JPH0350231B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a boiling water nuclear reactor with increased core integrity.

Generally, in a boiling water nuclear reactor, a cladding tube is filled with fuel pellets made of uranium oxide to form fuel rods, and a large number of these heating rods are loaded in a channel box to form a fuel assembly. A reactor core is constructed by arranging a large number of these fuel assemblies. In a boiling water reactor, coolant flows through the fuel assembly and voids are generated, but the void ratio is large in the upper part and small in the lower part. The presence of this void reduces the proportion of the coolant (light water) by that amount, and the proportion of fast neutrons converted into thermal neutrons decreases. Therefore, as the void ratio increases, the reactivity of that portion decreases.

For this reason, as shown in Fig. 1, the infinite multiplication factor (curve L in Fig. 1) at the lower part of the fuel loaded in the core
) is different from the infinite multiplication factor in the upper part (indicated by curve U in FIG. 1), and the infinite multiplication factor in the lower part becomes large from the beginning to the middle of the fuel combustion cycle. For this reason, the power distribution in the vertical direction of the core becomes nearly uniform, and a large power peak occurs at the bottom of the core. By the way, when the output of the fuel rods loaded in the reactor core increases excessively, the volume of the fuel pellets increases excessively due to swelling and thermal expansion, and the diameters of both ends of the fuel pellets increase, deforming them into a chain shape. The fuel pellets interfere with the inner surface of the cladding tube, causing so-called fuel pellet-to-cladding mechanical interaction (PCI), creating a risk of cladding tube breakage. In addition, the risk of PCI increased at the location of the large power peak that occurred in the lower part of the core, and there was a problem that the integrity of the core deteriorated. In order to eliminate such problems, in order to compensate for the difference in the infinite multiplication factor of fuel due to the difference in void ratio that occurs above and below the core, and to equalize the power distribution in the vertical direction of the core, the lower part of the fuel rod is By reducing the content of uranium-235,
In addition, efforts have been made to increase the content of burnable poison contained in the fuel pellets in the lower part, but none of these efforts have been able to achieve sufficient effects. In other words, in order to compensate for the effect of the difference in void fraction occurring above and below the core, straight line A is drawn in Figure 2.
As shown in the figure, the reaction difference between the top and bottom of the core is +1.5%.
In other words, it is necessary to give the fuel characteristics such that the reactivity is approximately 1.5% higher in the upper part of the core. However, in reality, as the burnup of the reactor core progresses, the adjustment control rods, which are inserted shallowly, are gradually withdrawn.

Therefore, in the early stages of combustion, these adjustment control rods suppress the reactivity in the lower core, and in the final stages of combustion, these adjustment control rods are pulled out, reducing the reactivity suppression effect in the lower core. do. Therefore, in consideration of the influence of the control rods for adjustment, the characteristics of the reactivity difference between the upper and lower portions of the core as fuel relative to the progress of core burnup must be set as shown by straight line B. By the way, when only fuel rods with a uniform uranium 235 content are loaded, the difference in reactivity between the upper and lower cores with respect to the progress of core burnup exhibits a characteristic as shown by curve C. If heating rods with different uranium-235 contents are loaded on the upper and lower sides, the characteristics shown in curve D will be obtained, but this curve D is simply a shift of curve C overall to the positive side. Since the pattern itself does not change, for example, as shown in FIG. 2, it is possible to obtain a characteristic that matches the straight line B only at the end of combustion, but it is not possible to match the straight line B over the entire combustion range. Furthermore, when fuel rods with different contents of burnable poisons are loaded, the change characteristic of the reactivity of burnable poisons decreases linearly as the burnup progresses, and the above straight line B The difference between curve D and curve D does not change linearly or monotonically as the burnup progresses, so we loaded fuel rods with different amounts of burnable poison at the top and bottom. It was also impossible to match the straight line B.

The present invention has been made based on the above circumstances, and its purpose is to completely compensate for the influence of the difference in void ratio that occurs above and below the core, and to make the power distribution in the vertical direction of the core uniform. The object of the present invention is to obtain a boiling water reactor that can make the power distribution uniform in the vertical direction and greatly improve the integrity of the reactor core.

