JPH0376435B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel assembly for a boiling water nuclear reactor that has been improved in order to effectively utilize neutrons.

[Technical background of the invention and its problems]

In a boiling water nuclear reactor, as shown in FIG. 1, a reactor core 3 is constructed by loading a large number of fuel assemblies 2 into the center of a reactor pressure vessel 1. Moreover, cooling water 4 is accommodated inside the pressure vessel 1 up to the upper part of the reactor core 3 . A steam separator 5 is placed above the pressure vessel 1.
and a steam dryer 6 are housed therein. Further, on the peripheral wall of the pressure vessel 1, there is a main steam outlet nozzle 7 at the top.
Water supply nozzles 8 are provided below.

The fuel assembly 2 includes a large number of fuel rods 10 in a rectangular cylindrical channel 9 as shown in FIGS. 2 and 3.
... are arranged in a matrix (usually 8 x 8 rods), and the upper and lower ends of these fuel rods 10 ... are supported by an upper tie plate 11 and a lower tie plate 12, respectively, and the upper and lower tie plates 11, 1
By supporting a plurality of locations between the fuel rods 10 with spacers 13, the mutual spacing between the fuel rods 10 is kept constant.

As shown in FIG. 4, the fuel rod 10 has a structure in which a large number of uranium fuels 15 formed as cylindrical pellets are piled up in a cladding tube 14 made of Zircaloy, for example, and sealed by a spring 16 from above. Each uranium fuel 15 is formed into a certain shape by sintering uranium oxide powder. In addition, 1 in the figure
Reference numeral 7 denotes a plug that seals the upper end opening of the cladding tube 14.

Therefore, by fission of the uranium fuel in the reactor core 3, the cooling water 4 in the pressure vessel 1 is boiled, and the steam generated thereby is passed through a steam separator 5 and a steam dryer 6 to the main steam outlet nozzle 7 of the power plant. Take out for use in turbine drive. Further, the steam that has passed through the turbine (not shown) is cooled and liquefied in a condenser (not shown), and is again supplied into the pressure vessel 1 from the water supply inlet nozzle 8 as cooling water 4 .

By the way, in a conventional fuel assembly, the enrichment of the uranium fuel 15 sealed in the fuel rods 10 is all the same, and the enrichment of the uranium fuel 15 in the vertical direction of the reactor core 3 is the same.
^{The 235} U enrichment distribution was uniform as shown in Figure 5.

On the other hand, inside core 3, steam (hereinafter referred to as void)
has occurred, and the void ratio (volume ratio of voids in two-phase flow) is as shown by curve V in Figure 6.
It gets higher towards the top. When the void ratio is high, the power decreases, so if we look at the core power distribution in the vertical direction, as shown by curve P in Figure 6, it decreases at the top, increases toward the bottom, and near the bottom. Become the best. However, at the bottom end, neutron leakage occurs, so the output does not come out like at the top end, and the output decreases rapidly.

By the way, the effective multiplication factor K _eff of the core 3 is proportional to the average infinite multiplication factor ∽. Also, when the core power at position Z from the top of the core 3 downwards is P, and the infinite multiplication factor is K∽, the average infinite multiplication factor ∽
can be approximately expressed as ∽=∫K∽・P・dZ/∫P・dZ.

Here, the infinite multiplication factor K∽ is almost proportional to the ²³⁵ U enrichment, but conventionally, the
Since the ²³⁵ U enrichment distribution was almost uniform, the average infinite multiplication factor ∽ was almost constant regardless of the core power. For this reason, the neutrons irradiated by the uranium fuel 15 are not effectively used for heating the cooling water in the lower output parts of the upper and lower ends of the reactor core 3, and therefore, it is difficult to increase the reactivity of the reactor core 3. It was a disadvantage.

[Purpose of the invention]

The present invention aims at effective use of neutrons by setting the ²³⁵ U enrichment distribution so that the vertical distribution of the infinite multiplication factor K∽ exhibits the same tendency as the vertical power distribution of the core, and thereby increases the average infinite multiplication factor ∽. An object of the present invention is to provide a fuel assembly for a boiling water reactor that can increase the core reactivity without increasing the amount of uranium fuel.

