FR2575580A1 - IMPROVED GRAY BODY BAR FOR A NUCLEAR REACTOR - Google Patents

Abstract

THE GRAY BODY BAR ACCORDING TO THE INVENTION COMPRISES A THIN WALL TUBE46 FORMING A FIRST LONGITUDINAL SECTION AND A SECOND LONGITUDINAL SECTION. WHEN THE GRAY BODY BAR IS INSERTED IN THE FUEL ASSEMBLY, THE FIRST LONGITUDINAL SECTION IS FOUND IN THE ARRANGEMENT OF FUEL BARS AND THE SECOND LONGITUDINAL SECTION IS FOUND OUTSIDE OF THIS ARRANGEMENT. THE FIRST LONGITUDINAL SECTION DEFINES A SPACE52 IN WHICH A STACK OF ANNULAR PELLETS48 OF A DIAMETER SUFFICIENT TO RETAIN THE SUPPORT OF THE WALL OF THE FIRST SECTION OF TUBE IS RETAINED. THE SECOND LONGITUDINAL SECTION DEFINES A FREE SPACE54 CONTAINING NO PELLETS AND CONTAINING ONLY A PARTIALLY COMPRESSED HELICOIDAL SPRING50 WHICH IS USED TO HOLD THE PILE OF PELLETS48 IN THE SPACE52 AND WHICH PROVIDES RADIAL SUPPORT TO THE WALL OF THE SECTION TUBE. THE TUBE IN WHICH THE GRAY BODY IS FOUND IS IN STAINLESS STEEL WITH A LOW EFFECTIVE NEUTRON CAPTURE SECTION AND THE ANNULAR PELLETS ARE PREFERABLY IN ZIRCALOY.

Description

Advanced gray body bar for a nuclear reactor.

  The present invention relates generally to reactions

  nuclear torers and, more particularly, an improved gray body bar, intended to be inserted into a nuclear fuel assembly. In conventional designs of pressurized water reactors, the reactor core has an excess of reactivity at start-up so that as this reactivity decreases during the existence of the core, there remains enough to maintain

  functioning the heart for an extended period of time. Any-

  times, since the core exhibits excess reactivity at the start of its existence, measures must be taken at this time to

check it properly.

  In a typical nuclear reactor, the core of the reactor is

  consisting of a multiplicity of long fuel assemblies con-

  each holding a multiplicity of long fuel elements or bars

  tible. A liquid coolant or coolant is pumped upward through the core to extract heat from it to produce useful work. The heat production of the heart is usually regulated by means of movable bars of various types, such as control bars, gray body bars, and / or water displacement bars. In pressurized water reactors, each

  fuel assembly typically includes a multiple

  multiplicity of guide tubes or socks in which said

  bars can be moved forward and backward.

  All of these three different types of bars have approximately

  the same dimensions but, due to the materials they are made of

  are used for somewhat different purposes. The displacement bars, either hollow or containing a material absorbing few neutrons, are used to move the heat transfer liquid from the reactor and in this way control the moderation of the reactor o The control bars containing a material absorbing a lot of neutrons, are used to control the reactivity of the heart, mainly by absorption of neutrons. Gray bars are similar to displacement bars, but use a medium neutron absorbing material, such as stainless steel, which has a higher reactivity value. They are used to control nuclear reactivity both by absorbing the neutrons, but this with a smaller cross section of neutron capture than the control rods, and by displacing the heat transfer liquid. Gray body bars are also known as fine adjustment auxiliary control bars. In the European patent application

  n 84 308376.9, we describe such an auxiliary control bar

  taking upper and lower stainless steel tubes and closed at their inner ends by an intermediate plug and at their outer ends by end plugs. The lower tube contains a stack of solid zirconium oxide pellets, placed one above the other between the intermediate plug and the end plug of the lower tube, this

  stack of pellets providing radial support to the lower tube.

  The upper tube is placed in communication with the outside, so that the heat transfer fluid can penetrate inside the

  tube to equalize the pressure exerted on the walls of this tube.

  This type of auxiliary control bar works reasonably

  well and fulfills its role in the range of operating conditions for which it was designed. However, it has been found that the solid zirconium oxide pellets reach high internal temperatures of between 300 and 6000 C.

  causing radial expansion causing the sheath to appear

  rition of damaging constraints.

  The main objective of the invention is to provide a bar of

  improved gray body overcomes this drawback.

