JP2953789B2 - Nuclear fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear fuel assembly loaded in a boiling water reactor and including a fuel rod having a short axial length (short fuel rod) as a fuel rod containing a nuclear fuel substance. The present invention relates to a nuclear fuel assembly capable of preventing a decrease in scram reactivity characteristics and improving safety of a nuclear reactor.

[0002]

2. Description of the Related Art FIG. 5 shows the structure of a nuclear fuel assembly 1 loaded in a boiling water reactor. In the channel box 2, a fuel rod 3 containing nuclear fuel material and a water rod 4 as a neutron moderating rod are housed in a grid of 8 rows and 8 columns with the axial direction being vertical while being separated from each other by a spacer 5. Is done. In the nuclear fuel assembly 1, the lengths of the fuel rods 3 are all equal. For this reason, both ends of the fuel rod 3 and the water rod 4 are supported and fixed by the upper tie plate 6 and the lower tie plate 7, respectively, but the positions (heights) of the upper end and the lower end of each fuel rod 3 can be aligned. .

[0003] By the way, recently, in order to improve the economy, the burn-up of fuel has been increasing. As a result, the peaking coefficient increases and the thermal characteristics per fuel rod 3 deteriorates. To alleviate this, the number of grids of the fuel rods 3 arranged in a grid in the nuclear fuel assembly is conventionally reduced. A design that increases the number of grids from 8 rows and 8 columns has been considered. However, an increase in the number of lattices causes problems such as an increase in pressure loss in the channel box 2 and a decrease in stability.

In order to solve such a problem, a fuel rod 3 having a conventional axial length (hereinafter referred to as a "long fuel rod") 3 shown in FIG. )
A short fuel rod 8 having a shorter axial length than the long fuel rod 3 has been devised. In both the long fuel rod 3 and the short fuel rod 8, the lower end of the cladding tube 9 is sealed with the lower end plug 10, and the fuel pellets 11 are sequentially filled from below. Then, an expansion spring 12 for absorbing the expansion of the fuel pellet 11 during combustion is disposed on the uppermost fuel pellet.
Seals the upper end of the cladding tube 9. The short fuel rod 8 has a shorter cladding tube 9 and a smaller number of fuel pellets 11 to be stored than the long fuel rod 3.

[0005] The short fuel rod 8 is connected to the long fuel rod 3.
FIG. 7 shows a nuclear fuel assembly arranged in a grid of 9 rows and 9 columns by mixing the above. Parts corresponding to those in FIG. 5 are denoted by the same reference numerals.

In the nuclear fuel assembly 14, the short fuel rods 8 (hatched) are attached to the lower tie plate 7, but do not reach the upper tie plate 6.

FIGS. 8 and 9 respectively show VIII-
It is a VIII line and IX-IX line sectional view. As shown in FIG. 8, the short fuel rods 8 are arranged in a total of 66 long fuel rods 3 arranged in 9 rows and 9 columns while surrounding two water rods 4 having a large diameter. A total of eight pieces are arranged in the second row and the second column, but a gap 16 is formed in the IX-IX line cross-sectional area where the short fuel rod 8 does not reach.

[0008]

The nuclear fuel assembly must achieve a flat power distribution as a whole from the viewpoint of the safety and soundness of the core fuel. Therefore, in Japanese Patent Application Laid-Open No. 61-240193, for a nuclear fuel assembly 1 including only long fuel rods 3 as fuel rods, the amount of fissile material, for example, uranium 235, is varied in the radial direction and the axial direction. Has proposed a nuclear design.

FIG. 10A is a radial sectional view of the nuclear fuel assembly 20 in this nuclear design. Each fuel rod is schematically shown in a grid pattern, and the numbers and symbols 21 to 21 in each fuel rod are shown.
25 and G ₅ are fissile material content contained a burnable poison amount is a reference code indicating the type of the different fuel rods. Symbol W represents a water rod.

