JPS6263888A - Fuel rod for nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION This invention relates generally to nuclear fuel rods, and more particularly, to the storage of stacked fuel within a fuel rod in cold conditions during handling and transportation of the fuel rod. Temperature-responsive spring clip that exhibits an expanded configuration that restrains axial movement of the pellet and a contracted configuration that allows axial movement of the fuel pellet within the fuel rod at high temperatures during reactor operation. It is related to.

Nuclear reactors currently in operation contain nuclear fuel in fuel elements, or fuel rods. A fuel rod consists of a shell consisting of an elongated, thin-walled, circular cross-section hollow cladding or tube and a stack of fuel pellets slidably loaded therein. Each fuel pellet is normal solid in form and cylindrical in shape;
It is composed of enriched uranium dioxide. The tubes are hermetically sealed at both ends with end plugs to isolate the fuel pellets from the surrounding environment and prevent contact and chemical reactions between the nuclear fuel and other materials, such as water, in the reactor core. It has become.

Additionally, spaces within the fuel rods between the top of the stack of fuel pellets and the top end plug are added to compensate for the accumulation of fission products and other gases released from the fuel pellets during reactor operation. It is standard practice to provide , ie, a Blenheim. To minimize the possibility of damaging the fuel pellets during transfer and loading of nuclear fuel into the reactor core, restraints, such as coil springs, are typically provided in the upper brenum. Coil spring restraints are generally designed to exert an axial force on the pellet stack within the fuel rod.

This axial force is large enough to prevent the fuel pellets from shifting during transportation, and is sufficient to protect the fuel rod cladding tubes and pellet stacks within them during reactor operation. small enough to accommodate differences in length changes.

As mentioned above, during operation of a nuclear reactor, the uranium dioxide fuel pellets generate and release gases that accumulate within the fuel rods. As the gas accumulates, the internal pressure of the fuel rods increases. The fact that the external dimensions of the fuel rods, such as length and diameter, are fixed, and that long-term use, i.e. combustion, requires that the fuel rods contain very large amounts of fission gas or other gases. In view of this, the volume of the internal cavity of the fuel rod becomes an important limiting factor in the design of the fuel rod. Since it is important to avoid overpressurizing the fuel rod, the volume of the internal cavity of the fuel rod is increased by reducing the volume within the fuel rod occupied by internal components such as coil springs. This is desirable.

The use of spring clips within fuel rods to restrain fuel pellets is disclosed in commonly assigned U.S. Pat. No. 4,080,2 by Gesinski.
No. 53 and US Pat. No. 4,460,540 to Funk et al. Recently, expanded spring clip (C-shaped lip) structures have been proposed to replace coil springs in fuel rod brenums. Such spring clips maintain the pellet stack in a fixed position during transportation by expanding against the inner diameter of the fuel rod tube and gripping the tube by frictional force. Pele 1
- Little or no axial force is exerted on the stack, but the pellet stack is geometrically confined in its proper position. One major disadvantage of this spring clip is the possibility of creating a binding condition, ie, potentially reducing the mobility of the pellet stack. Presumably, this spring clip would not accommodate the change in length between the pellet stack and the cladding tube; tightening of the spring clip would damage the tube, and shrinkage of the pellet stack would cause the fuel pellets to A gap will be formed between the spring clip and the spring clip. This type of problem is characterized by the presence of frictional forces between the inner surface of the sheathing tube and the outer surface of the splash clip.

Therefore, although the position of the pellet stack is well maintained within the tube of the fuel rod during handling and transportation, as previously achieved with coil springs, the position of the pellet stack during subsequent reactor operation is There is a need for an improved structure for spring clips that also allows for mobility.

SUMMARY OF THE INVENTION The present invention provides several embodiments of a spring clip that is a temperature-responsive variable shape member designed to meet the above-mentioned needs. In each embodiment, the spring clip is temperature responsive and has an expanded geometry that restrains axial movement of the stacked fuel pellets within the fuel rod at low temperatures during handling and transportation of the fuel rod. At high temperatures during reactor operation, the fuel pellets assume a contracted geometry that allows axial movement of the fuel pellets within the fuel rods.

