JPH052197B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a fuel assembly for a boiling water nuclear reactor;
In particular, it relates to a fuel assembly for a boiling water reactor with an improved water rod for flattening the axial power distribution.

[Conventional technology]

In a boiling water reactor, cooling water flows from the bottom of the reactor core to the top for the purpose of absorbing thermal energy from the nuclear fission reaction of the fuel and slowing down neutrons to contribute to the nuclear fission reaction. Cooling water flowing into the core from the bottom absorbs thermal energy as it flows to the top, so approximately 70% of the cooling water at the top of the core becomes bubbles, as shown in Figure 2.
The presence of these bubbles makes it insufficient for the cooling water to slow down the fast neutrons generated by the fission reaction to the level of thermal neutrons. Many fast neutrons that do not directly contribute are distributed, and the nuclear fission reaction rate is smaller than in the lower part, resulting in an uneven axial power distribution of the fuel.

Conventionally, fuel assemblies equipped with one or two hollow water rods have been put into practical use in order to reduce air bubbles and increase the amount of cooling water in the upper part of the fuel. Since this water rod does not generate heat by itself, its interior is filled with unboiled cooling water everywhere in the axial direction, and this reduces the neutron moderating effect of the upper part of the fuel assembly. This compensates for the reactivity at the top of the fuel. However, in conventional practical fuels, the amount of unboiled cooling water provided by the water rod is not necessarily sufficient, and flattening of the fuel axial power distribution remains insufficient. To solve this problem, it is conceivable to increase the amount of unboiled cooling water by making the water rod larger in diameter, but in that case, there is a drawback that the neutron moderation effect in the lower part of the core in the axial direction becomes excessive.

[Problems to be solved by the invention]

An object of the present invention is to eliminate the drawbacks of the prior art as described above, and to provide a fuel shaft that provides sufficient flattening of the fuel axial power distribution without excessive neutron moderating action in the axial lower part of the fuel. It is an object of the present invention to provide a fuel assembly for a boiling water reactor that can selectively provide sufficient neutron moderating action in the upper direction.

[Means for solving problems]

In order to achieve the above object, the fuel assembly for a boiling water nuclear reactor of the present invention is equipped with a water rod having a special configuration, and this water rod has a water rod that controls the flow in the pipe below the lower 12/24 nodes. It is characterized in that the cross-sectional area of the tube is reduced and that the reduced tube space is replaced by beryllium.

In one embodiment of the invention, the lower
A thick-walled beryllium tube is disposed within the water rod tube below the 12/24 node, and preferably a taper of 1/24 node is provided on the inside of both ends of the beryllium tube. The inner diameter of the beryllium tube may be the same from top to bottom, or may gradually widen from bottom to top.

In another embodiment of the invention, 9×
The upper Zircaloy-2 has a fuel rod arrangement in the form of a 9-square lattice, and the water rod has a cross-shaped cross-sectional outer peripheral shape corresponding to nine normal fuel rods.
It consists of a hollow tube and a lower beryllium cross having the same cross-sectional outer circumferential shape connected in the vicinity of the 12/24 node, and a cooling water inlet hole communicating with the hollow tube is provided in the center of the lower beryllium cross. ing. In this case as well, it is preferable that the upper end of the cooling water inflow hole gradually widens upward.

[Function] In the boiling water reactor fuel assembly of the present invention, the cross-sectional area of the tube is reduced below the lower 12/24 nodes, and the reduced tube space is replaced with water by beryllium. Because it has rods, this beryllium acts as a buffer to prevent sudden changes in neutrons from being applied to the fuel, and because of beryllium, which has a smaller neutron moderation ability than water, air bubbles in the upper part are used to reduce neutrons in the lower part. For example, even if this water rod occupies a volume equivalent to 9 to 15 fuel rods, the neutron moderating capacity in the lower part will not be excessively large. This means that the neutron moderating ability of the upper part can be sufficiently compensated for, the neutron spectra of the upper and lower parts of the fuel can be flattened, and the power distribution in the axial direction of the fuel can be made sufficiently uniform.

In the present invention, it goes without saying that the cross-sectional area of the pipe in the lower region of the water rod should be sufficient to ensure a flow rate that does not cause boiling of the cooling water in the upper part.

In order to make the above and other objects and features of the present invention more clear, preferred embodiments of the present invention will be described below with reference to the drawings.

