JPS6120886A - 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] [Technical field of invention] The present invention relates to a nuclear fuel assembly loaded into a nuclear reactor.

[Technical background of the invention]

A nuclear fuel assembly loaded in a conventional boiling water nuclear reactor will be explained with reference to FIGS. 4 and 5.

FIG. 4 shows a longitudinal sectional view of a conventional nuclear fuel assembly.

In FIG. 4, a nuclear fuel assembly 1 has a large number of fuel rods 3 and a plurality of water rods 6 arranged in a lattice of, for example, 8 rows and 8 cases in a rectangular cylindrical channel box 2. Connect the upper and lower ends of the water rod 6 to the upper tie plate 4.
and is supported by a lower tie plate 5.

Also, upper tie plate 4 and lower tie plate! -5, a plurality of spacers 7 are arranged at appropriate intervals so as to keep the distance between the fuel rods 3 constant.

The fuel rod 3 is constructed as shown in FIG. Here, FIG. 5 shows a longitudinal sectional view of the fuel rod. As shown in FIG. 5, the fuel rod 3 is constructed by loading a cladding tube 9 with a large number of fuel pellets 8 which are formed into a cylindrical shape by compacting uranium dioxide powder and sintering it at high temperature. An upper end plug 10 and a lower end plug 11 are welded to the upper and lower ends of. Also,
An upper plenum part 12 is formed in the upper part of the fuel rod 3, and the fuel pellets 8 are contained in the upper plenum part 12.
a plenum spring 1 which is crimped onto the lower end plug 11;
3 is provided.

[Problems with background technology]

In the above configuration, if the maximum thermal output of the nuclear fuel assembly 1 exceeds the cooling capacity of the coolant, the temperature of the cladding tube of the fuel rod 3 will rise and eventually burnout will occur.
The thermal output of the nuclear fuel assembly 1 must be kept below a certain value.

However, in the conventional nuclear fuel assembly 1, a plurality of spacers 7 are arranged in the axial direction of the fuel rods in order to maintain constant spacing between the fuel rods 3 and ensure a coolant flow path. Therefore, the flow of the coolant is resisted by the presence of the spacer 7, and the coolant is difficult to flow on the upstream side of the spacer 7, and the cooling effect is also poor. From the above, when the thermal output of the conventional nuclear fuel assembly 1 was increased, there was a risk that the fuel rods 3 would be burnt out near the upstream side of the spacer 7.

[Purpose of the invention]

An object of the present invention is to provide a nuclear fuel assembly capable of increasing the allowable maximum thermal output compared to conventional nuclear fuel assemblies.

[Summary of the invention]

A plurality of fuel rods are sealed with a lower end plug, an upper tie plate and a lower tie plate of the upper and lower cladding tubes of the fuel rods, and the upper and lower ends of the fuel rods are sealed with the upper end plug and the lower tie plate. A nuclear fuel assembly comprising a plurality of spacers arranged at appropriate intervals between each other to hold the fuel rods at appropriate intervals, and a channel box accommodating the plurality of spacers and the plurality of fuel rods. The length from the bottom surface of the fuel pellet loaded at the lowest end of the fuel pellet to the top side of each spacer was taken as 100%, and hollow fuel pellets were loaded between 98.5% and 100.5% based on this 100%. There is a nuclear fuel assembly characterized by the following.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to FIGS. 1 to 3. Here, FIG. 1 shows a longitudinal cross-sectional view of a fuel rod showing essential parts of an embodiment of the present invention, and FIG. 2 shows a cross-sectional view taken along the line A--A in FIG. 1. Note that the same parts as in FIGS. 4 and 5 are designated by the same reference numerals, and explanations of their configurations will be omitted. As shown in FIGS. 1 and 2, the spacers 7 are assembled in a grid pattern and are provided with spacer springs 21 to hold the fuel rods 20 at regular intervals. Hollow fuel pellets 22 are loaded near the upper side of the spacer 7 in the fuel rod 20, and solid fuel pellets 23 are loaded in the other portions.

In the above configuration, the flow of coolant within the nuclear fuel assembly 1 is obstructed by the spacer 7.