In a first embodiment, the spring clip is made from a conventional material known as a shape memory alloy. Spring clips made from this material are adapted to loosen, i.e., deform to a memorized shape, when heated in the core; in that deformed form, the spring clips with little or no force, the spring clip is adapted to slide axially within the tube with a small axial load. thus,
The mobility of the stack to the bellet 1 in the tube is improved - tightness of the spring clips is unlikely to occur during reactor operation.9 In a second embodiment, the spring clips are exposed to different temperatures. It is made using bimetallic plates that change their shape when By using bimetallic plates made of two materials with different coefficients of thermal expansion, the spring clip exerts a given force at one temperature and another force when heated or cooled above or below that temperature. It can be designed as follows. The bimetallic spring clip contracts away from one of the tubes of the fuel rod at reactor operating temperatures, eliminating any frictional interaction between the tube and the spring clip. As a result, when using bimetallic spring clips,
The mobility of the pellet stack within the tube is improved and tightness of the spring clips is less likely to occur during reactor operation.

Accordingly, the invention comprises a hollow tube, a sealing member such as an end plug sealing opposite ends of the tube, and nuclear fuel in the form of a stack of pellets slidably housed within the tube. It shows improvements in fuel rods for nuclear reactors. This improvement exhibits an expanded shape state that restrains nuclear fuel from sliding inside the hollow tube at low temperatures during fuel rod handling, and allows nuclear fuel to slide within the tube at high temperatures during reactor operation. It consists of a temperature-responsive variable shape member that exhibits a contracted shape state. More specifically, this temperature-responsive variable shape member is
The spring clip takes the form of a spring clip disposed in a pre-cum chamber defined within the hollow tube between the stack of fuel pellets and one of the end plugs to direct axial movement of the stack of fuel pellets within the tube. It is designed to provide sufficient frictional contact with the inner surface of the tube when expanded to provide restraint, and contracted to allow axial movement of the stack of fuel pellets within the tube. When this happens, it comes into contact with the inner surface of the tube with very little friction.

There are two embodiments of the spring clip. First implementation R
In one embodiment, the spring clip is made from a shape memory alloy, while in a second embodiment the spring clip is made from a bimetallic sheet.

These and other advantages of the present invention will become apparent to those skilled in the art from reading the following detailed description taken in conjunction with the drawings showing some preferred embodiments of the invention. .

In the following description, the same reference numerals indicate the same or equivalent parts throughout the drawings. In addition, in the following explanation, "front", "rear", "left side", "right side",
Terms such as "upper" and "lower" are words of convenience and are not to be construed as limiting terms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, and in particular to FIG. 1, there is shown in front view at 10 a fuel assembly in vertically foreshortened form. This fuel assembly 10 is a pressurized water reactor (PWR).
), which is basically a lower end structure for supporting a fuel assembly 10 on a lower core plate (not shown) in the core region of a nuclear reactor (not shown);
That is, it includes a lower nozzle 12 and a plurality of guide tubes, ie, guide simples 14, which project upward from the lower nozzle 12 and extend in the longitudinal direction. Additionally, the fuel assembly 10 includes a plurality of transverse grids 16 axially spaced apart along the guide simple 14 and elongated fuel rods 18 laterally separated from each other and supported by the grids 16. The organized sequence is DNL.

Further, the fuel assembly 10 includes a measuring tube 20 disposed at its center and an upper end structure, that is, an upper nozzle 22 attached to the upper end of the guide simple 14. By this arrangement of the parts, the assembly 10 forms an integral unit which can be handled normally without damaging the components.

As mentioned above, the array of fuel rods 18 in the fuel assembly 10 are held in a spaced relationship with each other by grids 16 spaced along the length of the fuel assembly 10. Each fuel rod 18 includes a hollow tube 24;
This tube 24 contains a stack of fuel rods 1-(nuclear fuel) 26, and both ends of the tube 24 are closed by sealing members, namely an upper end plug 28 and a lower end plug 30, so as to hermetically seal the fuel rod 18. It is being Generally, the stack of fuel pellets 26 will not completely fill the hollow tubes 24 of the fuel rods 18;
A plenum chamber 32 is provided above the fuel rods 18;
This plenum chamber 32 stores gases released by the fuel pellets 26 during reactor operation. A fuel pellet 26 made of fissile material is responsible for generating the reaction power of the pressurized water reactor. A liquid moderator/coolant, such as water or boron-containing water, is pumped upward through the core's fuel assemblies to extract the heat generated by the core and perform useful work.

