in the case of other types of reactors, the overall output of a nuclear power plant is no longer to be achieved with a single large reactor but instead with a plurality of small reactors. In this way, in the event of failure, inspection or maintenance of a reactor, the nuclear power plant can continue to be operated with reduced output. In comparison with the liquid metal cooled nuclear power plants heretofore constructed or planned, having only one reactor with an individual output of 1000 MW or more, nuclear power plants which include a plurality of modular reactors having an individual output that in each case is about 200 MW do admittedly have some disadvantages with respect to fuel exploitation and breeding rate, but also have appreciable advantages for construction and operation. In the event of failure of all of the active systems, a modular reactor can give off its residual heat to the outside completely through its surface. The applicant in German Patent DE 31 15 844 C2 has for the first time specified a sodium cooled nuclear reactor on the modular principle and presented the advantages of that principle. The configuration provided therein of the electro-magnetic pumps outside the reactor tank has appreciable advantages with respect to the installation, inspection and exchange of such pumps. However, it disturbs the unhindered residual heat removal from the reactor core through the tank wall to the outside if the normal heat removal through the secondary circuits to one or more steam circuits fails. However, the passive residual heat removal is a particular advantage of modular reactors and contributes considerably to the simple and inexpensive construction thereof. The reactor tank, which is filled with sodium, is disposed in the usual way in a second, inert-gas-containing so-called double tank, is supported at its lower end on a lower end ring and is guided at its upper end with a ring in a removable support ring, which rests on the upper edge of a reactor cavern of concrete. The double tank is intended to take up the liquid sodium in the event of damage occurring to the reactor tank. In Published European Application No. 0 118 016 A1, once again the advantages of the modular principle in transport and installation are described in detail and numerous structural details are explained more fully. Disadvantages of that construction are the penetrations of the outer wall of the tank which are provided there and the configuration underneath the reactor tank of a central electromagnetic pump, which results in an appreciable increase in the overall height and causes appreciable additional effort in the inspection and exchange of such pumps because the coolant first has to be drained off. Another disadvantage is that, in the event of failure of the secondary coolant circuits, the residual heat has to be accumulated in the biological shield, that is to say first of all in the concrete outside a double tank of steel. In U.S. Pat. No. 4,650,642, a nuclear reactor is described which is intended in particular to carry away the residual heat after a core meltdown, which in fact is extremely improbable, and to avoid damage to the environment. For that purpose, a thick shell of cast steel which is provided underneath the reactor tank is anchored in the ground and is intended to take up the reactor core melted through the reactor tank. The cylindrical side wall of the shell, which is provided as a double tank, is in contact either with a thermal insulation or with a cooling jacket, which is surrounded by a biological shield. Shells of such a type and size are very heavy and expensive and cannot be cast in one piece. They must therefore be welded together from more than one piece. An essential feature of that nuclear reactor is a water/steam cooling system underneath the reactor. The double tank and cavern are just one component. The double tank and cooling surfaces cannot be repaired from outside and it is scarcely possible to remove them. The applicant in Published European Application No. 0 308 691 A1 has specified a further liquid metal cooled nuclear reactor on the modular principle which, in comparison with the nuclear reactors heretofore constructed or planned, not only simplifies the structure but also offers advantages for operation, inspection and repair or exchange. In principle, that reactor is intended to avoid the repeatedly discussed possibility of cracks in the tank or in the structure from spreading, to avoid the core from dropping, and as a result, to avoid the control rods that are suspended from the cover from being withdrawn from the core by having the main parts superposed and resting on the base of the reactor cavern. Consequently, all of the relative movements between reactor core and the control rods in the event of an earthquake are reduced or completely avoided, so that considerable expenditure for safety measures can be avoided. In the event of a failure of the secondary coolant circuits thereof, the reactor can completely give off its residual heat through the surface of its reactor tank and through its double tank to cooling surfaces in the reactor cavern, although they have to be correspondingly reliable due to their great significance in terms of safety. The reliability is achieved by multiple redundancy of the complete cooling system, by emergency power supply and/or by operation in natural circulation. Such measures entail extra costs. It is accordingly an object of the invention to provide a removable liquid metal cooled nuclear reactor, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and in which the residual heat is initially accumulated and then given off to the surroundings. With the foregoing and other objects in view there is provided, in accordance with the invention, in a modular nuclear power plant having a reactor cavern with cooling surfaces, a liquid metal cooled nuclear reactor in the reactor cavern, comprising a reactor tank, a reactor core inside the reactor tank, at least one heat exchanger and primary pump inside the reactor tank, and a double tank of cast iron surrounding the reactor tank and serving as a heat accumulator, the double tank including a plurality of detachably interconnected and superposed or stacked rings and a base. In the base region, this double tank may also have approximately the same form as a double tank which was heretofore welded together from ferritic or austenitic steel sheets. However, it has a wall thickness which is eight to ten times greater, so that its heat capacity is adequate to accumulate the residual heat of the reactor for hours and to give it off to the cooling surfaces in the reactor cavern by radiation and convection. By virtue of the great accumulating effect, the subsequent cooling systems can start up slowly and they can even be switched off or repaired for a limited time. Consequently, the effort which was heretofore involved in making the cooling system immediately available can be considerably reduced, which results in a cost saving. This double tank can be removed with little effort for inspection or repair or for disposal. In accordance with another feature of the invention, the double tank is formed of a nodular graphite cast iron, which is also referred to as spheroidal cast iron. This material, according to DIN 1693, is authorized in nuclear engineering for the production of transfer casks for fuel assemblies. It has already been demonstrated in the case of such transfer casks that they can withstand a drop from a considerable height and elevated temperatures even for a lengthy time. As a cast structure, this heat accumulator can be provided with ribs to improve the heat transfer, without any great effort. The extra costs which arise from the much greater weight of the cast double tank are compensated by the considerably lower price of the cast iron in comparison with the price of welded sheet-steel tanks. By virtue of its great wall thickness, the cast double tank has appreciable advantages with regard to external influences. The critical buckling pressure under external shock pressure and penetration protection are many times higher than in the case of a thin-walled steel tank, so that additional protective measures outside the double tank can be dispensed with or reduced. In accordance with a further feature of the invention, the detachably interconnectable and superposable rings and the base are reinforced and can be screwed to one another by remote operation at their points of contact. In this way, the heavy double tank includes a plurality of parts which can each be individually produced, tested, transported and installed. In accordance with a concomitant feature of the invention, in each case the rings and the base have two annular seals at their points of contact, and a test bore disposed between the seals, which leads to the outside. Consequently, the seals can be monitored from outside. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a liquid metal cooled nuclear reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.