JPS59220674A - Fuel assembly - 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 the Invention] The present invention relates primarily to a fuel assembly used in a boiling water nuclear reactor.

[Technical background of the invention]

Generally, a boiling water nuclear reactor is equipped with a fuel assembly shown in FIGS. 1 and 2.

This fuel assembly consists of a plurality of fuel rods 3, a plurality of burnable poison fuel rods II (marked with the symbol G and indicated in °C), and one water rod between the upper tie plate and the lower tie plate. The rods S (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 AlIC. ,
It is housed within the Channel Hochsf. Then, the fuel assemblies are loaded into the reactor core along control rods S having a cross-shaped cross section, as shown in FIG.

The water rod 5 is a tube made of Zircaloy, and is formed so that the coolant flows from the bottom to the top.

In addition, the fuel rod 3 is equipped with a plurality of uranium dioxide pellets (not shown), which are made by burning and solidifying uranium dioxide (Uot) enriched with uranium-23S, into a pellet shape in the Zircaloy fuel cladding tube 3a. It is formed as follows. Further, the concentration of carbon dioxide in the axial direction of each fuel rod 3 is formed to be constant.

In addition, burnable poison fuel rods contain gadolinia, which is made by mixing uranium dioxide with a burnable poison such as gadolinia (GdxOs) at a concentration of several percent by weight and burning it into pellets in the Zircaloy fuel cladding tube. It is formed by loading a plurality of 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 fuel rod are set constant.

[Problems with background technology]

As combustion of the nuclear fuel in the fuel assembly progresses, the concentration of uranium λ3S inside the fuel assembly 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 ~% of the total fuel assemblies loaded in the reactor core are replaced with new fuel assemblies. .

However, this fuel exchange causes the nuclear 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 enrichment level of uranium-23 in each fuel rod is increased to 5 so that the infinite multiplication factor of the fuel assembly can be maintained at a value higher than the value required for operation over a long period of time. Furthermore, the surplus infinite multiplication factor at the beginning of the combustion operation period, which is caused by the increase in the enrichment of Uranko 33, is suppressed by the plurality of burnable poisonous fuel rods, as described above.

However, increasing the enrichment of each fuel rod 3,3 increases the cost of fuel production, and there is also a limit to the ability to suppress the infinite multiplication factor of the fuel assembly using the burnable poison button. However, the uranium enrichment could not be increased too much, and there were limits to the lengthening of refueling intervals.

It is also required to improve the combustion efficiency of nuclear fuel, but there are limits to the improvement of combustion efficiency in conventional fuel assemblies.

[Purpose of the invention]

The present invention has been made in view of these points, and is capable of maintaining the reactivity at a predetermined value or higher over a long period of time.
It is possible to lengthen the interval between fuel exchanges, enable long-term continuous operation of the reactor, and improve the operating rate of the reactor.
Another object of the present invention is to provide a fuel assembly that can improve fuel combustion efficiency.

[Summary of the invention]

The fuel assembly of the present invention has a distribution in the upper and lower halves of the authorized flammable substance amount of burnable poisonous fuel rods, so that the core power distribution has a lower peak at the beginning and middle of the combustion operation period, and the combustion It is characterized in that it is formed so that the core power distribution reaches an upper peak at the end of the operating period, and the reactivity is maintained at a predetermined value or higher over a long period of time.

[Embodiments of the invention]

Hereinafter, the present invention will be described with reference to embodiments shown in FIGS. 3 to 1/FIG.

3-3 show one embodiment of the present invention.

In FIG. 3, parts that are the same as those of the prior art are given the same reference numerals.

In this embodiment, two of the three burnable poisonous fuel rods (hereinafter referred to as Gd rods), as shown in FIG. 3 and FIG. A pellet containing approximately 5% by weight of gadolinia is loaded in the lower part of the pellet, and a pellet formed by mixing approximately 5% by weight of gadolinia into uranium dioxide is loaded in the lower part. In addition, the other six tubes with the symbol G are loaded with a mixture of about 5% by weight of gadolinia in uranium dioxide, which is baked and hardened into a pellet shape, over its entire length.