[0010] in FIG. 10 (B) shows the enrichment and burnable poison concentration in the axial direction of the uranium 235 in the elongated fuel rods 21 to 25 and G _5, which is accommodated in the fuel assembly 20.

The long fuel rods 21 to 25 and G ₅ are uranium 23
5 have different enrichments, and the fuel rods 21, 23, 2
4, 25, G ₅ are each enrichment 3.8,3.5,
3.0, 1.9 and 3.5%. The fuel rod 22
Indicates that the concentration differs in the axial direction (the center 22b is high (3.8%), and the upper 22a and the lower 22c are low (2.8
%)). As a result, the average enrichment in the axial direction of the entire fuel rod is 3.15, 3.35, 3.35 from the bottom, excluding the amount of uranium 235 contained in the upper and lower ends of the natural uranium blanket 26, 0.71% by weight. It becomes 15% by weight. Fuel rod G
₅ contains a burnable poison (gatolinia; Gd ₂ O ₃ ) having a concentration decreasing in three steps of 5.0, 4.0 and 2.5% by weight sequentially from the bottom in the axial direction. Burnable poisons absorb neutrons and reduce reactivity during the burning of nuclear fuel.

According to this fuel rod, a flat power distribution can be obtained in the axial direction throughout the operation cycle of the nuclear reactor.

However, when this nuclear design is applied to the nuclear fuel assembly 14 including the short fuel rods described above, the following problems occur. FIG. 11 shows the infinite multiplication factor in the nuclear fuel assembly 14 in relation to the burnup (a parameter of the burning time).

From this figure, in the region of low burnup (early combustion stage), there is no short fuel rod 8 and the amount of nuclear fuel material is relatively small (a relative section) (a section along line IX-IX in FIG. 7). The infinite multiplication factor of the upper cross-section is the radial cross-section (VI in FIG.
It can be seen that it is smaller than the infinite multiplication factor in the section taken along the line II-VIII; In both cross sections, the average concentration of fissile material and the amount of combustible material are equal in cross section.

[0015] The difference between the infinite multiplication factor which occurs even when the concentration of fissile material and the amount of burnable poisons are equal is because the lower cross section has a smaller passage area for the coolant than the upper cross section, and the amount of the moderator is small. This is caused by the low neutron spectrum and the small decrease in reactivity due to burnable poisons.

If there is a difference in the infinite multiplication factor in the axial direction at the initial stage of combustion, it is difficult to obtain a flat power distribution in the axial direction throughout the operation cycle of the reactor. Therefore, as shown in FIG. 12, the output becomes higher at the lower part in the axial direction. Then, at the end of the operation cycle, as shown in FIG. 13, the upper part in the axial direction where the combustion does not progress much at the beginning and the residual ratio of the fuel is high, the output from the lower part in the axial direction where the combustion of the fuel has advanced in the early part of the cycle. Will be higher.

In a boiling water reactor, a control rod is inserted from the bottom of the reactor during scram.
Therefore, when a scrum occurs at the end of the operation cycle, the output distribution is biased to the upper part in the axial direction, and therefore, even if the control rod is inserted from the lower part in the axial direction, the decrease rate of the reactivity is small. In other words, the scram reactivity characteristic deteriorates at the end of the cycle, which causes a problem in securing the safety of the reactor.

The present invention has been made in view of the above circumstances, and in a boiling water reactor, even when a short fuel rod is included, it is possible to prevent the deterioration of the scram reactivity characteristic and improve the safety of the reactor. It is intended to provide a nuclear fuel assembly that can be used.

