Patent Number: 051241137
Section: description

DETAILED DESCRIPTION As FIGS. 1 and 2 show, the vessel comprises four pressurized water light tubes 4 and four water outlet tubes 5. The tubes 4 are connected to the cold branches of the primary circuit of the reactor and the tubes 5 to the hot branches carrying the pressurized water heated by contact with the arrays 6 of the core 7 of the reactor to steam generators not shown. The internal equipments, comprising particularly a thimble 8 forming the casing of the core 7 and carrying at its lower part the support plate 9 of the core, are suspended within the vessel 1. The plate 9 is pierced apertures corresponding to the arrays of the core. As FIG. 3 shows, the core comprises one hundred and ninety-three arrays of square cross-section resting upon the plate 9. The upper internal equipments 11 of the reactor are located above the core 7. These upper internal equipments 11 comprise the guide tubes 12 of the rods, serving as spacing elements and effecting connection between an intermediate plate 13 and a core plate 10 to which they are attached. The internal equipments likewise comprise an upper plate 14, to which the upper parts of the tubes 12 are attached. The plates 13 and 14 are attached to thimbles coaxial to the thimble 8 and maintained, like the thimble 8, between the cover 2 and the vessel 1. The tubes 12 contain guide cards 15 and continuous guide devices permitting the rods to be maintained and guided during their vertical movements in the core. At their lower part the tubes 12 are pierced with orifices 16 to permit the exit of the water traveling through the arrays equipped with a cluster, to the outlet tubes of the vessel. These tubes 12 are constructed in two parts, the upper part of the tubes is suspended from the upper plate 14, and their lower part acts as a spacing element between the plates 10 and 13. The annular space included between the core 7, the cross-section of which is shown in FIG. 3, and the core casing 8 is filled by a massive partition 18 of stainless steel acting as a reflector particularly for the high energy neutrons produced in the core. The massive partition 18 occupies virtually the entire volume included between the core and the core casing. The fuel rods of the arrays consist of long zirconium tubes containing pellets of uranium oxide enriched with uranium 235. At each of their ends the zirconium tubes are filled for a length of approximately ten centimeters by pellets of uranium oxide (UO.sub.2) depleted of uranium 235 replacing the pellets of enriched uranium oxide. Thus two practically continuous layers 19 and 20 of depleted uranium are formed at the upper end and at the lower end of the core respectively. These layers 19 and 20 permit the absorption of the low energy neutrons and supply uranium 238 which can be transformed into plutonium 239 by the effect of the bombardment by the high energy neutrons. The stainless steel partition 18 and the depleted uranium layers 19 and 20 thus make it possible to limit to a minimum the escape of neutrons out of the core, which improves the efficiency of the reactor. The flow in the proximity of the vessel is also reduced by this means. FIG. 2 shows generally the guide tubes 12 permitting the guidance of the rods in the reactor core. For the entire core, ninety-six clusters of absorbing rods are used, each of these clusters being capable of being introduced into the guide tubes of an array. FIG. 1 shows such a cluster of absorbing rods or control rod 24 in the top position, it being attached to an actuating rod 25 itself movable in a tubular chamber 26 communicating with the interior of the vessel. A movement mechanism for the rod 25, not shown, is placed at the top of the chamber 26. Such a mechanism, of the conventional pawl type, permits the control rod 24 to be removed in the vertical direction, and with great precision, within the guide tubes of the array 6a arranged above the tube 12a. FIG. 1 likewise shows an assembly of neutron energy spectrum variation rods 27 fully inserted into the guide tubes of the array 6a. These neutron energy spectrum variation rods consist of a cluster of tubes of zirconium alloy filled with pellets of depleted uranium throughout their length. The control clusters, and also the neutron energy spectrum variation clusters, have the same length as the arrays. The neutron spectrum variation rods 27 can be in one or the other of two positions, one of these positions being the fully inserted position shown in FIG. 1, and the other position the fully extracted position equivalent to the position of the control cluster 24 shown in FIG. 1. A movement device associated with the spectrum variation rod 27 permits it to be moved from one to the other of these two positions. In their fully inserted position the absorbing rods forming the spectrum variation rod are engaged in the array 6 throughout their length. Referring to FIGS. 2 and 3, it is clear that for a core comprising one hundred and ninety-three arrays, ninety-six guide tubes 12 are used, each permitting the simultaneous guidance of a control rod 24 and of a spectrum variation rod 27. It is likewise clear that these guide tubes 12 are arranged above fuel arrays 30 arranged checkerboard-fashion in the cross-section of the core. One array in two is therefore equipped simultaneously with a control rod and with a spectrum variation rod of depleted uranium. The arrays 31 adjacent to those arrays 30 equipped with control rods and with spectrum variation rods are given neither the one nor the other of these two types of