The invention relates to fuel assemblies for nuclear reactors and more particularly to fuel assemblies for use in spectral shift reactors.

In typical nuclear reactors, reactivity control is accomplished by varying the amount cf neutron absorbing material (poisons) in the reactor core. Generally, neutron absorbing control rods are utilized to perform this function by varying the number and location of the control rods with respect to the reactor core. In addition to control rods, burnable poisons and poisons dissoived in the reactor coolant can be used to control reactivity.

In the conventional designs of pressurized water reactors, an excessive amount of reactivity is designed into the reactor core at start-up so that as the reactivity is depleted over the life of the core the excess reactivity may be employed to lengthen the core life. Since an excessive amount of reactivity is designed into the reactor core at the beginning of core life, neutron absorbing material such as soluble boron must be placed in the core at that time in order to properly control the excess reactivity. Over the core life, as reactivity is consumed, the neutron absorbing material is gradually removed from the reactor core so that the original excess reactivity may be used. While this arrangement provides one means of controlling a nuclear reactor over an expended core life, the neutron absorbing material used during core life absorbs neutrons and removes reactivity from the reactor core that could otherwise be used in a more productive manner such as in plutonium production. The consumption of reactivity in this manner without producing a useful product results in a less efficient depletion of uranium and greater fuel costs than could otherwise be achieved. In a spectral shift reactor the life of the reactor core is extended without suppressing excess reactivity with neutron absorbing material thereby providing an extended core life with a significantly lower fuel cost.

One such method of producing an extended core life while reducing the amount of neutron absorbing material in the reactor core is by the use of "Spectral Shift Control" In one such method the reduction of excess reactivity (and thus neutron absorbing material) is achieved by withholding a portion of the reactor coolant water until after some time of reactor operation so that the neutron spectrum at the beginning of core life is relatively hard. The shift from a "soft" to a "hard" spectrum results in more neutrons being consumed by U 238 in a useful manner rather than by poisons. As reactivity is consumed, water is gradually added so that the reactor core reactivity is maintained at a proper level. Thus, the reactor can be controlled without the use of neutron absorbing material and without the need for excess reactivity at start-up which results in a significant uranium fuel cost savings. The additional plutonium production also reduces the U²³⁵ enrichment requirements.

It is the principal object of the present invention to provide a fuel assembly suitable for use in connection with such a spectral shift reactor.

With this object in view, the present invention resides in a fuel assembly for a nuclear reactor comprising a top nozzle (54), a bottom nozzle (52), a plurality of guide tubes (56) extending between and interconnecting said top and bottom nozzles (54, 52), a plurality of grids (50) disposed in spaced relationship between said top and bottom nozzles (54, 52) and connected to said guide tubes (56), and a plurality of fuel elements (48) containing nuclear fuel arranged between said top and bottom nozzles (54, 52) and supported by said grids (50), characterized in that the top and bottom grids (58, 60) consist of Inconel and the intermediate grids (62) arranged between said top grid (58) and said bottom grid (60) consist of Zircaloy.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which:

• Figure 1 is a cross-sectional view in elevation of the reactor vessel;
• Figure 2 is a cross-sectional view in elevation of the top portion of the fuel assembly;
• Figure 3 is a cross-sectional view in elevation of the bottom portion of the fuel assembly;
• Figure 4 is a top view of an adapter plate;
• Figure 5 is a view in perspective of displacer rods and their respective fuel assembly;
• Figure 6 is a partial cross-sectional view in elevation of a fuel assembly;
• Figure 7 is a partial cross-sectional view in elevation of the lower portion of a fuel assembly;
• Figures 8 and 9 are partial cross-sectional views in elevation of the top portion of a fuel assembly;
• Figure 10 is a view along line X-X of Figure 8;
• Figure 11 is a view along line XI-XI of Figure 8; and
• Figure 12 is a view along line XII-XII of Figure 8.

