JPH06331766A - Fuel assembly - Google Patents

Abstract

PURPOSE:To obtain a gain of reactivity and the effect for improving fuel efficiency by increasing the accumulation of plutonium in a fuel assembly and enabling the accumulated plutonium to burn more effectively. CONSTITUTION:In a fuel assembly, the wall thickness of a channel box 1 is made thicker in the upper or lower part of a fuel assembly and thinner in the lower or upper part reciprocally by making the outer width of the channel box 1 changeable without changing the inner width of it vertically. During a certain period from the early stages of fuel loading, the thicker part of the channel box 1 is turned to the upper side of a fuel assembly to accumulate plutonium. During the remaining period, the thicker part is turned to the lower side to burn the accumulated plutonium.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly loaded in a boiling water nuclear reactor core, and more particularly to a fuel assembly suitable for improving combustion efficiency and fuel economy.

[0002]

2. Description of the Related Art FIG. 1 shows a conventional boiling water reactor (BWR).
The perspective view of the fuel assembly for vehicles is shown. The fuel assembly is composed of a plurality of fuel rods 2 containing a nuclear fuel material, one or more water rods 3, a fuel spacer 4 for fixing the fuel rods and the water rods, an outer tie plate 5 and a lower tie plate 6. Channel box 1
The channel box has a uniform thickness in the vertical direction. The channel box divides the fuel assembly into the inside of the fuel assembly and the outside of the fuel assembly when viewed in cross section. Although voids (vapor bubbles) are generated inside the fuel assembly due to heat transfer from the fuel rods, no voids are generated outside the fuel assembly, so that a pressure difference is generated outside the fuel assembly.

FIG. 2 shows a typical vertical distribution of pressure inside and outside the fuel assembly, where a is the internal pressure of the fuel assembly and b is the external pressure of the fuel assembly. As shown in this figure,
The pressure difference between the internal pressure and the external pressure of the fuel assembly in the core is the largest in the lower part of the core, becomes smaller toward the upper part of the core, and becomes the smallest in the upper end part of the core. Such a pressure difference causes creep deformation of the channel box 1. The creep deformation of this channel box is due to the smooth insertion of the control rod,
The life of the channel box is usually a few years, as it is an obstacle to pulling out.

It is necessary to take measures to reduce the amount of creep deformation. The simplest measure is to increase the thickness of the channel box. However, if the channel box is made thick, neutron absorption by the channel box increases, This leads to loss of reactivity and loss of burnup. Therefore, it is desirable to minimize neutron absorption by this channel box.

As one of the measures for this, Japanese Patent Laid-Open No. 53-
FIGS. 3 and 4 of Japanese Patent No. 43193 show an example in which the outer width dimension of the channel box is changed stepwise in the vertical direction, and the wall thickness of the channel box is increased only in the lower part of the core where the pressure difference is large. .

Similarly, in FIGS. 1 and 3 of Japanese Patent Application Laid-Open No. 63-261190, the inner width dimension of the channel box is changed in the vertical direction, and in particular, the wall thickness of the channel box is about 1 / from the lower end. An example in which the thickness is increased in the section of 3 to 1/2 is shown.

Further, as a method for obtaining the reactivity gain from the viewpoint of fuel economy, there is a spectrum shift operation by core flow rate control. It operates at a low flow rate from the beginning to the end of the operation cycle, and increases to a high flow rate at the end of the cycle to obtain a reactivity gain. When the flow rate is low, the void ratio in the core increases and Pu accumulation increases. A reactivity gain is obtained by increasing the flow rate and burning the accumulated Pu at the end of the cycle.

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention From the viewpoint of the life of the channel box, JP-A-53-43193 and JP-A-63
The wall thickness in the vertical direction is changed as shown in Japanese Patent No. 261190. Especially, the lower part is thicker than the upper part, but the effect of neutron spectrum shift cannot be expected.

There is also a spectrum shift operation by controlling the core flow rate, which is a low flow constant operation from the beginning of the cycle to a point before the end of the cycle, where Pu is accumulated, and the accumulated Pu is burned at the end of the cycle to increase the reactivity gain. Is to get.

An object of the present invention is to provide a fuel assembly in which the life of the channel box is maintained at the same level as in the conventional case and the spectrum shift operation is possible depending on the fuel shape.

[0011]

SUMMARY OF THE INVENTION In the fuel assembly, the above object is achieved by changing the outer width dimension of the channel box without changing the inner width dimension of the channel box in the vertical direction. The thickness is made thicker at the upper or lower part of the fuel assembly, and conversely thin at the lower part or upper part of the fuel assembly.For a certain period from the beginning of fuel loading, the thicker one of the channel box is made the upper part,
The remaining period is achieved by making the thicker wall of the channel box the lower part.

