JP3471192B2 - Fuel assembly - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a D-lattice core loaded with a plurality of fuel rods supported by an upper tie plate and a lower tie plate, respectively. A fuel assembly in which the distance between the fuel rods is held between the two tie plates by a plurality of fuel spacers, and the outer periphery thereof is surrounded by a channel box, and the upper tie plate and the channel box are connected by a channel fastener. In particular, the present invention relates to a fuel assembly in which the inclination of the fuel assembly generated by vibration such as an earthquake is suppressed, and a measure against scram caused by an increase in neutron flux generated in the D lattice core is taken. 2. Description of the Related Art Conventionally, there are channel fasteners in this type of fuel assembly as shown in FIGS. 5A and 5B. 5A and 5B, the channel fastener 1 generally includes a guard 2, a plate spring 3, a lock washer 4, and a cap screw 5. The guard 2 has a flat upper plate 6 formed in a substantially triangular shape, and a leg 7 extending downward with an L-shaped cross-section formed by the flat upper plate 6 by casting or steel plate. The leaf spring 3 has two spring legs 9 which are bisected from the upper plate portion 8 with the corner of the leg 7 of the guard 2 interposed therebetween. Is bent outwardly along the lower side of the outer surface of the
The tip is bent inward at the tip and is in contact with the leg 7 while ensuring a gap with the leg 7. The channel fastener 1 is mounted on a fuel assembly 10 as shown in FIG.
Is loaded in the core of the reactor and stands on a fuel support (not shown) as a cradle installed below the core, while its upper part is restrained from falling laterally by the upper grid plate Thereby, they are aligned in a state parallel to the vertical direction. That is, the fuel assembly 10 supports the upper and lower ends of a plurality of fuel rods with an upper tie plate 11 and a lower tie plate 12, respectively. Is held by a plurality of fuel spacers, and the outer periphery of the
The upper tie plate 11 and the channel box 13 are connected by the channel fastener 1. [0005] As shown in FIG.
Four fuel assemblies 10 are inserted in one lattice of
Adjacent fuel assemblies 10 are set at a height from the legs 7 so that the spring legs 9 of the plate spring 3 of the channel fastener 1 press against each other. It is pressed against the wall. In the channel fastener 1 thus configured, when a horizontal external force is applied to the spring leg 9, the spring leg 9 has its leg bent from the initial bending height H1 as shown in FIG. 7 can be moved downward while being in contact with the outer surface of the leg 7, and can bend until it becomes the same as the guard height H <b> 2 provided below and on the sides of the leg 7. [0007] A core in which a large number of fuel assemblies are loaded with the four fuel assemblies 10 as one set includes a C lattice core in which the intervals between the fuel assemblies 10 are uniform, and a non-uniform installation interval. The core is roughly classified into a D lattice core. As shown in FIG. 7, the D-lattice core is composed of a portion having a wide interval between the four fuel assemblies 10 (wide gap) and a portion having a narrow interval on the upper lattice plate 14 side (narrow gap). Have been. In FIG. 7, the control rod 15 is formed in a cross shape, and is inserted or pulled out between the four fuel assemblies 10. The shape of the channel fastener used in the C lattice core is shown in FIGS. 5A, 5B and 6, and the shape of the channel fastener used in the D lattice core is shown in FIG. ), (B) and FIG. 8 (A), (B) and FIG. 9, the same reference numerals are used for the same or corresponding portions as those of the channel fastener used for the C lattice core, and the description thereof is omitted. In the conventional fuel assembly 10 described above, when acceleration is applied in the horizontal direction due to vibration such as an earthquake, four fuel assemblies 10 are provided in the core lattice.
Is rocked, and one side row of the fuel assemblies 10 is pressed toward the opposing fuel assemblies 10 by the vibration in one direction. As a result, the spring leg 9 of the channel fastener 1 is crushed and the The gap disappears, the fuel assembly 10 tilts, and the spacing between the fuel assemblies 10 changes. When a D-lattice core is used in a boiling water reactor (BWR), the installation intervals of the fuel assemblies 10 are made uniform throughout the reactor core due to vibrations such as earthquakes.
That is, the distance between the upper grid plate 14 and the control rod 15 is increased.
