JPH02259493A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To contrive the effective utilization of plutonium by arranging first fuel rods in the circumference and the inner circumferential part in the neighborhood of large diameter hollow tube equivalent to a plurality of the fuel rods of an arrangement central part and second fuel rods in the other position. CONSTITUTION:Two regions of an assembly periphery part of high thermal neutron flux and the circumferenece of a water channel W are allowed to carry MOX fuel rods P (first fuel rods) and the region of a low thermal neutron flux sandwiched by the assembly circumference part and the circumference of the water channel W UO2 fuel rods U (second fuel rods). The neutrons which the fuel rods P absorb under the arrangement condition are allowed to adopt minimum number of neutron generation <239>Pu from lower energy than a resonance absorption energy region and a neutron rate decelerated down to a further lower energy region than a energy region which is smaller than that of a iotavalue of <235>U enlarges. Accordingly, the fuel rods P are allowed to escape the resonance absorption of <239>Pu and <240>Pu and furthermore an advantage in which a iota value of <239>Pu is larger than that of <235>U can be employed efficiently.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a fuel assembly for a boiling water reactor (BWR), and more specifically relates to a fuel assembly for a boiling water reactor (BWR), and more specifically, a fuel assembly for a uranium dioxide fuel rod and a plutonium-enriched fuel rod. This study concerns the effective use of plutonium in fuel assemblies containing plutonium.

[Prior art] Figure 4 shows an 8×
A cross-sectional view of a type 8 fuel assembly loaded into a reactor core is shown.

In the figure, an 8×8 type fuel assembly, designated as a whole by reference numeral 80, has a total of 62 wooden fuel rods 81, and 2 wooden hollow tubular rods (hollow tubes) that do not contain fuel material inside and are hollow and allow cooling water to flow through them. elements) wr are arranged in a square grid of 8 rows and 8 columns, and the outer periphery is surrounded by a channel box 10 made of Zircaloy. Here, the nuclear fuel loaded into the fuel rods 81 will be described later.

The 8×8 type fuel assembly 80 configured in this way is loaded with two sides adjacent to the control plate (control blade) la loaded with the neutron absorbing material of the cruciform control rod 1, as shown in the figure.

During power operation of the nuclear reactor, cooling water that also serves as a moderator flows from the bottom of the fuel toward the top, removing heat generated by the fuel rods 81. At this time, voids do not occur in the bypass area B outside the channel box 10 (the water gap area where the control rod 1, instrumentation tube i, etc. are arranged), but the cooling water flow path inside the channel box 10 (inch channel area )■ voids occur.

Therefore, the density distribution of the cooling water (moderator) in the cross section as shown in FIG. 4 is not uniform, and the density is high in the bypass region B, and low in the inch-thread channel region. Therefore, the thermal neutron flux distribution within the cross section of the fuel assembly 80 is also not uniform. In other words, the thermal neutron flux is high at the periphery of the assembly facing the bypass region B, where the moderator density is high, and conversely, at the center of the assembly, where the moderator density is low, the thermal neutron flux is low and the average energy of neutrons is high. There is.

Next, a conventional example in which the 8×8 type fuel 80 as described above is loaded with plutonium-enriched fuel will be described. In the following description, uranium-plutonium mixed oxide (MOX) fuel will be used as an example of plutonium-enriched fuel.

The 8X8 type fuel 80 in FIG. 4 also shows the fuel rod arrangement of the Atoll type fuel assembly disclosed in, for example, Japanese Patent Application Laid-Open No. 60-242391.

In the fuel rod 81 in Figure 4, the symbol P written in a circle indicates a fuel rod loaded with MOX fuel as nuclear fuel (hereinafter referred to as "MOX fuel rod"), and the symbol U is similarly written. 2 indicates a fuel rod (hereinafter referred to as a UO2 fuel rod) loaded with uranium dioxide (UO2) fuel, which mainly uses low-enriched uranium as nuclear fuel.

In this example of the Atoll type fuel assembly 80, a total of 32 MOX fuel rods are arranged in a relatively peripheral area of the assembly, and a total of 30 U02 fuel rods are arranged in other areas (relatively in the center of the assembly).
Fuel rods are located.

Therefore, the ratio of MOX fuel rods to all fuel rods per assembly is (32/62) x 100% = 52%, which accounts for about half.

In addition, as another conventional example, MOX fuel rods are arranged only in a part of the center area of the assembly, and U0
An island type fuel assembly in which two fuel rods are arranged and a discrete type fuel assembly in which all fuel rods are MOX fuel rods are known.

