JPH07110391A - Fuel assembly - Google Patents

Abstract

PURPOSE:To provide a highly reliable fuel assembly by setting the inner pressure of a MOX (mixed oxide fuel) rod nearly equal to that of a uranium fuel rod. CONSTITUTION:In the fuel assembly arranging 9X9 matrix fuel rods, 8 short- sized fuel rods currently adopted are made to be MOX fuel rods 100 and the other standard long size fuel rods are left to be uranium fuel rods 101. The MOX short-sized fuel rods 100 are to have longer plenum length that the conventional uranium short-sized fuel rods without changing the fuel effective length.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water reactor.

[0002]

2. Description of the Related Art In the core of a boiling water reactor, voids are generated in the coolant along the flow of the coolant from the lower side to the upper side. Therefore, the void ratio becomes higher toward the upper part of the core. Therefore, the pressure loss in the upper part of the core tends to increase. Also, in the upper part of the core, when the void fraction becomes zero when the reactor is stopped, the change in the moderator density from the time of operation becomes extremely large, the deceleration becomes too good, and the control rod keeps the core subcritical. The margin (called the reactor shutdown margin) becomes smaller.

In order to solve such a problem, the reactor fuel assembly 1 shown in FIGS. 4 and 5 has been conventionally designed. 4 is an axial sectional view of the nuclear reactor fuel assembly 1, and FIG. 5 is a sectional view taken along the line AA of FIG.

The nuclear reactor fuel assembly 1 is covered with a rectangular tube-shaped channel box 2 in which the fuel rods 3 are arranged in 9 rows and 9 columns. The upper and lower ends of the reactor fuel assembly 1 are supported by an upper support plate 4 and a lower support plate 5, and spacers 6 for holding the intervals between the fuel rods 3 are provided at several axial positions of the reactor fuel assembly 1. Are arranged. At the center of the cross section of the reactor fuel assembly 1, a water rod 7 through which cooling water flows is arranged.
Then, several partial length fuel rods (short fuel rods) 3b formed shorter than the other fuel rods 3a are arranged.

FIG. 6 is a vertical sectional view of a standard length fuel rod (long fuel rod) 3a. The long fuel rod 3a is a cladding tube 9
A plurality of sintered pellets 10 are loaded inside, and the upper and lower ends are sealed with an upper end plug 11 and a lower end plug 12. This sintered pellet loading section is referred to as a fuel section 13. A plenum portion 14 is provided above the fuel portion 13.
A spring 15 is disposed in the spring 15 to suppress the axial movement of the sintered pellet 10. Further, inside the spring 15, a getter 16 that absorbs moisture in the fuel rod 3a is incorporated.

FIG. 7 is a vertical sectional view of the short fuel rod 3b. The short fuel rod 3b has a plurality of sintered pellets 18 loaded in a cladding tube 17, and its upper and lower ends are sealed with an upper end plug 19 and a lower end plug 20. This sintered pellet loading section is referred to as a fuel section 21. Further, plenum portions 22 and 23 are provided on the upper and lower portions of the fuel portion 21, and a spring 24 that suppresses axial movement of the sintered pellet 18 is provided on the upper plenum portion 22.
Are arranged. In addition, a hollow tube 25 is arranged in the lower plenum portion 23, and a getter 26 that absorbs water inside the short fuel rod 3b is incorporated therein.

[0007]

In recent years, from the viewpoint of effective utilization of uranium resources, the movement of burning uranium-plutonium mixed oxide fuel at the same time as uranium fuel has been conspicuous. In order to increase the energy extracted from uranium, plutonium newly generated by combustion of uranium dioxide is recovered and mixed with natural uranium, recovered uranium or depleted uranium (hereinafter referred to as “M
The energy can be regained by the fission of plutonium.
However, since MOX fuel has lower thermal conductivity than normal uranium fuel, the temperature of the fuel rises, FP gas is easily released, and the amount of He gas produced is large. The MOX fuel rod filled with the MOX fuel pellets has a problem that the internal pressure becomes higher than that of a normal uranium fuel rod. Further, the range of increase in the internal pressure depends on the content rate of plutonium in the MOX fuel, and the internal pressure tends to increase as the content rate increases. FIG. 8 shows an example of changes in the internal pressure of the uranium fuel rods and changes in the internal pressure of the MOX fuel rods due to combustion, which are shown by solid lines 27 and dotted lines 28, respectively.

As described above, in the conventional technique, the internal pressure of the MOX fuel rod may be excessively higher than that of the uranium fuel rod, and the stress generated in the cladding tube may be excessive.

The present invention has been made in consideration of the above point, and an object thereof is to provide a highly reliable fuel assembly by making the internal pressure of the MOX fuel rod substantially equal to that of the uranium fuel rod.

[0010]

That is, according to the present invention, in a fuel assembly including a plurality of fuel rods having different lengths, a short one of the fuel rods is loaded with a mixed oxide fuel pellet of uranium and plutonium. It is characterized by being.

According to the present invention, in the above fuel assembly, the volume ratio of the plenum portion to the pellet loading portion of the short fuel rod loaded with the mixed oxide fuel pellets is such that the fuel rod loaded with uranium fuel pellets. It is characterized in that it is larger than the volume ratio of the plenum part to the pellet loading part.

