JPH11218592A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to prevent short fuel rods from being pulled out illegally by providing obstacles interfering with a cylindrical surface that is made when the short fuel rods are projected axially on cooling water flow channels above the short fuel rods. SOLUTION: Fuel assemblies 11 consist of such members as long fuel rods 13, short fuel rods 14, water rods 15, upper tie plates 12 and lower tie plates 16 that tie and support the upper ends and the lower ends of these three members respectively and a spacer 17 supporting the members mentioned above in order. By providing snags in cooling water flow channels in positions of the short fuel rods in the upper tie plates 12, it is physically impossible to illegally pull out the short fuel rods 14 to limit the access from a above the fuel assemblies 11 by jigs used to pull out the rods 14. This method can prevent the short fuel rods 14 from being pulled out illegally and can curb the pressure loss of the cooling water to a low degree.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a MOX fuel for a light water reactor, and more particularly to a safeguard method for a fuel assembly in which a part of a fuel rod is composed of a short fuel rod containing plutonium. The present invention relates to a fuel assembly having a structure capable of preventing unauthorized removal.

[0002]

2. Description of the Related Art A MOX fuel assembly for a light water reactor comprises MOX fuel rods loaded with pellets obtained by sintering a mixed oxide of uranium and plutonium in a grid pattern. Is partially used. In Japan, nuclear materials such as uranium and plutonium are managed under safeguards led by the International Atomic Energy Agency (IAEA), and it must be continuously shown that these materials have not been diverted to nuclear weapons. . In particular, in the case of MOX fuel, strict safeguards are required because plutonium, which is considered to be necessary for diversion to nuclear weapons, is used in small quantities.

To simplify safeguards, it is effective to apply monitoring / containment. As a means for containing MOX fuel for light water reactors, a method of attaching a seal to a fuel assembly is promising. In other words, the structure must be such that the assembly cannot be disassembled and the fuel rods cannot be removed unless the seal is destroyed, and the integrity of the assembly is confirmed by checking the integrity of the seal. Make sure. Such containment is achieved, in the case of fuel assemblies consisting of fuel rods of the same length, by placing a seal on the upper end plug extension of the combined fuel rod which is fixed to the upper tie plate with a nut. it can.

On the other hand, uranium fuel assemblies in which a part of fuel rods are replaced with short fuel rods for the purpose of increasing the burnup of fuel have been put to practical use, and are considered to be applied to MOX fuel in the future. However, such a fuel assembly has a structure in which there is no obstacle in the cooling water flow path above the short fuel rod, and has a structure in which the short fuel rod can be pulled out from the boss-pulled portion of the upper tie plate. I have. Therefore, when the short fuel rod is a MOX fuel rod, there is a problem that even if a seal is attached on the extension of the end plug of the combined fuel rod, the safety measures are incomplete. In addition, if the cooling water flow path above the short fuel is closed to prevent the short fuel rod from being pulled out, the pressure loss of the cooling water increases, and the fuel performance deteriorates.

[0005]

SUMMARY OF THE INVENTION It is a first object of the present invention to provide a fuel assembly in which a part of a fuel rod is composed of a short fuel rod containing plutonium. An object of the present invention is to provide a fuel assembly having a structure for preventing the fuel assembly. It is a second object of the present invention to provide the pull-out preventing structure in which the pressure loss of the cooling water is as small as possible.

[0006]

Means for attaining the first object is to provide a cooling water flow path above a short fuel rod, which interferes with a cylindrical surface formed when the short fuel is axially projected. Is to provide obstacles to By providing the obstacle, the pulling out of the short fuel rod can be made physically impossible.

Further, since the access from above the fuel assembly such as a jig used for illegally removing the short fuel rod can be restricted, the installation of the obstacle is a double protection means. The obstacle can be provided on the spacer, the fuel rod and the upper tie plate located above the short fuel rod.

[0008] The obstacle provided to achieve the first object may cause an increase in pressure loss of cooling water if the installation position is incorrectly selected, and may cause a decrease in the thermal fluidity of the fuel.
The pressure loss of the cooling water increases as the projected cross-sectional area of the obstacle in the cooling water flow path increases and the cooling water flow velocity at the obstacle position increases with respect to the obstacle having the same shape.
Therefore, the second object of the present invention of minimizing the pressure loss of the cooling water can be achieved by the following means.

That is, first, the obstacle is made as small as possible within a range where interference occurs when the short fuel rod is pulled out.
Second, the pressure loss of the cooling water can be minimized by disposing the obstacle on the outer peripheral surface side where the flow velocity of the cooling water is small. In a fuel assembly in which short fuel rods are arranged at the outermost periphery of the fuel assembly lattice, the effect of the second means is further enhanced. Further, the pressure loss can be further reduced by designing the obstacle to have a streamlined shape with respect to the inflow direction of water. Further, since the degree of fuel performance deterioration due to cooling water pressure loss is smaller at the upper part of the fuel assembly, it is more preferable to provide the obstacle in the upper tie plate.