The present invention will be explained below with reference to an embodiment shown in FIGS. 8 to 18. In the figure, numeral 1 is a control rod, which has a cross-shaped cross section. Four fuel assemblies 2 are arranged around the control rod 1 to form a unit lattice, and these unit lattices are arranged in a lattice to form a reactor core. Note that FIG. 3 shows only one fuel assembly 2. The fuel assembly 2 is constructed by loading 62 fuel rods 4 and two water rods into a channel box 3 having a substantially square cross section. And these fuel rods 4...
Cylindrical fuel pellets 5 formed by sintering uranium oxide are housed in a cladding tube 6, and the upper and lower ends of the cladding tube 6 are connected to an upper end plug 7 and a lower end plug 8.
A blemish 9 is formed in the upper part of the cladding tube 6. A spring 10 is housed within this blemish 9, and the fuel pellets 5 are elastically pressed and fixed by this spring 10. And these fuel rods 4
... is fissile material uranium-235 in the upper and lower parts.
Several types are used, including those with different contents of burnable poisons and those with different contents of burnable poisons. The Roman numerals attached to each fuel rod 4 in FIG. 3 indicate its type, and each type of fuel rod 4 is constructed as shown in FIG. 5. Type I fuel rods 4 are made of fuel pellets 5 having a uranium-235 content of 3.9 w/o (wt%) as schematically shown in Fig. 5.
It is filled with ..., and the content of uranium-235 is uniform in the upward direction. In addition, the fuel rod of type 4... has a uranium-235 content of 3.5w/
o fuel pellets 5... are accommodated, and the lower half has a smaller uranium-235 content than this.
It accommodates 3.3w/o fuel pellets 5... In addition, fuel rods 4 of type 4... accommodate fuel pellets 5... with a uranium-235 content of 2.9 w/o in the upper half, and fuel with a uranium-235 content of 2.5 w/o with a smaller uranium-235 content in the lower half. It contains pellets 5... In addition, fuel rods 4 of type , each have a uranium-235 content of 2.5w/o,
2.2w/o and 1.7w/o fuel pellets 5... are accommodated in the lower half, which contains small uranium-235 content fuel pellets 5 with a uranium-235 content of 2.5w/o.
..., and there is no difference in the uranium-235 content between the upper and lower parts. Moreover, the fuel rods 4 of the type are so-called poison rods containing fuel pellets 5 having a uranium-235 content of 2.5 w/o and containing 4.0 w/o of gadolinia as a burnable poison, and are filled with uranium at the top and bottom. 235 and gadolinia content are not given. Also, type 4 fuel rod... has uranium 235 at the top.
Contains fuel pellets 5 with a content of 2.9w/o,
There is no difference in the uranium-235 content between the upper and lower halves, but gadolinia is contained only in the lower half. Further, fuel rods of type 4... accommodate fuel pellets 5... containing 2.9 w/o of uranium-235 and 4.0 w/o of gadolinia in the upper half, and 2.9 w/o of uranium-235 in the lower half. o big and big
It contains fuel pellets 5 containing 5.0 w/o of gadolinia, and although there is no difference in the uranium 235 content between the upper and lower halves, the gadolinia content increases in the lower half. Moreover, the item with the symbol W is a water rod.

