JP2003185776A - Method of loading fuel into boiling water reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of loading fuel into a boiling water reactor capable of increasing reactivity gains as an entire operating cycle in the case of an integrated core loaded with a variety of fuel assemblies of different shapes or compositions. <P>SOLUTION: The method of loading fuel into a boiling water reactor, in which not less than two kinds of fuel assemblies 11U and 11M with different reactivity changes caused by combustion are mixedly loaded into the core, includes loading the first fuel assembly 11M with a relatively slow combustion- induced change in reactivity into an area where an average output is relatively higher than core average output density, and loading the second fuel assembly 11U with a relatively steep combustion-induced change in reactivity into an area where the average output is relatively lower than the core average output density. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel loading method for a boiling water reactor (hereinafter referred to as "BWR" as appropriate), and in particular, a plurality of fuel assemblies having different combustion changes in reactivity are combined into one reactor. The present invention relates to a fuel loading method for a nuclear reactor which is mixedly loaded in the core of the reactor.

[0002]

2. Description of the Related Art Generally, as a fuel loaded in a core of a BWR, fuel assemblies having various different shapes and fuel assemblies having different compositions are used. A core loaded with these multiple types of fuel assemblies is called a mixed core.

Prior arts relating to the fuel loading method in such a mixed core include, for example, Japanese Patent Laid-Open No. 2-232595 and Japanese Patent Laid-Open No. 60-262090.

In the boiling water reactor fuel loading method disclosed in Japanese Patent Laid-Open No. 2-232595, a fuel cell adjacent to a control rod to be inserted for a long period of time (for the purpose of improving controllability by the control rod, The uranium fuel assembly is always loaded in the control cell).

Further, in the reactor fuel loading method disclosed in Japanese Patent Laid-Open No. 60-262090, the MOX fuel assembly is used for the purpose of reducing the number of the MOX fuel assemblies as new fuel. A uranium fuel assembly is loaded in the central region of the core in the outer peripheral region of.

By the way, when the shapes of the fuel assemblies are different,
The volume ratio of water to fuel in the fuel assembly may differ. When the water-to-fuel ratio (so-called hydrogen-to-uranium atom number ratio and H / U ratio) is small, the amount of water is generally small at the early stage of combustion, and the neutron moderating effect is insufficient. Is less reactive,
As the combustion progresses, the energy spectrum of the neutron becomes harder, so that Pu is accumulated and the decrease in the reactivity of the fuel assembly is suppressed.

As a result, when comparing a fuel assembly having a low water-to-fuel ratio with a fuel assembly having a high water-to-fuel ratio, the former is a fuel assembly in which the combustion change in reactivity is relatively slow, and the latter is a reaction. The fuel assembly has a relatively steep combustion change.

On the other hand, it is assumed that the composition of the fuel assembly is different from that of uranium fuel, and that it is in the limelight in recent years in connection with the so-called pluthermal project. Plutonium extracted by spent fuel reprocessing was mixed with uranium. Uranium-plutonium mixed oxide fuel (hereinafter referred to as MO
There is X fuel). When this MOX fuel is loaded in the fuel assembly, the thermal neutron absorption cross section of fission nuclides plutonium-239 and plutonium-241 is larger than uranium-235, and the neutron absorption by plutonium-240 is larger than uranium-238. Based on the fact that the energy spectrum of neutron becomes harder, the accumulation of plutonium is promoted as compared with the uranium fuel assembly, and as a result, the decrease in reactivity due to the progress of combustion is suppressed.

Therefore, when the MOX fuel assembly and the uranium fuel assembly are compared, the former is a fuel assembly in which the combustion change in reactivity is relatively slow, and the latter is a fuel assembly in which the combustion change in reactivity is relatively slow. The fuel assembly becomes steep. Especially MO
In order to increase the burnup of the X fuel, it is necessary to increase the reactivity of the fuel, so that the plutonium enrichment of the MOX fuel is increased, and the above tendency is further strengthened.

Based on the above circumstances, when fuel assemblies having different shapes and fuel assemblies having different compositions are mixed and loaded into the core, the combustion changes in the reactivity of the fuel assemblies are mutually affected. Since they are different, various fuel loading methods have been proposed in consideration of those characteristics, and one example is JP-A-63-16292.