Displaces in the direction in which the side spacing decreases. At this time, even if the interval between the fuel assemblies 10 is slightly changed, the neutron flux density of the D-lattice core increases and the reactivity is positive, so that the reactivity scram occurs. There are cases. On the other hand, in a C lattice core in which the spacing of the fuel assemblies 10 is uniform over the entire core, the spacing between the fuel assemblies 10 slightly changes due to vibrations such as earthquakes because the arrangement is in a stable state with the highest reactivity. However, only the reactivity is reduced and no scrum occurs. Accordingly, there is no drastic measure for the reactivity application scram due to the neutron flux increase except for disposing the fuel assemblies 10 at equal intervals like a C-lattice core. Cannot be achieved without shutting down the operating plant for a long period of time, resulting in a large economic loss. Accordingly, the present invention has been made in view of the above-described circumstances, and has as its object to provide a fuel assembly which can surely suppress the collapse in the core and facilitates loading the core. And [0014] In order to solve the above-mentioned problems, a fuel assembly 1 according to the present invention is loaded into a D lattice core through a lattice of an upper lattice plate, and a plurality of fuel rods are provided. A fuel assembly in which the upper and lower ends are supported by an upper tie plate and a lower tie plate, respectively, and the outer periphery thereof is surrounded by a channel box, and the upper tie plate and the channel box are connected by a channel fastener having a plate spring and a guard. The leaf spring of the channel fastener has two spring legs, the width of the central portion below the spring legs is formed wider than the upper portion, and the pressing force of the leaf spring against the upper lattice plate is 5 kg. Over 25 kg or less, and the height of the guard for limiting the deflection of the leaf spring is set to 5.9. mm and 7.5 mm or less. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a side view showing a channel fastener in a first embodiment of the fuel assembly according to the present invention.
Note that the same or corresponding portions as those of the conventional configuration are shown in FIG.
6A and 6B and the same reference numerals as those in FIG. 6, and the description of the overlapping parts will be omitted. A fuel assembly 10 as an assembly body of the present embodiment supports upper and lower ends of a plurality of fuel rods with an upper tie plate 11 and a lower tie plate 12, respectively, as in FIG. The spacing between the fuel rods between the tie plates 11 and 12 is held by a plurality of fuel spacers, the outer circumference of which is surrounded by a channel box 13, and the upper tie plate 11 and the channel box 13 are connected by a channel fastener 1. The channel fastener 1 is provided at the upper end of the fuel assembly 10 and, as shown in FIG. 1, a plate spring is attached to the guard 2 via a guard 2, a plate spring 3, a lock washer 4, and a lock washer 4. The guard 2 holds the leaf spring 3 and has a flat upper plate 6 formed in a substantially triangular shape, and the flat upper plate 6 forms an L-shaped cross section and is downward. The extending leg 7 is integrally formed of a casting or a steel plate. The leaf spring 3 has two spring legs 9 which are bisected from the upper plate portion 8 with the corners of the legs 7 of the guard 2 interposed therebetween. The leg 7 is bent outward along a lower side of the outer surface, and a gap with the leg 7 is secured at a lower portion, while the tip is bent inward at the tip and is in contact with the leg 7. The fuel assembly 10 to which the channel fastener 1 is attached is loaded into the D lattice core of the nuclear reactor through the lattice of the upper lattice plate 14. 1 of upper lattice plate 14
Four fuel assemblies are inserted into one lattice, and adjacent fuel assemblies are fuel assemblies in which the spring legs 9 of the leaf springs 3 of the channel fastener 1 press against each other while the channel fastener 1 is not attached. The two sides including the diagonal corners of the body are pressed against the lattice wall surface of the upper lattice plate. The leaf spring 3 of the channel fastener 1
Has a pressing force of 5 kg in order to suppress the amount of change in the inclination of the fuel assembly 10 in the furnace due to a roll during an earthquake or the like.
