JPS62197793A - Nuclear fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to improvements in nuclear fuel assemblies, particularly fuel assemblies for boiling water reactors (hereinafter abbreviated as BWR fuel assemblies).

(Prior art) As shown in Fig. 5, a BWR fuel assembly is made by bundling a plurality of fuel rods (16) and one or two water rods (17) in a lattice shape with spacers (13). The expansion spring (l4) inserted in the upper end plug absorbs the elongation, and the upper tie plate (11) and the lower tie plate (l2)
It is assembled by connecting them with nuts etc. (15).

By the way, the water rod (17) is cylindrical like the fuel rod (16), and its size is approximately the same as the fuel rod (16), or about the size of a coffin, and the flow rate flowing through it is equal to that of the chamber. Considering the internal flow rate of Lubosoku, it is only 1 ~
It is only about 2%.

Moreover, as the diameter of the fuel rods becomes smaller, the water rods also become thinner, reducing the deceleration effect.

Therefore, in order to increase the neutron moderation ability in the upper part of the core and to prevent neutron thermalization, sufficient cooling is required even if voids do not occur in the bypass region and water rods. There has been a demand for a design that allows the material to flow.

However, with conventional water rods, the amount of water flowing inside the water rod is ensured, and the neutrons are decelerated by the water inside the water rod without generating voids in the rated output operation state in the rated output operation state. Flatten the distribution and make the void coefficient more positive, for example -1,05X
The purpose is to reduce 10-3 Δ/Δ■ to -1,02×10 Δ/Δ■, and it is hardly used for other purposes.

Under these circumstances, attempts have recently been made to purchase fuel with high burnup, and as a result, designs for increasing fuel enrichment have been considered.

At the same time, in the conventional water rod mentioned above, if the cross-sectional shape is the same due to the increase in concentration, H/23
It was found that 5 U decreased, the void coefficient became negative, and the operational feasibility decreased.

Therefore, as a countermeasure, increasing the number of water rods or increasing the area may be considered.One of these measures is to install coolant outlet ports at the positions of the water rods corresponding to the bottoms of the two upper spacers of the BWR fuel assembly. Japanese Patent Publication No. 59-52999 proposed a configuration in which a water rod is provided and a flow path is formed inside the water rod by a partition plate.

However, this proposal has a water rod configuration that is extremely difficult, and due to the arrangement of the water rods, the outflow of cooling water is limited to the vicinity where the water rods are located within the reactor core. It has not yet been determined whether it is sufficient as a high burnup fuel.

(Problems to be Solved by the Invention) The present invention deals with the above-mentioned facts, and in connection with the increase in the diameter of the water rod, it is assumed that a water box with a larger cross-sectional area is used. The purpose is to improve core characteristics, that is, axial power distribution and thermal margin, by providing one or more cooling water jets.

(Means for Solving the Problems) That is, the features of the present invention that meet the above objectives are summarized in FIG. The shape of the water box (2) is such that the east of the fuel rods (1), which are focused in a cross-sectional shape, is divided into a plurality of regions (at least in the center).And, in the axial direction of the water box (2), One or more jet ports (4) for spouting coolant are provided at an intermediate position between adjacent fuel rods in the partitioned region at an intermediate position of the effective length of the fuel.

(Function) With the above configuration, the coolant flowing upwardly in the water box from the lower part is ejected into the channel through the ejection port. However, a flow path must be created so that the coolant flows above the jet nozzle to ensure a sufficient flow rate to prevent voids from occurring in the water box during rated output operation.

Thus, below the spout, the flow rate in the water box is high and the flow rate in the channel is low, while at the top, the flow rate in the water box is low and the flow rate in the channel is high. At this time, the void ratio in the channel at the inlet position decreases as shown by the dotted line in Figure 3 due to the coolant, that is, the jetted water, which is jetted from inside the water box into the channel, and the power distribution is flattened in the axial direction. Suitable for high burnup fuel.