Next, the operation of this embodiment will be explained.

At the beginning of the combustion operation period (cycle), that is, when the burnup is θ~
, 2GWD/T, the trap as shown in Figure tA,
The number of Gd rods in the upper and lower halves is the same (3), but since the air concentration in the lower part of one Gd rod G1 is lower than that in the upper part, the overall Gd rod Jr.
The gadolinia concentration in the upper half of Gt is higher than in the lower half. Therefore, as shown in FIG. 6, the infinite multiplication factor A in the upper half of the entire fuel assembly is about the same or slightly lower than the infinite multiplication factor B in the lower half.

Then, at the beginning of this cycle, the two G
d Gadolinium at the bottom of rod G1 (Gd-/35 and Gd
-/! ;7) will be burned out.

Therefore, in the middle of the cycle, the burnup is 2 to 9aWD/
In the case of T, as shown in Figure 5-B, the upper half G
There are now 8 d-rods, more than the 6 in the lower half. As a result, as shown in FIG. 6, the infinite multiplication factor A in the upper half of the entire fuel assembly is about 3 inches lower than the infinite multiplication factor B in the lower half.

And in this way, during the early and middle stages of the cycle,
Since the infinite multiplication factors A and B change, the power distribution in the axial direction of the core has a lower peak during most of the cycle, as shown by the solid line in FIG.

As a result of such combustion, at the end of the cycle, i.e., at burnup 〉// GWD/T Ic, the amount of Ulanko 35 remaining in the upper half of the fuel assembly is relatively smaller than that in the lower half. The number is increasing. Therefore, as shown by the solid line in the figure, the output distribution in the axial direction at the end of the cycle has an upper peak.

In addition, looking at the void fraction in the reactor core, steam is generated from the lower part of the core during most of the early and middle stages of the cycle, and the average void fraction in the core, shown by the broken line in the diagram, is lower than that of conventional boiling water. Type atom 19' compared to about S~/
θ becomes higher. When this void fraction is high, the moderation effect of neutrons is suppressed, and the number of neutrons that have energy in the resonance region higher than thermal neutrons increases. Along with this, the resonance absorption of uranium-2311 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. 3 is lowered by about 5 to 10% compared to the conventional case. 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.

By making the amount of gadolinia mixed in the lower part of the fuel assembly smaller than that in the upper part, the core power distribution can be made to have a lower peak during most of the cycle and an upper peak at the end of the cycle. Furthermore, the amount of plutonium conversion increases, fuel can be burned more efficiently, and fuel economy can be improved.

In this embodiment, when the number of Gd rods is equal in the upper half and the lower half, at least one of the lower half has a smaller gadolinia concentration than the upper half.

In addition, in this example, the concentration distribution of gadolinia in the six Gd rods G is 5% by weight in the upper half and 6% by weight in the lower half.
You can also use it as This is to prevent the gadolinium in the lower half from burning out prematurely since the output is burned at the lower peak.

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 adjusted as follows. You should change the 5th button.

For example, if the combustion of uranium 2.3 in the lower half progressed extremely in the previous cycle and a large amount of uranium λ3s remained in the upper half, the number of Gd rods in the upper half was reduced at the beginning of the cycle. The amount of uranium 28 in the lower half is increased to promote the combustion of uranium-28 in the lower half in the early and middle stages of the cycle, resulting in a lower peak.

Conversely, if the combustion of uranium-235 in the lower half did not progress much in the previous cycle and a large amount of uranium and 23S remained, the combustion rate in the lower half should be made slightly faster than that in the upper half. It is necessary to maintain the thermal integrity of the reactor core.

At this time, as shown in Figures 9 to 1, the number N of Gd rods in the lower half is changed to the number N of Gd rods in the upper half.
1, and among the Gd rods in the lower half [: N, -
N, +/ ] or more Gd rods are preferably formed so that the gadolinia concentration is lower than that of the upper half.