  The invention therefore resides in a gray body bar intended to be inserted in a nuclear fuel assembly.

  including an organized set or arrangement of fuel rods

  tible, said gray body bar comprising a long tu-

  bular closed in a sealed manner at its opposite ends and formed of a material having a small effective cross-section for neutron capture, and pellets of a non-combustible material stacked end to end in the tubular element, said gray body bar, when '' it is inserted into the fuel assembly, having a first longitudinal section of the tubular element positioned inside the arrangement of fuel rods, and a second

  longitudinal section of the essential positioned tubular element

  also outside the fuel rod arrangement, ca-

  characterized by the fact that the stack of pellets is retained in a space for receiving pellets defined in said first longitudinal section and that each of the pellets has an annular shape so as to delimit an empty space passing through and has an external diameter sufficient to ensure a radial support to the adjacent wall part of said first longitudinal section, the second longitudinal section of the tubular element defining a free space in which there is no pellet and with which is associated a means intended to increase the radial resistance to crushing of

  said second longitudinal section with respect to the pressure ex-

  dull. By using annular pads instead of solid pads, the risk of damaging stresses in the sheath is minimized as the internal temperature

  ring pads tend to be much weaker, this re-

  sultat being further reinforced by the presence in each annular pellet of the internal vacuum which absorbs the little thermal expansion likely to occur. In addition, the annular pellets which weigh less than the solid pellets, reduce the mass of the gray body bar, which proves advantageous for the operation of the

  gray body bars during use.

  In a preferred embodiment of the invention, use is made of

  a helical spring to retain the stack of annular pellets

  inside the space for receiving pellets of the first longitudinal section of the tubular element, this helical spring being maintained under compression in the free space of the second longitudinal section and extending in the longitudinal direction substantially over the entire length of this free space. In addition, the coil spring has an outside diameter sufficient to radially support the wall of the second longitudinal section

  so that it does not flatten under the effect of external pressure

  better. Thus, the helical spring serves a double purpose in that it retains the stack of pellets in the tablet receiving space and that, in addition, it increases the radial resistance to crushing of the second longitudinal section. The fact that the helical spring, keeping the stack of pellets in place and safeguarding the integrity of the latter, fills this

  function in an elastic manner, thus allowing a certain dila-

  axial position of the pellets without inducing constraints, the steps

  annular pins are preferably formed of Zircaloy (registered trademark), which is a well known zirconium alloy which does not crumble

  not. In the preferred embodiment, the tubular member comprises

  taking said first and second longitudinal sections is a unitary element formed in one piece; this improves alignment

of the bar and its straightness.

  We will now describe by way of example, the mode of realization

  preferred position of the invention and one of its variants, with reference to the accompanying drawings, in which:

  Figure 1 is a partial view of a reactor body

  having, in elevation, a fuel assembly, a plate

  heart support frame extending transversely to the top of the fuel assembly, and a spider assembly which supports gray body bars, the heart plate being shown in section and partially torn off, and the fuel assembly being illustrated in a vertically shortened form and partially torn off to reveal a gray body bar according to the invention; Figure 2 is an enlarged top plan view of the spider assembly; Figure 3 is a sectional view through III-III of Figure 2 of the spider assembly;

  FIG. 4 is a large-scale view, vertically shown

  curved and partially sectioned, from the gray body bar re-

  showing the preferred embodiment of the invention; Figure 5 is an enlarged sectional view through V-V of Figure 4; and

  Figure 6 is a large-scale view and shortened verti-

  setting of a gray body bar representing a variant of

realization of the invention.

  In the description which follows, the same references desi-

  generate parts or the like or equivalent in all the figures and expressions such as "forwards", "backwards",

  "left", "right", "up", "down", and other expressions-

  analogous terms are used purely by way of illustration and without limitation. Referring now to the figures and more particularly to Figure 1, we see that there is shown a general combination of a fuel assembly referenced 10 as a whole, an upper core support plate 12 disposed above and across the top of the fuel assembly 10, and a

  spider assembly 14 disposed above the top plate

  12 heart support. The fuel assembly 10, which is shown in a shortened form in the vertical direction, basically comprises a lower end structure or lower nozzle 16, adapted to support the assembly on a lower core plate (not shown) in the region of heart-of

  nuclear reactor (not shown), a multiplicity of tubes-

  guides or socks 18 which extend longitudinally upwards from the lower nozzle 16, a multiplicity of transverse grids 20 spaced axially along the guide tubes 18, an organized assembly or arrangement of long fuel rods 22 spaced transversely and supported by the grids 20, an instrumentation tube 24 disposed in the center of the fuel assembly, and an upper end structure or upper end piece 26 fixed

  at the upper ends of the guide tubes 18. The assembly

  bustible 10 forms a unitary unit capable of being manipulated

  in a conventional manner without damaging the parts of the assembly.