The purpose of this prior art is to improve that the reactivity of the MOX fuel assembly is lower than that of the uranium fuel assembly in the early stage of combustion.
The MOX fuel assemblies are loaded in the peripheral region of the core, and the uranium fuel assemblies are loaded in the central region of the core with a high loading rate.

[0012]

However, the above-mentioned Japanese Patent Laid-Open No. 63-16292 has the following problems.

That is, as future technical trends, it is expected that the burnup of the core loaded with uranium fuel will be increased, the mixed loading of different types of fuels (uranium fuel and MOX fuel), and the partial loading of MOX fuel will increase. Therefore, it is becoming more and more necessary to improve the reactivity gain in mixed fuel loading by utilizing the combustion variation characteristic of the reactivity by the fuel assembly as described above.

Here, in general, in the case of a core of a power generation reactor, its purpose is to generate a predetermined amount of energy.
Achieving a certain value is required at the end of the operating cycle. Then, how each individual fuel composing the core bears the burnup to be achieved by the average of the whole core depends on the fuel loading pattern in the core. In order to improve the reactivity gain as described above, it is necessary to consider it as a total reactivity gain in the entire operation cycle.

However, in the above-mentioned Japanese Patent Laid-Open No. 63-16292, when the combustion change characteristic of the reactivity is utilized for improving the reactivity gain, only the reactivity gain in the initial stage of the operation cycle is aimed at, There was room for improvement because no consideration was given to the reactivity gain in the entire driving cycle (especially at the end of the driving cycle).

An object of the present invention is to provide a boiling water reactor capable of improving the reactivity gain in the entire operation cycle in the case of a mixed core loaded with fuel assemblies of various different shapes or compositions. It is to provide a fuel loading method.

[0017]

[Means for Solving the Problems] (1) In order to achieve the above object, the present invention mixes two or more types of fuel assemblies having different combustion changes of reactivity in the core of a boiling water reactor. In a fuel loading method for a boiling water reactor to be loaded, a first fuel assembly having a relatively slow combustion change in reactivity is loaded in a region where the average power is relatively higher than the core average power density,
The second fuel assembly, in which the combustion change in reactivity is relatively sharp,
The area where the average power is relatively lower than the core average power density is loaded.

Generally, in the case of a boiling water reactor core for power generation, the purpose is to generate a predetermined amount of energy, so the burnup, which is the time integral value of energy, has a constant value at the end of the operating cycle. Required. The fuel assembly loaded in the core is designed so that the reactivity becomes exactly 0 at the predetermined burnup at the end of the operating cycle of the entire core average.

At this time, when fuels having different shapes and compositions are mixed and loaded in the core, the combustion behaviors of the fuel assemblies are different from each other, and in the first fuel assembly in which the combustion change of reactivity is relatively slow. At a burnup lower than the predetermined burnup, the reactivity is greater than 0, but gradually decreases as the burnup approaches the predetermined burnup, and the reactivity becomes 0 as described above at the predetermined burnup. After that, the reactivity gradually decreases from 0 as the burnup further increases.

On the other hand, in the second fuel assembly in which the combustion change of the reactivity is relatively steep, the reactivity is much higher than 0 at the burnup lower than the predetermined burnup, but it approaches the predetermined burnup. Along with this, the reactivity decreases sharply, and after the reactivity reaches 0 as described above at the predetermined burnup, the reactivity sharply decreases more than 0 as the burnup further increases.

As a result of the above characteristics, when the burnup is lower than the predetermined burnup, the reactivity of both the first fuel assembly and the second fuel assembly becomes a positive value greater than 0, The absolute value of the two fuel assemblies is larger than that of the first fuel assemblies. Further, when the burnup is higher than the predetermined burnup, the reactivity of both the first fuel assembly and the second fuel assembly becomes a negative value smaller than 0, but the second fuel assembly is Its absolute value is larger than that of one fuel assembly.

Here, how each individual fuel assembly constituting the core bears the above-mentioned predetermined burnup of the entire core depends on the fuel loading pattern in the core. Therefore, by allocating the first and second fuel assemblies to the burnup higher than the predetermined burnup and the burnup lower than the predetermined burnup, respectively, and arranging them in the core, A positive reactivity gain can be obtained by using the behavior of the characteristic line.