Referring to Figure 1, the nuclear reactor is referred to generally as 20 and comprises a reactor vessel 22 with a removable closure head 24 attached to the top end thereof. An inlet nozzle 26 and an outlet nozzle 28 are connected to reactor vessel 22 to allow a coolant such as water to circulate through reactor vessel 22. A core plate 30 is disposed in the lower portion of reactor vessel 22 and serves to support fuel assemblies 32. Fuel assemblies 32 are arranged in reactor vessel 22 and comprise reactor core 34. As is well understood in the art, fuel assemblies 32 generate heat by nuclear fissioning of the uranium therein. The reactor coolant flowing through reactor vessel 22 in heat transfer relationship with fuel assemblies 32 transfers the heat from fuel assemblies 32 to electrical generating equipment located remote from nuclear reactor 20. A plurality of control rod drive mechanisms 36 which may be chosen from those well known in the art are disposed on closure head 24 for inserting or withdrawing control rods (not shown) from fuel assemblies 32. In addition, a plurality of displacer rod drive mechanisms 38 are also disposed on closure head 24 for inserting or withdrawing displacer rods 40 from fuel assemblies 32. Displacer rod drive mechanism 38 may be similar to the one described in copending United States Patent Application Serial No. 217,055, entitled "Hydraulic Drive Mechanism" and assigned to the Westinghouse Electric Corporation. For purposes of clarity, only a selected number of displacer rods 40 are shown in Figure 1. However, it should be understood, that the number of displacer rods 40 are chosen to correspond to the number of displacer rod guide tubes in fuel assemblies 32. Displacer rods 40 are arranged so as to be in colinear alignment with the guide tubes in fuel assemblies 32 so that displacer rods 40 may be inserted therein when it is desired. The insertion of displacer rods 40 into fuel assemblies 32 displaces water moderator from core 34 which reduces core moderation. A plurality of displacer rod guide structures 42 are located in the upper section of reactor vessel 22 with each being in alignment with a displacer rod drive mechanism 38 for guiding the movement of displacer rods 40 through the upper section of reactor vessel 22. A calandria 44 may be arranged between fuel assemblies 32 and displacer rod guide structures 42 and comprises a multiplicity of hollow stainless steel tubes arranged in colinear alignment with each displacer rod and control rod for providing guidance of the displacer rods and control rods through the calandria area and for minimizing flow induced vibrations in the displacer rods and control rods.

Referring now to Figures 2-5, fuel assemblies 32 comprise fuel elements 48, grids 50, bottom nozzle 52, top nozzle 54, and guide tubes 56. Fuel elements 48 may be elongated cylindrical metallic tubes containing nuclear fuel pellets and having both ends sealed by end plugs. Fuel elements 48 may be arranged in a substantially 20 x 20 rectangular array and are held in place by grids 50. When held by grids 50, fuel elements 48 may be arranged in a 12 mm. square pitch to form a 24 x 24 cm square fuel assembly. Guide tubes 56 which may number 25 are arranged in a generally 5 x 5 array within each fuel asssembly 32. Each guide tube 56 occupies the space of about four fuel elements 48 and extends from bottom nozzle 52 to top nozzle 54 and providing a means to support grids 50, top nozzle 54 and bottom nozzle 52. Guide tubes 56 may be hollow cylindrical metallic tubes manufactured from a metal such as Zircaloy tubing and capable of accommodating rods such as displacer rods 40 or control rods. Guide tubes 56 may have openings in the sides or in the bottom ends thereof for allowing reactor coolant to pass therethrough for cooling purposes. Displacer rods 40 and control rods are manufactured to be approximately the same size so that each guide tube 56 can equally accommodate either a displacer rod or a control rod. When not occupied by a rod, guide tubes 56 are filled with reactor coolant; however, when displacer rods 40 are inserted in guide tubes 56 displacer rods 40 displace the coolant therein.

Grids 50 which may number about 12 per fuel assembly are positioned at various locations along the length of fuel assembly 32 and serve to space fuel element 48 and guide tubes 56 at appropriate distances from each other and to allow the reactor coolant to circulate in heat transfer relationship with fuel elements 48. A more detailed description of grids similar to grids 50 may be -found in United States Patent Nos. 3,379,617 and 3,379,619. The grid 50 located closest to top nozzle 54 is referred to as top grid 58 and the grid 50 located closest to bottom nozzle 52 is referred to as bottom grid 60 with the ten grids 50 located therebetween being referred to as intermediate grids 62. Both top grid 58 and bottom grid 60 may be of the well-known spring-dimple type and may be made of Inconel for providing greater lateral support for fuel elements 48 when the grids are irradiated. Intermediate grids 62 may be of the dimple-type grids with no spring clips and made of Zircaloy for providing greater flow area for the reactor coolant.