[Summary of the invention]

The boiling water reactor fuel assembly according to the present invention includes:
The uranium-235 enrichment within the fuel rod is lowest at the upper and lower ends, and gradually increases downward from the upper end, reaching the highest near the lower end.
By providing such an enrichment distribution, the uranium fuel is irradiated with more neutrons in areas with higher core power, and neutrons can be used more effectively, making it possible to increase core reactivity.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 7 and 8.

FIG. 7 shows one of the many fuel rods constituting a fuel assembly for a boiling water reactor. This fuel rod 101 is formed as a cylindrical pellet inside an elongated cladding tube 102. A large number of uranium fuels 103A, 103B, .
The upper and lower ends of the cap are sealed with an upper plug 105 and a lower plug 106, respectively. The uranium fuels 103A, 103B, ..., 103N are formed into a columnar shape by sintering uranium oxide powder, but their ²³⁵ U enrichment is not constant, and there are differences as shown in Figure 8. It's attached. In other words, at the upper and lower ends, the content is about 0.71% by weight, which is as low as natural uranium, but as you go downwards from the top, it gradually increases to, for example, 1.5% by weight, 2.0% by weight, and 2.5% by weight. The content is set to reach a maximum of approximately 3.0% by weight in the vicinity. The above ²³⁵ U enrichment distribution exhibits the same tendency as the core power distribution shown by the virtual line in the figure.

With the above configuration, the inside of the fuel rod 101
Since ^{the 235} U enrichment distribution shows the same tendency as the core power distribution, the average enrichment of the fuel bundle can be kept low in the upper and lower ends of the core where the core power is low, resulting in fuel savings, and the core power is high. In some areas, neutrons will be used more effectively and the reactivity of the reactor core will be increased.

〔Effect of the invention〕

As described in detail above, the boiling water reactor fuel assembly according to the present invention is arranged such that the uranium-235 enrichment within the fuel rod is lowest at the upper and lower ends and highest near the lower end. ²³⁵ U in the vertical direction of the reactor core.
Since the enrichment distribution is configured to have the same tendency as the core power distribution, near the bottom of the core where the core power is high, that is, the neutron flux is high, neutrons are effectively used to increase the core power due to the high enrichment. It is used to increase core power. Furthermore, since the enrichment is low at the upper and lower ends of the core where the neutron flux is low, the average enrichment of the fuel bundle can be kept low.
Therefore, not only is there no need to increase the amount of fuel, but it is also possible to save fuel, which provides excellent effects. In addition, in the present invention, uranium
Since the enrichment distribution of uranium-235 shows the same tendency as the axial power distribution of the reactor core, uranium-235
Uranium-235 can prevent unburned remains.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a boiling water reactor, Figure 2 is a sectional view of a fuel assembly, Figure 3 is a perspective view of a fuel assembly, and Figure 4 is a partially cutaway side view of a fuel rod. ,
Figure 5 is a ²³⁵ U enrichment distribution diagram in the core of a conventional boiling water reactor, Figure 6 is a void ratio and power distribution diagram in the core of a boiling water reactor, and Figure 7 is an embodiment of the present invention. FIG. 8 is a longitudinal sectional view of the fuel rod showing the uranium-235 enrichment distribution of the uranium fuel within the fuel rod in the same example. 101...Fuel rod, 103A, 103B,...,
103N...Uranium fuel.

Claims (1)

[Claims] 1. The fuel rod is characterized in that the uranium-235 enrichment within the fuel rod is lowest at the upper and lower ends, and gradually increases downward from the upper end so that it is highest near the lower end. Fuel assembly for boiling water reactors. 2. The fuel assembly for a boiling water nuclear reactor according to claim 1, characterized in that the uranium-235 enrichment at the upper and lower ends of the fuel rod is 0.71% by weight.