JP3894784B2 - Fuel loading method for boiling water reactor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel loading method for a boiling water reactor (hereinafter referred to as “BWR” where appropriate), and in particular, a plurality of fuel assemblies having different combustion changes in reactivity are mixed and loaded in a reactor core. The present invention relates to a fuel loading method for a nuclear reactor.
[0002]
[Prior art]
In general, the fuel loaded into the BWR core is a fuel assembly having various different shapes and fuel assemblies having different compositions. A core loaded with these multiple types of fuel assemblies is called a mixed core.
[0003]
As a conventional technique related to a fuel loading method in such a mixed core, for example,
There are JP-A-2-232595 and JP-A-60-262090.
[0004]
In the boiling water reactor fuel loading method described in JP-A-2-232595, a fuel cell (hereinafter referred to as a control cell) adjacent to a control rod to be inserted for a long period of time is intended to improve controllability by the control rod. A uranium fuel assembly is always loaded to the cell.
[0005]
Further, in the nuclear reactor fuel loading method described in Japanese Patent Application Laid-Open No. 60-262090, the MOX fuel assembly is disposed in the outer peripheral region of the core for the purpose of reducing the number of loaded bodies as a new fuel of the MOX fuel assembly. In addition, uranium fuel assemblies are loaded in the central region of the core.
[0006]
By the way, if the shape of the fuel assembly is different, the volume ratio of water and fuel in the fuel assembly may be different. When the water-to-fuel ratio (so-called hydrogen to uranium atom ratio, almost corresponding to the H / U ratio) is small, the amount of water is generally small at the initial stage of combustion, and the neutron moderating effect is insufficient. However, since the energy spectrum of neutrons becomes harder as combustion progresses, Pu is accumulated, and a decrease in reactivity as a fuel assembly is suppressed.
[0007]
As a result, when comparing a fuel assembly with a low water-to-fuel ratio and a fuel assembly with a large water-to-fuel ratio, the former is a fuel assembly with a relatively slow change in reactivity, and the latter is a combustion with reactivity. The change is a relatively steep fuel assembly.
[0008]
On the other hand, the composition of the fuel assembly is different from that of uranium fuel, and uranium / plutonium in which plutonium extracted from spent fuel reprocessing is mixed with uranium is recently attracting attention in connection with the so-called pull thermal project. There is a mixed oxide fuel (hereinafter referred to as MOX fuel as appropriate). When this MOX fuel is loaded into the fuel assembly, the thermal neutron absorption cross sections of the fission nuclides plutonium-239 and plutonium-241 are 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 neutrons becomes harder, the accumulation of plutonium is promoted as compared with the uranium fuel assembly, and as a result, the decrease in reactivity accompanying the progress of combustion is suppressed.
[0009]
Therefore, when comparing MOX fuel assemblies with uranium fuel assemblies, the former is a fuel assembly with a relatively slow change in reactivity, and the latter is a fuel with a relatively sharp change in reactivity. It becomes an aggregate. In particular, when increasing the burnup of MOX fuel, it is necessary to increase the reactivity of the fuel, which increases the plutonium enrichment of the MOX fuel, thus further strengthening the above tendency. .
[0010]
Based on the above circumstances, when fuel assemblies of different shapes and fuel assemblies of different compositions are mixed and loaded into the core, the combustion changes in the reactivity of the fuel assemblies are different from each other. Various fuel loading methods that take these characteristics into consideration have been proposed. For example, Japanese Patent Laid-Open No. 63-16292 is available.
[0011]
In this prior art, in order to improve that the reactivity of the MOX fuel assembly is lower than the reactivity of the uranium fuel assembly in the early stage of combustion, the MOX fuel assembly is placed in the peripheral region of the reactor core. The fuel assembly is loaded in the central region of the core at a high loading rate.
[0012]
[Problems to be solved by the invention]
However, the following problems exist in the above Japanese Patent Laid-Open No. 63-16292.
[0013]
In other words, as future technological trends, it is expected that the uranium fuel loaded core will have higher burnup, mixed loading of different types of fuels (uranium fuel and MOX fuel), and even partial loading of MOX fuel. There is a growing need to improve the reactivity gain at the time of mixed fuel loading by utilizing the combustion change characteristic of the reactivity due to the fuel assembly as described above.
[0014]
Here, in general, in the case of the core of a power generation nuclear reactor, the purpose is to generate a predetermined energy, so that a certain value can be achieved at the end of the operation cycle with respect to the burnup which is a time integral value of energy. Required. The manner in which the individual fuels constituting the core bear the burnup that should be achieved for the overall average of the core depends on the fuel loading pattern in the core. Even in the case of improving the reactivity gain as described above, it is necessary to consider the total reactivity gain as seen in the entire operation cycle.