That is, the first fuel assembly represented by a gradual downward-sloping characteristic line is loaded in a region where the average power is relatively higher than the average power density of the core, such as the central region of the core, and a sharp downward slope is made. The second fuel assembly represented by the characteristic straight line is loaded in a region where the average power is relatively lower than the core average power density, such as a region around the core where neutron leakage is large.
In this case, the burnup at the end of the operation cycle of the first fuel assembly becomes larger than the above-mentioned predetermined burnup and its reactivity becomes a negative value, but as described above, it has a gradual right-down characteristic. Its absolute value is small. On the other hand, the burnup at the end of the operation cycle of the second fuel assembly becomes smaller than the above-mentioned predetermined burnup, the reactivity becomes a positive value, and the absolute value of the absolute value is obtained because of the sharp right-down characteristic as described above. Is big.

At this time, the reactivity as the core average is
(Core loading ratio of the first fuel assembly) x (reactivity at the final burnup of the first fuel assembly) and (core loading ratio of the second fuel assembly) x (at the final burnup of the second fuel assembly) As described above, (reactivity at the final burnup of the first fuel assembly) is a negative value, and (reactivity at the final burnup of the second fuel assembly) is a positive value. However, as described above, the absolute value of the former is smaller than the absolute value of the latter when comparing absolute values. Therefore,
When viewed from the whole core, the core average reactivity is above the predetermined burnup rather than the effect of the negative reactivity due to the first fuel assembly in which the combustion progressed more than the core average above the predetermined burnup. The effect of the positive reactivity due to the second fuel assembly which has not reached the value becomes larger. As a result, a positive reactivity gain utilizing the difference in combustion behavior as described above can be obtained in the entire operation cycle (particularly at the end of the cycle).

(2) In the above item (1), preferably,
The first fuel assembly is loaded in the central region of the core as an area having a relatively higher average power than the core average power density, and the second fuel assembly has a relative average power from the core average power density. It is loaded in the peripheral region of the core as a relatively low region.

(3) In the above (1) or (2), and preferably, the first fuel assembly comprises a uranium-plutonium mixed oxide fuel, and the second fuel assembly comprises a uranium fuel.

(4) In the above (1) or (2), and preferably, the water-fuel ratio of the first fuel assembly is relatively small, and the water-fuel ratio of the second fuel assembly is relatively small. Large.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

A first embodiment of the present invention will be described with reference to FIGS. In the present embodiment, a uranium fuel assembly is used as a fuel assembly having a relatively steep change in reactivity and a MOX fuel assembly is used as a fuel assembly having a relatively slow change in reactivity as a boiling water atom. It is an embodiment in the case of mixed loading in the reactor core.

FIG. 2 is a vertical cross-sectional view showing the overall structure of the fuel assemblies 11U and 11M loaded by the fuel loading method of this embodiment, and FIG. 3 is a cross-sectional view of the uranium fuel assembly 11U as described later. FIG. 4 is a cross-sectional view of the MOX fuel assembly 11M as described later.

In these FIG. 2 and FIG. 3 or FIG.
The fuel assemblies 11U and 11M of this embodiment include a fuel rod 12, a channel box 13, a water rod 14, and an upper tie plate 1.
5, the lower tie plate 16 and the fuel spacer 17 and the like.

The upper and lower ends of the fuel rod 12 and the water rod 14 are held by an upper tie plate 15 and a lower tie plate 16. Several spacers 17 are arranged in the axial direction of the fuel rods 12 and appropriately maintain the gap between the fuel rods 12 and the water rods 14. The channel box 13 is attached to the upper tie plate 15 and the fuel rods 12 held by the spacers 17
It surrounds the outer circumference of the bundle. Cross-shaped control rod 19
Is adjacent to the channel box 13. The fuel rod 12 is made by filling a large number of fuel pellets in a cladding tube whose both ends are sealed by an upper end plug and a lower end plug. Water rod 14
The cooling water does not fill the fuel substance and does not boil inside. The size of the water rod is fuel rod 7
It is equivalent to the main amount. Short fuel rod (partial length fuel rod) 18
Eight (fuel rod symbol P) are provided in the horizontal direction at the second position from the outer layer in the fuel rod array, including the corner portion.

In FIG. 3, in the fuel assembly 11U, all the fuel pellets constituting the fuel rods 12 are composed of UO2 which is a fuel substance, and contain 235U which is a fission material, but does not contain PuO2. . Some of the fuel pellets are uranium fuel pellets containing gadolinia and are composed of UO2 which is a fuel substance and gadolinia (Gd2O3) which is a burnable poison contained therein. The number of gadolinia-containing uranium fuel rods (fuel rod symbol G) used is twelve.