[0019]

According to the present invention, as a means for achieving the above object, a fuel rod containing a nuclear fuel substance and a fuel rod containing a nuclear fuel substance and a burnable poison are arranged in a square grid in a channel box. In a nuclear fuel assembly in which the fuel rods are composed of long fuel rods and short fuel rods having different axial lengths and having different amounts of nuclear fuel material in the axial direction, the channel box The position of the burnable poison-containing fuel rods arranged in the second row from the outer layer of the inner fuel rod array is changed to the axis in which the amount of nuclear fuel material is relatively large in the axial region where the amount of nuclear fuel material is relatively small. In comparison with the arrangement position of the burnable poison-containing fuel rods in the direction region, the position is averagely separated from the corners of the four corners of the channel box, and the axial region in which the amount of nuclear fuel material is relatively small. The number of fuel rods containing a burnable poison in a radial cross section is the same as the number of the fuel rods containing a burnable poison in a radial cross section in an axial region where the amount of nuclear fuel material is relatively large. I do.

[0020]

In the nuclear fuel assembly according to the present invention, a region having a large amount of nuclear fuel material and a region having a small amount of nuclear fuel material in the axial direction, and a region having a small amount and a large amount of coolant passage area are generated in the axial direction. It appears as

By the way, since the thermal neutron flux is larger near the corners of the four corners of the channel box than in the other radial areas, if many fuel rods containing burnable poisons are arranged near the corners of the four corners, the amount of decrease in reactivity will be large. Become.

Therefore, in the present invention, the arrangement position of the burnable poison-containing fuel rods arranged in the second row from the outer layer is changed in the axial region where the amount of nuclear fuel material is relatively small, in other axial regions. Compared with the position of the burnable poison-containing fuel rods, the position is set at an average distance from the corners at the four corners of the channel box to reduce the decrease in reactivity due to the burnable poison. Thereby, a flat axial power density distribution is obtained, and at any time of the operation cycle, the control rod inserted from below the fuel rod in the axial direction,
It is possible to enhance the safety of the reactor even during scram without causing deterioration of the reactivity characteristics.

[0023]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2A is a radial sectional view showing a nuclear fuel assembly according to the present invention.
Is constructed by arranging and arranging fuel rods in a grid of 9 rows and 9 columns in a channel box 40.

The numerals / symbols 31 to 34 and P, G ₁ , G ₂ , G ₃ in each fuel rod having a circular cross section represent the types of fuel rods having different amounts of nuclear fuel material (uranium) and burnable poisons. And the symbol W represents a water rod.

FIG. 2B shows the nuclear fuel assembly 3
0 long fuel rods 31 to 34 and G ₁ ,
The graph shows the uranium enrichment (amount of nuclear fuel material) in the axial direction of G ₂ , G _{3 and the} short fuel rod P, and the content of gadolinium as a burnable poison. The magnitude of the uranium enrichment a, b, c, d is
a>b>c> d, and the gadolinium content X, Y, Z
Is X>Y> Z.

In the nuclear fuel assembly 30 according to the present embodiment, a total of 66 long fuel rods 31 to 34 and G ₁ , G ₂ , G ₃ , eight short fuel rods P, and water are provided in the channel box 2. It is configured to house two rods W. The long fuel rods 32 have different uranium enrichment in the axial direction. The lower part is c, the central part is a, and the upper part is c. In addition, long fuel rod G
The gadolinium content in ₁ also differs in the axial direction, with X being the lower part, Y being the central part, and Z being the upper part.

[0028] In the present embodiment, the axial length fuel rods P height l ₃ for containing (uranium enrichment b), (from the height l ₁ to a height l ₃₎ the lower part of the fuel assembly region, uranium quantity is larger than the axial region from a height l ₃ the length fuel rods not present up to a height l _6. Conversely, height l ₃
The axial area cross section from l to l ₆ is from height l ₁ to height l
The passage area of the moderator is larger than that of the axial section up to ₃ .

However, in this embodiment, there are long fuel rods G ₂ and G ₃ . Uranium enrichment from a height l ₁ of the elongated fuel rods G ₂ to a height l ₃ is c, uranium enrichment from a height l ₃ to the height l ₅ is a. Moreover, gadolinium content of the long fuel rods G ₂ is, the lower is X, in the region of the central portion from the height l ₂ to a height l ₃ is Y, in other regions burnable poison is contained Absent. That is, in the long fuel rods G _2, in the axial region from the height l ₁ to a height l _3, it is identical to the long fuel rod G _1, l from a height l _3
In the axial region up to _5, it is the same as the long fuel rod 31.