[0015]
However, in the above Japanese Patent Laid-Open No. 63-16292, when utilizing the combustion change characteristic of the reactivity to improve the reactivity gain, only the reactivity gain at the initial stage of the operation cycle is intended, The reactivity gain as seen in the end of the driving cycle was not considered and there was room for improvement.
[0016]
An object of the present invention is to provide a fuel loading method for a boiling water reactor capable of improving the reactivity gain as seen in the entire operation cycle in the case of a mixed core loaded with fuel assemblies of various different shapes or compositions Is to provide.
[0017]
[Means for Solving the Problems]
(1) In order to achieve the above object, the present invention provides a fuel for a boiling water reactor in which two or more types of fuel assemblies having different combustion changes in reactivity are mixed and loaded in the core of the boiling water reactor. In the loading method, the first fuel assembly, which has a relatively slow change in reactivity, is loaded mainly in the central region of the core where the average power is relatively higher than the core average power density. The second fuel assembly having a relatively steep combustion change is loaded mainly in a peripheral region of the core, which is a region having an average power relatively lower than the core average power density, and the first fuel assembly is uranium / plutonium. The fuel assembly includes a mixed oxide fuel, and the second fuel assembly is a fuel assembly including uranium fuel.
[0018]
In general, in the case of the core of a boiling water reactor for power generation, the purpose is to generate a predetermined energy, so that the burnup, which is the time integral value of energy, needs to be a constant value at the end of the operation cycle. . The fuel assembly loaded in the core is designed so that the reactivity is exactly 0 at the predetermined burnup at the end of the average operation cycle of the entire core.
[0019]
At this time, when fuels having different shapes and compositions are mixed and loaded into the core, the combustion behaviors of the fuel assemblies are different from each other, and the first fuel assembly in which the change in the combustion of the reactivity is relatively slow is described above. Although the reactivity is greater than 0 at a burnup lower than the above burnup, it gradually decreases as the predetermined burnup approaches, and further burns after the reactivity reaches 0 as described above at the predetermined burnup. As the degree increases, the reactivity gradually decreases from zero.
[0020]
On the other hand, in the second fuel assembly in which the change in the combustion of the reactivity is relatively steep, the reactivity is much higher than 0 at a burnup lower than the predetermined burnup, but suddenly approaches the predetermined burnup. After the reaction rate becomes zero as described above at the predetermined burn-up rate, as the burn-up rate further increases, the reactivity sharply decreases to greater than 0.
[0021]
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 is a positive value greater than 0, and the second fuel set The absolute value of the body is greater than that of the first fuel assembly. When the burnup is higher than the predetermined burnup, the reactivity of both the first fuel assembly and the second fuel assembly is a negative value smaller than 0, but the second fuel assembly is the first. Its absolute value is larger than that of one fuel assembly.
[0022]
Here, how the individual fuel assemblies composing the core bear the predetermined burnup of the entire core average burnup depends on the fuel loading pattern in the core. Therefore, by arranging the first and second fuel assemblies in the core so as to distribute and bear each of the burnup higher than the predetermined burnup and the burnup lower than the predetermined burnup. A positive reactivity gain can be obtained by utilizing the behavior of the characteristic line.
[0023]
That is, the first fuel assembly represented by a gentle 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, for example. The represented second fuel assembly 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 is greater than the predetermined burnup and the reactivity becomes a negative value. 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 predetermined burnup, and the reactivity becomes a positive value. Is big.
[0024]
At this time, the reactivity as the average of the cores is: (core loading ratio of the first fuel assembly) × (reactivity in the final burnup of the first fuel assembly) and (core loading ratio of the second fuel assembly) × (Reactivity at the final burnup of the second fuel assembly), and as described above, (Reactivity at the final burnup of the first fuel assembly) is a negative value, The reactivity at the final burnup) is a positive value, but as described above, the absolute value of the former is smaller than the absolute value of the latter in the comparison between the absolute values. Therefore, when viewed in the entire core, the core average reactivity is higher than the predetermined average burnup than the predetermined fuel burn-up effect of the first fuel assembly burned more than the predetermined average burnup. The effect of the positive reactivity due to the second fuel assembly that has not reached the burnup is greater. As a result, it is possible to obtain a positive reactivity gain using the difference in the combustion behavior as described above as seen in the entire operation cycle (particularly at the end of the cycle).