In FIG. 4, in the fuel assembly 11M, the fuel pellet is composed of a mixed oxide of UO2 and PuO2 which are fuel substances, and 239Pu and 241P which are fission substances.
u and 235U. Some of the fuel pellets are uranium fuel pellets containing gadolinia and are composed of UO2 which is a fuel substance and gadolinia (Gd2O3) which is a burnable poison contained therein. The number of uranium fuel rods with gadolinia (fuel rod symbol G) used is 1
Eight.

FIG. 1 shows the two types of fuel assemblies 11U,
It is a fuel assembly layout drawing which shows one quadrant of the boiling water reactor core (1/4 rotation symmetrical core) comprised using 11M.

In FIG. 1, in this core, the combustion change in reactivity is relatively different from that of the uranium fuel assembly 11U shown in FIG. 3 as the second fuel assembly in which the combustion change in reactivity is relatively sharp. M shown in FIG. 4 as a slow first fuel assembly
The OX fuel assembly 11M is mixed and loaded. More specifically, as shown in FIG. 1, the average output of the uranium fuel assembly 11U is mainly the peripheral side (outer peripheral side) region of the core and the fuel cell (so-called control cell) region adjacent to the control rod for a long period of time. A relatively low region (specifically, a region lower than the core average power density) is loaded, and the MOX fuel assembly 11M is mainly a region on the center side of the core, etc., where the average power is relatively high (specifically, the core average). It is loaded in a region higher than the power density) (however, in this example, the MOX fuel assemblies 11M are also loaded in the outermost circumferential region in some numbers in consideration of the power distribution characteristics).

The numbers in FIG. 1 indicate the number of core stay cycles. For clarity, the uranium fuel assembly 11U is represented by a circle, and the MOX fuel assembly 1M is not enclosed. It is represented by numbers.

Next, the operation of the present embodiment configured as described above will be described below.

FIG. 5 shows the relationship between the reactivity and the burnup in a general boiling water type fuel assembly, with the horizontal axis representing burnup and the vertical axis representing reactivity.

Generally, in the case of the core of a boiling water reactor for power generation, its purpose is to generate a predetermined amount of energy, so the burnup, which is the time integral value of energy, has a constant value at the end of the operating cycle. Required. The point E0 in FIG. 5 represents the burnup at the end of the operating cycle of the entire core average, and the fuel assembly loaded in the core is designed so that the reactivity becomes exactly 0 at this burnup E0.

At this time, when fuels having different shapes and compositions are mixed and loaded into the core, the combustion behavior of each fuel assembly is different from each other. In FIG. 5, a symbol a indicates, for example, the first fuel assembly 11M in which the combustion change in reactivity is relatively slow, and is represented by a relatively gentle straight line to the right passing through E0. At a burnup lower than the above E0, the reactivity is greater than 0, but gradually decreases as it approaches E0, the reactivity becomes 0 at the burnup E0 as described above, and as the burnup exceeds E0 and the burnup increases. The reactivity gradually decreases further than 0.

In FIG. 5, the symbol b represents, for example, the second fuel assembly 11U in which the combustion change in reactivity is relatively steep, and is represented by a relatively steep straight line passing through E0. At a burnup lower than the above E0, the reactivity is much higher than 0, but it rapidly decreases as it approaches E0, the reactivity becomes 0 as described above at the burnup E0, and the burnup increases further beyond E0. As the temperature increases, the reactivity sharply decreases to more than 0.

As a result of the above two characteristic lines, as shown in FIG. 5, when the burnup is lower than E0, the reactivity of either the first fuel assembly 11M or the second fuel assembly 11U is detected. Also becomes a positive value larger than 0, and the absolute value of the second fuel assembly 11U is larger than that of the first fuel assembly 11M. Further, when the burnup is higher than E0, the reactivity of both the first fuel assembly 11M and the second fuel assembly 11U becomes a negative value smaller than 0, but the second fuel assembly 11U is As a result of the absolute value thereof being larger than that of the first fuel assembly 11M, the reactivity of the first fuel assembly 11M becomes larger.

Here, the average burnup E0 of the whole core
How the individual fuel assemblies constituting the core bear the load depends on the fuel loading pattern in the core. A fuel assembly loaded in a high power region such as the central region of the core reaches a higher burnup than a point E0 such as point E2 in Fig. 5, but a low power region such as the peripheral region of the core. The fuel assembly loaded in the fuel cell has a lower burnup than the point E0 as shown by the point E1 in FIG. In other words,
Even under the condition of the burnup E0 at the end of the operating cycle of the average of the entire core, some fuels have a burnup higher than E0 and some have a burnup lower than E0.