And set to 25 kg or less. The thickness of the leaf spring 3 is set in a range of 1.4 to 1.7 mm,
Further, the bending height of the leaf spring 3 before loading on the core is set in a range from 4.8 to 5.8 mm to 5.9 to 7.5 mm. Next, the operation of the present embodiment will be described. In the thus configured channel fastener 1 of the fuel assembly 10 according to the present embodiment, as shown in Table 1, the plate thickness of the plate spring 3 and the initial bending height H1 of the plate spring 3 are increased. Since the spring constant and the pressing force are increased, the spring is less likely to be crushed than the spring of the conventional channel fastener. Further, by setting the guard height H2 higher than in the past, the amount of deflection of the leaf spring 3 during close contact of the guard is reduced, and the stress generated in the leaf spring 3 is reduced. [Table 1] As shown in FIG. 4, the conventional D-grid channel fastener has a pressing force of 5 k at the time of loading.
g, but in the channel fastener 1 according to the present embodiment, the pressing force exceeds 5 kg and the loadability is limited to 2
By increasing the pressure to 5 kg or less, a change in inclination of the fuel assembly in the furnace can be suppressed. The shape of the channel fastener 1 for avoiding the scram due to the neutron flux rise in the D-lattice core is not the current shape for the D-lattice as shown in FIGS. Based on the shape for the C lattice as shown in (A) and (B). This is because the channel fasteners for the D lattice are easily affected by variations due to differences in thermal expansion during operation and dimensional tolerances of furnace equipment, and it is difficult to increase the pressing force. Assuming that the spring constant of the leaf spring 3 is k and the amount of deflection of the leaf spring 3 is δ, the pressing force W satisfies the following relationship: W = k · δ. Is increased by increasing the plate thickness of the leaf spring 3 to increase the spring constant k, or increasing the amount of deflection δ of the leaf spring 3. That is, as shown in Table 1, the leaf spring 3
There is a method of increasing the initial bending height H1 or using both of them. The spring constant k and the amount of deflection δ are
When the pressing force W is more than 5 kg and not more than 25 kg, it can be changed so as to satisfy Expression 1. Here, when the pressing force W exceeds 25 kg, the loading limit of the fuel assembly to the core is exceeded. When the spring constant k of the leaf spring 3 is increased by increasing the thickness of the leaf spring 3, the thickness of the conventional channel fastener for the C lattice is 1.3 mm in nominal value, but the channel according to the present embodiment is not limited to 1.3 mm. The fastener 1 has a plate thickness of 1.4 mm, and the spring constant k is approximately twice as large as 1.7 mm.
Of the range. If the plate thickness exceeds 1.7 mm, the loading limit of the fuel assembly to the core will be exceeded. Further, if the spring constant k is simply doubled,
Since W = 10 kg, pressing force W is 25 kg at maximum.
It is necessary to increase the amount of deflection δ so that Therefore, if δ = 5 / k = δmin from Equation 1, it is necessary to increase the bending height of the leaf spring 3 before loading so that δmin <δ <2.5δmin. On the other hand, as the pressing force W increases, the stress generated in the leaf spring 3 increases, which poses a problem from the viewpoint of soundness. The leaf spring 3 of the channel fastener 1 bends until it comes into close contact with the opposing guard 2, and cannot be further deformed. Therefore, in the channel fastener 1 according to the present embodiment, the guard height H2 of the channel fastener 1 is set to be higher than the conventional range of 4.8 to 5.8 mm to 5.9 to 7.5 mm so that the guard is H. The amount of deflection of the leaf spring 3 during close contact with the plate spring 2 is reduced, and the stress generated in the leaf spring 3 is reduced. Here, the guard height H2
Exceeds 7.5 mm, the load limit of the fuel assembly to the core is exceeded. As described above, according to the present embodiment, even when an external force is applied in the horizontal direction during an earthquake or the like, in the channel fastener 1 of the fuel assembly 10 having the above-described configuration,
Since the deflection of the spring leg 9 is suppressed, the fuel assembly 1
The inclination of zero is alleviated, and the intervals between the fuel assemblies 10 are also maintained substantially uniform, so that it is possible to avoid the generation of the nuclear reactivity scram due to the neutron flux rise in the D-lattice core. FIG. 2 is a side view showing a channel fastener in a second embodiment of the fuel assembly according to the present invention.