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0029]
A first embodiment of the present invention will be described with reference to FIGS. In this embodiment, a uranium fuel assembly is used as a fuel assembly with a relatively steep change in reactivity and a MOX fuel assembly is used as a boiling water atom as a fuel assembly with a relatively slow change in reactivity. This is an embodiment in the case of mixed loading in the reactor core.
[0030]
FIG. 2 is a longitudinal sectional view showing the overall structure of the fuel assemblies 11U and 11M loaded by the fuel loading method of the present embodiment, and FIG. 3 is a transverse sectional view of the uranium fuel assembly 11U as will be described later. 4 is a cross-sectional view of the MOX fuel assembly 11M as will be described later.
[0031]
2, 3, or 4, the fuel assemblies 11U and 11M of the present embodiment include a fuel rod 12, a channel box 13, a water rod 14, an upper tie plate 15, a lower tie plate 16, a fuel spacer 17, and the like. It is composed of
[0032]
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 rod 12 and appropriately maintain a gap between the fuel rod 12 and the water rod 14. The channel box 13 is attached to the upper tie plate 15 and surrounds the outer periphery of the bundle of fuel rods 12 held by the spacer 17. A control rod 19 having a cross-shaped cross section is adjacent to the channel box 13. The fuel rod 12 is one in which a large number of fuel pellets are filled in a cladding tube sealed at both ends by an upper end plug and a lower end plug. The water rod 14 is not filled with fuel material, and cooling water that does not boil inside passes through. The size of the water rod is equivalent to seven fuel rods. The short fuel rods (partial fuel rods) 18 (fuel rod symbol P) are provided with eight horizontal positions including the corner portion in the second layer from the outer layer in the fuel rod array.
[0033]
In FIG. 3, in the fuel assembly 11U, all fuel pellets constituting the fuel rod 12 are composed of UO2 that is a fuel material, and contain 235U that is a fission material, but do not contain PuO2. Some fuel pellets are composed of gadolinia-filled uranium fuel pellets made of UO2 as a fuel material and gadolinia (Gd2O3) as a combustible poison contained therein. The number of uranium fuel rods with gadolinia used (fuel rod symbol G) is twelve.
[0034]
In FIG. 4, the fuel assembly 11M is composed of a mixed oxide of UO2 and PuO2 whose fuel pellets are fuel materials, and contains 239Pu, 241Pu and 235U which are fission materials. Some fuel pellets are composed of gadolinia-filled uranium fuel pellets made of UO2 as a fuel material and gadolinia (Gd2O3) as a combustible poison contained therein. The number of gadolinia-filled uranium fuel rods (fuel rod symbol G) used is 18.
[0035]
FIG. 1 is a fuel assembly layout diagram showing one quadrant of a boiling water reactor core (1/4 rotationally symmetric core) constructed using the two types of fuel assemblies 11U and 11M.
[0036]
In FIG. 1, in this core, the uranium fuel assembly 11U shown in FIG. 3 as the second fuel assembly having a relatively steep change in reactivity and the second fuel assembly having a relatively slow change in reactivity. The MOX fuel assembly 11M shown in FIG. 4 as one fuel assembly is mixedly loaded. Specifically, as shown in FIG. 1, the average output of the uranium fuel assembly 11U mainly in the peripheral side (outer peripheral side) region of the core and the fuel cell (so-called control cell) region adjacent to the control rod inserted for a long period of time is A relatively low region (specifically, a region lower than the core average power density) is loaded, and the MOX fuel assembly 11M is a region where the average power is relatively high, such as a central region of the core (specifically, the core average) (In this example, the MOX fuel assembly 11M is loaded in the outermost peripheral region slightly in consideration of the power distribution characteristics).
[0037]
The numbers in FIG. 1 indicate the number of core stay cycles. For the sake of clarity, the uranium fuel assembly 11U is represented by a circled number, and the MOX fuel assembly 1M is represented by a blank number. ing.
[0038]
Next, the operation of the present embodiment configured as described above will be described below.
[0039]
FIG. 5 shows the relationship between reactivity and burnup in a general boiling water fuel assembly, with the burnup on the horizontal axis and the reactivity on the vertical axis.
[0040]
In general, in the case of the core of a boiling water reactor for power generation, the purpose is to generate a predetermined energy, so that the burnup, which is the time integral value of energy, needs to be a constant value at the end of the operation cycle. . The point E0 in FIG. 5 represents the burnup at the end of the operation cycle of the average of the entire core, and the fuel assembly loaded in the core is designed so that the reactivity is exactly 0 at the burnup E0.
