JPS6249946B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nuclear reactor fuel assembly, and particularly to a nuclear reactor fuel assembly suitable for use in a boiling water nuclear reactor (hereinafter abbreviated as BWR). Conventionally, the operating period of a nuclear reactor is approximately one year, and approximately one quarter of all fuel assemblies are replaced with new fuel assemblies every year. However, with the above operating method, the period during which the reactor is shut down for periodic inspections and fuel changes is approximately 25% of the total operating period.
%, efforts are being made to increase the plant utilization rate by extending the operating period and reducing the ratio of the reactor shutdown period to the total operating period. In order to extend the operating period, it is necessary to increase the amount of fissile nuclides at the initial stage of combustion, that is, the average core enrichment. There are two ways to increase the average core enrichment: loading highly enriched fuel and increasing the number of fuel exchangers; however, the latter requires a longer time for fuel exchange, so the former is generally preferred. method has been adopted. However, the use of highly concentrated fuel has the disadvantage of reducing reactor shutdown margin. What is the reactor shutdown margin?
In a cold state (approximately 20℃), one control rod with the highest reactivity value is in the withdrawn state, and all other control rods are in the fully inserted state, from the critical value (1.0) of the effective multiplication factor. It's the difference. This depends on the average infinite multiplication factor of the four fuel assemblies surrounding the control rod, which has the highest reactivity value. In BWR, during reactor operation (hereinafter referred to as operation), voids occur and the neutron moderating ability decreases due to a decrease in moderator density, and uranium in the fuel
Since there is a negative reactivity due to the Doppler effect of 238, the infinite multiplication factor of the fuel is smaller than when it is cold.
Generally, the difference between the infinite multiplication factor at cold temperature and the infinite multiplication factor during operation increases as the degree of concentration increases. Furthermore, if the amount of gadolinia, which is a burnable poison used to suppress excess reactivity, is increased, this tendency will be exacerbated. Therefore, in a fuel that is enriched to extend the operating period and has a large amount of gadolinia added to suppress excess reactivity, the average infinite multiplication factor of the four fuel assemblies mentioned above during operation is Even if it is the same as in the case of cooling, the average infinite multiplication factor increases when the temperature is cold, and the margin for reactor shutdown decreases. The present invention has been made in view of the above, and its purpose is to provide a nuclear reactor fuel assembly that can extend the operating period and ensure sufficient margin for reactor shutdown. It's about doing. A feature of the present invention is that the average enrichment of fissile material in the first layer fuel rods adjacent to the channel box of the fuel assembly is compared with that of the second layer fuel rods located inside the first layer fuel rods and adjacent to the first layer fuel rods. The point is that the average enrichment of fissile material has been increased by more than 1.5 times. First, the principle of the present invention will be explained in detail. Figure 1 shows an example of the structure of a BWR fuel assembly.
It has a structure in which 62 fuel rods A and 2 water rods B are bundled in a channel box C in an 8×8 square grid. Figure 2 shows an example of the fuel rod enrichment distribution of a conventional fuel assembly, in which two water rods W are placed in the center of the fuel assembly, and eight gadolinia (burnable poison) rods G are placed in the center of the fuel assembly. It is equipped as shown in the figure. The enrichments of the other fuel rods numbered 1-9 and gadolinia rod G are shown in Table 1.