The same parts as those in the first embodiment will be described with the same reference numerals. The same applies to the following embodiments. In the channel fastener 1 of the second embodiment, a guard projection 18 as a deformation restraining member is provided on the guard 2 itself between the spring leg 9 and the guard 2 in order to suppress the flat spring 3 from being crushed. is there. In the channel fastener 1 constructed as described above, when vibration is applied during an earthquake, the upper grid plate 1
4, a counterforce is generated in the horizontal direction on the spring leg 9 of the opposed channel fastener 1 in the lattice of 4, and the tip of the spring leg 9 moves downward while being in contact with the leg 7 of the guard 2. In the channel fastener 1 according to the present embodiment, the provision of the guard projection 18 makes the channel fastener 1 harder to collapse than the spring of the conventional channel fastener. As described above, according to the second embodiment, the provision of the guard projection 18 restrains the crush of the leaf spring 3 and suppresses the crush of the leaf spring 3. This makes it possible to suppress the amount of change in the tilt of the fuel assembly in the reactor, which causes the neutron flux to rise and cause the nuclear reactivity scram. Other configurations and operations are the same as those of the first embodiment, and therefore, description thereof will be omitted. FIG. 3 is a side view showing a channel fastener in a third embodiment of the fuel assembly according to the present invention. In the embodiment shown in FIG. 3, the width below the leaf spring 3 is increased. Thus, the local maximum stress generated in the leaf spring 3 can be reduced, and by combining with the above-described first and second embodiments that suppress the collapse of the leaf spring 3. In addition, the inclination of the fuel assembly can be suppressed. As described above, according to the first to third embodiments, the inclination of the fuel assembly in the furnace due to vibration is reduced by reducing the deflection of the plate spring 3 of the channel fastener 1 as compared with the conventional one. Can be suppressed. As described above, according to the first aspect of the present invention, the pressing force of the channel fastener against the upper lattice plate by the plate spring is set to be more than 5 kg and 25 kg or less, and this plate spring is set. By setting the height of the guard that limits the deflection of the channel to more than 5.9 mm and not more than 7.5 mm, even if a horizontal reaction force acts on the plate fastener of the channel fastener during an earthquake, the amount of change in the inclination in the furnace As a result, the possibility of the nuclear reactivity scram can be reduced. Further, the fuel assemblies can be easily loaded into the core.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view showing a channel fastener in a first embodiment of a fuel assembly according to the present invention. FIG. 2 is a side view showing a channel fastener in a second embodiment of the fuel assembly according to the present invention. FIG. 3 is a side view showing a channel fastener in a third embodiment of the fuel assembly according to the present invention. FIG. 4 is a diagram showing a relationship between a spring force and a fall of a fuel assembly during an earthquake. 5A and 5B are a plan view and a side view of a conventional channel fastener structure for a C grating. FIG. 6 is a side view showing a fuel assembly to which a channel fastener for C lattice is attached. FIG. 7 is a partially enlarged plan view showing a state where the fuel assembly of FIG. 6 is loaded in a reactor core. 8 (A) and (B) are a plan view and a side view of a conventional channel fastener structure for a D grating. FIG. 9 is a side view showing a fuel assembly to which a channel fastener for a D lattice is attached. [Description of Signs] 1 Channel fastener 2 Guard 3 Plate spring 4 Lock washer 5 Cap screw 6 Flat upper plate 7 Leg 8 Upper plate 9 Spring leg 10 Fuel assembly (assembly body) 11 Upper tie plate 12 Lower tie plate 13 Channel box 14 Upper lattice plate 15 Control rod 18 Guard protrusion (deformation restraint member)

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Akira Tanabe 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Pref. -5991 (JP, A) JP-A-61-258190 (JP, A) JP-A-1-212389 (JP, A) JP-A-5-73597 (JP, U) (58) Fields investigated (Int. . ^7, DB name) G21C 3/00 - 3/34

Claims (1)

(57) [Claim 1] A D-lattice core is loaded through a lattice of an upper lattice plate, and upper and lower ends of a plurality of fuel rods are supported by an upper tie plate and a lower tie plate, respectively. In a fuel assembly in which the outer periphery is surrounded by a channel box and the upper tie plate and the channel box are connected by a channel fastener having a leaf spring and a guard, the leaf spring of the channel fastener is 2
A plurality of spring legs, a width of a central portion below the spring legs is formed wider than above, and a pressing force of the plate spring against the upper lattice plate is set to be more than 5 kg and 25 kg or less. The height of the guard, which limits the deflection of the leaf spring, exceeds 5.9 mm and 7.5 m
m or less.