Patent Number: 048428132
Section: summary

FIELD OF THE INVENTION The present invention relates to nuclear reactors and their internal equipments, and more particularly to pressurized water nuclear reactors, called PWR. However the invention may concern any type of reactors comprising a pressure resistant vessel for containment of a pressurized coolant, a removable cover for said vessel, coolant inlet nozzle means provided through said vessel and coolant outlet nozzle means provided through said vessel located in proximity of the cover, a core located within the vessel, arranged to be upwardly traversed by the coolant in operation and comprising a plurality of mutually adjacent fuel assemblies, a plurality of clusters of control elements vertically movable by drive shafts projecting through the cover of the reactor for moving the control elements into and out of said core, and internal equipments including upper internal equipments located between the core and the cover. The invention also relates to reactors having internal equipments comprising lower internal equipments located between the core and the bottom of the vessel and having probe guiding means for guiding elongated probes into and out of the core and penetrating into the core through the lower part of the assemblies. STATE OF THE ART Different types of nuclear reactors with their respective internal equipments are already known. European Pat. No. 0,125,326, relating to upper internal equipments for pressurized water reactors, discloses an architecture having the coolant entering the reactor through one or more coolant inlet nozzles, flowing along the inner surface of the vessel for penetrating into the core through the bottom part of the core of the reactor, traversing said core therefore removing some of its heat, and then coming out of said core for penetrating into a chamber including guide tubes for clusters of control elements before entering a coolant distribution calender which deflects the coolant towards coolant outlet nozzles. The guide tubes are for guiding clusters and their drive rods or drive shafts. The coolant leaving the core is flowing through the guide tubes of the clusters or clusters guides. This solution has different drawbacks. The cluster guides being traversed by the coolant, are subjected to shocks and turburlences due to said coolant passage. This for example, may damage the drive rods of the clusters of control elements. Moreover, if the primary coolant system is accidentially damaged in its cold part before entering the vessel, the volume of water situated above the core in the upper internal equipments will poorly contribute to the emergency cooling of the core because said volume of water, on one hand will be at a high temperature (the core outlet temperature) and on the other hand will tend to disappear shortly through the breach. Concerning lower internal equipments of nuclear reactors of the prior art, they typically include in the low part of the vessel, a structure formed of guide tubes for guiding elongated probes into an out of the assemblies from underneath the core. The guide tubes for elongated probes sealingly project into the reactor vessel from underneath through the vessel bottom and are disposed in the prolongation of the guide tubes. This forms weak points, which in case of several breaks are likely to cause emptying of the vessel and dewatering of the core. Another solution consists in connecting the core monitoring instrumentation through the cover of the vessel. This requires disconnecting said instrumentation whenever it is necessary to remove the vessel cover. However, it might seem to a man skilled in the art that the internal equipments such as defined above were sufficient. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved reactor; it is a more particular object to provide a reactor the internal equipments of which provide: a better protection for the cluster guides for guiding the clusters of control elements and their drive shafts, by avoiding to have coolant gone out of the core traverse said cluster guides, PA1 an improved safety in case of a primary coolant system breaking on the cold part of the circuit located upstream of the coolant inlet nozzle, by creating a substantial water reserve formed by the volume situated above the core and through which the coolant does not pass, the emptying time of which volume may be controlled, its temperature being close to the coolant inlet temperature into the vessel. PA1 the use of the differential pressure existing between the sky of the vessel and the chamber including the cluster guiding means due to the hydraulic pressure and to the head losses; this creates a flow directed downwardly which forms an aid to clusters falling and may also be used for cooling fertile control elements, PA1 a minimization of the number of penetrations of mechanisms driving the drive shafts of the clusters of control elements through the vessel cover. PA1 separating means directly located above said core for collecting the coolant flowing out of said core and comprising a lower perforated plate and an upper perforated plate, each having a plurality of holes for passage of said coolant through said plates, each of said holes in said lower plates being connected to a respective one of said holes in said upper plate by a spacer tube for guidance of the coolant within and along said spacer tube through said separating means, said separating means further comprising a plurality of cluster guides each one located along said spacer tubes for guiding a respective one of said clusters and their corresponding drive shaft through said separating means, and an external envelope connected to said upper and said lower plates, PA1 a plenum chamber for collecting the coolant flowing out of said spacer tubes through the holes in said upper plate and directing said coolant toward the coolant outlet nozzle means, and PA1 a plurality of guiding tubes in said plenum chamber aligned with said spacer tubes for guiding the drive shafts of said clusters through said plenum chamber. PA1 at least one sleeve sealingly projecting through and fixed to said vessel above the coolant