absorbent rods. Referring to FIG. 4, this shows the arrangement of the guide tubes 32 intended to receive control rods, and of the guide tubes 33 intended to receive spectrum variation rods. Each array comprises fifty-six guide tubes, of which sixteen tubes are intended to receive control rods and forty tubes intended to receive spectrum variation rods. The guide tubes of the arrays 30 actually receive these rods during the operation of the reactor, whereas the corresponding guide tubes of the arrays 31 receive a cluster of plugs to create a head loss equivalent to that due to the rods introduced into the tubes 32 and 33 of the arrays 30. The whole of the cluster 24 forming the control rod reproduces the transverse distribution of the guide tubes 32 illustrated in FIG. 4, whereas the whole of the cluster 27 reproduces the distribution of the guide tubes 33 likewise illustrated in FIG. 4. At the start of the fuel cycle, that is to say after a charge, the movement mechanisms of the spectrum variation rods are used to place all the rods of depleted uranium in the position of maximum insertion in the arrays 30, so that the volume of moderator compared to the volume of fuel in the core of the reactor is considerably reduced. Furthermore, each of the depleted uranium rods of the spectrum variation rods absorbs the low energy neutrons locally, which causes an increased shift of the neutron spectrum towards the high energies. This local effect repeated in the whole of the core by the spectrum variation rods distributed regularly in one array in two causes an overall effect of hardening the spectrum within the reactor core. In this manner a sufficient spectrum shaft to produce an appreciable conversion of the uranium 238 of the fuel into plutonium 239 is achieved, both by the reduction in the volume of the moderator and by the absorption of the low energy neutrons. The depleted uranium rods absorbs the slow neutrons surplus to those which are required to maintain the chain reaction in the core, and give rise to the formation of plutonium 239 by the effect of the bombardment by the higher energy neutrons. After operating with the spectrum variation rods inserted during an appreciable part of the cycle of the reactor, these rods are extracted. After the extraction of the spectrum variation rods, the fissile material formed by the high energy neutron bombardment during the first phase is consumed within the core of the reactor. By the increased effect of spectrum shift by the absorbing depleted uranium rods, and by the use of a stainless steel peripheral reflector and of absorbing layers of depleted uranium on each side of the core, at its lower part and its upper part, it is possible to equip only one fuel array in two in the core with an assembly comprising both a control rod and a spectrum variation rod. FIG. 3 shows that, of the hundred and ninety-three fuel arrays forming the core, there are ninety-six arrays 30 which receive a control rod and a spectrum variation rod and ninety-seven arrays 31 which receive only a cluster of plugs. It will therefore be possible to install above the cover 2 of the vessel 1 the movement mechanisms for the control rods comprising a coaxial insertion and extraction mechanism for the spectrum variation rods, whereas the arrangement of such mechanisms above each of the arrays would have been extremely difficult and would compromise the conception of the vessel, its cover and its internal equipments. The device according to the invention therefore has the advantage of permitting all the control rods and spectrum variation rods to be positioned without necessitating any major modification of the vessel of a nuclear reactor of the pressurized water type. Moreover, by virtue of the reflector effect of the circumferential partition of the core of the absorption effect of the upper and lower layers of depleted uranium of the core, and of the increased spectrum shift effect of the neutron spectrum variation rods, the conditions of utilization of the fuel of the reactor core can be considerably improved. These substantial improvements, which engender major economies in the cost of the fuel, are obtained at the cost of a relatively minor modification to the structure of the reactor. The invention is not limited to the embodiment which has just been described; on the contrary, it comprises all variants thereof. For instance, a different distribution of the guide tubes 32 and 33 receiving the control rod and the spectrum variation rod in each of the arrays 30 may be imagined. A different distribution of the arrays receiving the control rods and the spectrum variation rods may likewise be contemplated. Configurations may also be imagined, in which these two types of rods are not associated with the same assemblies of the core. Clusters forming the control and spectrum variation rods comprising a different number of absorbing rods may likewise be imagined. As to the spectrum variation rods, the use of different absorbent materials from depleted uranium, containing or not containing fertile material capable of being transformed into fissile material by the effect of the neutron bombardment, may be imagined. Devices of any type, mechanical, hydraulic or pneumatic, for the movement of the control rods and for the total insertion or extraction of the spectrum variation rods may be imagined. The invention is applicable to any nuclear reactor cooled and moderated by pressurized light water comprising fuel arrays arranged vertically within which control rods are moved vertically for the operation of the reactor.