Twenty-four stainless steel sleeves 64 are brazed to each top grid 58 and each bottom grid 60 and twenty-four Zircaloy sleeves 66 are welded to each intermediate grid 62. All guide tubes 56 except center guide tube 68 are mechanically attached by internal bulging to each grid 50. Having twenty-four guide tubes 56 attached to all twelve grids 50 produces a fuel assembly with significantly greater lateral stiffness than most existing fuel assemblies. Additionally, twenty-four stainless steel sleeves 70 are welded to each top adapter plate 72 which forms part of top nozzle 54. Each top adapter plate 72 may be constructed as shown in Figure 5 but are preferably formed as shown in Figure 4. Stainless steel sleeves 70 are welded to each adapter plate 72 by four axial welds. A bottom adapter plate 74 which may be similar to top adapter plate 72 forms part of bottom nozzle 52. A plurality of stainless steel screws 76 penetrate through -bottom adapter plate 74 and are used to attach guide tubes 56 to bottom nozzle 52. Screws 76 may also have a channel therethrough for allowing the reactor coolant to enter the guide tubes for cooling purposes. A locking pin (not shown) may be used to secure screws 76 to bottom adapter plate 74 by welding the pin to bottom adapter plate 74. As an alternative, a locking cup (not shown) may be used to secure screws 76.

Referring now to Figure 6, core plate 30 has a plurality of guide pins 80 mounted thereon that are formed to fit into semi-circular notches 82 of bottom nozzle 52. Guide pins 80 are arranged to fit into four adjacent fuel assembly notches 82 so as to provide alignment of fuel assemblies 32 on core plate 30. Similarly, top nozzle 54 may also have semi-circular notches 82 for accommodating such guide pins from a top core plate (not shown) if so desired.

Referring now to Figures 6-12, a fuel assembly locking mechanism 84 is provided in each fuel assembly 32 for removably attaching each fuel assembly to core plate 30 thereby eliminating the need for holddown springs that were used in the prior art. For clarity the fuel assemblies shown in Figures 6-12 have been simplified; however, it should be understood that these fuel assemblies are identical to those shown in Figures 2-4. Thus, all fuel assemblies 32 may be provided with a locking mechanism 84. Locking mechanism 84 comprises a stainless steel lower member 86 which is slidably disposed in the center of bottom adapter plate 74. Lower member 86 has a first bore 88 therein for allowing reactor coolant to pass therethrough and has external threads 90 around the outside of its lower portion for engaging anchor mechanism 92. A Belleville spring washer 94 is disposed in a notch in bottom adapter plate 74 and is compressed when lower member 86 is forced downwardly relative to bottom adapter plate 74 as shown in Figure 6. Spring washer 94 prevents loss of axial preload due to small unlocking rotation of external threads 90.

Anchor mechanism 92 comprises a stainless steel lower anchor 96 which may be screw attached or welded to core plate 30. A removable insert 98 is captured and held by lower anchor 96. Insert 98 may be made of an anti- galling and wear-resistant, austenitic, stainless steel and is provided with internal threads 100 for engaging external threads 90 thereby locking lower member 86 to core plate 30. Insert 98 may also have a second bore 102 therein for allowing reactor coolant to pass therethrough and into first bore 88 of lower member 86. A locking collar l04 may be provided with internal threads (not shown) that mate with external threads (not shown) on insert 98 so that locking collar 104 may be screwed onto insert 98 thereby pulling both insert 98 and locking collar 104 into tight contact with lower anchor 96. In addition, a locking pin 106 may be provided for locking insert 98 to locking collar 104 thereby preventing inadvertent unlocking of the two members.

Center guide tube 68 which may be made of Zircaloy is bulged attached to lower member 86 and extends into top nozzle 54. A ratchet mechanism 108 is attached to the top center guide tube 68 for locking center guide tube 68 to top nozzle 54. Ratchet mechanism 108 comprises a slotted member 110 that is welded to the upper end of center guide tube 68. A slider member 112 having lugs 114 on its inner surface is slidably disposed over slotted member 110 such that lugs 114 are slidably disposed in slots 116 of slotted member 110. This arrangement allows axial movement but not rotation of slider member 112 relative to slotted member 110. Slider member 110 also has a first set of ratchet teeth 118 mounted around the outside thereof. A second set of ratchet teeth 120 is mounted on retainer housing 122.in a manner so as to mate with first set of ratchet teeth 118. Retainer housing 122 is disposed around slider member 112 and is attached to top adapter plate 72 so that slider member 112 may slide relative to retainer housing 122. A biasing mechanism 124 which may be a coil spring is disposed between top adapter plate 72 and first set of ratchet teeth 118 and around slider member 112 for urging slider member 112 upwardly thus moving first set of ratchet teeth 118 into contact