Therefore, the two types of fuel assemblies 11 described above are used.
When the M and 11U are arranged so as to be distributed to either a burnup higher than the E0 or a burnup lower than the E0, a positive reactivity gain or a negative reactivity gain occurs depending on the behavior of the characteristic line. It will be.

First, as a comparative example, a second fuel assembly 11U represented by a sharp right-down characteristic straight line is loaded in a high power region of the core, and a first fuel assembly 11M represented by a gentle right-down characteristic straight line. Let us consider the case where is loaded in the low power region of the core. In this case, the burnup at the end of the operation cycle of the second fuel assembly 11U becomes E2, which is larger than E0, and its reactivity K2b becomes a negative value. The value | K2b | is large and K2b is 0.
Much smaller than On the other hand, the burnup at the end of the operation cycle of the first fuel assembly 11M becomes E1 which is smaller than E0, and its reactivity K1a becomes a positive value, but since it has the gradual right-down characteristic as described above, its absolute value is absolute. The value | K1a | is small, and K1a does not become much larger than 0.

At this time, the first fuel assembly 11M and the second fuel assembly 11M
The loading rate in the core of the fuel assembly 11U is W, respectively.
a, Wb (Wa + Wb = 1), the core average burnup E0 =
Since E2 × Wa + E1 × Wb, the reactivity as a core average is represented by K2a × Wa + K1b × Wb. Here, as described above, K2b is a negative value and K1a is a positive value. However, as described above, in comparing absolute values, | K2b |> | K1a |
Has become. Therefore, when viewed from the whole core,
As for the core average reactivity, the effect of the negative reactivity by the second fuel assembly 11U, which has advanced combustion more than the core average burnup E0, is more positive by the first fuel assembly 11M which has not reached the core average burnup E0. Is greater than the effect of
As a result, the reactivity gain becomes negative.

On the other hand, in the present embodiment, the first fuel assembly 11M represented by a gentle downward-sloping characteristic straight line
Is loaded in the high power region of the core, and the second fuel assembly 11U represented by a sharp downward-sloping characteristic line is loaded in the low power region of the core. In this case, the burnup of the first fuel assembly 11M at the end of the operation cycle becomes E2 which is larger than E0 and the reactivity K2a becomes a negative value, but as described above, it has a gentle right-down characteristic. The absolute value | K2a | is small, and K2a does not become much smaller than 0.
On the other hand, the burnup at the end of the operation cycle of the second fuel assembly 11U becomes E1, which is smaller than E0, and its reactivity K1b becomes a positive value. Moreover, as mentioned above, the absolute value of the absolute value is | K1b | is large, and K1b is a value considerably larger than 0.

At this time, the first fuel assembly 11M and the second fuel assembly 11M
The loading rate in the core of the fuel assembly 11U is W, respectively.
a, Wb (Wa + Wb = 1), the core average burnup E0 =
Since E2 × Wa + E1 × Wb, the reactivity as a core average is represented by K2a × Wa + K1b × Wb. Here, as described above, K2a has a negative value and K1b has a positive value. However, as described above, when comparing absolute values, | K2a | <| K1b |
Has become. Therefore, when viewed from the whole core,
The core average reactivity is positive by the second fuel assembly 11U which has not reached the core average burnup E0 due to the effect of the negative reactivity by the first fuel assembly 11M which has burned more than the core average burnup E0. The effect of the reactivity of becomes larger.
As a result, in the present embodiment, it is possible to obtain the positive reactivity gain utilizing the difference in combustion behavior as described above.

Further, if this positive reactivity gain is utilized,
The uranium fuel assembly 11U is loaded into a core of a normal boiling water reactor (not a mixed core loaded with a plurality of fuel assemblies having different shapes or compositions, a core loaded with a single fuel assembly) The average enrichment of the MOX fuel assembly is reduced as compared with the uranium fuel assembly, and the MOX fuel assembly 11M is also loaded in a normal MOX fuel single core.
It can also reduce the average plutonium enrichment compared to fuel assemblies and improve fuel economy.

