FR2679061A1 - Dismantleable nuclear fuel assembly - Google Patents

Abstract

The fuel assembly, usable in a light-water reactor, comprises a support having two end parts (12, 14) connected by guide tubes (16) and having grills (20, 22). Each grill has three sets of interlocked platelets defining cells. The platelets are fitted with means for clamping the rods, which hold these rods at the nodes of a regular triangular network. The upper end part comprises a bottom part having a bell for transition from a hexagonal cross-section to a cylindrical segment, fitted with a removable adapter plate and a top part having a cylindrical sleeve capable of sliding on the cylindrical segment and a holding plate. Tubular bracing members limit the mutual spacing of the adapter plate and of the holding plate. Springs exert a force on the plate which tends to separate them.

Description

REMOVABLE NUCLEAR FUEL ASSEMBLY

  The invention relates to nuclear fuel assemblies for a reactor cooled and moderated by light water, of the type comprising a support skeleton having

  two end pieces connected by guide tubes and grids

  parts along the assembly, each grid having three sets of interleaved plates delimiting cells crossed one by the guide tubes and the other by fuel rods, the plates being provided with rod clamping means which hold these rods to the nodes of a regular triangular network and being

  attached to at least some of the guide tubes.

  Such fuel assemblies are already known, at least one end grid of which is made up of a

  material retaining its mechanical properties under irradia-

  tion and helps to hold the pencils vertically

  say that the intermediate grids are made of a zirconium-based alloy (whose neutron absorption is much

  lower than that of the end grids) and are mutated

  means, such as bosses and springs cut in the plates, exerting on the rods a transverse holding and supporting force which is likely to decrease during irradiation. The invention aims in particular to provide an assembly of the type defined above, the upper end piece of which is formed so as to transmit an anti-flight force exerted by an upper core plate by allowing differential expansions between assemblies and between the 'assembly and the reactor structure and to allow the exit of water to passages provided in the upper core plate between the supports of the upper end pieces. To this end, the invention proposes in particular an assembly of the type defined above, characterized in that the upper end piece comprises: a lower part having a bell for transition from a hexagonal section to a cylindrical section, provided with a transverse adapter plate having a removable connection with some of the guide tubes; and an upper part having a cylindrical sleeve capable of sliding on the cylindrical section of the lower part and a retaining plate, tubular spacers limiting the mutual spacing of the adapter plate and the retaining plate being provided in alignment other guide tubes and springs being provided between the plates to exert a force on them

  which tends to separate them from each other.

  The guide tubes placed opposite the spacers can be simply supported on the latter, while the other guide tubes, much more numerous, are

  detachably attached to the adapter plate The or-

  grooves in the bell may have an ap-

  approximately semi-circular and each open on most of a hexagonal face The guide tubes are

  for example fixed to the lower end using block screws

  rotated after tightening on end sleeves

  guide tubes Screws and plugs may be loose

  with a central hole to allow the cooling of control rods or spectrum variation rods

  when these pencils are inserted into the assembly.

  The invention will be better understood on reading the

  description which follows of a particular embodiment,

  given by way of nonlimiting example The description is

  refer to the accompanying drawings, in which:

  Figure 1 is a block diagram in elevation

  tion of an assembly, a single fuel rod and a single guide tube being shown;

  Figure 2 is a detail view in large scale.

  the, showing the upper part of the assembly, in cross-section

  in front of a plane passing through the axis; Figure 3 is a detail view in elevation, showing the method of fixing the major part of the guide tubes to the upper and lower ends, as well as a fraction of the lower end grid; Figure 4 is a top view of a fraction of one of the grids of the assembly of Figure 1; Figures 5 A, 5 B and 5 C schematically show

  several possible modes of connection between internal platelets

  laughs of a grid; Figure 6 is a top view of a fragment of Figure 4, showing an embodiment of grids

  allowing the admission of light diameter guide tubes -

  higher than that of fuel rods; Figures 7 and 8 show, respectively in top view and in section along the line VIII-VIII of Figure 7, a possible constitution of elastic means for transverse support and support in an end grid; Figure 9, similar to Figure 8, is a section along the line IX-IX of Figure 6; Figure 10 is a detail view showing a

