Patent Number: 042808723
Section: summary

The present invention relates to a core catcher associated with a fast nuclear reactor of the liquid-metal cooled type having a pressure vessel. This device is intended to collect reactorcore elements which are liable to melt either partially or completely as a result of a fast temperature rise caused by a major operational accident condition and then to fall under gravity to the bottom of the reactor pressure vessel which contains the liquid metal. A number of different designs of so-called core catchers or catchpots are already known. A device of this type generally consists of a structure of mechanically welded sheet metal plates mounted beneath a diagrid which supports fuel assemblies of the reactor core, the diagrid being in turn supported by a metallic structure which bears on the bottom wall of the pressure vessel. In a typical design, a core catcher is constituted by an array of cups or the like which are separated from each other and each capable of collecting part of the molten reactor core. The core catcher is so arranged that the liquid metal coolant contained within the reactor vessel is caused by an effect of convective heat transfer to circulate around the core catcher while producing a cooling action on this latter. As disclosed in particular in the U.S. Patent Application Ser. No. 589,881, now abandoned, filed on June 24, 1975, provision has already been made in a core catcher of this type for passages which extend through the diagrid support structure. Thus the cooled liquid metal collected within the pressure vessel at the outlet of heat exchangers of the reactor is circulated through said passages in order to produce a powerful cooling action on the core catcher while preventing any unacceptable temperature rise by removal of residual heat from the portions of molten reactor core which have thus been collected. This accordingly affords a satisfactory degree of protection of the other reactor structures contained in the pressure vessel. The present invention is concerned with another design concept of a core catcher in which the circulating liquid metal coolant is confined within an enclosed space, said core catcher being so arranged as to prevent any collected core debris from coming into contact with the pressure vessel. To this end, the device under consideration essentially comprises a collecting tray having a large area which is placed beneath the reactor core and pierced by a central chimney, and a bearing structure for said collecting tray extending in the form of a shell which is substantially parallel to the bottom wall of the pressure vessel, there being delimited between the bottom wall, the diagrid support structure and the diagrid an enclosed space which contains the collecting tray and in which a natural circulation of liquid metal can be induced under the action of the temperature differences arising from the fuel deposited on the collecting tray and by virtue of the presence of the chimney, said natural circulation being such as to ensure sufficient cooling of said fuel. During normal operation, the liquid metal contained in the enclosed space thus defined is practically stagnant except for leakages at the bottom end-fittings of the fuel assemblies through the diagrid and collected within said enclosed space, the leakage flow being intended to serve in a manner known per se to supply an annular region which is located between the pressure vessel and a parallel baffle wall and communicates with said enclosed space. In the event of a major accident which results in particular in partial or total melt-down of the reactor core, the core is collected on the tray within said enclosed space and produces an appreciable rise in temperature of the liquid metal in which said tray is immersed. A circulation of liquid metal is consequently established between the hot zone constituted by the collecting tray and the colder zones constituted by the walls which delimit said space, i.e. the walls forming the diagrid and the diagrid support structure and especially that surface of the diagrid support structure which is in contact externally with the mass of colder liquid metal contained in the pressure vessel. By virtue of these arrangements, the tray which serves to collect the debris from the molten reactor core can be placed beneath the core in the lowest portion of the enclosed space in such a manner as to ensure that said collecting tray has the largest possible area and extends in particular beyond the lateral limits of the reactor core. Preferably and in accordance with a particular feature of the invention, the collecting tray which is placed beneath the reactor core has a circular peripheral contour and a slightly conical shape from the periphery to the center in order to facilitate the circulation of liquid metal within the enclosed space. As an advantageous feature, the central chimney for the circulation of liquid metal is provided with an inclined top cover-plate carried by small columns, said cover-plate being intended to protect the pressure vessel against elements of the molten reactor core which fall onto the collecting tray. In accordance with a secondary feature, the collecting tray can have two walls separated if necessary by a clearance space. The top wall is advantageously formed by successive adjacent sheet metal elements provided with an overlapping zone from one element to the next. In accordance with a further distinctive feature, the collecting tray is supported on the bearing surface by means of a rigid structure constituted by vertical radial ribs which carry the collecting tray and are braced with respect to each other by means of circumferential stiffening members. The ribs are preferably provided with notches in order to reduce thermal stresses on those edges of said ribs which are in contact with the collecting tray and with openings for the circulation of liquid metal between said ribs. In accordance with yet another distinctive feature, the bearing surface which supports the collecting tray is suspended from the diagrid support structure by means of suspension members, that face of said bearing surface which is directed towards the pressure vessel being provided with shoes which are capable of coming into contact with said vessel in the event of failure of said suspension members.