FR2784221A1 - Water-cooled nuclear reactor fuel assembly has tubular guide sleeve fixed in aperture in adapter plate - Google Patents

Abstract

The adapter plate aperture (11) receiving the lower end of the guide tube is fitted with a tubular guide sleeve (12) forming an extension of the guide tube (7) which projects below the adapter plate and faces the core support plate (3). The guide sleeve has a lower flared conical end (12b) for an instrumentation conduit and is fixed in the adapter plate by a threaded upper end and a weld (13). The fuel assembly consists of a supporting frame for a bundle of parallel fuel pencils with transverse spacer plates and instrumentation guide tubes (7), at least one of which has its lower end in an adapter plate (2a) with feet (2b) resting on a reactor core support plate (3).

Description

The invention relates to a fuel assembly for a reactor.

  water-cooled nuclear system comprising means for guiding an instrumentation conduit. Pressurized water nuclear reactors have a core formed of vertically arranged and repositioned prismatic assemblies

  health on a support plate inside the nuclear reactor vessel.

  Fuel assemblies have a hand frame

  like a bundle of fuel rods parallel to each other, arranged

  in an axial direction of the fuel assembly. The framework

  carries a plurality of transverse spacer plates for holding the rods and a plurality of axial direction guide tubes replacing certain rods in the fuel assembly and two end caps fixed to the ends of the guide tubes. One of the end pieces, or lower end piece, via which the fuel assembly rests on the core support plate has an adapter plate transverse to the assembly guide tubes and attached feet

  on the adapter plate intended to come to rest, in the position of

  defect in fuel assembly, on the core support plate.

  During the operation of the nuclear reactor, it is necessary to periodically carry out flow measurements inside the core. Very small fission detectors are used for this, which are moved by remote control using teleflex cables, inside instrumentation conduits constituted by tubes closed at one of their ends, called glove fingers. Gloves are introduced

  in certain core assemblies arranged in a pre-

  determined, inside the instrumentation guide tubes of these assemblies

  fuel wounds. By moving the flow detectors inside the thermowells introduced into the fuel assemblies, flow measurements can be made along the entire height of the core. The thermowells must be able to be extracted from the core assemblies, for example to facilitate the reloading operations of the core; for that, we exercise

  pulling on the end of the thermowell from an instrument room

  tation disposed laterally with respect to the reactor vessel well.

  The thermowells pass through the curved bottom of the tank and reach the base of the core, to penetrate the guide tubes.

  instrumentation of fuel assemblies, each through

  diary of a straight crossing passage formed by a set of guide

  dage comprising in particular a cuff for crossing the bottom of the tank, a vertical column fixed to the underside of the core support plate and a rectilinear opening passing through the core support plate,

  in alignment with the instrumentation guide tube of the assembly

matching bustible.

  The end portion of the instrument conduit pass-through

  tation is generally constituted by a socket fixed on the upper face

  of the core support plate, in the extension of the opening passing through this plate. However, a free space is left between the end of the bushing and the inlet of the fuel assembly instrumentation guide tube to facilitate the fitting of the assembly,

  during the reloading of the reactor and to allow the displacements ac-

  accompanying the differential expansion of the fuel assembly by

  compared to the lower internal equipment of the reactor.

  It is also possible to design the guide sleeve of the con-

  instrumentation in telescopic form, the sleeve comprising a part movable in the axial direction returned by spring, intended for

  absorb displacements and expansions while ensuring continuous guidance

  bare of the instrumentation duct between the passage hole passing through the plate

  that upper heart and the instrumentation guide tube of

Fuel assembly.

  Nevertheless, the guide sleeves of instrumentation conduits fixed on the upper core plate, at certain positions of fuel assemblies, whether made in rigid form or in telescopic form, must be adapted to the dimensions of I '' lower end of fuel assemblies. Indeed, the distance

  between the inlet part of the instrumentation guide tube on the lower surface

  of the adapter plate and the upper surface of the support plate

  heart port is variable with the height of the feet of the lower end of

fuel assembly.