[Problem to be solved by the invention] Among the conventional examples mentioned above, in the island type fuel, MO
Since the X fuel rod is arranged far from the control rod, it is possible to prevent a decrease in the reactivity control effect (control rod value) of the control rod, which will be described later. However, since the MOX fuel rods are arranged only in a part of the center of the assembly, the amount of plutonium loaded per assembly is reduced as a result. Therefore, it does not necessarily meet the recent demand for plu-thermal utilization, which is an attempt to actively utilize plutonium in thermal neutron reactors.

On the other hand, although a large amount of plutonium can be loaded in the discrete type fuel or the Atoll type fuel 80, the value of the control rod decreases because the MOX fuel rod is located near the control rod. This is because plutonium has a larger thermal neutron absorption cross section than uranium, resulting in a smaller thermal neutron flux.

Furthermore, in island-type fuels and discrete-type fuels, MOX fuel rods are placed in the center of the assembly, but as described in the prior art above, the average energy of neutrons in the center of the assembly, where moderator density is low, is high. Such an arrangement is unfavorable for effective use of plutonium because neutron economics deteriorates.

The present invention has been made in view of the problems of the prior art described above, and its purpose is to provide a fuel assembly for BWR that can effectively utilize plutonium without deteriorating neutron economy. It is. Further, in this BWR fuel assembly, it is also part of the object of the present invention to prevent a decrease in the value of the control rods.

[Means for Solving the Problems] A BWR fuel assembly according to the present invention includes a hollow tubular element filled with a moderator and a plurality of first fuels loaded with plutonium-enriched fuel as unburned nuclear fuel. A rod and a plurality of second fuel rods loaded with uranium dioxide fuel as unburned nuclear fuel are arranged in a square grid of 9 rows and 9 columns and the outer periphery is surrounded by a channel box, wherein the hollow As the tubular element, one or more large-diameter hollow tubular elements such as one or more large-diameter quarterronds or large-diameter two-channels corresponding to the plurality of fuel rods in the central part of the array are used, and the channels in the array are The above problem is solved by arranging the first fuel rods in the peripheral part adjacent to the box and the inner peripheral part adjacent to the large-diameter hollow tubular element, and arranging the second fuel rods in other parts. This has been achieved.

In this case, among the first fuel rods arranged in the peripheral part, the first fuel rods are arranged in a row or column facing a control plate of a cruciform control rod inserted outside the channel box in the peripheral part. A configuration may be adopted in which part or all of the first fuel rods are replaced with the second fuel rods.

Furthermore, the plutonium-enriched fuel loaded into the first fuel rod is typically a uranium-plutonium mixed oxide fuel.

In addition, "unburnt nuclear fuel" means the nuclear fuel loaded during the manufacture of the first and second fuel rods, and the types of fissile material produced as these fuel rods burn , shall not be particularly limited. Furthermore, the addition of burnable poisons, etc. to nuclear fuel is also optional.

[Function] Nuclear properties of uranium and plutonium Prior to a detailed explanation of the present invention, the nuclear properties of UO2 fuel and MOX fuel will be described.

FIG. 5 graphically shows the characteristics of isotopes contained in Uo2 fuel and MOX fuel.

In the figure, each graph (a), (b), (c)
The horizontal axis is the neutron energy, and the vertical glaze is the nuclear fission cross section.

Absorption cross section! The number of neutrons generated per # neutron absorption (η) is taken.

Among the plutonium isotopes contained in MOX fuel, the fissile materials that cause nuclear fission by thermal neutrons are 2!9 pu and 241 pu, but as is clear from Figure 5,
The fission cross section and η value of 23 e p u are larger than those of 235U contained in UO2 fuel. Therefore, when comparing fuels that have experienced combustion with the same burnup in the same core (in other words, the same amount of fission), under the optimal deceleration condition, MOX fuel has a larger infinite multiplication factor than UO2 fuel. There are advantages that can be obtained.

On the other hand, 240 p u does not cause nuclear fission due to thermal neutrons, and as is clear from the graph (b) in Figure 5, it is in the energy region (
Below, ″°resonant absorption energy region°°.

).

In addition, since 239 p u also has a resonance absorption energy region around 0.3 eV, the η value of 239 p u is
There it drops sharply.

9x9 Type Fuel Assembly Next, the type of fuel assembly to which the present invention is applied and its thermal neutron flux will be described.