[0041]
At this time, when fuels having different shapes and compositions are mixed and loaded into the core, the combustion behaviors of the fuel assemblies are different from each other. In FIG. 5, the symbol a indicates, for example, the first fuel assembly 11M in which the change in the combustion of the reactivity is relatively slow, and is represented by a relatively gentle right-downward straight line passing through E0. At a burnup lower than E0, the reactivity is greater than 0, but gradually decreases as E0 is approached. At the burnup E0, the reactivity becomes 0 as described above, and further exceeds E0 and the burnup increases. The degree of reactivity gradually decreases further than zero.
[0042]
In FIG. 5, symbol b represents, for example, the second fuel assembly 11U having a relatively steep change in reactivity, and is represented by a relatively steep right-downward straight line passing through E0. At a burnup lower than E0, the reactivity is much higher than 0, but it decreases rapidly as it approaches E0, and at burnup E0, the reactivity becomes 0 as described above, and the burnup increases beyond E0. As it goes on, the reactivity decreases sharply more than zero.
[0043]
As a result of the two characteristic lines as described above, as shown in FIG. 5, when the burnup is lower than E0, the reactivity of both the first fuel assembly 11M and the second fuel assembly 11U is less than 0. It becomes a large positive value, and the absolute value of the second fuel assembly 11U is larger than that of the first fuel assembly 11M. When the burnup is higher than E0, the reactivity of each of the first fuel assembly 11M and the second fuel assembly 11U is a negative value smaller than 0, but the second fuel assembly 11U has the first value. As a result of the absolute value being greater than that of the one fuel assembly 11M, the reactivity of the first fuel assembly 11M is greater.
[0044]
Here, how the individual fuel assemblies constituting the core bear the burnup ratio E0 of the entire core average 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 burnup higher than a point E0 such as point E2 in FIG. 5, but a low power region such as the peripheral region of the core. Thus, the fuel assembly loaded on the vehicle stays at a burnup lower than the point E0 as indicated by a point E1 in FIG. In other words, even under the condition of the burnup rate E0 at the end of the operation cycle of the entire core average, there are fuels with a burnup higher than E0 and fuels with a low burnup.
[0045]
Therefore, when each of the two types of fuel assemblies 11M and 11U described above is allocated to either a burnup higher than the E0 or a burnup lower than the E0, a positive reaction occurs depending on the behavior of the characteristic line. Degree gain or negative reactivity gain will occur.
[0046]
First, as a comparative example, the second fuel assembly 11U represented by a sharp right-down characteristic line is loaded in the high power region of the core, and the first fuel assembly 11M represented by a gentle right-down characteristic line is loaded into the core. Consider the case of loading in a low output area. In this case, the burnup at the end of the operation cycle of the second fuel assembly 11U becomes E2 greater than E0, and the reactivity K2b becomes a negative value. The value | K2b | is large and K2b is considerably smaller than zero. On the other hand, the burnup at the end of the operation cycle of the first fuel assembly 11M becomes E1 smaller than E0, and the reactivity K1a becomes a positive value, but since it has a gentle downward slope characteristic as described above, its absolute The value | K1a | is small, and K1a is not so much larger than zero.
[0047]
In this case, Wa a loading ratio in the core of the first fuel assembly 11M and the second fuel assemblies 11U, respectively, when Wb (Wa + Wb = 1) , is a core average burnup E0 = E 1 × Wa + E 2 × Wb Therefore, the reactivity as the core average is expressed by K 1a × Wa + K 2b × Wb. Here, as described above, K2b is a negative value and K1a is a positive value. However, as described above, in the comparison between absolute values, | K2b |> | K1a |. Accordingly, when viewed in the whole core, the average reactivity of the core is equal to the core average burnup E0 due to the negative reactivity effect by the second fuel assembly 11U that has advanced combustion than the average burnup E0. The effect of the positive reactivity due to the first fuel assembly 11M that has not been reached becomes larger, and as a result, the reactivity gain becomes negative.
[0048]
On the other hand, in the present embodiment, the first fuel assembly 11M represented by a gentle downward-sloping characteristic line is loaded in the high-power region of the core, and the second fuel assembly represented by an abrupt downward-sloping characteristic line. The body 11U is loaded in the low power region of the core. In this case, the burnup at the end of the operation cycle of the first fuel assembly 11M is E2 greater than E0, and the reactivity K2a is a negative value. Its absolute value | K2a | is small, and K2a is not so much smaller than 0. On the other hand, the burnup at the end of the operation cycle of the second fuel assembly 11U is E1, which is smaller than E0, and the reactivity K1b is a positive value. | K1b | is large, and K1b has a value considerably larger than zero.