inlet and outlet nozzle means, and PA1 probe guide ducts each arranged for receiving at least one elongated probe, said guide ducts following a path penetrating into the vessels through said sleeve, projecting downwardly along and fixed to said cylindrical barrel, extending below the lower supporting plate of the core toward which said probe guide ducts are transversely distributed and directed for traversing said supporting plate and terminating in close proximity of the lower part of the assemblies. PA1 a nuclear reactor further comprising a cylindrical external barrel located within the vessel and spaced from an inner surface of said vessel, said barrel containing the core, the separating device and the plenum chamber, and being arranged to direct a major part of the coolant introduced into the vessel through coolant inlet nozzle means into a space under the core, wherein the upper internal equipments further comprise means for driving some of said coolant, introduced through said inlet nozzle means, above the plenum chamber and said guiding tubes for downwardly supplying a flow of coolant through said guiding tubes into the separating means. PA1 the spacer tubes are vertical and distributed at nodal points of a hexagonal network therefore defining vertical spaces having a substantially hexagonal cross-section between each group of six adjacent ones of said spacer tubes, and wherein each of said vertical spaces contains one of said cluster guides and has a axis coinciding with the axis of said cluster guide, the transversal perforated plates thereof being held in radial abutment against each of the six spacer tubes defining said space. PA1 the separating means form the lower part of the upper internal equipment suspended or maintained in the vessel by static clamping between the cover and the vessel. PA1 the reactor whose fuel assemblies each comprises an upper end piece; the lower plate of the separating means comprises passages for individual control elements of said clusters into and out of the core; and said passages comprise first connecting means for slidably connecting the lower plate to a respective end piece of a fuel assembly and second connecting means for connecting the corresponding cluster guide to said lower plate. PA1 the nuclear reactor further comprises water coolant injection means including at least one pipe whose outlet is located in proximity of the lower part of the separating means. PA1 the guide tubes are curved with radii of curvature having a sufficient value for allowing an elongated probe to be slidably engaged and withdrawn from outside the reactor into and out of said guide ducts. PA1 the internal equipments further comprise lower internal equipments having a bottom wall forming with the lower supporting plate of the core a remote and low disturbed enclosure for the coolant, said bottom wall and said lower supporting plate having a plurality of holes for passage of the coolant through said enclosure into the core, each of said holes in said wall being respectively connected to a respective one of said holes in said lower supporting plate by a duct for driving the coolant within and along said duct into the core assemblies, said enclosure containing the terminating parts of the probe guide ducts which are extending below the lower supporting plate. It is therefore possible to omit the safety injection accumulators external to the vessel which are used in prior art in case of such breaking accident. To this end, the invention provides a nuclear reactor comprising: (a) a pressure resistant vessel for containment of a pressurized coolant, PA0 (b) a removable cover for said vessel, PA0 (c) coolant outlet nozzle means provided through said vessel in close proximity of said cover, PA0 (e) a core located in said vessel arranged to be upwardly traversed by said coolant in operation and having a plurality of mutually adjacent fuel assemblies, PA0 (f) a plurality of clusters of control elements, each vertically movable into and out of said core by a drive shaft, PA0 (g) upper internal equipments located in said vessel between said core and said cover having: PA0 (a) a pressure resistant vessel for containment of a pressurized coolant, PA0 (b) a removable cover for said vessel, PA0 (c) coolant inlet nozzle means provided through said vessel and coolant outlet nozzle means provided through said vessel in close proximity of said cover, PA0 (d) a core located in said vessel arranged to be upwardly traversed by said coolant in operation, having a plurality of mutually adjacent fuel assemblies and supported by a perforated supporting plate having a plurality of passage holes for passage of said coolant into the core, PA0 (e) upper internal equipments located above said core, PA0 (f) a cylindrical external barrel located within said vessel and spaced from the inner surface of said vessel, said barrel containing the core and the upper internal equipments, and PA0 (g) probe guiding means for slidably guiding flexible elongated probes into and out of said core through the lower part of the assembly, comprising: Above described internal equipments are particularly advantageous in reactors having a large number of clusters such as spectrum variation reactors called RVS and/or undermoderated reactors called RSM which use clusters of regular control elements and clusters of elements for varying the neutron energy spectrum. The invention also provides a nuclear reactor comprising: This arrangement permits to suppress penetrations occuring below the coolant inlet and outlet nozzles, for example in the vessel bottom, and this in addition to the results already mentioned. The invention also provide some preferred embodiments as follows: The guiding means for the clusters or "cluster guides" are units formed as mechanically welded parts. They include transverse perforated plates or "guide cards" connected together by continuous guide sleeves and by square bars extending over the whole height of the cluster guide. Some guide cards further have a lateral radial abutment against the spacer tubes so as to provide good transverse rigidity for the guiding means and more generally the separating means.