Further, in the fuel assembly of the present invention, the volume of the plenum portion of the short fuel rod loaded with the mixed oxide fuel pellets is set to be large in accordance with the plutonium content of the loaded fuel pellets. Characterized by

[0013]

In the present invention, the MOX is used for a short fuel rod which has a greater degree of freedom in setting the volume of the plenum as compared with a long fuel rod.
Fuel pellets are loaded and a large plenum volume is secured depending on the plutonium content.

As a result, even if a larger amount of FP gas and He gas than the uranium fuel is released from the MOX fuel, the internal pressure of the MOX fuel rod during the reactor operation is maintained at the same level as that of the uranium fuel rod. It is possible to significantly reduce the risk that excessive stress will occur in the fuel cell and cause fuel damage.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a fuel loading pattern of one embodiment of the fuel assembly of the present invention.
The eight short fuel rods used in the fuel assembly in which the fuel rods are arranged in rows and nine columns are referred to as MOX fuel rods (P marked cells in the figure) 100, and other standard long fuel rods are conventionally used. The uranium fuel rod (an unmarked cell in the figure) 101 is used as it is.

However, the short MOX in this embodiment is used.
As shown in FIG. 2, the length of the fuel rod (hereinafter, referred to as MOX short fuel rod) 100 is slightly smaller than that of the conventionally designed short uranium fuel rod (hereinafter referred to as uranium short fuel rod) 3b (B).
Only) long. This is the active fuel length (length of the fuel section 21)
This is because the plenum length is made longer than that of the uranium short fuel rod 3b without changing.

As a result, even when MOX fuel pellets having an FP gas release rate and a He gas generation amount larger than that of uranium fuel are loaded, the ratio of the volume of the upper and lower plenum portions 22 and 23 to the volume of the fuel portion 21 is increased. Therefore, it is possible to maintain an internal pressure almost equal to that of the conventional uranium short fuel rod.

FIG. 3 shows a fuel loading pattern of another embodiment of the fuel assembly of the present invention. The eight short lengths adopted in the fuel assembly in which the fuel rods are arranged in 9 rows and 9 columns. High enrichment MO with fuel rod loaded with high Pu enrichment pellets
Using X fuel rods (mass marked with P _{H in the} figure) 110, a portion of the standard long fuel rods has a relatively low degree of enrichment and the increase in fuel rod internal pressure is considered to be similar to that of uranium fuel. Degree MO
(In the figure, P _L sign of mass) X fuel rods have applied 111.

Also in this case, the high enrichment MOX short fuel rod 1
10 is the fuel part 2 as compared with the short fuel rod 3b of the conventional design.
The length of the plenum is long without changing the length of 1. Further, even when compared with the MOX short fuel rod 100 shown in FIGS. 1 and 2, if the Pu enrichment is large, the plenum length is set to be long according to the Pu enrichment.

As is clear from the above description, according to the above-described embodiment, in the fuel assembly employing the partial length fuel rod, which is an improvement plan for thermal hydraulic power, the short length fuel rod is loaded with the MOX fuel pellets. However, by adjusting the plenum volume according to the plutonium content, the FP gas release rate and the He gas production rate are larger than those of uranium fuel, and the fuel rod internal pressure is high, without shortening the conventional active fuel length. It is possible to solve the problem of using MOX fuel in a light water reactor.

[0022]

As described above, the MOX fuel pellets are loaded into short fuel rods and the plenum volume is adjusted according to the plutonium content, so that the internal pressure of the MOX fuel rods becomes equal to that of the uranium fuel rods. This can contribute to improving the reliability of the fuel assembly.

[Brief description of drawings]

FIG. 1 is a diagram showing a fuel loading pattern of a fuel assembly according to an embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view showing a comparison between a short fuel rod (a) according to the present invention and a conventional short fuel rod (b).

FIG. 3 is a diagram showing a fuel loading pattern of a fuel assembly according to another embodiment of the present invention.

FIG. 4 is a vertical sectional view showing a conventional fuel assembly.

5 is a sectional view taken along the line AA of FIG.

FIG. 6 is a vertical sectional view of a conventional long fuel rod 3a.

FIG. 7 is a vertical sectional view of a conventional short fuel rod 3b.

FIG. 8 is a graph showing a difference in internal pressure change due to combustion of a conventional uranium fuel rod and a MOX fuel rod.

[Explanation of symbols]

 1 ... Fuel assembly 3a ... Long fuel rod 3b ... Uranium short fuel rod 7 ... Water rod 21 ... Fuel part 22, 23 ... Plenum part 100 ... MOX short fuel rod 110 ……… High enrichment MOX short fuel rod 111 ………… Low enrichment MOX long fuel rod

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Claims (4)

[Claims] 1. A fuel assembly including a plurality of fuel rods having different lengths, wherein a short one of the fuel rods is loaded with a mixed oxide fuel pellet of uranium and plutonium. body. 2. The fuel assembly according to claim 1, wherein the volume ratio of the plenum portion to the pellet loading portion of the short fuel rod loaded with the mixed oxide fuel pellets is the fuel loaded with uranium fuel pellets. A fuel assembly characterized in that it is larger than the volume ratio of the plenum portion to the pellet loading portion of the rod. 3. The fuel assembly according to claim 1 or 2, wherein the plenum volume of the short fuel rod is set to be large in accordance with the plutonium content of the loaded fuel pellets. Fuel assembly. 4. The fuel assembly according to claim 1, wherein the length of the pellet loading portion of the short fuel rod is constant regardless of the plutonium content of the loaded fuel pellets. A fuel assembly characterized by being set.