In order to control the nuclear fission process, several control rods 34 are
A reciprocating movement is possible within the guide simple 14 which is placed in a predetermined position in the fuel assembly 10 .

In particular, the one-part nozzle 22 includes a bar cluster control mechanism 36 having an internally threaded cylindrical member 38 with a plurality of radially extending fluke arms 40. There is. Each arm 40 is connected to a control rod 34, and a control mechanism 36 acts to immobilize the control rod 34 vertically within the guide thimble 14, thereby causing the fission process in the fuel assembly 1o, all in a known manner. is designed to be controlled.

fLt'tJ (')-'? ”-1〈-9 2 single chi x
:/ N, (')u S,,, i-Na-4-Ne=Pra Y-Goo core operation Oη Only during handling, transportation, and loading of each fuel rod 18, the fuel rods 18 In order to restrain the movement of the stack of fuel pellets I 26 within the hollow tube 24 at Provides an improved restraining member (on the temperature-responsive shape) that fits within the plenum chamber 32 of the fuel rod 18 at the bottom of the stack of fuel pellets 26. Two embodiments of the spring clip 42 are shown in the specifications provided herein. The spring clip 42 of each embodiment is temperature responsive and the fuel rod 18
a) Approximately 93°C previously experienced during handling and transportation
At low temperature conditions in the range up to (below 200 degrees), the spring clips 42 assume an expanded configuration that restrains axial movement of the fuel pellets 26 within the fuel rods 18, and which are also experienced during reactor operation. from about 93°C (below 200) to about 427
At high temperature conditions, in the range of 800" F., the spring clip 42 assumes a contracted configuration that allows axial movement of the fuel pellet 26 within the fuel rod 18. fuel pellet 26 in tube 24 of
When in the expanded state, it is designed to be in superior frictional contact with the inner surface 44 of the hollow tube 24 to restrain movement of the fuel bellet 1-26 within the tube 24. When in the contracted state for clearance, it contacts the inner surface of tube 24 with negligible friction.

In a first embodiment, as shown in FIGS. 4-6, the spring clip 42' is made from a conventional metallic material known as a shape memory alloy. An example of such a material is Nitinol. By first fabricating a spring clip 42' from such material while heating it to the annealing temperature, the selected geometry is f & t, -,' (71N Fl frs
a5 m fr 1Tff -A +Y! :M
It will be recalled that when #a W +, -ad A is used, for example, when 42 spring clips "42' are used in the reactor core. In this invention,
The spring clip 42' contacts the inner surface 44 of the tube 24 with little or no force, causing the spring clip 42' and the stack of fuel pellets 26 to slide axially within the tube 24 with a small axial load. Spring clip 4
The shape of this memory of 2' is determined.

Once its "memory" shape is set, the spring clip 42' is then exposed to cold conditions comparable to those experienced in the past during fuel rod handling and transportation, e.g. about -17,8 h.
It is cooled from C (below 0) to 93 degrees C (below 200). At this temperature condition, the spring clip 42' is deformed from the previously described "memory" shape to an expanded configuration and contacts the inner surface 44 of the tube 24 with sufficient friction 2 to cause the fuel bell I to
- The 6 spring clips 42', which are designed to restrain the axial movement of the 26 stacks, are elastically deformable approximately C
: has a flange 46' projecting inwardly from each end of the C-shaped part and spaced apart from each other by a constant gap 48'.

When the spring clip 42' is in the expanded state, the width of the gap 48' is X', as shown in FIG.

When the spring clip 42' is removed from the hollow tube 24, as shown in FIG.
As shown in FIG. 6, the width of the gap 48' is less than °"x".

In a second embodiment, as shown in FIGS. 7-9, the spring clip 42'' is made from an elastically deformable bistal plate that deforms when exposed to different temperatures. As in the first embodiment, the spring clip 42'' is generally C-shaped;
projecting inwardly from each end of the
It has a flange 46'' on which is placed the flange 46''. Examples of metals constituting the two-layer bimetal plate include stainless steel, Inconel alloy, and carbon steel.