[Table] Figure 3a shows the difference in output distribution between cold and high temperatures for each fuel rod in percent units in this example. Now, as shown in Figure 3b, the outermost fuel rod (channel box C)
The fuel rods adjacent to the first layer) are referred to as the first layer fuel rods, and the adjacent fuel rods inside the second layer fuel rods are referred to as the third and fourth layer fuel rods. In this case, the output at cold temperatures is relatively larger than at high temperatures, and on the contrary, it is smaller in the second layer fuel rods. This is because at cold temperatures, the neutron spectrum in light water becomes softer than at high temperatures (the proportion of low-energy neutrons increases), and the proportion of neutrons generated by nuclear fission increases; This is because the ratio of the surrounding light water region per fuel rod becomes large in the first layer fuel rods adjacent to the fuel rods. Therefore, at cold temperatures, the output of the first layer fuel rods becomes relatively larger than the output of the second layer fuel rods. Since the fourth layer fuel rods are surrounded by water rods, they show the same tendency as the first layer fuel rods, and the third layer fuel rods are less affected by the water rods, so the changes are small. The variation in this power distribution approximately represents the variation in the reactivity value of the fuel rod at each fuel rod location. Therefore, in order to reduce the change in reactivity between cold and high temperatures, it is effective to minimize the enrichment of the first layer fuel rods and increase the enrichment of the second layer fuel rods as much as possible. It turns out that it is. It is also effective to lower the enrichment of the 4th layer fuel rods and increase the enrichment of the 3rd layer fuel rods that are far from the water rods, but this only has 2 rods and the degree of contribution is small. . The above is the principle of the cold-temperature-high-temperature reactivity change reduction method according to the present invention, but in actual fuel assembly design, there is a restriction that the local power peaking within the fuel assembly must be kept within the design reference value. That is, it is necessary to suppress the output ratio of the maximum output fuel rod to the average output of the fuel assembly to, for example, 1.15 or less. Therefore, when applying the principles of the present invention to a design, it is necessary to optimize the enrichment distribution within this criterion. Now, the ratio of the average enrichment of the second layer fuel rods to the average enrichment of the first layer fuel rods is α, and the ratio of the average output of the second layer fuel rods to the average output of the first layer fuel rods is β. The relationship between the two is shown in FIG. In FIG. 4, curve a is a relational curve for a fuel assembly having eight gadolinia rods, and curve b is a relational curve for a fuel assembly having no gadolinia rods. In the fuel assembly shown in Figure 2, the average enrichment is 3.03% by weight and the local power peak is 1.15, the average enrichment of the second layer fuel rods is 2.9% by weight, and the average enrichment of the first layer fuel rods is 2.9% by weight.
Since it is 2.5% by weight, α=1.16, and the average output of the first layer fuel rods is greater than the average output of the second layer fuel rods. Difference in infinite multiplication factor between cold and high temperatures (Δk
_cpld-hpt ) is 6% Δk at a burnup of 0 Gwd/st, decreases with the combustion of gadolinia, then recovers, has a similar value at 10 Gwd/st, and then decreases monotonically. In this manner, in the combustion assembly shown in FIG. 2, the power sharing ratio of the first layer fuel rods is increased, so that Δk _cpld-hpt is relatively large. However, because the fuel assembly average enrichment is relatively low (approximately 2.9% by weight), it has conventionally been relatively easy to satisfy the design conditions for reactor shutdown margin even with the above-mentioned fuel assembly design. FIG. 5 is a diagram showing the relationship between the cold effective multiplication factor (k _cpld ) and α when one control rod is withdrawn for two types of fuel assemblies with different average enrichments. The c line represents a fuel assembly with an average enrichment of 2.8% by weight, and the d curve represents a fuel assembly with an average enrichment of 3.0% by weight. The difference from the critical value of k _cpld corresponds to the reactor shutdown margin, and as a normal design condition, 1%Δ
Consider the reactor shutdown margin for k. That is, k _cpld is
0.99 or less is a necessary condition for core design. Although the value of k _cpld differs depending on the fuel loading method of the core, FIG. 5 shows a normal 1/4 core mirror symmetrical loading pattern. Fifth
The figure shows that as the average enrichment of the fuel assembly increases, the reactor shutdown margin decreases. Although the fuel assembly in Figure 2 does not meet the design conditions, it can be seen that if a fuel assembly with a large value of α is designed, even a high-enrichment fuel assembly can satisfy the design conditions, and the average enrichment can be reduced. This shows that for fuel assemblies containing more than 3% by weight, it is necessary to design fuel assemblies with α=1.5 or more. The present invention will be described in detail below using examples shown in Tables 2 and 3. Table 2 shows an example of the enrichment distribution of each fuel rod in the fuel assembly of the present invention, and shows channel boxes C of water rods W, gadolinia rods G, and other fuel rods 1 to 9 (see Fig. 1). ) is the same as in Figure 2.