It is to be noted that a major object of the present invention is to effectively apply containment, which is one of safeguarding measures, to a MOX fuel assembly, and the above-described means is applied to a sealed fuel assembly. Can be expected to have a greater effect.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below.

FIGS. 1 (a) to 1 (d) show configuration examples in which the present invention is applied to a fuel assembly including a MOX short fuel rod.
(B) The fuel assembly 11 is composed of a long fuel rod 13 and a short MO.
The upper tie plate 12 for restraining and supporting the X fuel rod 14, the water rod 15, and their upper and lower ends, respectively, the lower tie plate 16 and the spacer 17 for aligning and supporting them.
And the like. The short fuel rods 14 containing plutonium are arranged in the fuel assembly as shown in the cross section AA as an example.

In addition, as shown in the cross section BB of FIG. 1C, the short fuel rods 14 have a lattice structure in which the positions of the short fuel rods 14 are left vacant in the upper part of the fuel assembly, and a preferable water-to-uranium ratio in terms of fuel characteristics. It is designed to be.

In this embodiment, as shown in the cross-section C-C of FIG. 3D, the protrusion 121 is provided in the cooling water flow path at the position of the short fuel rod of the upper tie plate 12 so that the short fuel rod can be pulled out. In addition to making it physically impossible, access from above the fuel assembly such as a jig used for illegally removing the short fuel rod is restricted. Note that with respect to the corner portion that interferes with the handle of the upper tie plate, consideration for pulling out the short fuel rod is unnecessary.

FIG. 2 shows details of a cross section of the upper tie plate 12 to which the present invention is applied. The protrusion 121 is provided so as to interfere with the axial projection cross section 141 of the short fuel rod.

FIG. 3 is a cross-sectional view of a typical conventional upper tie plate, but nothing interferes with the short fuel rod projection cross section 141, so that a structure capable of improperly pulling out the short fuel rod can be performed. The embodiment shown in FIG. 2 solves this problem. 4A and 4B are an enlarged view of the protrusion 121 shown in FIG. 2 and a side cross-sectional view taken along a cross section AA. As described above, the protrusion is provided at a position that interferes with the axial projection cross section 141 of the short fuel rod. Such protrusions are shown in FIGS.
However, in order to reduce the pressure loss of the cooling water as much as possible, it is more preferable to install the cooling water on the outer peripheral surface side of the fuel assembly on the surface (b) where the cooling water flow rate is small.

Further, in order to increase the effect of reducing the pressure loss, it is conceivable to form the projection into a streamlined shape with respect to the inflow direction of water as shown in section AA. On the other hand, if the cooling water pressure loss does not cause a problem in fuel characteristics, a cross-shaped structure or a bridge-type structure is provided instead of the projection as shown in FIGS. It is also conceivable to adopt a structure that can more reliably prevent the short fuel rod from being pulled out than the protrusion shown in FIG.

FIG. 5 shows a design example in which a part of the short fuel rod containing plutonium is arranged at the outermost peripheral portion of the fuel assembly. FIG. 5A is a cross section of the lower portion of the fuel assembly, and a part of the short fuel rod is arranged at the outermost peripheral portion. FIG. 5B shows an example of the upper tie plate design in this case.
As shown in detail in (c), by providing the protrusion on the outermost periphery of the upper tie plate, it is possible to prevent the pullout of the short fuel rod while minimizing the pressure loss.
Therefore, in order to effectively apply the present invention, a design in which the short fuel rods are arranged at the outermost periphery of the fuel assembly is more preferable.

Although an example in which various obstacles are provided in the upper tie plate 12 has been described above, the upper tie plate having such a structure can be easily manufactured by conventional casting and machining.

The fuel assembly has a structure to which the present invention can be applied besides the upper tie plate, and typical application examples are shown below.

FIG. 6 shows, as one means for preventing the short fuel rod from being pulled out, a projection 181 on a locking washer tab 18 which is a metal fitting for securing the upper tie plate and the combined fuel rod.
This is an embodiment provided with the following. The locking washer tab is a washer that locks between the upper tie plate and the nut when fixing the combined fuel rod to the upper tie plate with a nut, and has a function to prevent rotation of the nut by bending the claw after tightening the nut. Have. Fig. 6 (a), (b)
And (c) show a locking washer tab corresponding to the fuel rod arrangement shown in FIGS. 1 and 5, and a mounting position thereof.

FIG. 7 shows the spacer 17 above the short fuel rod.
FIG. 3 is a cross-sectional view of FIG. The spacer is a spacer band on the outer periphery
173 and a spacer cell 172 for holding the fuel rod are the main elements. In the present embodiment, a bridge type obstacle 1 is attached to a pair of spacer cells adjacent to the position of the short fuel rod.
71 is provided to prevent the short fuel rod from being pulled out. As a specific method of installing the obstacle, a method in which a zirconium alloy, stainless steel, or the like is used as a raw material and the obstacle is attached to the spacer cell by spot welding can be considered.