  In particular, in order to increase the thermal power of nuclear reactors, it has been proposed to extend the fissile column of the fuel assemblies constituted by the fuel rods. In order not to modify the height of the reactor vessel containing the core, we have had to modify the structure of the core support plate to produce this plate in the form of a thick plate whose size in height is less than the size of the upper heart plates used

  previously which included two elementary braced plates.

  Due to the greater heating of the thick plate compared to the braced plates of the previous solutions, it was necessary to increase the distance between the fissile column and the plate to reduce the flow of radiation received by this plate and guarantee its resistance to l 'irradiation. So we proposed

  extend the feet of the lower end caps of the fuel assemblies

  tible with a length close to 7 cm.

  Generally speaking, it may be necessary to charge

  ment of a water-cooled nuclear reactor with fuel assemblies having lower nozzles having different foot heights. In this case, the core support plate must be fitted with sockets of different characteristics, depending on the position of these sockets and the loading plan of the core in fuel assemblies having

different lower end caps.

  This results in a great complexity of the maintenance and recharging operations of the core of nuclear reactors, in particular due to the difficulty of replacing guide bushings in a highly

irradiated from the nuclear reactor.

  The object of the invention is therefore to propose an assembly of

  bustible for a water-cooled nuclear reactor with bone-

  saturation of maintaining a bundle of parallel fuel rods between

  them, arranged in an axial direction, comprising a plurality of plates

  ques transverse spacers for holding the rods, a plurality of axial direction guide tubes replacing certain rods of the fuel assembly, including a guide tube for an instrumentation duct at the central part of the assembly of fuel and two end fittings attached to the ends of the guide tubes, one of

  end caps or bottom end cap with a transverse adapter plate

  dirty with respect to the guide tubes of the assembly and the feet integral with the adapter plate intended to come to rest in the service position of

  the fuel assembly, on a reactor support plate

  nuclear torch crossed by an opening for passage of the instrumentation conduit, this fuel assembly making it possible to ensure very good guiding of the instrumentation conduit to the entry of the instrumentation guide tube, whatever the characteristics dimensional

  the lower end of the fuel assembly.

  For this purpose, a tubular guide sleeve is fixed in an or-

  groove crossing the adapter plate of the lower nozzle, at its central part, in the extension of the instrumentation guide tube, so

  to be projecting from a lower face of the adapter plate

  trice intended to come vis-à-vis the heart support plate.

  In order to clearly understand the invention, we will now describe by way of example, with reference to the attached figures, an assembly

  of fuel according to the invention comprising an insulating guide sleeve

trumentation.

  FIG. 1 is a perspective view of a fuel assembly

  ble resting on the core support plate of a nuclear reactor

  cooled by pressurized water and the contour of a neighboring assembly in

  the heart having different characteristics.

  FIG. 2A is a view in axial section of the lower end piece of a fuel assembly according to the invention comprising a guide sleeve. FIG. 2B is a sectional view of an alternative embodiment of the

guide sleeve.

  In Figure 1, a fuel assembly is shown

  affected by the reference 1 which rests by means of its lower end piece 2 on a core support plate 3 of a nuclear reactor cooled by pressurized water. The fuel assembly 1 comprises a bundle of fuel rods 4 in which the rods 4 are all parallel

  between them and arranged in the axial direction 5 of the assembly of

  bustible. The fuel rods 4 are held by a frame comprising spacer grids 6 distributed in the axial direction 5, following the length of the fuel assembly ensuring the maintenance of the fuel rods 4 according to a regular network generally with mesh square visible in the upper part of Figure 1. The fuel assembly 1 has a straight prismatic shape with square section, and the spacer grids are square. Guide tubes 7 are substituted

  to certain pencils 4, in the bundle of pencils; the guide tubes 7 pre-

  feel a length greater than the length of the rods 4 and have parts projecting in the axial direction at each of the ends of the bundle. The end caps of the fuel assembly such as

  The lower end piece 2 are fixed to the ends of the projecting parts of the tu-

bes-guides 7.