By using a bimetallic plate made of two metals with different coefficients of thermal expansion, the spring clip 42'' can exert a predetermined force at one temperature, and when heated or cooled above or below that temperature, will produce a different force at the new temperature. It can be designed to apply force.In Figure 8,
When the spring clip 42'' is in the expanded state within the tube 24, the spring clip 42'' has a gap 4 of a width equal to ``y'''.
8". This is due to the spring clip during handling and transportation of the fuel rod 18 (albeit slightly compressed from its natural state when removed from the tube 24 as shown in FIG. 7). 42'', in which the spring clip 42'' contacts the inner surface 44 of the tube 24 with sufficient friction to restrain movement of the 1-1 fuel Pel/Watt 26 stack. When the spring clip 42'' is removed from the tube 24, the gap 48'' is
As shown in the figure, it increases to a width greater than "y". Thereafter, when the fuel rods 18 are loaded into the core and the core is at operating temperature, the temperature increase causes the diameter of the spring clips 42'' to be reduced and the inner surface 42 of the fuel rod tube 24 to be
be separated from In this contracted state of the spring clip 42'' shown in FIG. 9, the width of the gap 48'' is "y
” smaller than.

The temperature responsive variable geometry spring clip of the present invention and the many advantages associated therewith will be appreciated from the above description. In addition, changes may be made in the form, construction and arrangement of the invention without departing from the spirit and scope of the invention and without sacrificing any of its important advantages, and the form described herein is merely It should be clear that this is only a preferred embodiment.

[Brief explanation of drawings]

FIG. 1 is a partially sectional front view of a fuel assembly including fuel rods using the improved spring crimp of the present invention;
The fuel rods are shown truncated and vertically truncated for clarity. FIG. 2 is an enlarged vertically shortened front view of the fuel rod taken out from the fuel assembly of FIG. FIG. 3 is similar to FIG. The figure shows the contracted shape state of the fuel assembly after irradiation, i.e., the spring 1 rusob contacts the inner surface of the fuel rod tube with little or no force, and the axial fuel beam The spring crimp is shown moved toward the upper end plug of the fuel rod in response to thermal expansion of length l/). FIG. 4 is an enlarged plan view of a first shape memory alloy embodiment of a spring clip according to the invention, showing the spring clip removed from a fuel rod. FIG. 5 is an enlarged cross-sectional view of the fuel rod taken along line 5--5 in FIG. This figure shows a spring clench made of shape memory alloy. Figure 6 is an enlarged view of the fuel rods along line 6-6 in Figure 3.
FIG. 6 is a diagram illustrating a fuel rod installed in a tube and placed in a contracted state such that a space is formed between the spring clip and the inner surface of the tube such that the spring clip has little or no frictional contact with the inner surface of the tube; Figure 5 shows the shape memory alloy spring clip of Figure 4; FIG. 7 is an enlarged plan view of a second bimetallic plate embodiment of a spring clip according to the invention, showing the spring clip removed from a fuel rod. FIG. 8 is an enlarged sectional view of the fuel rod similar to FIG. 5, but
Figure 8 shows the bimetallic plate spring clip of Figure 7 installed within a fuel rod tube and placed in an expanded configuration in frictional contact with the inner surface of the tube; FIG. 9 is an enlarged view of a fuel rod similar to FIG. 6, with the fuel rod provided within the tube and with a spring clip on the inner surface of the tube for most or all of <R1! 8 shows the bimetallic plate spring clip of FIG. 7 placed in a contracted state with a space formed between the spring clip and the inner surface of the tube to avoid rubbing contact; FIG. In the figure 10: Fuel assembly 18: Fuel rod 24: Pipe 26: Fuel pellet (nuclear fuel)
28.30 Two-end plug (sealing member) Jokichi of the drawing (no change in content) 32: Plenum chamber 42.42', 42'': Spring clip (temperature response variable shape member), -1-7 Patent applicant representative Person So Wado Teru□゛￥し）中
□□ −□ FIG 2 FIG, 3 FIG
, 6 Procedural Amendment Writing Method) September 24, 1986

Claims (1)

Claims: (a) an elongated hollow tube having opposed ends and having nuclear fuel slidably loaded therein; and (b) said opposed ends for sealing said nuclear fuel within said tube. (c) exhibiting an expanded shape state that restrains the sliding of the nuclear fuel within the tube at low temperatures during fuel rod handling; a temperature-responsive variable shape member disposed within the tube and exhibiting a contracted shape state that allows the nuclear fuel to slide in the tube.