In recent years, high burnup fuel (for example, an average discharge burnup of 30 to 32 G w
d/t to 45 Gw d/t), a 9x9 type fuel assembly is being developed. An example of the cross-sectional configuration of this 9×9 type fuel is shown in FIG. In this type of 9x9 type fuel, the above 8x8 type fuel 80
Since the number of fuel rods is increased compared to that in the previous example, the average linear power density per fuel rod can be reduced. Furthermore, the resulting margin can be used to increase the area of the non-boiling portion. That is, it is possible to increase the diameter of the hollow tubular element through which non-boiling water passes. For example, the 9×9 type fuel 9 shown in FIG.
0, in addition to a total of 72 fuel rods 91, a rectangular water channel W occupying a 3×3 area in the center of the assembly is adopted. In addition, a configuration example has been proposed in which one or more large-diameter water rods, which are obtained by enlarging the diameter of the water rod wr, are provided.

In the following explanation, the term 9x9 type fuel simply refers to a 9x9 type fuel equipped with this kind of large-diameter hollow tubular element, and the large-diameter hollow tubular element is also referred to as a water channel. This will be explained using W as a representative example.

Now, in the 9X9 type fuel, by employing the water channel W, the water density at the center of the assembly can be increased, and the thermal neutron flux distribution is also improved accordingly. As an example, Figure 7 shows the thermal neutron flux distribution in the AA' direction of the cross section of the assembly in Figure 6. The thermal neutron flux is relatively high in the region around the channel W, and the thermal neutron flux is relatively low in the region sandwiched between the two.

Trouble IL Go! In the fuel assembly according to the present invention, the placement positions of the first and second fuel rods are determined by focusing on the nuclear properties of uranium and plutonium described above and the thermal neutron flux distribution in the 9x9 type fuel. decide. That is, MOX fuel rods (
A U2O2 fuel rod (second fuel rod) is placed in a low thermal neutron flux region sandwiched between the periphery of the assembly and the quarter channel W.

Considering the neutrons absorbed by the MOX fuel rod under this arrangement condition, it falls into the following energy range (A) to (B).
The proportion of neutrons slowed down to

(^) An energy region lower than the above resonance absorption energy region.

(B) The η value of 239 p u takes the minimum value, and 23
An energy region even lower than the energy region smaller than the η value of 5U.

Therefore, the MOX fuel rod under the above arrangement condition is 239
can escape the resonance absorption of p u and 240 pu,
Moreover, it is possible to take advantage of the fact that the η value of 23' l p u is larger than that of 235U.

On the other hand, the UO□ fuel rod arrangement area under the above arrangement conditions is
It is unsuitable for locating MOX fuel rods. This is because the thermal neutron flux in this region is low and the neutron energy is high, so there is insufficient moderator for MOX fuel. Therefore, in the present invention, UO2 fuel rods are arranged in this area.

As described above, neutron economy can be improved by arranging MOX fuel rods in regions where thermal neutron flux is relatively high and arranging 002 fuel rods in regions where thermal neutron flux is relatively low.

In this case, since the MOX fuel rods are arranged both around the assembly and around the water channel, the amount of plutonium loaded per fuel assembly can be increased.

However, if the MOX fuel rods are placed in the area facing the control plate 1a of the control rods 1 in the periphery of the assembly (corresponding to the first row and first column shown in Figure 6), the thermal neutron absorption cross section of plutonium will be Since it is larger than uranium, its thermal neutron flux is smaller. Therefore, the value of the control rod is somewhat lower than when the U02 fuel rod is placed at this location. In order to take this measure, some or all of the MOX fuel rods placed in the area facing the control board 1a are replaced with UO2 fuel rods. This makes it possible to suppress the decline in control rod value.

By the way, in the above explanation, plutonium-enriched fuel is
Although the description has been made using the X fuel as a representative, the characteristics of the MOX fuel with respect to the U02 fuel are due to the difference in the nuclear properties of plutonium and uranium, as described above. Therefore, plutonium-enriched fuels other than MOX fuel also have the same characteristics as described above. Therefore, the first fuel rod in the present invention is not limited to the MOX fuel rod, and it is also possible to use other types of plutonium-enriched fuel.

However, the 32 wood fuel rods 91 arranged in the area around the 9 sides of the assembly and the 16 wood fuel rods 91 arranged in the area around the water channel W are all MOX fuel rods. on the other hand,
The 24 fuel rods 91 placed between these three areas are
This is a UO2 fuel rod.

M occupied by all fuel rods per 90A of this 9×9 type fuel
The ratio of OX fuel rods is as high as ((32+16)/72)x100%=67%. Therefore, compared to the conventional island type fuel or atole type fuel 80, the amount of plutonium loaded can be significantly increased.

