Patent Number: 048428132
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a pressurized water reactor 1 includes a pressure resistant vessel 2 containing a coolant closed by a removable cover 3, coolant inlet nozzles 4 and coolant outlet nozzles 5, a core 6 with a lower supporting plate 7 perforated with holes 8 for allowing coolant to pass therethrough upwardly into the fuel assemblies 9 and clusters 10 of control elements vertically movable by drive shafts 11 through the cover 3 of reactor 1. The reactor also includes upper internal equipments 12 located between the core 6 and the cover 3. A cylindrical external barrel 13, spaced from the internal surface 14 of vessel 2 and having an upper collar 15 maintained between cover 3 and vessel 2, drives the coolant flow entering through inlet nozzle 4 and directs it along the inner surface 14 of the vessel 2 so that it penetrates into the core 6 through the low part of the assemblies 9. Passage means 16 at the level of the collar 15 are provided for by-passing the core, a small part of the coolant. The coolant is directed to a volume 17 defined by the cover 3 of the reactor 1 and by the top part 18 of the upper internal equipments 12 of the reactor. A plenum chamber 19 collects the coolant coming out of the core and directs it transversely towards the coolant outlet nozzle 5, out of the vessel. The coolant traverses the cylindrical barrel 13 through holes 20 disposed in line with the outlet nozzles 5. Guiding tubes 21 guide the drive shafts 11 of clusters 10 through the chamber 19 and protect said drive rods from coolant turbulences. The upper internal equipments 12 further include separating means or device 22 having guides 23 for clusters 10 and for the shafts 11. The separating device is disposed between the core 6 and the plenum chamber 19. Said device 22 further comprise a lower plate 24 perforated with holes 25 allowing the coolant to flow from the core 6 into said device 22, and with holes 26 for movement of clusters 10 and their control rods 11 through the lower plate into an out of the assemblies 9 of the core. The device 22 has an upper plate 27 perforated with holes 28 for the coolant to pass into the chamber 19 and holes 29, placed in line with the guiding tubes 21 of the drive shafts 11 of clusters of control elements. Both plates of the device 22 are connected together at their periphery by an envelope 30 therefore forming an enclosure. The envelope constitutes the radial external limit of the separating device 22. A minimum clearance between device 22 and the cylindrical external barrel 13 is provided. Spacer tubes 31 connect together the coolant passage holes 25 of the lower plate 24 with the coolant passage holes 28 of the upper plate 27 and are disposed between the cluster guides 23. The cluster guides 23 may preferably be open mechanically welded, tubular units formed by perforated plates or "guide cards" 32 spaced apart by continuous guide sleeves 33 and square bars 34 extending over and along the whole height of the cluster guides (FIG. 5). FIGS. 4 and 5 show a preferred embodiment with spacer tubes 31 disposed according to a triangular pitch. The spacer tubes are symetrically disposed at the six apices of a hexagon, the center of each hexagon being located on the axis of a corresponding cluster guide 23. FIG. 5 shows more particularly in cross-section one half of a "card" 33 and three of its six associated spacer tubes 31. The cluster guide 23 is in radial abutment against each of the six surrounding tubes through bosses 35 spaced apart along the tubes; it is fixed in a way known perse to the upper plates 27 of device 22. FIG. 7 shows a partial top view of the upper plate 27. The cluster guide 23 is shown schematically by its dotted contour. It is rigidly fixed by means of three screws 39 to plate 27. Hole 29 is for the passage of the drive rod 11 of the cluster guided by the cluster guide 23. In the embodiment shown by way of non limitative example of FIG. 6, the cluster guide 23 is centered resiliently and may expand into the lower plate 24, being slidely fitted in holes 26. These holes receive from underneath the upper end pieces 37 of the corresponding fuel assemblies 9. The spacer tubes 31 are moreover rigidly fixed thereto by screws 36 in the lower plate 24. Reinforcements 38 are finally formed in the low part of the spacer tubes 31 conveying the coolant. In another preferred embodiment of the invention, the separating means 22 form the lower part of the upper internal equipment 12 suspended by its collar 15 supported by the vessel and pressurely maintained by the cover 3. External cooling water may be introduced by water coolant injection means 40 directly into the separating means through at least one pipe 41 whose outlet is advantageously located in proximity of the lower part of the separating means, near the upper part of the core. According to the invention, the internal equipments of the reactor 1 further include probe guiding means or device 42 for guiding elongated probes into and out of the core through the lower part 43 of the fuel assemblies 9. This device includes probe guide ducts 44 each intended to receive at least one elongated probe (not shown). These guide ducts 44 sealingly penetrate the vessel 2 of the reactor 1 through sleeves 45 shown with dotted lines on FIG. 1 and on FIG. 3. These sleeves 45 are situated above the coolant inlet 4 and outlet 5 nozzles. Ducts 44 then extend downwardly to below the lower plate of core 7 along the external barrel 13 which fixedly supports said tubes, the clearance existing between device 22 and said barrel allowing their passage. Fluid tightness of plate 24 with respect to barrel 13 is carefully provided so as to prevent the coolant coming from the assemblies from taking the path existing between the envelope 30 and barrel 13. The volume which is defined there is relatively tranquilized. The guide ducts 44 are then transversely distributed are directed for traversing the supporting plate 7 of the core from underneath and for terminating in