That is, in the examples shown in FIGS. 7 to 1/1, the gadolinia concentration in the upper half is ! i: 3 Gd rods G of 1lti% and 7% by weight in the lower half, and 5 in the upper and lower halves.
Gd rod G with weight %, Gd rod G with 0 weight % in the upper half and S weight ff % in the lower half.

is considered to be a co-book.

In this case, the number of Gd rods G with a low gadolinia concentration in the lower half is seven more than the number of Gd rods Gs with a high gadolinia concentration in the lower half, so the fuel assembly as a whole burns as a lower peak. The degree of the peak can be suppressed to a low level. Then, from the beginning to the middle of the cycle, the gadolinium in the lower half of the Gd rod G is completely burned out, so top-firing is performed. In addition, in this case, K to further reduce the degree of the lower peak at the early stage of the cycle is Gd
The gadolinia concentration of rods G2 and G may be 4% by weight.

〔Effect of the invention〕

Since the fuel assembly of the present invention is constructed and operates in this way, 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. It enables continuous operation of the nuclear reactor for a long period of time, improves the operating rate of the reactor, and further improves the combustion efficiency of the fuel.

[Brief explanation of drawings]

Part 7 is a partially cutaway perspective view showing a conventional fuel assembly, Part 2
The figure is an enlarged sectional view taken along the line ■-■ in Figure 1, Figures 3 to 1/Figures show embodiments of the fuel assembly of the present invention, and Figures 3 to 8 show examples of the fuel assembly of the present invention. Figure 3 is a diagram similar to Figure 2, Figure G is an explanatory diagram showing the types of Gd rods and their gadolinia concentrations in the axial direction of the core, and Figure 5A is the number of Gd rods in the axial direction at the beginning of the cycle. Line diagram showing 3rd
Figure B is a diagram showing the number of Gd rods in the axial direction at the middle and end of the cycle, Figure 6 is a characteristic diagram showing the change in infinite multiplication factor (Kω) with respect to burnup (GWD/T), and Figure 7 is at the beginning of the cycle. FIG. 8 is a diagram similar to FIG. 7 at the end of the cycle, and FIGS. 9 and 10 show other embodiments of the present invention. FIG. 11 is a similar view to FIG. 3 showing the Gd rod in FIG. 9 and FIG. 1O. 3...Fuel rod, '1 v Gt r G3...Gd
rod (burnable poison fuel rod). Fig. 3 Fig. 4 Fig. 5A Fig. 5B [¥] Gd mouth, number of g Gd mouth, ... 1 sheet 6th
Illustration! 4 degrees (GWD/T) Fig. 7 Hoyt (%) d Shika (J eye) Dimension I Hatake) Fig. 8 Hoyt wheel (%) Exit saw (4) Eye boo 1 I J cover) No. Figure 9 Figure 10 Figure 34 4 trees 2 trees

Claims (1)

[Claims] l In a fuel assembly formed by arranging a plurality of fuel rods and a plurality of combustible recessed fuel rods, the approved amount of burnable poison in the burnable poison type fuel rod The distribution in the upper and lower halves is shaped to such an extent that the core power distribution has a lower peak at the beginning and middle of the combustion operation period, and an upper peak at the end of the combustion operation period. fuel assembly. 2. The number of burnable poison fuel rods in the upper and lower halves is equal, and the concentration of burnable poison in the lower half of at least 7 burnable poison fuel rods is lower than the concentration in the upper part. A fuel assembly according to claim 1, characterized in that: 3. The fuel assembly according to claim 1, wherein the number of burnable poison fuel rods in the upper half is greater than the number of burnable poison fuel rods in the lower half. The number N of burnable poison fuel rods in the lower half is greater than the number N1 of burnable poison fuel rods in the upper half! Increase (N2-N1
+/) The fuel assembly according to claim 1, characterized in that the concentration of the burnable poison in the lower half of the burnable poison fuel rods is lower than the concentration in the upper half.