JPH0555836B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates primarily to a fuel assembly used in a boiling water nuclear reactor.

[Technical background of the invention]

In general, boiling water reactors are shown in Figures 1 and 2.
The fuel assembly shown in the figure is installed.

This fuel assembly includes a plurality of fuel rods 3, a plurality of burnable poison-containing fuel rods 4 (designated with the symbol G), and two fuel rods between an upper tie plate 1 and a lower tie plate 2. Water rods 5 (marked with the symbol W) are arranged in a square lattice of 8 rows and 8 columns as shown in FIG. 2, and are supported at several locations in the middle by spacers 6. and is housed in the channel box 7. The fuel assembly is then loaded into the reactor core along the control rod 8, which has a cross-shaped cross section, as shown in FIG.

The early Waterrod 5 was a Zircaloy tube.
It is formed so that the coolant flows from the bottom to the top.

In addition, the fuel rod 3 has a Zircaloy fuel cladding tube 3a loaded with a plurality of uranium dioxide pellets (not shown) in which uranium dioxide (UO ₂ ) enriched with uranium 235 is sintered into pellets (not shown). It is formed by The enrichment degree of uranium dioxide in the axial direction of each fuel rod 3 is formed to be constant.

In addition, the burnable poison-containing fuel rod 4 is made by mixing uranium dioxide with a burnable poison such as gadolinia (Gd ₂ O ₃ ) at a concentration of several percent by weight in a Zircaloy fuel cladding tube, and baking the mixture into a pellet shape. It is formed by loading a plurality of gadolinia-mixed pellets in the axial direction. The enrichment degree of uranium dioxide and the amount of gadolinia mixed in the axial direction of each burnable poison-containing fuel rod 4 are set constant.

[Problems with background technology]

As the nuclear fuel in the fuel assembly progresses,
The concentration of uranium-235 inside decreases, the infinite multiplication factor of the fuel assembly decreases, and the reactivity decreases.

Therefore, in order to operate the reactor under high reactivity, the reactor is shut down at regular intervals and 1/4 to 1/3 of the total fuel assemblies loaded in the core are replaced with new fuel assemblies. are being exchanged.

However, this fuel exchange causes the reactor to stop for a long period of time, reducing the operating rate, so it is desired that the interval between fuel exchanges be as long as possible.

For this reason, conventionally, the uranium-235 enrichment of each fuel rod is increased so that the infinite multiplication factor of the fuel assembly can be maintained over a long period of time at a value greater than that required for operation. Furthermore, the surplus infinite multiplication factor at the beginning of the combustion operation period, which occurs due to the increase in the enrichment of uranium-235, is suppressed by the plurality of burnable poison-filled fuel rods 4, 4, as described above. .

However, increasing the enrichment of uranium-235 in each fuel rod 3 increases the cost of fuel production.
Furthermore, there was a limit to the ability of burnable poison rods to suppress the infinite multiplication factor of fuel assemblies, so the uranium enrichment could not be increased too much, and there was also a limit to the lengthening of fuel replacement intervals.

There is also a need to improve the combustion efficiency of nuclear fuel, but there is a limit to the improvement in combustion efficiency in conventional fuel assemblies.

[Purpose of the invention]

The present invention has been made in view of these points, and it is possible to maintain the reactivity at a predetermined value or higher for a long period of time, and it is possible to extend the interval between fuel exchanges, so that the reactor can be used for a long period of time. The purpose of the present invention is to provide a fuel assembly that enables continuous operation of the nuclear reactor, improves the operating rate of the reactor, and improves the combustion efficiency of the fuel.

[Summary of the Invention] The present invention provides a fuel assembly formed by arranging a plurality of fuel rods and _two burnable poison-containing fuel rods in which at least the lower half of the fuel rods are mixed with a burnable poison. , out of N ₂ fuel rods with burnable poison mixed in the lower half, N ₁ fuel rod with burnable poison mixed in the upper half, and at least the lower half A burnable poison was mixed into the unit ( _N2
-N ₁ +1) Burnable poison concentration in the lower half of more than one fuel rod containing burnable poison is calculated as the burnable poison concentration in the upper half of the fuel rod containing burnable poison in the upper half of the fuel rod. It is characterized by being formed smaller than the concentration.