  fragment of plaque to which a tongue is attached-

  fastening on a guide tube; FIG. 11 is a top view on a large scale of a fragment of grid, showing the camber of the internal plates at their connection with the belt. The fuel assembly 10 shown diagrammatically in FIG. 1 comprises a skeleton having an upper endpiece 12 and a lower end 14 connected by guide tubes 16 and having grids 20 and 22 regularly distributed along the guide tubes The grids define cells 35 distributed at the nodes of a regular network, some of which are crossed by guide tubes 16 or a tube central instrumentation (not shown) and the others by fuel rods 24 These rods can rest on

  the lower end piece 14 They are retained axially and main-

  held at the nodes of the network by the grids, as will be seen below The grids are themselves fixed at

at least some of the guide tubes.

  During operation of the reactor, the lower tip

  laughing 14 is placed on the core support plate 26, pierced

  refrigerant inlet passages in the assemblies.

  The upper end piece 12 receives the thrust from the upper plate.

  heart 28, pierced with 30 outlet openings

refrigerant between the tips.

  We will describe first, with reference to

  Figures 2 and 3, a possible constitution of the tip

  and a possible method of detachable attachment of this end-piece allowing it to be removed and replaced.

  for example to replace pencils.

  The end piece 12 comprises two parts which can slide vertically relative to each other and holding means intended to limit the mutual spacing of the two parts and to transmit the thrust of the plate 28 from

to each other.

  The lower part is that which is fixed to the guide tubes It consists of an adapter plate 32 in the form of a disc fixed to the cylindrical upper section of a transition bell 34 The lower part of the bell 34 is fixed to a stop plate 36 of hexagonal section,

  drilled through holes for guide tubes 16 and open

  Refrigerant passage tures The stop plate 36 is at a distance from the fuel rods sufficient to allow the elongation of the latter (FIG. 1) and constitutes a stop limiting the lifting of the rods. The bell 34 is for example made of stainless steel sheet, welded to the plates 32 and 36 Large notches

  38 in each side allow the coolant to exit towards the interval between the upper nozzles.

  The upper part is designed to slide on the cylindrical wall formed by the periphery of the plate

  adapter 32 and the upper section of the bell 34.

  It comprises a cylindrical sleeve 40 in which is transversely welded a retaining plate 42, provided in

  its center with a coolant passage nozzle.

  The holding means comprise several sets of

  three struts-spring, for example.

  Each set includes a tubular spacer 44

  gned with a guide tube 16 and at least one helical spring

  dal 46 surrounding the spacer and compressed between the plates

  32 and 42 In the case illustrated in FIG. 2, each

  seems to have two springs mounted concentrically one

to the other, of different stiffness.

  Each tubular spacer ends with a bri-

  attached to the adapter plate 32 The upper part of

  the spacer is threaded to receive a nut 48 for

  setting of the maximum spacing between plates 32 and 42 and

  therefore of the precompression of the springs 46 The central bore

  spacer 44 has the same diameter as the tubes

  guides 16 to authorize the insertion of pencils of a grap-

  eg control or spectrum variation in tubes

  guides aligned with tubular spacers.

  The guide tubes 16 placed in alignment with the

  trestles 44 are simply in abutment against the flange of the latter The other guide tubes are each fixed

  on the adapter plate 32 by removable means.

  In the case illustrated in FIG. 3, these means include

  a threaded sleeve 50 with an internal diameter sufficient to allow the passage of control rods or spectrum variation pencils These sockets are screwed into

  threaded sleeves 52 welded to the ends of the tu-

  bes guides 16 Once screwed in, the sockets 50 are locked

  quées in rotation by deformation of a thin skirt of which

  they are provided in indentations machined in the adapter plate 32.