  The lower end piece 2 comprises a flat adapter plate 2a of transverse direction relative to the axis 5 of the fuel assembly and four feet 2b secured to the adapter plate 2a, according to each of the

  angles of the square adapter plate.

  One of the guide tubes 7, disposed at the central part of the bundle and engaged in an opening passing through the adapter plate 2a at its central part, is intended to receive an instrumentation conduit passing through the core support plate 3 by a passage opening 8 located in

  The axial alignment of the instrumentation guide tube.

  In Figure 1, there is shown the outline of an assembly of

  bustible 1 'placed adjacent to the fuel assembly 1 in the core of the nuclear reactor which has a lower nozzle 2' by means of which the assembly 1 'rests on the core support plate 3, the feet 2 of which 'b are substantially shorter than the feet of the end piece 2 of the fuel assembly 1. As a result, in the lateral bearing plane of the end pieces 2 and 2', in the loading position in the core,

  there is an opening 9 delimited by the upper face of the plate adapted

  header of the end piece 2 ', the underside of the adapter plate of the end piece 2 and the feet 2b of the end piece 2 of greater height than the feet 2'b of

end piece 2 '.

  The cooling water of the reactor which circulates in the axial direction inside the fuel assemblies can pass through the opening 9, so that transverse currents are produced which can

  vibrate the fuel assembly rods.

  Therefore, in the lower part of the fuel assembly,

  tible 1 'with feet of lower height, a belt or pads-

  closing tabs on the lateral sides of the bundle of fuel rods.

  This belt or these closing plates ensure the closing of the

  grooves such as 9, in the case where the fuel assembly 1 'is

  juxtaposed with one or more fuel assemblies such as 1 com-

  carrying a lower end piece 2 having feet 2b of greater height

as the feet of assembly 1 '.

  We can thus solve the problem of the distribution of hydraulic flows in fuel assemblies with heights of

different feet.

  As shown in Figure 1, the distance between the upper surface

  lower of the core support plate 3 onto which opens the opening 8 for passage of an instrumentation conduit and the underside of the adapter plate 2a in which is engaged the instrumentation tubeguide of the fuel assembly 1 depends on the height of the feet 2b of

  fuel assembly. When using guide bushings

  of instrumentation ducts fixed on the upper face of the plate

  than the core support 3, it is necessary to use sockets of a higher height in the case of a fuel assembly such as the assembly 1 than in the case of a fuel assembly such as the having short feet 2'b. This results in difficulty in terms of maintenance and

nuclear reactor loading.

  In FIG. 2A, the lower end piece 2 of an assembly has been shown.

  fuel blocking according to the invention constituted by an adapter plate

  trice 2a secured to feet 2b intended to come to rest on the upper surface

  lower of the adapter plate 3. The fuel assembly whose lower end 2 is shown in FIG. 2A is a fuel assembly intended to receive a thermowell in a guide tube

  instrumentation 7 fixed in an opening 11 passing through the adapter plate

  trice 2a of the nozzle 2 in its central part. In its loading position, shown in Figure 2A, the fuel assembly is arranged

  above an opening 8 for glove fingers to pass through the plate

  that of core 3 whose axis is aligned with axis 5 of the assembly of

  bustible along which the opening 11 and the guide tube are arranged

instrumentation 7.

  According to the invention, a guide sleeve 12 is fixed in the opening 11, in an arrangement coaxial with the instrumentation guide tube 7 and the opening 8 for the passage of a thermowell through the support plate. 3. The sleeve 12 has a cylindrical tubular part

  12a having a thread on its external surface allowing the screw to be screwed

  guide lug 12 in a threaded part of the opening 11 of the adapter plate 2a. The guide tube 12 also includes a flared end portion 12b having a frustoconical shape to facilitate the engagement of a

  thermowell in the guide sleeve 12.