On the other hand, uranium enrichment from a height l ₁ of the elongated fuel rods G ₃ to a height l ₃ is a, the height l ₅ from a height l ₃
The uranium enrichment in the axial region up to is c. The gadolinium content is Z in the upper part, Y in the region from the height l ₃ to the height l ₄ in the center, and no burnable poison in other regions. That is, in the long fuel rods G _3, in contrast to the long fuel rods G _2, the height l ₃ from the height l ₁
In the axial region up to the same as the long fuel rod 31,
In axial region from a height l ₃ to the height l ₅ it has become equal to the long fuel rod G _1.

As described above, the arrangement of the burnable poison-containing fuel rods arranged in the second row from the outer layer of the axial region having a large passage area of the moderator from height l ₃ to height l ₅ is as follows. From the arrangement of the burnable poison-containing fuel rods arranged in the second row from the outer layer of the axial region having a small moderator passage area from height l ₁ to height l ₃ , from the corners at the four corners of the channel box, It will be located at an average distance.

FIG. 1A is a cross-sectional view of an axial region from height l ₃ to height l _5, and shows only the arrangement of the fuel rods containing burnable poisons.

FIG. 1B is a sectional view of an axial region from height l ₁ to height l _3, and shows only the arrangement of the fuel rods containing burnable poisons.

In the examples shown in FIGS. 1A and 1B, of the burnable poison-bearing fuel rods arranged in the second row from the outer layer, the corners at the two corners where no control rod exists on the diagonal line are shown. As for the burnable poison-bearing fuel rods in the vicinity, the arrangement of the burnable poison-bearing fuel rods in FIG. 1 (A) is at an average distance from the corner compared to FIG. 1 (B). Are located. In other words, in the second row from the outer layer in the nuclear fuel assembly, the arrangement position of the burnable poison-containing fuel rods in the axial region where the amount of nuclear fuel material is relatively small is the flammability in other axial regions. Compared to the position of the poison-containing fuel rods, the nuclear design is arranged on the average farther from the corners at the four corners of the channel box.

FIG. 3 shows the neutron infinite multiplication factor of the nuclear fuel assembly in relation to the burnup (a parameter of the burning time) as in FIG.

As is clear from FIG. 3, the nuclear fuel assembly 30 of the present embodiment has a radial cross section (FIG. 2) in which the short fuel rods P do not exist and the amount of nuclear fuel material is small in the first half of combustion.
(B) is a cross section in the region from height l ₃ to l ₆ (upper cross section) also in the radial cross section (height l in FIG. 2 (B)) where the short fuel rods P are present and the amount of nuclear fuel material is large. The cross section of the region from ₁ to l ₃ ; lower cross section) also has almost the same infinite multiplication factor. This is because the arrangement of the burnable poison-containing fuel rods in the axial region where the amount of the nuclear fuel material is small, that is, the axial region where the reactivity decrease amount due to the burnable poison is large because the moderator passage area is large is different from the other regions. This is because the thermal neutron flux is arranged at a position lower than that of the thermal neutron flux to offset the effect.

Therefore, the nuclear fuel assembly 30 of the present embodiment
The power density in the axial direction is always almost flat, including the initial and final stages of the reactor operation cycle, as shown in FIG. 4, because the difference in the combustion rate in the axial direction throughout the reactor operation cycle is small.

Therefore, in this embodiment, even when the scrum occurs at any time during the operation cycle, the reactivity of the control rod inserted from below in the axial direction is excellent, and the safety of the reactor is improved. Will be kept.

