Abstract:
An assembly of the type having a water channel extending along a longitudinal axis and having an upper section of larger cross-section area than a lower section and at least one fuel rod receiving groove extending longitudinally on the outer surface of the lower section, fuel rods extending longitudinally and disposed around the water channel and fixing members for fixing at least one fuel rod to the water channel in the at least one groove below the upper section. 
     According to one aspect of the invention, the at least one groove extends along the upper section such that a fuel rod received in fixing members is longitudinally extractable or insertable from the upper end side of the fuel assembly.

Description:
[0001]    The present invention relates to a nuclear fuel assembly for boiling water reactors, of the type comprising a water channel extending along a longitudinal axis and having an upper section of larger cross-section area than a lower section and at least one fuel rod receiving groove extending longitudinally on the outer surface of the lower section, fuel rods extending longitudinally and disposed in an array around the water channel and fixing members for fixing at least one fuel rod to the water channel in the at least one groove below the upper section. 
       BACKGROUND 
       [0002]    A conventional nuclear fuel assembly for boiling water reactor comprises a bundle of fuel rods and a water channel arranged in a fuel channel, the water channel being surrounded by the fuel rods. In operation, water flows through the water channel and through the fuel channel, from lower end to upper end of the fuel assembly. 
         [0003]    Water serves as a moderator for the nuclear reaction and as a coolant. Water is progressively heated, so that water is in vapour-liquid phase nearby the upper end of the fuel assembly. 
         [0004]    The water channel enables to increase the moderator (water) to fuel ratio and the coolant amount near the central region of the fuel assembly. 
         [0005]    The moderator (water) to fuel ratio tends to decrease toward the upper end of the fuel assembly as the proportion of vapour increases in the water flow, namely in the water channel. The decrease of said ratio leads to a less efficient burning of the fuel in the upper region. 
         [0006]    Providing fuel rods of shorter length enables to reduce the amount of nuclear fuel in the upper region of the fuel assembly and to improve burning of the nuclear fuel in said upper section. 
         [0007]    Providing a water channel having an upper section of larger cross-section area enables to increase the amount of water in the upper region, to compensate the increase of vapour in the water flow, and to thus improve burning of the fuel rods. 
         [0008]    U.S. Pat. No. 5,202,085 describes a nuclear fuel assembly having a water channel surrounded by fuel rods. The water channel has a lower section having a cruciform cross-section occupying a region equivalent to five fuel rod cells and an upper section of square cross section occupying a region equivalent to nine fuel rod cells. The fuel rods comprise shorter fuel rods disposed adjacent the water channel beneath the upper region. 
         [0009]    However, such an arrangement makes it difficult to catch the shorter fuel rods located beneath the upper section, e.g. in view of replacing one of these fuel rods. 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the invention is to provide a water channel for a nuclear fuel assembly for a boiling nuclear reactor which enables to improve burning of the fuel while making handling of the fuel rods more convenient. 
         [0011]    To this end, the invention provides a water channel of the above-mentioned type, wherein the at least one groove extends along the upper section such that a fuel rod received in fixing members is longitudinally extractable or insertable from the upper end side of the fuel assembly. 
         [0012]    In other embodiment of the invention, the water channel comprises one or more of the following features, taken in isolation or in any technically feasible combination: 
         [0013]    the or each groove is of constant width along the lower section and the upper section; 
         [0014]    the or each groove has a depth larger in the lower section than in the upper section; 
         [0015]    the or each groove is adapted to receive at least one fuel rod; 
         [0016]    the or each groove is adapted to receive at least two fuel rods side-by-side; 
         [0017]    the water channel has a main duct and at least two lateral ducts in fluid communication with the main duct along the whole length of the water channel, lateral ducts defining between them at least one fuel rod reception groove on the outer surface of the water channel; 
         [0018]    each lateral duct has a cross section area which is constant along the length of the water channel, and the main duct has a larger cross section area in the upper section than in the lower section; 
         [0019]    each lateral duct replaces at least one fuel rod in the array; 
         [0020]    each lateral duct replaces one fuel rod in the array; 
         [0021]    the main duct replaces at least one fuel rod in the array; 
         [0022]    the main duct replaces a square unit of several fuel rods in the array; 
         [0023]    the main duct replaces a four fuel rods square unit in the array; 
         [0024]    the water channel has four lateral ducts defining between them four fuel rod reception grooves around the water channel; 
         [0025]    the water channel has a cruciform cross section replacing fuel rods disposed in the diagonals of a fuel rods square unit of the array; 
         [0026]    the water channel replaces fuel rods disposed in the diagonals of a sixteen fuel rods square unit of the array. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention and its advantages will be better understood upon reading the following description, given solely by way of example and with reference to the appended drawings, in which: 
           [0028]      FIG. 1  is a schematic partial view of a nuclear fuel assembly for boiling water reactor, the fuel assembly comprising a water channel; 
           [0029]      FIG. 2  is a sectional view of the fuel assembly along II-II on  FIG. 1 ; 
           [0030]      FIG. 3  is an enlarged view of the central zone of  FIG. 2 ; 
           [0031]      FIG. 4  is a partial perspective view of the water channel; and 
           [0032]      FIG. 5  is a partial view of the fuel assembly of  FIG. 1  illustrating maintenance operations on the fuel assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    As illustrated on  FIG. 1 , the nuclear fuel assembly  2  is elongated along a longitudinal central axis L. Only an intermediate section of the fuel assembly  2  is represented on  FIG. 1 . 