close proximity of the lower part of the assemblies 9. For removing and engaging an enlongated probe from the outside of the reactor, a sufficient value for the radii of curvature is provided for the guide ducts 44. In a preferred embodiment of the invention, the lower part of the lower internal equipments 46 includes an enclosure formed by the lower support plate 7 of the core and a wall 47 perforated with coolant passage holes 48. Ducts 49 connect said passage holes 8 in wall 47 to lower plate 8 and channel the coolant toward the core. A volume 51 protects from turbulences is thus defined, in which the probe guide ducts 44 are distributed before terminating in close proximities of the lower part of the assemblies. The enclosure is filled through holes 50 formed for this purpose in wall 47 and air purged during the initial filling of the reactor by means provided in the support plate of the core. Normal operation of the nuclear reactor with internal equipments such as described in the invention is described hereafter. Black arrows on FIGS. 1 and 2 indicate the path followed by the main coolant flow in the reactor. The cold coolant enters reactor 1 through the inlet nozzles 4. The main coolant flow is deflected by external barrel 13 and for its most part downwards to the bottom of the reactor wherein it penetrates into the ducts 49 through which it flows before upwardly traversing the assemblies 9 of the core 6 of the reactor. The flow of coolant coming from each assembly is then deflected by the upper end piece 37 of said assembly towards the inlet holes 25 of the spacer tubes 31 of device 22. The collant flow, heated by the core, then traverses device 22 through said tubes 31 and arrives in the plenum chamber 19 where it is again deflected transversely among the guide tubes 21 of drive shafts 11 toward the outlet nozzles 5 after traversing the cylindrical barrel 13 through holes 20 disposed in line with the outlet nozzles. The coolant coming from the assemblies is therefore isolated along all its travel path, from the coolant volumes surrounding the clusters and their drive shafts contained in device 22 and in guide tubes 21. These volumes are directly connected to the capacity 17 situated under the cover of the reactor. This capacity is overpressurized with respect to the coolant leaving the assemblies. Therefore an axial downward current, shown by white arrows in FIGS. 1 and 2, is existing in tubes 21 and device 22. This downward current constitutes an aid to clusters falling which is an important safety advantage. It may also provide cooling of the clusters if some of them are for example fertile clusters as in case of spectrum variation reactors. Passage means 16 at the level of collar 15 of the cylindrical barrel 13 are provided to direct a reduced cold coolant flow coming from the inlet nozzle, toward capacity 17 situated under the cover of the reactor. This is indicated on FIG. 1 by a thin black arrow. During a reactor shutdown for refueling, the device for guiding the elongated probes does not need to be removed when the upper internal equipements are removed for access to the fuel assemblies. The probe guide ducts 44 penetrates into the vessel through sleeves 45 located under the inlet and outlet coolant nozzles. Then, they are supported by the external barrel 13 down to below the core. Therefore they are independent of the upper internal equipments. In some accidental conditions, a nuclear reactor according to the invention is particularly advantageous. One of the most penalizing situation with regard to safety is the loss of coolant due to a rupture in the primary coolant circuit. Internal equipments of a reactor according to the invention will permit the above-described flow shown with white arrows in FIGS. 1 and 2, to enter the core downwardly, even after a rupture of said primary coolant circuit just before the coolant inlet nozzle into the vessel. Relatively cold coolant existing in the separating means and the dead volume 17 under cover 3 via the guide tubes 21 of the drive shafts 11, will directly fill the core in case of accident. The gravitation force and the overpressure which reign at the beginning of an accident in device 22 and volume 17 direct the coolant to the core through cluster guides 23 and lower plate of device 22. Immediately after the accident, the coolant volume having the vessel inlet temperature, and which is available in device 22 and in capacity 17 will provide cooling of the core for a time of the order of 30s, sufficient to allow the medium pressure safety injection pumps 52, MPSI, known in the prior art and forming part of the injection means 40, to start up and to reach their nominal delivery rate. In an advantageous embodiment, but in no way limitative, the MPSI pumps 52 inject the coolant in the lower part of device 22. Two penetrations 53 pierced in the vessel flange and six pipes 41 will distribute the coolant in close proximity of the upper part of the core. To inject coolant at the lower part of device 22 limit heat exchanges between the cold fluid delivered by the MPSI pumps and the fluid at the vessel inlet temperature or the vapor which are contained in device 22 and capacity 17, thus avoiding a pressure drop, which could result in slowing down, even preventing, the flow of safety injection fluid to reach the core for cooling said core. The delivery rate of the MPSI pumps and the coolant passage sections through plate 24 and guiding tubes are determined to maintain a certain level of coolant in device 22, therefore authorizing an equal distribution of the coolant delivered by the MPSI pumps into all the assemblies of the core. It is thus possible to maintain the temperature of the elements of the fuel assemblies at an acceptable level, until the core is again refilled with water at the end of the accident, by means known per se, such as low pressure safety injection pumps (LPSI). External accumulators are no more necessary for reducing the rewatering time and can be avoided. Dimensions of the LPSI pumps may even be reduced.