In the above-described embodiment, a case has been described in which a nuclear fuel assembly having two types of axial lengths, ie, one type of long fuel rod and one type of short fuel rod, is used. However, even if the fuel assembly has three or more axial lengths, the fuel containing burnable poisons can obtain a uniform axial power density corresponding to the amount of nuclear fuel material in the axial direction. Adjusting the arrangement of the bars is within the scope of the present invention.

[0040]

As described above, in the nuclear fuel assembly of the present invention, by including the short fuel rods, a flat axial power density distribution can be obtained even if the amount of nuclear fuel material varies in the axial direction. At any time in the operation cycle, the reactivity characteristics of the control rod inserted during the scram do not deteriorate, and the safety of the reactor can be improved.

[Brief description of the drawings]

1 (A) and 1 (B) are explanatory diagrams respectively showing the arrangement of burnable poison-containing fuel in radial cross sections at different positions in the axial direction of a nuclear fuel assembly according to one embodiment of the present invention. .

FIG. 2A is a radial cross-sectional view of a nuclear fuel assembly according to one embodiment of the present invention. (B) is a graph showing the amount of nuclear fuel substance and the amount of burnable poison in the axial direction of each fuel rod.

FIG. 3 is a graph showing a relationship between a burnup and a neutron multiplication factor in an upper cross section and a lower cross section of the nuclear fuel assembly according to the present embodiment.

FIG. 4 is a graph showing the axial power densities of the nuclear fuel assembly according to the present embodiment at the beginning and end of the reactor operation cycle.

FIG. 5 is an axial sectional view of a nuclear fuel assembly in which all fuel rods are composed of long fuel rods.

FIG. 6A is an axial sectional view of a long fuel rod. (B) is an axial sectional view of a short fuel rod.

FIG. 7 is an axial sectional view of a nuclear fuel assembly in which fuel rods are composed of long fuel rods and short fuel rods.

8 is a sectional view taken along line VIII-VIII in FIG.

FIG. 9 is a sectional view taken along line IX-IX of FIG. 7;

FIG. 10A is a radial cross-sectional view when a conventional nuclear design is applied to a nuclear fuel assembly having only long fuel rods. (B) is a graph showing the amount of nuclear fuel substance and the amount of burnable poison of each fuel rod in this nuclear design.

FIG. 11 is a graph showing a relationship between burnup and neutron multiplication factor in an upper cross section and a lower cross section of a nuclear fuel assembly to which the nuclear design is applied.

FIG. 12 is a graph showing an axial power density of a nuclear fuel assembly to which the nuclear design is applied at an early stage of a reactor operation cycle.

FIG. 13 is a graph showing the axial power density of a similar nuclear fuel assembly at the end of a reactor operation cycle.

[Explanation of symbols]

Reference Signs List 30 Nuclear fuel assembly 40 Channel box 31 Long fuel rod 32 Long fuel rod 33 Long fuel rod 34 Long fuel rod G ₁ Long fuel rod G ₂ Long fuel rod G ₃ Long fuel rod P Short fuel rod

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G21C 3/328 GDB G21C 3/326 G21C 3/62

Claims (1)

(57) [Claims] A fuel rod containing a nuclear fuel substance and a fuel rod containing a nuclear fuel substance and a burnable poison are bundled in a square grid in a channel box. In a nuclear fuel assembly composed of long fuel rods and short fuel rods having different lengths and having different amounts of nuclear fuel material in the axial direction, the flammable fuel arranged in the second row from the outer layer of the fuel rod array in the channel box is used. In the axial region where the amount of nuclear fuel material is relatively small, the arrangement position of the fuel rod containing the toxic poison is compared with the arrangement position of the burnable poison-containing fuel rod in the axial region where the amount of nuclear fuel material is relatively large. The number of fuel rods containing burnable poisons in the radial cross section in the axial region where the amount of nuclear fuel material is relatively small, with the positions being averagely separated from the corners of the four corners of the channel box, Nuclear fuel assembly, characterized in that the amount of fuel material was equal to the number of fuel rods containing burnable poison in the radial cross section of the relatively large axial region.