         [0034]    In use, the fuel assembly  2  is placed in the core of a nuclear reactor with the axis L extending substantially vertically. In the following, the terms “lower” and “upper” refer to the position of the fuel assembly  2  in the reactor. 
         [0035]    The fuel assembly  2  comprises a bundle of nuclear fuel rods  4 ,  6  and a water channel  8  arranged in a fuel channel  10 . The fuel rods  4 ,  6 , the water channel  8  and the fuel channel  10  are elongated and extend longitudinally parallel to axis L. 
         [0036]    Each fuel rod  4 ,  6  comprises a tubular cladding filled with stacked nuclear fuel pellets. 
         [0037]    The water channel  8  is surrounded by the fuel rods  4 ,  6 . The water channel is tubular and elongated along direction L. The water channel  8  comprises a tubular lower section  14  and a tubular upper section  16  of larger cross section area than the lower section  14 . 
         [0038]    The fuel rods  4 ,  6  are arranged in an array and the water channel  8  replaces some of the fuel rods in the array. 
         [0039]    Fuel rods  4  are shorter than fuel rods  6 . The shorter fuel rods  4  extend only along the lower section  14  without extending along the upper section  16 . The longer fuel rods  6  extend along the lower section  14  and the upper section  16 , i.e. substantially along the whole length of the water channel  8 . 
         [0040]    The fuel assembly  2  comprises fuel rod supporting spacers  18  distributed along the length of water channel  8 , only one spacer  18  being illustrated on  FIG. 1 . 
         [0041]    The spacer  18  is latticed and comprises a plurality of cells  20  for receiving the longer fuel rods  6  therethrough, and a central passage  22  for receiving the water channel  8  and the shorter fuel rods  4 . 
         [0042]    The spacer  18  is guided by the water channel  8  and its axial movement is restricted, e.g. by welded spacer stops. 
         [0043]    The fuel assembly  2  comprises fixing members  24  for receiving the shorter fuel rods  4 . The fixing members  24  are fixed along the lower section  14  of water channel  8  for fixing the shorter fuel rods  4  adjacent to the water channel  8 . 
         [0044]    As illustrated on  FIG. 2 , each longer fuel rod  6  extends through a respective cell  20  and is supported transversally by the spacer  18  to maintain the transversal gap between the longer fuel rods  6 . 
         [0045]    To this end, each cell  20  has protrusions protruding from the lateral walls of the cell  20  to contact the outer surface of the longer fuel rod  6  inserted through the cell  20 . 
         [0046]    In the represented embodiment, the spacer  18  is arranged such that the cells  20  are disposed in a 10×10 lattice with the passage  22  replacing the sixteen cells square unit at the centre of the network. 
         [0047]    As illustrated on  FIG. 3 , the lower section  14  and the upper section  16  of the water channel  8  have cross sections of the same shape, upper section  16  having however a larger cross section area that lower section  14 , as it will be explained later. 
         [0048]    The water channel  8  has a cruciform cross section and replaces eight fuel rods in the array, the branches of the cross corresponding to the two diagonals of a sixteen fuel rods square unit in the array. 
         [0049]    The water channel  8  has a main duct  26  and four lateral ducts  28  in fluid communication with the main duct  26  through transverse openings  30  extending along the whole length of the water channel  8 . 
         [0050]    The main duct  26  has a substantially square section and replaces a four fuel rods square unit in the array, and each lateral duct  28  has a substantially square section and replaces one fuel rod in the array. 
         [0051]    Each lateral duct  28  is disposed at one of the four corners of the main duct  26  and aligned with the diagonals of the main duct  26 . 
         [0052]    The water channel  8  has four grooves  32  provided on the outer surface  34  thereof and distributed around the longitudinal axis of the water channel  8 . Each groove  32  is adapted to receive two shorter fuel rods  4  side-by-side. 
         [0053]    Each groove  32  is defined between two lateral ducts  28 . Each groove  32  has a U-shaped cross section defined by a bottom wall  36  and two lateral walls  38 . 
         [0054]    The bottom wall  36  externally defines the groove  32  and internally defines the main duct  26 . Each lateral wall  38  externally defines the groove  32  and internally defines a lateral duct  28 . The main duct  26  is thus defined by the bottom walls  36  of the grooves  32 . 
         [0055]    The walls of the water channel  8  have a constant thickness along the length of the water channel  8 . The varying cross section area of the water channel  8  is obtained via deformations of the walls of the water channel  8 . 
         [0056]    In view of obtaining a larger cross section area in the upper section  16 , the main duct  26  has a larger cross section area in the upper section  16  than in the lower section  14 . 