  All the guide tubes 16 are fixed in the same way

  way on the lower end piece This lower end piece 14,

  usually made of stainless steel, includes an adapter plate 54 fixed to a hexagonal section body and pierced with coolant passage openings This plate serves as a support for the fuel rods 14 It is pierced with shouldered holes for the passage of threaded sockets 56 , intended to be screwed into end plugs 58 of the guide tubes 16 The sockets are pierced with a calibrated hole 56 a for supplying the guide tubes with water for cooling the control rods or the variation of

  spectrum insertable in the guide tubes.

  The grids can have the constitution of main-

  cipe shown in Figure 4 and be formed by three sets of plates having assembly slots, the grids of one of the games being parallel to each other and 1200 of the plates of the other two games As shown in Figure 4, the plates can be assembled two by two at mid-iron. In the case illustrated in FIG. 4, each grid consists of sixty internal plates and six external plates. The external plates can be extended by folded guide tabs 60

  (figure 3) avoiding the risk of snagging between assemblies

age during handling.

  At least the two end grids 20 are

  with platelets made of an alloy retaining properties

  mechanical tees under irradiation, for example in Inconel 718 The plates can then be fixed two by two

  by soldering Other materials can also be used

  rials, for example martensitic steel.

  In the embodiment shown in FIG. 3, stainless steel caps 62 are fixed by brazing to the plates of the low end grid. These caps receive the lower end of the guide tubes 16, fixed to the plate 54 of the lower end by screws 56 The upper grid 20 can, similarly, carry brazed sleeves each intended to surround a guide tube. The upper end grid thus equipped can, once inserted on the guide tubes, either be free or be axially retained by zirconium alloy rings, welded on at least some of the guide tubes, on either side of the sleeves A set

  can be provided to avoid constraints.

  The intermediate grids 22 can be made

  either in a zirconium-based alloy or (if maintenance of the mechanical characteristics after irradiation is sought) in a stainless alloy, such as the Inconel 718 The outer plates can also be fitted with deflectors intended to avoid snagging between

  adjacent assemblies during handling.

  The wafers can be assembled in several different ways, depending on whether we are mainly looking for the ease of manufacturing the wafers or the reliability of the

welds at the intersections.

  In the case illustrated in FIG. 5 A, the slots for assembling a plate 64 with a plate 66 of a set at 1200 are formed at an angle, with an inclination

  This solution has the drawback of making the

  complex cutting, but it facilitates welding.

  In the variant shown in Figure 5B, the slots

  are straight, which simplifies the manufacturing of the pads-

  but is less favorable from a welding point of view.

  FIG. 5C constitutes a compromise between the two previous embodiments: the slots of a plate are straight, but provided with a chamfer for supporting the plates of the

other games.

  The grid plates are provided with rod clamping means which include support bosses formed on the external plates and boss-spring assemblies on the internal plates In the particular case illustrated in FIG. 6, o the guide tubes

  16 have a diameter greater than that of pencils, the bos-

  Support plates must be removed near the cells containing the guide tubes, as will be seen below. In common cells, pencils can

  be supported in five points, distributed in three directions

  tions at 1200 Two of the pads 64 and 66 (fig 5 to 7) which delimit the cell are provided with bosses 68 located at two different levels One of the pads of the third set 70 has a cut and deformed spring 72 The bosses 68 being close slots, it will often be advantageous to constitute them by parts cut into the shape of a bridge of small width, the opening under the bridge being in the longitudinal direction of the plates.

  dimples and springs can be the same width.

  However, other types of dimples can be used, for example in the form of a half-button, as described.

  in French patent application No. 90 09446.

  As noted above, the pads should

  may be locally deformed to delimit cells of sufficient size to receive the guide tubes 16 (FIG. 6) When the difference in diameter between the

  guide tubes 16 and the fuel rods 24 is important

  aunt, this deformation must extend over the cells adjacent to those which receive the guide tubes In this case, the deformation of the wafer replaces a set of two bosses aligned in the longitudinal direction of the rods, as shown in Figures 6 and 9, and the pencil is held at four points, two being formed by the arching of the plates 74 and the other two by

springs 72.

  When the intermediate grids are made of alloy

  zirconium-based, like guide tubes 16, they can

  wind be fixed by welding to the latter For this purpose the plates can be extended, around the guide tubes, by cambered tabs 76 (Figure 10) which are

  resistance welded on the guide tubes.