Therefore, the flow rate of the coolant is slow, and the cooling effect in that area is poor. However, according to the present invention, since the hollow fuel pellets 22 having a small thermal output are arranged near the upper side of the spacer 7 of the fuel rod 20, it is possible to prevent the cladding tube 9 from burning out in this area.

Note that it is preferable that the number of hollow fuel pellets 22 is small in order to increase the thermal output of the entire nuclear fuel assembly. For this reason, the presence of the spacer 7 is limited to a certain area where the cooling effect of the coolant is reduced. The region where the cooling effect of the coolant is reduced due to the presence of the spacer 7 is the spacer portion and the vicinity upstream of the spacer.

Now looking at the change in the axial direction due to the combustion of fuel pellets, the third
The result will be as shown in the figure. Here, in Figure 3, the horizontal axis is the fuel burnup (G
The characteristic curve of the fuel rod is shown with WD/T) and the effective fuel length increase rate on the vertical axis. Incidentally, the fuel effective length increase rate is calculated by dividing the amount of increase in the axial direction of all the fuel pellets inserted into the fuel rod during combustion by the axial length of all the fuel pellets (hereinafter referred to as the fuel effective length). In Figure 3,
The reason why the effective length of the fuel decreases one step as the burnup increases is due to burning stripes in the fuel pellets, and the subsequent increase is due to swelling of the pellets.

Considering the fuel effective length increase rate shown in FIG. 3 and the thermal expansion of fuel pellets during combustion, the area shown in FIG. If the length from the bottom surface of the fuel pellet 23 loaded at the bottom end of the fuel rod 20 to the top side of the first spacer 7 is 11, then 0.985 and 1.0051 from the right.
Areas up to 1 are possible.

Also, the second. In the case of the third spacer, it is determined similarly. By arranging the hollow fuel pellets 22 in this region, the present invention can achieve the best effect.

〔Effect of the invention〕

According to the present invention, since hollow pellets with low thermal output are arranged near the upper side of the spacer, burnout of the fuel rods can be prevented even when the cooling capacity decreases near the upper side of the spacer, and furthermore, the fuel rods can be prevented from being burnt out. Allowable heat output can be increased.

[Brief explanation of drawings]

Fig. 1 is a vertical cross-sectional view of a fuel rod showing the main parts of an embodiment of the present invention, Fig. 2 is a cross-sectional view taken along the line A-A in Fig. 1, and a side view, and Fig. 3 is a characteristic curve diagram of the fuel rod. , FIG. 4 is a longitudinal sectional view of a conventional nuclear fuel assembly, and FIG. 5 is a longitudinal sectional view of the fuel rod shown in FIG. 4. 1...Nuclear fuel assembly 2...Channel box 3.20...Fuel rod 4...Upper tie plate 5...Lower tie plate 6...Water rod 7
...Spacer 8...Fuel pellet 9...
Cladding tube 10... Upper end plug 11... Lower end plug 12... Upper plenum portion 13... Plenum spring 21... Spacer spring 22...
Hollow fuel pellet 23...Solid fuel pellet agent
Patent Attorney Noriyuki Chika (and 1 other person) Figure 1 Figure 2 - Figure 3 Tweet Figure 4 Figure 5

Claims (1)

[Claims] A plurality of fuel rods are formed by loading a large number of solid fuel pellets into a cladding tube and sealing the upper and lower ends of the cladding tube with an upper end plug and a lower end plug, and the upper and lower ends of the fuel rods are sealed. an upper tie plate and a lower tie plate to support; a plurality of spacers arranged at appropriate intervals between the upper tie plate and the lower tie plate to hold the fuel rods at appropriate intervals; and the plurality of spacers and the plurality of spacers. In a nuclear fuel assembly consisting of a channel box that accommodates fuel rods, the length from the lower surface of the solid fuel pellet loaded at the lowest end of the fuel rod to the upper side of each spacer is 10
0%, and from 98.5% to 1 based on this 100%.
A nuclear fuel assembly characterized in that hollow fuel pellets are loaded between 0.00.5% and 0.00.5%.