An embodiment of the present invention configured as described above includes fuel rods 4 of a type in which the content of uranium 235 in the lower part is smaller than the content of uranium 235 in the upper part, and fuel rods 4 of a type in which gadolinia is contained only in the lower part... , a type of fuel rod containing gadolinia in the upper and lower parts and a large gadolinia content in the lower part 4... and a type in which the uranium-235 content is uniform in the upper and lower parts...
, fuel rods 4... are loaded in the fuel assembly 2, so when this fuel assembly 2 is loaded into the reactor core, the fuel rods 4... are loaded into the reactor core, and as shown in FIG. The difference between the upper infinite multiplication factor (shown by curve U in Figure 6) and the lower infinite multiplication factor (shown by curve L in Figure 6) is small, and the average of the differences is extremely small, so the core output peak The power distribution above and below the core is uniform, and the health of the core is improved. Next, the reason will be explained. The change in the infinite multiplication factor of the fuel polymer 2 as a fuel assembly as the burnup progresses is as shown in Figure 6, but for example, the number of fuel polymers loaded in the entire reactor core changes every year. 1/
3 each will be exchanged. Therefore, the fuel assemblies have been loaded in the core for 8 years, and from the perspective of the entire core, 1/3 of the fuel assemblies 2 with burnup progression in 3 stages are loaded at all times. The infinite multiplication factor for the entire core is the average. The relationship between the difference in reactivity between the upper and lower parts of the core as a whole, which is caused by the equal difference in void fraction actually occurring in the upper and lower parts of the core, and the burnup of the core is as shown in FIG. The curve C shown in Figure 7 is the case when only heating rods are used in which the uranium-235 content and gadolinia content do not change vertically. The reactivity is larger, and the difference in reactivity between the upper and lower sides is non-linear. In addition, straight line A shows the characteristics necessary to equalize the power distribution in the vertical direction of the core by compensating for the effect of the difference in void ratio occurring above and below the core, and the difference in reactivity above and below the core is +1.5.
%, that is, the reactivity is required to be higher in the upper part by about 1.5%. However, in reality, the adjustment control rods, which are shallowly inserted into the lower core, are gradually withdrawn in response to the progress of the burnup of the reactor core. As the reactivity suppressing effect in the lower part of the core gradually decreases, the characteristics required for the fuel become as shown by straight line B. In this example, the fuel rods 4 of the types whose uranium 235 content in the lower part is smaller than the uranium 235 content in the upper part are loaded in the fuel assembly 2, so the reactivity of the lower part of the core as a whole is decreases, and a characteristic as shown by curve D is obtained. By appropriately setting the difference between the upper and lower uranium-235 contents in the fuel rods 4 of these types, this curve D can be made into a straight line at the end of the furnace combustion cycle (approximately 9 GWD/T in this example). Characteristics matching B can be obtained. However, this curve D is like an upward translation of the curve C, and the pattern of the curve C does not change, so just loading fuel rods 4... with different types of uranium-235 content at the top and bottom will result in straight line B. The desired characteristics as shown cannot be obtained. However, there is uranium in this fuel assembly 2 above and below.
In addition to fuel rods 4 with different 235 content, fuel rods 4 with different gadolinia content in the upper and lower parts, and fuel rods 4 with gadolinia only in the lower part are loaded. Thereby, it is possible to match the target straight line B of the core upper and lower reactivity difference with respect to the core burnup. That is, the difference between straight line B and curve D in FIG.
When AGd is determined, it becomes as shown schematically by a straight line F in FIG.
By compensating by , the characteristic of straight line B can be achieved. Generally, when fuel pellets 5 contain gadolinia, the reactivity as a poisonous substance is influenced by its content as shown in Figure 10a, and the higher the gadolinia content, the more effective the reactivity is as a poisonous substance. In addition, as shown in FIG. 10b, the burnup until gadolinia burns out and disappears increases as the gadolinia content increases. Moreover, this gadolinia is burned as the burn-up progresses, and its reactivity as a poison decreases linearly. Therefore, when the above type of gadolinia content differs between the upper and lower parts is loaded into the reactor core, the reactivity of gadolinia as a poison in the upper part decreases as shown by the straight line G in Figure 8, and the lower part The reactivity of gadolinia as a poison decreases as shown by the straight line H, and these straight lines G and H have different odor patterns. Therefore, by appropriately setting the absolute value of the gadolinia content in the upper and lower parts and the difference between the contents in the upper and lower parts, the difference in the reactivity of the upper and lower gadolinia as a poison can be adjusted to the above-mentioned straight lines G and H, as shown in Figure 8. As the burnup progresses, the removal increases due to the difference in scent, and at the end of combustion, the upper gadolinia burns out first,
After this upper part of gadolinia is burned out, the straight line H
According to the scent, the difference in the reactivity of the upper and lower gadolinia as a poison decreases.
Note that the characteristics shown in FIG. 8 are average values obtained when combustion assemblies with three levels of burnup are loaded into the core, one third each, as in the case of FIG. 7. Therefore, by loading fuel rods 4 of this type into the combustion assembly 2, AGd ₁ of the characteristic difference AGd shown in FIGS. 7 and 9 can be reduced.
Compensation for the portion shown by is performed, and the characteristics shown by curve E in Figure 7 are obtained, and the characteristics of the difference in reactivity above and below the core that match the target straight line B from the middle to the late stages of the reactor combustion cycle are obtained. It will be done. Next, in fuel rods 4 that contain gadolinia only in the lower part and have a relatively small content, the gadolinia contained therein decreases in reactivity as a poison as the burnup progresses, as shown by the straight line in Figure 8. The characteristic is that the fuel decreases linearly and the fuel is exhausted early in the furnace combustion cycle. Therefore, this type of fuel rod 4 compensates for the difference AGd ₂ in characteristics between the straight line B and the curve E in FIG. It is possible to obtain characteristics of the reactivity difference between the upper and lower cores that match the above.

