Patent Number: 046719229
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a nuclear reactor cooled by a liquid metal and more specifically relates to supporting the liquid metal-filled vessel and the contained reactor core. It is known that for the purpose of providing biological protection the vessel of a fast neutron nuclear reactor is placed in a concrete vessel shaft and its upper part is sealed by a metal slab, whose gaps and openings are filled with concrete. In solutions used on the Rhapsodie, Phenix and Super Phenix reactors, the slab rests on an annular bearing surface formed in the upper part of the vessel shaft and the vessel is directly suspended on said slab. The vessel is filled with a primary liquid metal such as sodium, which cools the reactor core by transferring the heat given off in the reactor core to exchangers in which a secondary fluid circulates, which is generally also sodium. The circulation of the liquid metal in the core and in the exchangers is brought about by pumps. The primary sodium temperature is close to 540.degree. C. at the outlet from the core, i.e. in the upper part of the vessel, whilst it drops to about 400.degree. C. at the outlet from the exchangers, i.e. in the lower part of the vessel. Hereinafter the term "hot" corresponds to the temperature of the liquid metal leaving the core, the term "tepid" to the temperaure of the liquid metal leaving the exchangers and reentering the core and the term "cold" to a temperature close to ambient temperature, but which may for example reach the fusion temperature of the liquid metal, i.e. approximately 100.degree. C. for sodium. In the best known constructions the reactor vessel is suspended from the upper slab and transmits to it a considerable load. To prevent excessive creep of the upper part of the vessel walls, it is necessary to cool the same by circulating tepid sodium along the said walls. This leads to a certain thermodynamic loss and this arrangement also requires the use of baffles which are difficult to construct. In addition, suspended vessel are to a certain extent sensitive to possible seismic movements. It has also been previously proposed to place the tepid vessel bottom on the bottom of the cold vessel shaft by means of distributed supports permitting radial differential expansion movements. These supports have, for example, been constituted by rollers or rods, or even Stellite or graphite blocks. This solution has not been adopted because it did not appear to offer sufficient reliability. Another solution which has in fact been used consists of supporting the tepid bottom vessel in its lower peripheral part by radially displaceable supports. The supports then receive high individual stresses, but can be more easily inspected. However, compared with suspended vessels there is a need to radially transfer internal loads. BRIEF SUMMARY OF THE INVENTION The present invention relates to the construction of a fast neutron nuclear reactor in which the support procedure adopted for the vessel makes its manufacture easier and less expensive than that of the aforementioned, known solutions. In addition, said vessel has safety and reliability characteristics which are at least as good as those of existing vessels. Therefore the present invention proposes a nuclear reactor cooled by a liquid metal comprising a liquid metal-filled vessel, which contains the reactor core, a sealing slab sealing off the upper part of the vessel and a vessel shaft in which the vessel is located, wherein the bottom of the vessel rests on the bottom of the vessel shaft, wherein means are provided for cooling the bottom of the vessel to a temperature close to ambient temperature and wherein the inner areas of the vessel traversed by a forced flow of liquid metal are entirely positioned above a horizontal limiting plane which, at the transition height between said plane and the bottom of the vessel, leads to a thermal stratification of the liquid metal and consequently to an appropriate limitation of the thermal stresses in the vessel walls and the internal structures. Preferably the transition height is equal to at least 1/10 of the vessel diameter. The use of a cold vessel bottom makes it possible to support the vessel and all the internal loads on the vessel shaft bottom by means of supports which are very simple because they are submitted to low thermal gradients and stresses. Preferably the space formed between the vessel shaft bottom and the vessel bottom and which is traversed by the supports is used for the circulation of a fluid for cooling the vessel bottom. This space can also be vertically divided into a lower space ensuring a centripetal circulation of the fluid and an upper space ensuring a centrifugal circulation in contact with the vessel bottom, the two spaces being separated by a fairing which can have detachable parts to facilitate inspection. The vessel supports traverse this fairing. The two spaces are able to communicate in the vicinity of the vessel axis by a baffle which can be filled with liquid metal in the case of a leak, stopping the circulation of the cooling fluid which is liable to react with the liquid metal. In order to limit shape defects and faults during the construction of the vessel shaft bottom and the vessel bottom and in order to permit the centering of the vessel resting by gravitation on its supports, a substantially conical, downwardly pointing shape is advantageously chosen for the two bottoms. The transition height corresponding to the temperature gradient at the vessel bottom leads to a heat loss, which is acceptable in a high power reactor. However, this loss can be reduced by placing within the said transition height thermal insulating inclusions having a thermal conductivity below that of the liquid metal, so as to limit the downward heat flux over the said height. Finally, in view of the fact that the vessel rests on the vessel shaft bottom, the connection between the upper part of the vessel and the slab merely serves to seal the primary confinement area, whilst permitting expansions of the vessel. To this end the vessel side wall can be connected to the slab via an expansion bellows with bending corrugations.