FIG. 8 is a sectional view showing the upper portion of the fuel assembly and a diagram showing a method of providing an obstacle on the upper portion of the fuel rod.
In the present embodiment, the connecting rod washer 131 is provided as an obstacle at the upper end plug of one set of fuel rods adjacent to the short fuel rod, thereby preventing the short fuel rod from being pulled out.

By applying the above embodiment to a sealed fuel assembly, a containment means effective as a safeguard for a fuel assembly in which a part of the fuel rod is constituted by a short fuel rod containing plutonium. Is completed.

That is, by applying the present invention,
Unless the seal is destroyed, the assembly can be dismantled and the fuel rods cannot be taken out, and it is possible to confirm the integrity of the seal by confirming the integrity of the seal. it can.

Such containment, as in the case of a fuel assembly consisting of fuel rods of the same length, is achieved by the upper end of the combined fuel rod secured to the upper tie plate by a nut as shown in FIG. This can be achieved by attaching a seal on the stopper extension.

[0027]

According to the present invention, in a fuel assembly in which a part of a fuel rod is constituted by a short fuel rod containing plutonium, a structure for preventing the short fuel rod from being illegally pulled out is provided. In addition, it is possible to provide the pull-out preventing structure in which the pressure loss of the cooling water is minimized.

[Brief description of the drawings]

FIG. 1 is a side sectional view, a sectional view taken along line AA, a sectional view taken along line BB, and a sectional view taken along line CC of a fuel assembly including a short MOX fuel rod to which the present invention is applied.

FIGS. 2A, 2B, 2C, and 2D are cross-sectional views of an upper tie plate of a fuel assembly to which the present invention is applied.

FIG. 3 is a cross-sectional view of an upper tie plate of a conventional fuel assembly.

FIG. 4 is an enlarged plan view of a portion to which the present invention is applied to an upper tie plate, and a cross-sectional view and a plan view taken along line AA of FIG.

FIGS. 5 (a) and (b), (c) and (d) are plan views showing an example of application to a fuel assembly including a short MOX fuel rod at the outermost periphery.

6 (a) to 6 (c) are a plan view of an embodiment of the present invention using a locking tab washer and a cross-sectional side and plan views and a cross-sectional view of FIG. 6 (a).

FIGS. 7A, 7B, 7C, and 7D are explanatory views showing an embodiment of the present invention using a spacer.

FIG. 8 is an explanatory view showing an embodiment of the present invention using a fuel rod bridge.

FIG. 9 is a side sectional view showing a sealed fuel assembly.

[Explanation of symbols]

11 fuel assembly, 12 upper tie plate, 13 long fuel rod, 14 short fuel rod, 15 water rod,
16 Bottom tie plate, 17 Spacer, 18 Locking tab washer, 19 Seal, 121 Projection, 1
41: cylindrical projection of the short fuel rod in the axial direction, 171: spacer bridge, 172: spacer cell, 173: spacer band, 181: obstacle provided on the locking tab washer.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Kanamori 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Pref. Hitachi, Ltd. Hitachi Plant (72) Inventor Nobuo Tsutsumi 3-chome, Sachicho, Hitachi-shi, Ibaraki No. 1 Inside Hitachi, Ltd. Hitachi Plant

Claims (9)

[Claims] 1. A fuel assembly comprising a plurality of fuel rods arranged in a lattice, an upper tie plate, a lower tie plate and a spacer for supporting the fuel rods, wherein a part of the fuel rods contains plutonium. In a fuel assembly composed of short fuel rods, an obstacle that interferes with a cylindrical surface formed when the short fuel is projected in the axial direction is provided on the cooling water flow path above the short fuel rod. A fuel assembly, characterized in that: 2. The fuel according to claim 1, wherein the obstacle is a protrusion, a bridge-type structure, or a cross-type structure provided in a short fuel rod upper cooling water flow path of an upper tie plate. Aggregation. 3. The fuel assembly according to claim 1, wherein the obstacle is provided integrally with a fitting for securing the combined fuel rod and the upper tie plate. 4. The spacer according to claim 1, wherein the obstacle is provided in a cooling water flow path above the short fuel rod of the spacer.
The fuel assembly as described. 5. The fuel assembly according to claim 1, wherein the obstacle is a structure that connects a pair of fuel rods adjacent to and adjacent to a short fuel rod upper cooling water flow path in a bridge shape. 6. The fuel assembly according to claim 1, wherein the obstacle is located on the outer peripheral surface side of the cooling water flow path above the short fuel rod. 7. The fuel assembly according to claim 1, wherein the fuel assembly has a streamlined shape with respect to a water inflow direction. 8. The fuel assembly according to claim 1, wherein the short fuel rod containing plutonium is located at the outermost periphery of the lattice arrangement. 9. The fuel assembly according to claim 1, wherein the upper connection end plug or an extension thereof has a sealing structure.