Embodiment A FIG. 1 shows a design example of a 9X9 type fuel assembly according to an embodiment of the present invention.

In the figure, the general structure of the 9x9 type fuel, which is indicated by the reference numeral 90A as a whole, is the same as the 9x9 type fuel 90 shown in FIG. 6 above.

FIG. 2 also shows a design example of a 9×9 type fuel assembly according to another embodiment of the present invention.

In the 9x9 type fuel 90B shown in the figure, fuel rods 91 arranged in rows and columns facing the control plate 1a of the control rods 1 at the periphery of the assembly are fueled with UO2 fuel except for two corner rods C1 and C2. It is a stick.

As mentioned in the section of the effect, by adopting the configuration shown in Fig. 2, it is possible to suppress the decline in control rod value. For comparison in which the disadvantage of reduction is eliminated, the control rod values in the design examples of Examples A and B are shown in FIG.

In the figure, solid line A is Example A (9X9 type fuel 90A)
, dotted line B is the control rod value of Example B (9X9 type fuel 90B). Comparing Examples A and B, Example A is more advantageous in terms of plutonium loading per assembly, but Example B is more advantageous in terms of control rod value.

Incidentally, in each of the above embodiments, a 3×3 type ota-channel W of -wood is shown as a large-diameter hollow tubular element surrounded by MOX fuel rods at the center of the assembly, but instead of this, a part or a plurality of A large-diameter autoroller may also be used.

[Effects of the Invention] As explained above, according to the BWR fuel assembly according to the present invention, plutonium-enriched fuel rods are provided around the large-diameter hollow tubular element and the periphery of the fuel assembly where the thermal neutron flux is relatively high. By arranging it, plutonium can be used effectively without deteriorating neutron economy.

On the other hand, if the plutonium-enriched fuel rods arranged in the rows or columns facing the control rods are replaced with uranium fuel rods, it is possible to suppress the decline in the value of the control rods due to the use of plutonium-enriched fuel rods. .

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view of a 9x9 type fuel according to one embodiment of the present invention, Fig. 2 is a cross-sectional view of a 9x9 type fuel according to another embodiment, and Fig. 3 is a cross-sectional view of a 9x9 type fuel according to another embodiment. Diagram showing the control rod value; Figure 4 is a cross-sectional view showing the conventional Atoll type 8x8 type fuel loaded in the reactor core; Figure 5 (a)
, (b) and (c) are diagrams showing the relationship between the fission cross section, absorption cross section, and number of neutrons generated per unit neutron absorption (η) with respect to neutron energy in uranium and plutonium, and Fig. 6 is a diagram to which the present invention is applied. FIG. 7 is a cross-sectional view for explaining the structure of the 9×9 type fuel, and FIG. 7 is a diagram showing the thermal neutron flux distribution in the previous figure. [Explanation of symbols of main parts...Cross-shaped control rod 1a...Control board 10...Channel boxes 90A, 90B...9x9 type fuel 91... ...
Fuel rod P... MOX fuel rod (first fuel rod) U...
...UO2 fuel rod (second fuel rod) W...
・・3×3 type two-way channel (large-diameter hollow tubular element) In each figure, the same reference numerals indicate the same or equivalent parts. Agent: Patent Attorney Masaru Sato

Claims (3)

[Claims] (1) A hollow tubular element filled with a moderator, a plurality of first fuel rods loaded with plutonium-enriched fuel as unburned nuclear fuel, and a plurality of first fuel rods loaded with uranium dioxide fuel as unburned nuclear fuel. In a fuel assembly formed by arranging second fuel rods in a square lattice of 9 rows and 9 columns and surrounding the outer periphery with a channel box, the hollow tubular element includes a plurality of fuel rods in the center of the arrangement. one or more large diameter hollow tubular elements corresponding to one or more large diameter hollow tubular elements, and a peripheral portion adjacent to the channel box in the array and an inner peripheral portion adjacent to the large diameter hollow tubular elements are provided with the first fuel rods. A fuel assembly for a boiling water reactor, characterized in that the second fuel rod is arranged in another part. (2) In the boiling water reactor fuel assembly according to claim 1, among the first fuel rods arranged in the peripheral portion,
Part or all of the first fuel rods arranged in rows or columns facing the control plate of the cross-shaped control rods inserted outside the channel box are replaced with the second fuel rods. Fuel assembly for boiling water reactors. (3) The boiling water reactor fuel according to claim 1 or 2, wherein the plutonium-enriched fuel loaded into the first fuel rod is a uranium-plutonium mixed oxide fuel. Aggregation.