  As mentioned above, the fuel rods 22 are held spaced from each other by the grids 20 which are spaced along the fuel assembly. Each fuel rod 22 contains nuclear fuel pellets (not shown) and is closed at its opposite ends by means of end caps (not shown). Fuel pellets made of fissile material are the source of the reaction energy generated in the reactor. A liquid moderator / coolant, such as water, or water containing boron, is pumped upward through the fuel assemblies of the core to extract the

  heat for the production of useful work.

  The upper heart support plate 12 extends trans-

  versally to the upper part of all the fuel assemblies located in the core and it includes coolant flow passages, such as, for example, passage 28, formed through this plate. At least some of these passages 28 are axially aligned with the guide tubes 18 to allow the downward movement of gray body bars, such as gray body bars 30, as well as other control bars (not shown ), through the passages 28 of the heart plate 12, and

  this up to the respective guide tubes 18.

  As can best be seen in FIGS. 2 and 3, the spider assembly 14 comprises a central hub 32, a multiplicity of fins 34 extending radially from the hub and a plurality of fingers 36 disposed on the fins 34 and connected to the upper ends of the bars 30 of gray body. Preferably, the central hub 32 is in the form of a long tube having on its upper end a segment 40 internally threaded for connection to the drive mechanism (not shown) used to raise and lower the spider 14, as is well known in the art. The tubular hub 32 houses a load absorption mechanism which includes a helical spring 42 kept compressed and a nipple 44 adapted to be housed in a shallow cavity (not shown) formed in the upper surface of the core support plate 12 so as to contribute to the appropriate centering of the bars 30 of gray body in the passages 28 of this plate as well as in the guide tubes 18. As is well known, the main purpose of the load absorption mechanism is to prevent the application of a shock to the core support plate 12 and to the fuel assembly 10 when the spider 14 strikes the plate 12 when the gray body bars 30 are fully inserted in

the guide tubes 18.

  Referring now to Figures 4 and 5, we see that the

  gray body bar 30 according to the present invention comprises an element

  tubular ment 46, a stack of annular support pellets 48 disposed in the tubular element, a helical spring 50 having a length sufficient to maintain the stack of pellets 48 in the space 52 for receiving pellets located inside a lower longitudinal section of the tubular element 46, the helical spring 50 being disposed, in a state of partial compression, inside a hollow space 54 which is in an upper longitudinal section of the tubular element and in which has no pellet, the helical element further having an outside diameter sufficient to provide radial support to the wall of this upper longitudinal section. It will be noted that the helical spring 50 extends in the axial direction

  substantially over the entire length of the hollow space 54.

  More specifically, the tubular member 46 is a cylindrical tube 56 whose opposite ends are closed by means of end caps 58 and 60, respectively. With the upper end plug 58 an integral part is a rod 62 which extends upwards and which has an end part 64 externally threaded for its connection with an end part

  lower 66 internally threaded with one of the spider fingers 36.

  The lower end plug 60 is conical. The tube 56 is in the form of a thin-walled sheath which is made of stainless steel, as are the end plugs 58 and 60. The sheath, because it is made of stainless steel, has a small effective neutron capture section and, because of its thin wall, minimizes the weight and, therefore, the moment

  of inertia of the gray body bar 30.

  Each of the pellets 48 of the stack has a hollow annular shape which allows it to fit concentrically inside the tube 56 and ensures an empty central space 68 passing through the pellet 48. The use of hollow pellets 48 reduced to a minimum the mass of the bar 30 of gray body while maintaining sufficiently high the crushing resistance of the tube 56. Due to the lower mass of the bar 30, the internal temperature of the bar remains at an acceptable level, this which reduces the stresses exerted on the sheath. In addition, the interior void space 68 of each pellet 48 contributes to maintaining the internal temperature of the pellet itself at a lower level, which further reduces the radial expansion and, consequently, the stress exerted on the sheath. Preferably, each pellet is formed from Zircaloy, a material which does not absorb neutrons to an appreciable extent and which does not crumble during the gradual displacement of the bar 30 of gray body forward and

  backwards in the fuel assembly 10.