Note that in order to achieve such characteristics, fuel rods 4
It is necessary to appropriately set the difference in the uranium-235 content between the upper and lower parts of ..., the difference in the gadolinia content between the upper and lower parts, the gadolinia content contained in the lower part, etc., and these are set as follows. First, in Figure 7, calculate the difference in core reactivity between the target straight line B and the curve.
If Avoid, then Avoid=A _G d+A _E ...(1). Here, A _G d is the integral value of the difference in reactivity between the upper and lower cores compensated by the inclusion of gadolinia, that is, the area surrounded by straight line B and curve D in Figure 7, and A _E is the area between the upper and lower uranium The integrated value of the difference in reactivity between the upper and lower cores compensated for by the difference in 235 content, ie, the area of the portion surrounded by curves D and C in FIG. 7, is shown. Then, Ag 1 is the reactivity effect as a poison rod in the lower part of the fuel rod 4 that contains gadolinia only in the lower part, and Ag ₁ is the reactivity effect as a poison rod in the upper part of the fuel rod 4 that has a difference in gadolinia content between the upper and lower parts. reactivity effect
Ag ₂ is the reactivity effect as a poison rod in the lower part of this type of fuel rod 4..., and Ag ₃ is the number of fuel rods 4 that contain gadolinia only in the lower part, and m is the difference in gadolinia content between the upper and lower parts. If the number of fuel rods 4 of a certain type is n, then the above A _G
d is A _G d=mAg ₁ +n(Ag ₃ -Ag ₂ )...(2). In addition, in the formula (2), Ag ₁ , Ag ₂ ,
Ag ₃ is the average value of the entire reactor core when combustion assemblies with N stage burnup are loaded in the reactor core at a rate of 1/N, and the reactivity of the above various fuel rods 4 as Poins rods is generally Letting AgK be AgKΔKg・Egk/2N...(3). In other words, AgK in the above equation is expressed by the area of the triangular part surrounded by each line segment and the X and Y axes shown in Figure 8, but gadolinia exists only in the first-year burnt aggregate. Therefore, the overall average is the value divided by the fuel drumstick size N.

In addition, ΔKg is the reactivity effect per fuel rod at the beginning of the cycle, and EgK is the reactivity effect of this fuel rod 4...
This is the burnup at the end of the first year of combustion. Moreover, the above A _E is given by A _E C・Δ〓・Eo...(4). However, C10 (%Δk/w/o) Δ〓: Difference in uranium 235 content between upper and lower parts (w/o) Eo: Burnup of the reactor core. In other words, this difference in reactivity between the upper and lower sides is almost proportional to the difference in the uranium-235 content between the upper and lower sides, and this difference in the uranium-235 content hardly changes as the burnup progresses, so as shown in Figure 7, the reactivity of the reactor is It is considered to be almost constant from the beginning to the end of the combustion cycle.

Then, the difference between the uranium content and gadolinia content between the upper and lower parts can be set appropriately so as to satisfy these equations (1) to (4). The gadolinia content in the lower part of the rod 4 is smaller than the gadolinia content in the upper part of the fuel rod 4, which has a difference in gadolinia content between the upper and lower parts, and the gadolinia content is 1 to 2 w/o, and If there is a difference in gadolinia content, the difference in gadolinia content between the upper and lower sides of fuel rod 4... is 1.0~. 5w/
It is preferable that it is o. In order to confirm the effects of the present invention, the results of determining the relative power distribution in the vertical direction of the reactor core for the above-mentioned example are shown in Figs. 11 to 13, and Fig. 11 shows the combustion cycle of the reactor. The initial one, FIG. 12 shows the middle stage of the furnace combustion cycle, and FIG. 18 shows the final stage of the furnace combustion cycle. Curve J in the figure shows an example of the present invention, and curve K shows a conventional product in which the uranium-235 content differs between the top and bottom, and the gadolinia content is uniform on the top and bottom. . As is clear from these results, in the conventional system, the output peak occurs at the bottom of the core in the early and middle stages of the furnace combustion cycle, but in the example of the present invention, the power output peaks at the bottom of the core throughout the entire combustion cycle of the furnace. There is no power peak at the bottom, and the power distribution pattern is nearly constant during the entire combustion cycle of the reactor, greatly improving the health of the core.