[Table] From Table 2, the average enrichment of the fuel assembly is 3.03% by weight, the average enrichment of the second layer fuel rods is 3.4% by weight, the average enrichment of the first layer fuel rods is 2.1% by weight, and α = 1.62.
That is, the fuel assembly average enrichment is the same as in Table 1 and satisfies the same local power peaking of 1.15. Moreover, the enrichment of the second layer fuel rods is
Since the enrichment is relatively higher than that of the bed fuel rods, α=1.62, and from Figure 5, the reactor shutdown margin is approximately 1.5% Δk, which satisfies the design conditions. In this embodiment, as can be seen from FIG. 2, there are eight gadolinia rods G, but this number may be increased or decreased as necessary. Also, although the average enrichment is set at 3.03% by weight, this may be increased or decreased depending on the operating time and the number of fuel assemblies replaced.In the conventional system, the average enrichment is 3.03% by weight or more. In this case, the design standard could not be satisfied, but according to the embodiment of the present invention, α can be increased accordingly, and the design standard can be satisfied. Table 3 shows other embodiments of the present invention,
The arrangement of each fuel rod, water rod, etc. in channel box C is the same as above.

[Table] From Table 3, the average enrichment of the fuel assembly is 3.03% by weight, the average enrichment of the second layer fuel rods is 3.8% by weight, the average enrichment of the first layer fuel rods is 2.0% by weight, and α=1.90.
In Table 3, the concentration of gadolinia rod G is higher than in Table 2, but when α = 1.90, the reactor shutdown margin is 2.3% Δk from Figure 5, which fully satisfies the design conditions as in Table 2. are doing. Furthermore, with this enrichment distribution, it is possible to increase the fuel assembly average enrichment to about 3.3% by weight. Note that the discussion so far has been based on the 1/4 core mirror-symmetrical fuel loading method, but in a method in which fuel assemblies of different fuels are loaded uniformly throughout the core, the reactor shutdown margin is generally increased by an additional 1% Δk. Therefore, the upper limit of the fuel assembly average enrichment can be further increased. Now, when comparing the conventional fuel assembly shown in Table 1 and the fuel assembly of the present invention shown in Table 3, the difference in k _cpld is approximately 3% Δk, which is the same design condition of a reactor shutdown margin of 1% Δk. In the case of Table 3, the average concentration can be increased by about 0.4% by weight.
This corresponds to a difference of about 4,000 Mwd/t in the average take-off burnup, and if the number of refueling bodies is designed to be constant, the refueling cycle length will increase by about 1,000 Mwd/st, improving the plant utilization rate by about 10%. do. Furthermore, if the plant is designed to have a constant utilization rate, the number of fuel replacement units will be reduced by approximately 10%, improving fuel economy and reducing the time required for replacement. As explained above, according to the present invention, there is an effect that the plant utilization rate can be improved by extending the operation period, and that a sufficient margin for reactor shutdown can be secured.

[Brief explanation of the drawing]

Figure 1 is a structural diagram of a BWR fuel assembly, Figure 2 is a structural diagram of a conventional fuel assembly, and Figure 3 is a diagram of the fuel rods in the fuel assembly when the core condition changes from high temperature to cold temperature. Figure 4 is a diagram showing the difference in local output in percentage units, Figure 4 is a diagram showing the relationship between α and β, and Figure 5 is a diagram showing the relationship between α and the cold effective multiplication factor when one control rod is pulled out. It is a line diagram showing a relationship. A, 1 to 9... Fuel rod, B, G... Gadolinia rod, C... Channel box, W... Water rod.

Claims (1)

[Claims] 1. The average fissile material enrichment of the first layer fuel rods adjacent to the channel box of the fuel assembly loaded in the reactor is compared to the average enrichment of the second layer fuel rods adjacent to the first layer fuel rods. A nuclear reactor fuel assembly characterized by having an average enrichment of fissile material of 1.5 times or more.