In a similar way, the positive reactivity gain described above is used to reduce the number of replacements of the uranium fuel assembly 11U at the end of the cycle so that the uranium fuel assembly 11U can be taken out more than when it is loaded in a normal uranium fuel single core. The average burnup is increased, and the MOX fuel assembly 11M has a normal MO
The number of replacements at the end of the cycle is reduced to increase the average burnup of the fuel taken out, compared with the case where the X-fuel single core is loaded.
It can also improve fuel economy.

A second embodiment of the present invention will be described with reference to FIG. 6 and the above-mentioned FIGS. 1 and 3. In this embodiment, the uranium fuel assembly 11 having a relatively large water-to-fuel ratio is used as the second fuel assembly having a relatively steep change in reactivity.
U ′ is a first fuel assembly having a relatively slow change in the reactivity and a uranium fuel assembly 11U having a relatively small water-to-fuel ratio is mixedly loaded in the boiling water reactor core. It is an embodiment.

FIG. 6 is a transverse cross-sectional view showing the structure of the main part of the uranium fuel assembly 11U 'as the second fuel assembly, which corresponds to FIGS. 3 and 4 in the first embodiment. is there.

In FIG. 6, the uranium fuel assembly 11U '
Is similar to the uranium fuel assembly 11U shown in FIG. 3 above, all the fuel pellets are composed of UO2 which is a fuel substance, and contain 235U which is a fission substance.
Does not contain O2. Some of the fuel pellets are uranium fuel pellets with gadolinia, which is the fuel material UO2.
And gadolinia (Gd) which is a burnable poison contained in this
2O3). The number of uranium fuel rods containing gadolinia (fuel rod symbol G) used is 12. No short fuel rods are arranged.

The uranium fuel assembly 11U 'is provided with a so-called rectangular water rod (water channel) 14' as a water rod which is not filled with a fuel substance and through which cooling water which does not boil inside passes. The size of the water rods 14 ′ corresponds to nine fuel rods 12, and the size of the water rods 14 and 14 of the uranium fuel assembly 11U shown in FIG. 3 (for seven fuel rods 12). ) Is larger than.

As can be seen from the above configuration, when the uranium fuel assembly 11U 'and the uranium fuel assembly 11U are mixed and loaded in the core as in the present embodiment, the ratio of water to fuel is relatively large. The uranium fuel assembly 11U 'is a second fuel assembly having a relatively steep combustion change in reactivity, and corresponds to the uranium fuel assembly 11U of the first embodiment described above. On the other hand, the above-mentioned uranium fuel assembly 11U having a relatively small water-to-fuel ratio becomes the first fuel assembly in which the combustion change of the reactivity is relatively slow, and becomes the MOX of the above-described first embodiment.
It corresponds to the fuel assembly 11M. In the present embodiment, based on such a correspondence relationship, in the core arrangement configuration shown in FIG. 1 described above, the uranium fuel assembly 1 of the first embodiment described above is used.
The uranium fuel assembly 11U 'having a relatively large water-to-fuel ratio is arranged at the position where 1U was arranged, and the water pair was arranged at the position where the MOX fuel assembly 11M of the above-described first embodiment was arranged. A uranium fuel assembly 11U having a relatively small fuel ratio is arranged.

Also in this embodiment, the same effect can be obtained based on the same principle as that of the first embodiment.

That is, the first fuel assembly (the uranium fuel assembly 11U in this embodiment) whose combustion characteristic is represented by a straight line descending to the lower right is loaded in the high power region of the core, and is represented by the straight line descending to the right. The second fuel assembly (uranium fuel assembly 11U 'in this embodiment) is loaded in the low power region of the core. As a result, in the same manner as described above, the core average reactivity does not reach the core average burnup more than the effect of the negative reactivity due to the first fuel assembly 11U that has burned more than the core average burnup. The effect of the positive reactivity by the fuel assembly 11U 'becomes larger, and the positive reactivity gain can be obtained.

As described above with reference to FIG. 1, in the first and second embodiments, basically, the second fuel assembly whose combustion change in reactivity is relatively steep is mainly provided around the core. Side (outer peripheral side) region and a fuel cell (so-called control cell) region adjacent to a control rod to be inserted for a long period of time, etc., while the first fuel assembly whose combustion change in reactivity is relatively slow is mainly used in the core. Although it was loaded in the central region and, in exceptional cases, the first fuel assembly was also loaded in the outermost peripheral region in consideration of the power distribution characteristics, it is not limited to this.