  The length of the guide tube 12 is chosen so that the dis-

  tance between the end of the engagement cone 12b of the sleeve 12 and the upper surface of the support plate 3 on which opens

  opening 8 is as small as possible, taking into account the imperatives

  nant loading and operation of the fuel assembly in the core of the nuclear reactor. The sleeve 12 has a protruding part under the underside of the adapter plate 2a, located opposite the upper surface of the core support plate 3 whose length

  is slightly less than the height of the feet 2b of the end piece 2. The length

  This protruding part can be adjusted very precisely when the threaded part of the sleeve 12 is screwed into the opening 11. After fitting and adjusting the sleeve 12 in the adapter plate 2a,

  makes the sleeve unscrewable and captive by a point or a gasket

lasts 13.

  As can be seen in FIG. 2B, it is also possible to use a guide sleeve 12 whose threaded tubular part is extended by a deformable sleeve 12c which can be expanded inside at least one recess 14 machined in the opening 11, of the adapter plate 2a. It is thus possible to block the rotation of the sleeve 12 to make it

unscrewable and captive.

  The fuel assembly according to the invention therefore makes it possible to

  read an efficient guide of a thermowell to its entry inside a fuel assembly instrumentation guide tube, by a guide sleeve whose length protrudes under the adapter plate

  of the lower end of the fuel assembly is always perfect-

  adapted to the characteristics of the nozzle and in particular to the height

feet of the nozzle.

  The invention is not limited to the embodiment which has been described.

  It is thus possible to imagine fixing the guide sleeve in the adapter plate of the nozzle of the fuel assembly in a different manner from that which has been described. The guide sleeve can also have a different shape to be adapted to the conditions

  insertion and guiding of the thermowell.

  The invention can be applied to any fuel assembly of a nuclear reactor cooled by light water comprising an instrumentation guide tube.

Claims (5)

  1.- Fuel assembly for a water-cooled nuclear reactor comprising a framework for holding a bundle of rods   fuel parallel to each other arranged in an axial direction com-   taking a plurality of transverse spacer plates (6) for holding the rods (4), a plurality of guide tubes (7) of axial direction replacing certain rods (4) of the fuel assembly, including a central tube of guiding an instrumentation duct and two end fittings (2) fixed to the ends of the guide tubes (7), one of the fittings (2) or lower fitting comprising an adapter plate (2a) transverse to the guide tube (7) of the assembly (1) and feet (2b) intended to come to rest, in the service position of the fuel assembly (1) on a core support plate (3) of the reactor nuclear passage through an opening (8) for passage of the instrumentation conduit, characterized   by the fact that a tubular guide sleeve (12) is fixed in an or-   groove (11) passing through the adapter plate (2a) of the lower end piece (2) at its central part, in the axial extension of the instrumentation guide tube (7), so as to be projecting from a underside of the adapter plate (2a) intended to come opposite the heart support plate (3).   2.- fuel assembly according to claim 1, characterized   in that the guide sleeve (12) has a cylindrical tubular barrel   that (12a) and a frustoconical end portion (12b) for engaging the con- instrumentation.   - 3.- Fuel assembly according to any one of the res-   dications 1 and 2, characterized in that the guide sleeve (12) comprises   carries a threaded engagement and fixing part in a threaded part   of the opening (11) passing through the adapter plate (2a).   4.- fuel assembly according to claim 3, characterized   by the fact that the guide sleeve (12) is made unscrewable and imper-   dable by a joint or a welding point (13).   5. Fuel assembly according to claim 3, characterized in that the guide sleeve (12) comprises, in the extension   axial of its threaded part screwed into the opening (11), a deformed cuff   table (12c) to make the guide sleeve (12) unscrewable and waterproof   dable by deformation of the deformable cuff (12c) in at least one   recess (14) machined in the opening (11) of the adapter plate (2a).