[Embodiments of the invention]

The present invention will be described below with reference to embodiments shown in FIGS. 3 to 11.

3 to 8 show one embodiment of the present invention.

In FIG. 3, the same parts as in the prior art are given the same reference numerals.

As shown in FIG. 3 and FIG.
Two of the burnable poison-containing fuel rods (hereinafter referred to as Gd rods) with the code _G1 were loaded with uranium dioxide mixed with about 5% by weight of gadolinia and baked into pellets. The lower part is filled with uranium dioxide mixed with about 1% by weight of gadolinia, which is baked and hardened into pellets. In addition, the other six tubes with reference numeral _G2 are loaded with uranium dioxide mixed with about 5% by weight of gadolinia in the form of sintered pellets throughout their length.

That is, at the beginning of the fuel operation period (cycle), that is, when the burnup is between 0 and 2 GWD/T, the number of Gd rods in the upper and lower halves is both eight, as shown in Figure 5A. The same number, but two
The gadolinia concentration in the lower part of Gd rod G ₁ is lower than the upper part, and the gadolinia concentration in the upper half of Gd rods G ₁ and G ₂ as a whole is higher than that in the lower half. The infinite multiplication factor A in the upper half is set to be about the same level or slightly lower than the infinite multiplication factor B in the lower half of the whole.

Then, in the middle of the cycle, when the gadolinium (Gd155 and Gd157) at the bottom of the two low-concentration Gd rods _G1 has been completely burned out, that is, the burnup is between 2 and 2.
In the case of 9GWD/T, as shown in Figure 5B,
The number of Gd rods in the upper half is 8, which is more than the 6 in the lower half, and as shown in Figure 6, the infinite multiplication factor B in the lower half of the entire fuel assembly is greater than the infinite multiplication factor B in the lower half. The magnification A is set to be about 3% lower.

As a result of such combustion, at the end of the cycle, that is, at a burnup of 9 to 11 GWD/T, the remaining amount of uranium-235 in the upper half of the fuel assembly becomes relatively larger than that in the lower half. The infinite multiplication factor A in the half section is made to gradually approach the infinite multiplication factor B in the lower half section.

However, due to the loading of such fuel assemblies, during the combustion operation period of the reactor, the power distribution in the axial direction of the reactor core reaches a lower peak from the beginning to the middle of the cycle, as shown by the solid line in Figure 7. At the end of the cycle, the output distribution in the axial direction reaches an upper peak as shown by the solid line in FIG.

In addition, when looking at the void rate in the core, in most of the early and middle stages of the cycle,
Steam is generated from the lower part of the core, and the average void fraction in the core, indicated by the broken line in Figure 7, is about 5 to 10% higher than in conventional boiling water reactors. When this void fraction is high, the moderation effect of neutrons is suppressed, and the number of neutrons having energy in a resonance region higher than that of thermal neutrons increases. Along with this, uranium 2
The resonance absorption of 38 increases, and the amount of conversion to plutonium increases. This improves fuel combustion efficiency and, in turn, improves fuel economy.

As the cycle progresses toward the end, the steam generation region moves upward in the axial direction. As a result, the average void ratio shown by the broken line in FIG. 8 is lowered by about 5 to 10% compared to the conventional method. Then, the moderation effect of neutrons is promoted, and the core reactivity is increased. As a result, the reactivity of the reactor core can be maintained at a required value or higher for a long period of time, and the reactor can be operated for a long period of time.

In this way, by making the amount of gadolinia mixed in the lower part of the fuel assembly smaller than that in the upper part, and by making the infinite multiplication factor in the upper half smaller than the infinite multiplication factor in the lower half, the core power distribution was changed over the cycle. It can be a lower peak for the most part and an upper peak at the end of the cycle. Furthermore, the amount of plutonium conversion is increased, and the fuel can be burnt economically, improving fuel economy.

In this example, when the number of Gd rods in the upper half and lower half is equal, the gadolinia concentration of at least one of the lower half is made lower than the gadolinia concentration of the upper half mixed with gadolinia. .