Note that the present invention is not limited to the above embodiment.

For example, the difference and absolute value between the upper and lower contents of fissile materials such as uranium-235, the difference and absolute value between the upper and lower contents of burnable poisons such as gadolinia, the number of these various fuel rods and their arrangement, etc. are as described above. It is not limited to one embodiment.

In addition, other than fuel rods with different amounts of fissile material in the upper and lower parts, fuel rods with different contents of burnable poison in the upper and lower parts, and fuel rods in which only the lower part contains burnable poison, other fuel rods are the same as above. In this example, the content of fissile material is uniform on the top and bottom, but other fuel rods are not necessarily limited to those in this example, and may have configurations that give various nuclear characteristics to change the characteristics of the reactor. You may also use one.

As described above, the present invention provides a fuel rod in which the content of fissile material is different between the upper and lower parts, and the fissile content in the lower part is smaller than that in the upper part, and a fuel rod in which the content of burnable poison in the lower part is lower than that in the upper part. A fuel assembly is loaded with fuel rods that have a larger toxic content and fuel rods that contain burnable poison only in the lower part. Therefore, these various fuel rods provide a difference in reactivity between the top and bottom of the core, compensate for the difference in reactivity between the top and bottom of the core due to the difference in void fraction between the top and bottom of the core, and increase the output between the top and bottom of the core throughout the entire fuel cycle. The effects are great, such as being able to maintain uniform distribution and greatly improving the health of the reactor core.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the burnup of a conventional fuel assembly and the infinite multiplication factor, and FIG. 2 is a diagram showing the general relationship between the core burnup and the difference in reactivity above and below the core. 3 to 13 show an embodiment of the present invention, FIG. 3 is a cross-sectional view of a fuel assembly, and FIG. 4 is a cross-sectional view of a fuel assembly.
Figure 5 is a vertical cross-sectional view of a fuel rod, Figure 5 is a diagram schematically showing the vertical distribution of fissile material and burnable poison in various fuel rods, and Figure 6 is a diagram showing the burnup and infinite increase in fuel assemblies. A diagram showing the relationship between the magnification and Figure 7 is a diagram showing the relationship between the core burnup and the difference in reactivity between the upper and lower cores.
Figure 8 is a diagram showing the relationship between burnup and gadolinia reactivity, Figure 9 is a diagram schematically showing the relationship between burnup and gadolinia reactivity difference above and below, and Figure 10a is gadolinia content. Figure 10b is a diagram showing the relationship between the gadolinia content and the burnup until it burns out, and Figures 11 to 13 are diagrams showing the relationship between the gadolinia content and the reactivity effect of gadolinia. FIG. 2 is a diagram showing the relative power distribution in the vertical direction of the core of an embodiment of the present invention and a conventional one at the beginning, middle, and end of the combustion cycle. 1...Control rod, 2...Combustion assembly, 4...Fuel rod, 5...Fuel pellet.

Claims (1)

[Claims] 1. In a reactor core configured by arranging a large number of fuel assemblies each consisting of a large number of fuel rods, the fuel assemblies include fuel rods in which the content of fissile material in the lower part is smaller than the content of fissile material in the upper part. , a fuel rod containing a burnable poison, in which the burnable poison content in the lower part is greater than the burnable poison content in the upper part, and the difference between the burnable poison content in the upper part and the lower part is 1.0 to 1.5% by weight; The burnable poison content in the upper part of the fuel rod contains burnable poison only in the fuel rod, and this burnable poison content is higher than the burnable poison content in the upper part. 1. A boiling water nuclear reactor, characterized in that it is loaded with fuel rods whose content is 1 to 2% by weight and other fuel rods.