  To facilitate assembly between the inner pads and the outer pads, constituting a

  belt, it is preferable to bend the termi-

  nales 79 of the inner pads so that they intersect at 90 the outer pads constituting the belt 78 (Figure 11) This arrangement allows the inner pads to be provided with pins which engage in

  slots provided in the belt, before welding.

  The outer plates can be connected in the corners of the belt, as shown in Figure 11, or in the faces. This fixing can be carried out, like that of the inner plates together, by

  electron beam or laser welding.

  As mentioned above, we can also

  use intermediate grids made up of plates-

  your alloy retaining its properties under irradiation, for example Inconel In this case, the wafers may have a slightly smaller thickness than in the case of using Zircaloy The wafers can then

  be brazed together at points of entanglement.

  In a particular embodiment, the assembly was intended to receive three hundred and twelve pencils arranged

  at the nodes of a 12.75 mm pitch network The structure sup-

  port had fifteen guide tubes disassembled

  table with end caps and three guide tubes placed in the

  spacer spacers The end grids were in Inconel 718 and the intermediate grids in Zircaloy These dimensions led to adopting a common width of 1.8 mm for the bosses and the pressed springs The

  the height of the inserts was the same for the

  intermediate and end grids A thickness of the inner plates of the order of 0.4 mm has been found

  satisfactory, the thickness being advantageously a little higher for the outer plates in the case of Zircaloy grids.

Claims (8)

  1 Nuclear fuel assembly for reactor   cooled and moderated by light water, including a sque-   support strip having two end pieces (12,14) connected by guide tubes (16) and having grids (20,22) distributed along the assembly, each grid having three sets of tangled plates delimiting cells traversed by one by the guide tubes (16) and the others by fuel rods, the plates being provided with means (68,72) for clamping the rods which hold these rods at the nodes of a regular triangular network and being fixed to at least some guide tubes,   characterized in that the upper end piece (12) comprises: a lower part having a bell (34)   tion of a hexagonal section to a cylindrical section, provided with an adapter plate (32) having a demon-   table with some of the guide tubes (16); and an upper part having a cylindrical sleeve (40) capable of sliding on the cylindrical section of the lower part and a retaining plate (42), tubular spacers (44) limiting the mutual spacing of the adapter plate (32 )   and the retaining plate (42) being provided in section   of other guide tubes and springs (46) being provided between the plates to exert a force on them   which tends to separate them from each other.   2 assembly according to claim 1, characterized in that the springs are placed around the spacers (44).   3 Assembly according to claim 1 or 2, charac-   terized in that said other guide tubes are only those placed in alignment with the tubular spacers   laires (44) and in that said spacers have the same internal bore as the guide tubes to allow the insertion of rods belonging to clusters of control-35 or spectrum variation.   4 assembly according to claim 3, characterized in that said other guide tubes are supported on the spacers and in that the latter have a support flange on the adapter plate and an end portion   thread for receiving a nut (48) for adjusting the gap   maximum between the adapter plate (32) and the retaining plate (42).   Assembly according to any one of the claims   tions, characterized in that the means of fi   fixing the adapter plate with at least some of the guide tubes include screws locked in rotation after   clamping on end sleeves (52) of the guide tubes.   6 Assembly according to any one of the claims   tions above, characterized in that the plates are tangled two by two at mid-iron and in that the   rod clamping means include flat bosses   ceded on cell walls at 1200 with a household spring managed on another wall.   7 assembly according to claim 6, characterized in that the bosses and the springs are cut in the wall of the plates and give support in five points with pencils.   8 Assembly according to any one of the claims   above, characterized in that, the guide tubes   having a diameter larger than that of the fuel rods   the platelets are deformed around the tube receiving cells, said deformation extending over the adjacent cells.   9 Assembly according to any one of the preceding claims, characterized in that the inserts   The tails of each grid are arched (79) at their ends so that their intersection with the outer plates is made at 900. 10 Assembly according to any one of the preceding claims, characterized in that the plates   have, at the interlocking locations, straight chamfered slots.