  As indicated above, the helical spring, or compression spring 50, fulfills a double function. Firstly, its length is sufficient to keep it in a state of

  compression between the stack of pellets 48 and the plug of ex-

  upper end 58, thereby constantly pushing the stack of pellets against the lower end plug 60 while allowing a small increase, of thermal origin, in the length of the individual pellets. Thus, the helical spring 50 securely holds the stack of pellets 48 in the pad receiving space 52 of the lower tube section which is the section inside the bar arrangement.

  combustible when the gray body bar is inserted in the assembly

  - combustible wiring 10, the upper tube section which delimits the free space 54 being essentially outside the arrangement of fuel rods and inside the associated passage 28 of the upper core support plate 12 when the gray body bar is in the inserted position. The second function fulfilled by the helical spring 50 is to provide internal support for the thin-walled tube 56 over the entire length of the space.

  free without adding any weight other than that of the light spring.

  FIG. 6 shows an alternative embodiment of a bar 30 'of gray body which is similar to the bar 30 of gray body of the preferred embodiment in that it comprises a tubular element 46' containing a stack of annular pellets hollow 48 '. However, unlike the gray body bar 30, the long tube 56 'of this gray body bar 30' is composed of upper and lower tube sections 70, 72 which are separated from each other and which are connected to each other by means of a solid or solid plug 74 interposed between these tube sections 70 and 72 whose adjacent ends are assembled to this plug 74. The lower tube section 70 delimits a space 52 ′ for receiving pads which contains the stack of pads 48 'and the upper tube section 72 defines a space 54' in which there is no internal structure. The upper tube section 72 has

  openings 76 formed through its wall to allow the calo-

  carrier of the reactor to reach the free space 54 ′ in order to equalize the pressure prevailing in this space and the external pressure prevailing outside the gray body bar. The two tube sections 70 and 72 are thin-walled sheaths and, together with the flat or solid plug 74, are formed of stainless steel. The pellets 48 ′ are formed from a non-combustible material having a negligible coefficient of thermal expansion and can be formed from zirconium oxide; more especially, since the pellets are annular and voids 68 are formed therein, they do not increase in length appreciably and, therefore, they can be kept firmly stacked between the stopper

  solid or solid 74 and the end plug 60.

Claims (8)

  1. Gray body bar intended to be inserted in an assembly   nuclear fuel armor comprising an organized assembly or arrangement of fuel rods, said gray body rod comprising a long tubular element sealed at its opposite ends and formed of a material having a small effective cross-section for neutron capture, and pellets made of a non-combustible material and stacked end to end inside the tubular element, said gray body bar when inserted in the fuel assembly, having a first longitudinal section of the tubular element disposed inside said   fuel rod arrangement and a second long section   tudinal of the tubular element arranged essentially at the ex-   inside of the fuel rod arrangement, characterized in that the stack of pellets (48 or 48 ') is retained inside the space (52 or 52') for receiving pellets, defined in said first longitudinal section, and that each of the pellets has an annular shape, so as to defuse a through empty space (68), and has an external diameter sufficient to provide radial support to the adjacent wall portion of said first longitudinal section, the second longitudinal section of the tubular element (46 or 46 ') delimiting a free space (54 or 54') which does not contain a tablet and to which is associated a means (50 or 76) intended to increase the radial resistance to crushing of the   second longitudinal section vis-à-vis the external pressure.   2. Gray body bar according to claim 1, characterized in that said long tubular element (46 or 46 ') is a thin-walled sheath (56 or 56 ').   3. Gray body bar according to claim 2, characterized   by the fact that said sheath (56 or 56 ') is formed of stainless steel stainless. -   4. Gray body bar according to any one of claims   1, 2 or 3, characterized in that said means serving to increase the radial resistance to crushing of the second longitudinal section is a helical spring (50) arranged and maintained under partial compression inside said free space (54 ) to resiliently retain the stack of pellets (48) in the space (52) receiving the pellets, the helical spring (50) extending in the longitudinal direction substantially over the entire length of the free space (54) and having an external diameter sufficient to support in the radial direction the wall of said second longitudinal section.   5. Gray body bar according to any one of claims   1, 2, 3 or 4, characterized in that the said pellets (48) are formed of Zircaloy.   6. Gray body bar according to any one of claims   1, 2 or 3, characterized in that said first and second longitudinal sections are separate tube sections (70 and 72) joined to each other by means of a solid plug (74) disposed between these two sections, said stack of annular pellets (48 ') being retained in said space (52') receiving pellets by the flat plug (74).   7. Gray body bar according to claim 6, characterized in that the annular pellets (48 ') are formed of oxide of zirconium.   8. Gray body bar according to claim 6 or 7, character-   laughed at by the fact that said means serving to increase the resistance   radial crushing force of said second longitudinal section comprise at least one hole (76) formed through a wall portion of the second longitudinal section so as to allow a pressure equalization between the free space (54 ') and the outside the gray body bar (30 ').