[0012]

The present invention reduces the water region at the upper part of the fuel assembly, which is a high void generation region, by making the thicker one of the channel boxes the upper part of the fuel assembly for a certain period from the beginning of fuel loading. Since the void fraction increases, the neutron spectrum is hardened and the fission reaction at the upper part of the fuel assembly is suppressed. In addition, large ²³⁸ U of neutron absorption, which accounts for the majority of the fuel nuclide is a species that does not fission also absorb the neutron ²³⁹ Pu fission nuclides repeat the β decay
become. Therefore, ²³⁹ Pu in the upper part of the fuel assembly is accumulated with almost no fission. This accumulated
^In order to burn ²³⁹ Pu efficiently, by lowering the thicker part of the channel box in the remaining period, the water region in the upper part of the fuel assembly is increased and the neutron spectrum is softened to promote fission. It is to be.

As described above, by making the thicker one of the channel box the upper part of the fuel assembly for a certain period from the beginning of fuel loading and the lower part of the fuel assembly for the remaining period, spectrum shift operation becomes possible. , Reactivity and burnup are improved.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS. FIG. 3 is a sectional view of a high burnup fuel assembly which is an embodiment of the present invention. As shown in FIG. 4, the fuel arrangement is
Based on the lattice type, two large diameter water rods are arranged in the center, and the feature is that the number of fuel elements is larger than that of conventional fuel.

FIG. 5 shows the structure of a channel box used in this fuel assembly. The channel box has a length L of about 4 m and has a square cross-section structure, and FIG. 5 schematically shows the vertical cross section thereof. In this embodiment, the inner width D of the channel box is about 13.25 cm.
However, the outer width of the channel box differs from the upper end by the length L / 3 to L / 2 of the total length L from the upper end, and the upper outer width d1 is about 13.86c.
m, the outer width d2 of the lower part is about 13.66 cm, the wall thickness of the channel box is about 3.0 mm at the upper part and 2.0 mm at the lower part, and the upper part is thicker than the lower part. .

As described above, as a result of making the thickness of the upper part of the channel box thicker than that of the lower part, the water region in the upper part of the fuel assembly becomes smaller, so that the neutron moderating effect is small and the neutron spectrum is hardened. On the other hand, FIG. 6 shows the core axial power distribution of this embodiment. As shown in the figure, the output distribution X1 is obtained when the conventional channel box is used, and the output distribution X2 is obtained when the channel box of this embodiment is used, and the output distribution X2 is higher in the lower portion in this embodiment. Therefore, the void ratio also becomes larger in this embodiment, the neutron moderating effect is reduced in the upper part of the fuel assembly, and the neutron spectrum is hardened.

Therefore, in the upper part of the fuel assembly, ²³⁸ U, which has a large neutron absorption, absorbs neutrons and repeats β decay to generate ²³⁹ Pu, which is a fissile nuclide. Accumulated without.

It is necessary to effectively burn the ²³⁹ Pu accumulated on the upper part of the fuel assembly, but in this embodiment, as shown in FIGS. 7 and 8, the thicker the fuel assembly is with respect to the front channel box. By increasing the water region in the lower part of the body and in the upper part of the fuel assembly, the neutron spectrum can be softened and the fission reaction can be increased.

As described above, in the spectrum shift operation by the channel box, ²³⁹ Pu was accumulated during the fixed period from the beginning of fuel loading, and ²³⁹ Pu was accumulated during the remaining period ^.
Try to burn Pu.

As described above, by using the channel boxes of the present embodiment having different upper and lower wall thicknesses, the spectrum shift operation becomes possible, the reactivity gain is obtained, and the fuel size is improved.

[0021]

According to the present invention, since the accumulation of Pu in the fuel assembly can be increased and the accumulated Pu can be effectively burned, the reactivity gain can be obtained and the effect of improving the fuel economy can be obtained. There is.

[Brief description of drawings]

FIG. 1 is a perspective view of a conventional fuel assembly.

FIG. 2 is an axial distribution diagram of pressure applied to a fuel assembly.

FIG. 3 is a sectional view showing the structure of a fuel assembly according to an embodiment of the present invention.

FIG. 4 is a cross-sectional view of the fuel assembly shown in FIGS. 3 and 7.

5 is a vertical cross-sectional view of the channel box in FIG.

FIG. 6 is an axial distribution diagram of relative output and void fraction in the present invention.

FIG. 7 is a sectional view showing the structure of a fuel assembly according to another embodiment of the present invention.

8 is a vertical cross-sectional view of the channel box in FIG.

[Explanation of symbols]

1 ... Channel box, 2 ... Fuel rod, 3 ... Water rod, 4 ... Fuel spacer, 5 ... Upper tie plate, 6 ...
Lower tie plate, 7 ... Large diameter water rod.

Claims (2)

[Claims] 1. In a fuel assembly loaded in a core of a boiling water reactor, an inner width dimension of a channel box of the fuel assembly is the same from an upper portion to a lower portion of the fuel assembly, The outer width is changed between the upper part and the lower part of the fuel assembly, and the wall thickness of the channel box is also made different between the upper part and the lower part, and the wall thickness of the channel box is large for a certain period from the initial loading of the fuel assembly. The fuel assembly is characterized in that the upper side is the upper side and the thicker side of the channel box is the lower side for the rest of the period. 2. The fuel assembly according to claim 1, wherein a thickness of the channel box is thicker in a section of about 1/3 to 1/2 from an upper end or a lower end of the fuel assembly, compared with a section other than the above. body.