〔Example〕

FIG. 1 is a schematic sectional view showing only the improved water rod of a fuel assembly having an 8×8 square lattice arrangement according to the present invention, showing the hollow tube 1 made of Zircaloy-2 and its It consists of a beryllium tube 2 inserted inside over a certain length of the lower part. This water rod has a structure in which a beryllium tube 2, whose outer diameter is slightly smaller than the inner diameter of the hollow tube, is inserted into a hollow tube 1 manufactured by a normal process. The beryllium tube 2 has both ends tapered inside, is inserted into the lower 9/24 position of the hollow tube 1, and is fixed at the lower end by TIG welding or the like. The length of the tapered portions at both ends of the beryllium tube 2 is 1/24 or less of the total length, and functions to smoothly guide the cooling water flowing in from the lower part, and to prevent turbulence from occurring at the upper part when the internal flow path widens. This taper also serves to facilitate continuous variation of the neutron spectrum in the axial direction. Although the inner diameter of the beryllium tube 2 is uniform in the illustrated example, it may gradually widen from the bottom to the top.
Beryllium is less effective at moderating neutrons than water without air bubbles, and in terms of its effect on moderating neutrons, it exhibits the same properties as cooling water in the upper part of the reactor core, which contains air bubbles. As a result, the neutron energy distribution in the upper part of the reactor core and the neutron energy distribution in the lower part become approximately uniform as shown in FIG. 3, and the power distribution in the axial direction of the fuel is flattened as shown in FIG. 4.

FIG. 5 is a schematic perspective view showing only the improved water rod of a fuel assembly with a 9x9 square lattice arrangement according to the present invention, and this water rod has a volume equivalent to 9 to 15 normal fuel rods. occupies the top
The 15/24 nodes are made of a cross-shaped hollow tube 10 made of Zircaloy-2, and the lower 9/24 nodes are made of a beryllium cross 11. As shown in FIGS. 6 and 7, the beryllium cross 11 has a cooling water inlet in the center of the cross with a diameter sufficient to ensure a sufficient flow rate to prevent the cooling water from boiling within the hollow tube 10. Hole 1
2, and each of the arms of the cross has auxiliary cooling water inlet holes 13. These five through holes 12 and 13 preferably have a taper that gradually widens at their upper and lower ends, and each has a shape that gradually widens in diameter from the bottom to the top. The font 11 has an inclined shape such that it is higher toward the outside of the cross arm and lower toward the center.

In a 9 x 9 fuel assembly with cross-shaped water rods, the proportion of water rods in the fuel assembly is more than 9 fuel rods, so the distribution of neutron energy in the lower part of the fuel is It is easy to make the distribution approximately equal to that of the neutron spectrum, and it is possible to make the neutron spectrum more uniform in the fuel axis direction and to make the power distribution in the fuel axis direction even more flat.

〔Effect of the invention〕

As described above, according to the fuel assembly for a boiling water reactor of the present invention, the cross-sectional area of the pipe in the lower 12/24 nodes and below is reduced, and this reduced space in the pipe is filled with beryllium. With the water rods displaced by the fuel, there are enough neutrons in the upper axial portion of the fuel to provide sufficient flattening of the fuel axial power distribution without excessive neutron moderation in the lower axial portion of the fuel. It is possible to selectively apply a deceleration effect.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view showing a water rod of a fuel assembly for a boiling water reactor according to an embodiment of the present invention, FIG. 2 is a diagram showing the axial void fraction distribution of conventional fuel, and FIG. Figure 3 is a diagram showing a comparison of the average neutron energy in the axial direction between the present invention and the conventional example, Figure 4 is a diagram showing a comparison of the power distribution in the axial direction between the present invention and the conventional example, and Figure 5 is a diagram showing a comparison of the power distribution in the axial direction between the present invention and the conventional example. FIG. 6 is a schematic perspective view showing a water rod of a fuel assembly for a boiling water reactor according to another embodiment of the present invention.
FIG. 7 is a cross-sectional view taken along the line B--B in FIG. 5. 1...Water rod main body hollow tube, 2...Beryllium tube, 10...Cross-shaped hollow tube, 11...Beryllium cross shape, 12, 13...Cooling water inflow hole.

Claims (1)

[Claims] 1. A boiling tube characterized by having a water rod in which the cross-sectional area of the pipe is reduced below the lower 12/24 nodes, and the reduced space in the pipe is replaced by beryllium. Fuel assembly for water reactors. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein a thick-walled beryllium tube is disposed inside the tube below the lower 12/24 nodes. 3. The fuel assembly for a boiling water reactor according to claim 2, wherein the beryllium tube has a taper of 1/24 node on the inside of both ends thereof. 4. The fuel assembly for a boiling water reactor according to claim 2, wherein the inner diameter of the beryllium tube gradually increases toward the top. 5 It has a fuel rod arrangement in the form of a 9x9 square lattice, and the water rods have the same cross section as the upper Zircaloy-2 hollow tube, which has a cross-shaped cross-sectional outer circumferential shape corresponding to nine fuel rods. It consists of a lower beryllium cross in the shape of a surface outer periphery connected near the 12/24 node, and a cooling water inflow hole leading into the hollow tube is provided in the center of the lower beryllium cross. A fuel assembly for a boiling water nuclear reactor according to claim 1. 6. The fuel assembly for a boiling water reactor according to claim 5, wherein the upper end of the cooling water inlet hole is gradually widened upward.