(Embodiments) Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIGS. 1(A) and 1(II) are examples of the water box (2) of the BWR fuel assembly (A) according to the main part of the present invention,
A water box (2) with a cross-shaped cross section in the center of a 9 x 9 array of fuel rods (11 bundles) housed in a channel box (3) corresponds to 9 fuel rods in the center, In addition, the fuel rod (1) is installed with a width larger than the diameter of the fuel rod (1).

At the middle of the effective length of the cross-shaped water box (2) in the axial direction of the fuel rods, the jet ports (4) are arranged symmetrically at the central position of each branch, adjacent to the water box.
It is located towards the middle between the main fuel rods.

As a result, as shown by the arrow in FIG. 1 ([y)], water flowing from the lower part of the water box is ejected from the spout (4) into the channel (5). At this time, as shown in the same figure (o), there is a channel (6) through which water flows upward from the spout (4).
Create a flow rate and flow it as shown by the arrow to ensure a flow rate that does not cause voids.

In addition, in the above configuration, the number of jet ports (4) is not limited to one, but it is also possible to install a plurality of jet ports (4) in the middle of the effective fuel length. Of course, in this case as well, a flow path flowing upward should be formed.

Further, although the spout (4) is provided at the center of each branch in Fig. 1, it is also possible to arrange the spout at the tip as shown in Fig. 2, or in other words, as shown in Fig. 3. The spout (4) is connected to the channel (
5) Open inward.

In addition, although the above explanation uses a 9x9 array as an example, it is not limited to a 9x9 array, but an 8x8 array is also possible. It is only necessary to make modifications such as creating a water box corresponding to four fuel rods in the center.

Thus, the BWR fuel assembly configured as described above becomes as shown in FIG. 3 due to the above-described effects, the core void fraction is reduced at the ejection port position, and the power distribution is flattened in the axial direction.

(Effects of the Invention) The present invention is as described above. A spout is provided in the center between the fuel rods, and the coolant flows into the water box from the bottom near the axial middle of the water box, which has a wider area than previous water boxes. Coolant, that is, cooling water, is spouted into the channel, and the flow rate in the water box is high below the spout and the flow rate in the channel is low, while the flow rate in the water box is low above the spout. is small, the flow rate in the channel is large, and the core void ratio is also reduced at the jet nozzle position, preventing voids from forming in the water box, ensuring a sufficient amount of coolant, and improving the axial power distribution. This has the remarkable effect of suppressing the lower part of the surface and achieving flattening.

Furthermore, it is expected that the thermal margin will be increased by directing the jet nozzle toward the fuel rod where dryout is likely to occur.

4. 'AN $' explanation of the drawings Figures 1 (A) and (It) are a schematic cross-sectional view showing the arrangement of the walk box according to the main part of the present invention, and a schematic vertical cross-sectional view of a part of the spout part; Fig. 2 is a schematic cross-sectional view of another embodiment showing a modified example of the position of the water outlet, Fig. 3 is a schematic partial longitudinal cross-sectional view showing another embodiment of the water box, and Fig. 4 is a collection of the present invention. Power distribution in the axial direction of the body (solid line) and void fraction (
FIG. 5 is a longitudinal sectional view showing an example of the configuration of a BWR fuel assembly.

(A)...Fuel assembly.

(1)...Fuel rod.

(2)...Water box.

(3)...Channel Bonx.

(4)... spout.

(5)...Channel.

(6)...Flow path.

Aoi Iun (b) (b) 3rd figure + Man Lord 1 inner helmet 5th map

Claims (1)

[Claims] 1. In a fuel assembly for a boiling water reactor having a water box, the water box has a cross-sectional shape that divides a bundle of fuel rods into a plurality of regions at the center. , characterized in that one or more coolant jet ports are provided toward an intermediate portion between the fuel rods adjacent to the water box in each of the partitioned regions at an intermediate position of the fuel effective length in the axial direction of the fuel rod. Nuclear fuel assembly. 2. The nuclear fuel assembly according to claim 1, wherein the water box has a flow path at a position above the ejection port inside the water box, through which the coolant flows upward within the water box. 3. Claim 1 or 2, wherein the water box has a cross shape that divides the bundle of fuel rods into four equal parts at the center of the channel, and has a size corresponding to nine fuel rods. The nuclear fuel assembly described.