That is, in the case of mixed cores loaded with fuel assemblies having various different shapes or compositions, as long as the basic effect of the present invention, that is, the improvement of the reactivity gain over the entire operation cycle, is obtained, It is not necessary to load the outermost peripheral region of the first fuel assembly. In this case,
Since one fuel assembly is loaded only in the central region, a clear boundary is formed with the first fuel assembly in the peripheral region. According to the study by the inventors of the present application, in a large reactor, the power distribution in the central region of the core is nearly flat, and the position of the boundary between the central region and the peripheral region is the range of the peripheral region where the power decreases. It was found that the distance from the center of the core is about 7/10 or more of the core radius. The arrangement shown in FIG. 1 also satisfies this condition, excluding the outermost first fuel assembly.

[0061]

According to the present invention, the first fuel assembly whose combustion change in reactivity is relatively slow is loaded in a region where the average power is relatively higher than the core average power density, and the combustion change in reactivity is changed. Is loaded in a region where the average power is relatively lower than the core average power density. As a result, when viewed from the entire core, the core average reactivity is higher than the predetermined average burnup than the predetermined average burnup due to the negative reactivity of the first fuel assembly that has advanced combustion. The effect of the positive reactivity due to the unreachable second fuel assembly is greater. As a result, a positive reactivity gain utilizing the difference in combustion behavior as described above can be obtained in the entire operation cycle (particularly at the end of the cycle).

[Brief description of drawings]

FIG. 1 is a target to which the first embodiment of the present invention is applied;
It is a layout drawing which shows one quadrant of the boiling water reactor mixed core (1/4 rotation symmetrical core) comprised using the kind of fuel assembly.

FIG. 2 is a vertical cross-sectional view showing the overall structure of a fuel assembly loaded by the fuel loading method according to the first embodiment of the present invention.

FIG. 3 is a cross-sectional view of the uranium fuel assembly shown in FIGS. 1 and 2.

FIG. 4 is a cross-sectional view of the MOX fuel assembly shown in FIGS. 1 and 2.

FIG. 5 is a diagram showing the relationship between reactivity and burnup in a general boiling water fuel assembly, with the horizontal axis representing burnup and the vertical axis representing reactivity.

FIG. 6 is a cross-sectional view showing the structure of a main part of one of the fuel assemblies loaded by the fuel loading method according to the second embodiment of the present invention.

[Explanation of symbols]

11M MOX fuel assembly 11U, U'Uranium fuel assembly 12 fuel rods

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yasushi Hirano             Kanagawa Prefecture Yokosuka City 23-13-1 Kawa             Ceremony company Global Nuclear Fue             Within Le Japan (72) Inventor Shingo Fujimaki             Kanagawa Prefecture Yokosuka City 23-13-1 Kawa             Ceremony company Global Nuclear Fue             Within Le Japan (72) Inventor Manabu Yoshida             Kanagawa Prefecture Yokosuka City 23-13-1 Kawa             Ceremony company Global Nuclear Fue             Within Le Japan

Claims (4)

[Claims] 1. A fuel loading method for a boiling water nuclear reactor in which two or more kinds of fuel assemblies having different burning changes in reactivity are mixedly loaded in the core of a boiling water nuclear reactor, and the burning changes in reactivity are The relatively slow first fuel assembly,
The second fuel assembly, which is loaded in a region where the average power is relatively higher than the core average power density and in which the combustion change in reactivity is relatively sharp,
A fuel loading method for a boiling water reactor characterized by loading in a region where the average power is relatively lower than the core average power density. 2. A fuel loading method for a boiling water reactor according to claim 1, wherein the first fuel assembly is provided in a central region of the core as a region having a relatively higher average power than the core average power density. A fuel loading method for a boiling water nuclear reactor, comprising loading and loading the second fuel assembly in a peripheral side region of a core as a region having an average power relatively lower than the core average power density. 3. A fuel loading method for a boiling water reactor according to claim 1 or 2, wherein the first fuel assembly comprises a uranium-plutonium mixed oxide fuel and the second fuel assembly comprises a uranium fuel. A method for loading fuel into a boiling water reactor, comprising: 4. A fuel loading method for a boiling water reactor according to claim 1 or 2, wherein the water-fuel ratio of the first fuel assembly is relatively small, and the water-fuel of the second fuel assembly is relatively small. Fuel loading method for boiling water reactors, characterized in that the proportion is relatively large.