         [0057]    To this end, the main duct  26  has a diverging section  40  at the junction between the lower section  14  and the upper section  16 , in which the bottom walls  36  are inclined relative to the longitudinal axis of the water channel  8  and diverge from lower section  14  towards upper section  16 . 
         [0058]    Each lateral duct  28  has a constant cross section area along the length of the water channel  8 . 
         [0059]    Each groove  32  has a constant width W along the length of the water channel  8  (i.e. along the lower section  14  and the upper section  16 ) and a depth D larger along the lower section  14  than that d along the upper section  16 . 
         [0060]    The water channel  8  is obtainable e.g. by extrusion. 
         [0061]    Each fixing member  24  comprises a tubular sleeve  42  adapted to receive a shorter fuel rod  4  therein, and inwardly protruding projections  44  adapted to contact the outer surface of a shorter fuel rod  4  passing trough the sleeve  42 . 
         [0062]    Each fixing member  24  is fixed on the lower section  14  in one of the grooves  32 , namely on the outer surface of the bottom wall  36  of the groove  32 . 
         [0063]    Several fixing members  24  are distributed and aligned longitudinally along the lower section  14  for receiving a shorter fuel rod  4 . 
         [0064]    As illustrated on  FIG. 3 , fixing members  24  are arranged in pairs, the fixing members of each pair being disposed side-by-side in the same groove  32  for receiving two shorter fuel rods  4 . The fixing members  24  of each pair are separate. In an alternative, two adjacent fixing members  24  are connected one to the other and form a 2×1 fixing device. 
         [0065]    The depth of the groove  32  at the upper section  16  and the fixing members  24  are adapted such that the outer surface of each short fuel rod  4  does not radially interfere with the bottom wall  36  and lateral walls  38  of the groove  32 . 
         [0066]    In operation, water flows through the water channel  8  and also through the fuel channel  10  between the fuel rods  4 ,  6 . 
         [0067]    The water serves as a moderator for the nuclear reaction: it slows down the neutrons emitted by the fuel pellets contained in the fuel rods  4 ,  6 . The water also serves as a coolant and exchanges heat with the fuel rods  4 ,  6 . 
         [0068]    The water channel  8  surrounded by the fuel rods  4 ,  6  enables to increase the moderator (water) to fuel ratio and the coolant amount in a central region of the fuel assembly  2 . 
         [0069]    The water flow is heated from the lower end to the upper end of the fuel assembly  2 . At the upper end, the water flow is in vapour-liquid phase, the quantity of vapour increasing from lower end to upper end. 
         [0070]    Vaporized water is less efficient than liquid water as a moderator, and efficiency of fuel burning depends among others on the moderator to fuel ratio. 
         [0071]    The shorter fuel rods  4  reduce the amount of nuclear fuel in the upper region of the fuel assembly  2 . The upper section  16  of larger cross section of the water channel  8  provides a greater amount of water in the upper region of the fuel assembly. 
         [0072]    As a result, it is possible to compensate a decrease of moderator to fuel ratio due to increasing amount of vaporized water. 
         [0073]    As illustrated on  FIG. 5 , each shorter fuel rod  4  can be gripped from the upper end of the fuel assembly  2  and slide upwardly through the groove  32  along the upper section  16  for extracting the short rod  4  without removing the water channel  8 , e.g. in view of maintenance operations. A shorter fuel rod  4  can be inserted in the corresponding fixing member  24  in the same manner. 
         [0074]    This is due to the fact that the grooves  32  provided in the lower section  14  extend along the upper section  16  with a depth adapted so that a shorter fuel rod  4  will not interfere with the walls  36 ,  38 , and specifically the bottom walls  36  upon longitudinal extraction or insertion. 
         [0075]    Maintenance operations can thus be conducted more easily. It is namely possible to replace a damaged shorter fuel rod  4 , without extracting the water channel  8 . 
         [0076]    The fixing members  24  dedicated to the shorter fuel rods  4  adjacent to the water channel  8  enable to simplify the shape of the spacers  18  despite the grooves  32 , since it is possible to provide a passage  22  of square cross section. The spacers  18  can be the same along the lower section  14  and the upper section  16 . 
         [0077]    The cruciform shaped cross section of the water channel  8  provides a good distribution of the water in a transverse plane of the fuel assembly  2 . The main duct  26  is adjacent to eight fuel rods  4 . Each lateral duct  28  is adjacent to seven fuel rods  4 ,  6 . 
         [0078]    This distribution is obtained with a single water channel. The fuel assembly  2  can thus be manufactured economically. 
         [0079]    The water channel  8  is obtainable e.g. by extrusion. 
         [0080]    In the illustrated embodiment, the longitudinal axis of the water channel coincides with the longitudinal central axis of the fuel assembly. In an alternative, the longitudinal axis of the water channel is offset with respect to the longitudinal central axis of the fuel assembly  2 . 
         [0081]    In alternative embodiments, the fuel rods  4 ,  6  are arranged in a square array having a different number of cells (e.g. 8×8, 9×9 . . . to 11×11 or more), the array has a different outer shape (e.g. hexagonal), the water channel has a different shape and replaces a different number of fuel rods in the array, and/or the water channel passage has a different shape and replaces a different number of fuel rods in the array.