[0049]
At this time, assuming that the loading ratios of the first fuel assembly 11M and the second fuel assembly 11U in the core are Wa and Wb (Wa + Wb = 1), respectively, the core average burnup E0 = E2 × Wa + E1 × Wb. The reactivity as the core average is expressed as K2a × Wa + K1b × Wb. Here, as described above, K2a is a negative value and K1b is a positive value. However, as described above, | K2a | <| K1b | Accordingly, when viewed in the whole core, the core average reactivity is higher than the average average burnup E0 than the effect of the negative reactivity by the first fuel assembly 11M that burns more than the average burnup E0. The effect of the positive reactivity due to the second fuel assembly 11U that has not reached is greater. As a result, in this embodiment, it is possible to obtain a positive reactivity gain using the difference in combustion behavior as described above.
[0050]
Further, if this positive reactivity gain is utilized, the uranium fuel assembly 11U is made 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, MOX loaded in a single MOX fuel core is reduced in average enrichment compared to the uranium fuel assemblies loaded in the core). Compared to the fuel assembly, the average plutonium enrichment can be reduced and the fuel economy can be improved.
[0051]
Alternatively, using the above-described positive reactivity gain, the number of replacement bodies at the end of the cycle of the uranium fuel assembly 11U is reduced, and the average burnup taken out from the case where the uranium fuel single core is loaded is used. In addition, the MOX fuel assembly 11M also increases the average burnup by reducing the number of replacement bodies at the end of the cycle, compared with the case where the MOX fuel assembly 11M is loaded into a normal MOX fuel single core, thereby improving the fuel economy. You can also.
[0059]
Note that, as described above with reference to FIG. 1, in the above implementation mode, basically, the second fuel assemblies combustion variation is relatively steep reactivity mainly core near side (outer peripheral side) region and While loading a fuel cell (so-called control cell) region etc. adjacent to a control rod inserted for a long period of time, the first fuel assembly whose reactivity combustion change is relatively slow is loaded mainly in the central region of the core, Furthermore, exceptionally, the first fuel assembly is slightly loaded in the outermost peripheral region in consideration of the power distribution characteristics, but is not limited thereto.
[0060]
That is, in the case of a mixed core loaded with fuel assemblies of various different shapes or compositions, as long as the basic effect of the present invention to improve the reactivity gain as seen in the entire operation cycle is obtained, the first fuel It is not necessary to load the outermost peripheral region of the assembly. In this case, since the first 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 nuclear reactor, the power distribution in the central region of the core is almost flat, and the position of the boundary between the central region and the peripheral region is the range of the peripheral region where the output decreases. It was found that the distance from the center of the core was a position where the core radius was about 7/10 or more. The arrangement shown in FIG. 1 also satisfies this condition, except for the first outermost fuel assembly described above.
[0061]
【The invention's effect】
According to the present invention, the first fuel assembly whose reactivity combustion change is relatively slow is loaded in a region where the average output is relatively higher than the core average power density, and the combustion change in reactivity is relatively steep. The second fuel assembly is loaded in a region where the average power is relatively lower than the core average power density. As a result, when viewed in the entire core, the core average reactivity is higher than the predetermined burnup of the core average than the negative reactivity effect of the first fuel assembly that has burned more than the predetermined average burnup. The effect of the positive reactivity due to the second fuel assembly not reaching the degree is greater. As a result, it is possible to obtain a positive reactivity gain using the difference in the combustion behavior as described above as seen in the entire operation cycle (particularly at the end of the cycle).
[Brief description of the drawings]
FIG. 1 shows one quadrant of a boiling water reactor mixed core (¼ rotation symmetric core) constituted by using two types of fuel assemblies, to which the first embodiment of the present invention is applied. FIG.
FIG. 2 is a longitudinal sectional view showing the entire structure of a fuel assembly loaded by the fuel loading method according to the first embodiment of the present invention.
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] The reactivity and the burnup relationships in general boiling water fuel assembly burnup in the horizontal axis, Ru FIG der shown in vertical axis reactivity.
[Explanation of symbols]
11M MOX fuel assembly
11U, U 'Uranium fuel assembly
12 Fuel rod

Claims (1)

In a boiling water reactor fuel loading method in which two or more types of fuel assemblies having different combustion changes in reactivity are mixed and loaded in the core of a boiling water reactor,
The first fuel assembly, which has a relatively slow change in the combustion of the reactivity, is loaded mainly in the central region of the core, where the average power is relatively higher than the core average power density,
The second fuel assembly having a relatively sharp change in the combustion of reactivity is loaded mainly in the peripheral region of the core, which is a region where the average power is relatively lower than the core average power density,
The first fuel assembly is a fuel assembly including a uranium / plutonium mixed oxide fuel, and the second fuel assembly is a fuel assembly including a uranium fuel. Loading method.

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