In addition, at the beginning of the cycle, especially in the replacement cycle, in order to adjust the axial power distribution of the core according to the combustion history of the previous cycle, the distribution of the amount of gadolinium in the upper and lower halves of the Gd rod was determined as follows. It would be better to change it like this.

For example, if the combustion of uranium-235 in the lower half did not progress much in the previous cycle and a large amount of uranium-235 remains, the combustion rate in the lower half is set to be slightly faster than in the upper half. Thermal integrity must be maintained.

At this time, as shown in Figures 9 to 11, gadolinia was mixed into at least the lower half.
The number of Gd rods N ₂ is greater than the number N ₁ of Gd rods with gadolinia mixed in the upper half, and the number of Gd rods with Gd rods mixed with gadolinia in the lower half (N ₂ - _{N 1} + 1) is lower. It is preferable that the concentration of gadolinia in the half part is lower than the concentration of gadolinia in the upper half of the Gd rod in which gadolinia is mixed in the upper half. That is, in the example shown in FIGS. 9 to 11, three Gd rods _G1 with a gadolinia concentration of 5% by weight in the upper half and 1% by weight in the lower half are used. 4 pieces of Gd rod G ₂ with 5% weight, the upper half is 0% by weight and the lower half is 5% by weight Gd rod G ₃
There are said to be two.

In this case, the gadolinia concentration in the lower half is small.
Since the number of Gd rods G ₁ is one more than the number of Gd rods G ₃ in the lower half where the gadolinia concentration is high, the fuel assembly as a whole burns as a lower peak, but the degree of the peak is suppressed to a low level. Then, in the early to middle cycle, Gd rods
Since the gadolinium in the lower half of _G1 is completely burnt out, the number of Gd rods in the upper half becomes one more than in the lower part, and from then on, combustion with the lower peak occurs in the same manner as described above. In this case, in order to further reduce the degree of the lower peak at the beginning of the cycle,
The gadolinia concentration of Gd rods _G2 and _G3 may be 6% by weight.

〔Effect of the invention〕

Since the fuel assembly of the present invention is constructed and operates in this manner, it is possible to maintain the reactivity above a predetermined value over a long period of time, and the interval between fuel exchanges can be extended. Enables long-term continuous operation of nuclear reactors, improves reactor availability,
Furthermore, it has effects such as improving fuel combustion efficiency.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway perspective view showing a conventional fuel assembly, FIG. 2 is an enlarged sectional view taken along the line - in FIG. 1, and FIGS. 3 to 11 are implementations of the fuel assembly of the present invention. For example, Figures 3 to 8 show an embodiment of the present invention, Figure 3 is similar to Figure 2, and Figure 4 is a diagram similar to Figure 2.
An explanatory diagram showing the types of Gd rods and their gadolinia concentrations in the axial direction of the core, Figure 5A is a diagram showing the number of Gd rods in the axial direction at the beginning of the cycle, and Figure 5B
The figure shows the number of Gd rods in the axial direction in the middle and end of the cycle, and Figure 6 shows the burnup (GWD/
Figure 7 is a characteristic diagram showing the change in the infinite multiplication factor (K∞) with respect to T). Figure 7 is a characteristic diagram showing the core power distribution and void generation distribution in the core axis direction at the early and middle stages of the cycle. FIG. 7 is a similar view, FIG. 9 and FIG. 10 are similar views to FIG. 3, and FIG.
FIG. 1 is a diagram similar to FIG. 4 showing the Gd rod in FIGS. 9 and 10. 3...Fuel rods, _G1 , _G2 , _G3 ...Gd rods (fuel rods containing burnable poison).

Claims (1)

[Claims] 1. In a fuel assembly formed by arranging a plurality of fuel rods and _two fuel rods containing burnable poison, at least the lower half of which is mixed with burnable poison,
Of N ₂ fuel rods with burnable poison mixed with burnable poison in the lower half, N ₁ fuel rod with burnable poison mixed in the upper half, at least in the lower half. burnable poison was mixed into (N ₂ −N ₁ +
1) The burnable poison concentration in the lower half of one or more fuel rods containing burnable poison is lower than the burnable poison concentration in the upper half of the fuel rod containing burnable poison in the upper half. A fuel assembly characterized in that it has been formed.