Patent Number: 039309398
Section: summary

BACKGROUND OF THE INVENTION This invention relates to gas-coolant nuclear breeder reactors. Such a reactor is totally enclosed by a steel containment vessel. The reactor includes a core enclosed by a pressure vessel, the core including fuel elements each comprising a bundle of fuel rods which, if operated at excessive temperatures, may melt and fall. Therefore, a melt core intercept basin is positioned beneath the core. In the event the melted core falls into this basin, the core's heat must be dissipated rapidly because if uncontrolled, there may be a excessive pressure increase inside of the steel containment vessel which might damage the integrity of the latter. With adequate heat dissipation, such a pressure rise may be suppressed to a safe level. The prior art has suggested a basin inside of the pressure vessel directly beneath the core and which is provided with indirect cooling means in the form of water pipes. The idea is to dissipate the heat, presumably through evaporation of the water forming steam, but the prior art has made no provision for handling the resulting steam in a practical manner. A concept of this kind is suggested by the German Offenlegungsschrift 2,035,089. SUMMARY OF THE INVENTION An object of the present invention is to provide a basin beneath the core with means for dissipating the heat of a melted core in the basin in a practical manner and at a rate preventing an excessive pressure increase within the steel containment vessel. According to the invention, the basin may be located outside or inside of the pressure vessel, providing it is beneath the core. When the core melts, it can burn through the bottom of the pressure vessel and fall into the basin if on the latter's outside. If the basin is inside of the pressure vessel and can dissipate the melted core's heat with adequate efficiency, it may be possible to prevent the bottom of the pressure vessel from being burned through. When outside of the pressure vessel, the basin may be very much larger transversely than the inside dimension of the pressure vessel. In this case the invention provides means, such as suitable ducts or conduits, for conducting a fluid coolant cooling the metal core in the basin, from the latter to an extended or large area of the inside of the steel containment vessel for cooling by conduction of the heat through the latter's wall to its outside which is exposed to the atmosphere. The large outside surface area of the containment vessel provides extensive heat dissipation both by convection and radiation. This external heat dissipation may be considerably increased by spraying water on the outside of the containment vessel. Using ducts, the heated coolant is carried to the inside of the top of the containment vessel where the coolant gives up its heat, the heat loss being rapid if the top of the containment vessel is water-sprayed on its outside, the cooled fluid coolant then falling within the containment vessel for return to the basin beneath the pressure vessel. Thus, a circulation is established. The basin may be positioned inside of the pressure vessel beneath the core and provided with bottom cooling means, such as a chamber beneath the basin inside of the pressure vessel and containing water converting to steam when heated by the melted core, via conduction through the basin, and thereby forming the fluid coolant conducted by the conducting means to the inside of the containment vessel, preferably to the top of the containment vessel. In the first instance, when the basin is outside of the pressure vessel, the fluid coolant may consist of the gas atmosphere normally maintained inside of the containment vessel during normal reactor operation. This gas is capable of circulating, rising when heated, and falling when cooled. When the basin is on the inside of the pressure vessel, the steam rises when hot and falls when cooled or as condensate, effecting circulation via the chamber beneath the basin on the inside of the pressure chamber. In the latter instance, the top of the containment vessel may be double-walled to form a chamber into which the steam is sent and from which the condensate is returned to the chamber beneath the basin. When the basin is inside of the pressure chamber, a refractory, such as graphite, is indicated as the basin material, and in this case, particularly, the uniformity and efficiency of the necessary heat transfer may be increased by the use of a low melting temperature metal positioned either inside the basin or in passages formed in the basin or in the chamber beneath the basin or combinations of these possibilities, such a metal forming a molten bath providing both uniformity and efficiency for the heat transfer to the coolant by conduction through the molten metal and uniform transfer relative to the basin which may be possibly of low thermal conductivity.