JPH0472595A - Reactor core and loading method for fuel - Google Patents

Abstract

PURPOSE:To increase thermal allowance to perform good operations by arranging new fuel assemblies of MOX fuel assemblies in which the change in reactivity is slow, in the area of small importance, thereby restricting the output. CONSTITUTION:Fuel assemblies 21 having lower reactivity out of new fuel assemblies containing plutonium are loaded in the central part of a reactor core of large importance, and highly reactive fuel assemblies 25 out of the new fuel assemblies are arranged in the peripheral part (area A) of the core of small importance, and in the area B for surrounding control cells C. By arranging the new fuel assemblies of MOX fuel assemblies 25 in which the change in reactivity is slow, in the areas A and B in this way, a decrease in thermal allowance due to distortion of the distribution of radial output with respect to the reactor core is prevented to smooth the distribution of output. Thus, output can be kept constant throughout the operating period, and thermal allowance can be increased.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a boiling water type atomic reactor in which a uranium fuel assembly and a uranium/plutonium mixed oxide (hereinafter referred to as MOX) fuel assembly are loaded into a reactor core. This article relates to a method of loading fuel into a furnace.

[Conventional technology]

Generally, boiling water reactors use enriched uranium, which is enriched from natural uranium, as a fuel material.

The core of a boiling water reactor is constructed by loading a fuel assembly containing enriched uranium as a fuel material. After the reactor has operated for a certain period of time, the used fuel assembly is replaced with a new, unburned fuel assembly. Because this fixed period of operation and fuel exchange are repeated many times, a plurality of fuel assemblies with different core residence periods coexist in the reactor core.

FIG. 5 shows a cross section of a fuel assembly 21 using enriched uranium obtained from natural uranium as a fuel material. The fuel assembly 21 includes several types of fuel rods 22 with different enrichments, 8×
The channels are bundled in a square grid shape of 8 and housed inside the channel box 23. In the figure, Fzt to F19 and G1 to G3 shown inside each fuel rod 22 indicate the uranium enrichment level of each fuel rod 22. In the fuel rods 22 marked 61 to G3, gadolinia, which is a burnable poison, is added to the fuel pellets. The rod marked W in the figure is a large diameter water rod.

FIG. 6 shows an example of the arrangement of fuel assemblies 21 in the reactor core.

Since the core has quarter-rotational symmetry in the cross section, FIG. 6 shows only the quarter quadrant of the cross section. The numbers in the figure indicate the number of fuel cycles that the fuel assembly 21 experiences within the reactor core. That is, 1 is the 1st cycle, 2 is the 2nd cycle
Indicates that the cycle is... The longer the fuel assembly 21 has a larger number of fuel cycles, the longer it stays in the core. As shown in FIG. 6, fuel assemblies 2 with different stay periods in the reactor
1 are arranged so as to be uniformly distributed within the core.

An example of such a nuclear reactor fuel loading method is shown in Japanese Patent Publication No. Sho 63-16292.

[Problem to be solved by the invention]

When a MOX fuel assembly is used as part of a fuel assembly for a boiling water reactor, the following problems arise due to the difference in the nuclear properties of uranium and plutonium. This M.O.
The X fuel assembly is equipped with fuel rods (MOX fuel!) filled with fuel pellets containing MOx and is loaded into the reactor core.As the amount of MOX fuel in the MOX fuel assembly increases, the number of uranium fuel Differences will appear in the reactivity characteristics of MOX fuel assemblies.Generally, MOX fuel assemblies are
As the X-fuel mixture increases, the neutron spectrum becomes harder compared to the uranium fuel assembly. For this reason, the plutonium conversion rate is improved and the reduction in reactivity due to combustion is reduced, so that the combustion change in the infinite multiplication factor becomes gradual. This hardening of the neutron spectrum reduces the ability to control the reactivity of burnable poisons such as gadolinia, and the infinite multiplication factor of the MOX fuel assembly at the initial stage of combustion becomes larger than that of the uranium fuel assembly. This increases the thermal margin.

There is a risk that the reactor shutdown margin will decrease.

An object of the present invention is to be able to increase the thermal margin in a reactor core loaded with MOX fuel assemblies for a predetermined period of time.

It is an object of the present invention to provide a nuclear reactor core and a fuel loading method that enable good nuclear reactor operation.

[Means to solve the problem]

The above object can be achieved by arranging the new fuel assembly containing plutonium in the first region of the second layer from the outermost periphery of the reactor core and in the second region adjacent to the control cell.

[Effect]

In the present invention, MOX fuel assemblies with high reactivity are arranged in the first and second regions where neutron importance is low, so these MOX fuel assemblies with slow reactivity changes can be used to protect the surrounding area of the core and the control cells. The surrounding output can be kept almost constant throughout the operating period,
Good reactor operation can be continued without causing extreme distortion of power distribution.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The core of a boiling water reactor, which is a preferred embodiment of the present invention, will be explained below with reference to FIGS. 1 to 3.

FIG. 2 shows a cross section of the MOX fuel assembly 25 used in this example.

Each fuel rod 26 arranged in the channel box 27 of the MOX fuel assembly 25 includes MOX fuels 1 to F5 having different plutonium enrichments and a fuel rod G containing gadolinia, which is a burnable poison.

As the number of the MOX fuel rod-1 to F5 suffix increases, the plutonium enrichment decreases. M.O.
In the X fuel assembly 25, 48 out of 60 fuel rods 26 are MOX fuel 6.

The MOX fuel assembly 25 is a new MOX fuel assembly.
including. The above-mentioned fuel assembly 21 contains uranium and does not contain plutonium in a new fuel assembly state.

MOX fuel assembly 25 and fuel assembly 21 shown in FIG.
FIG. 3 shows a comparison of the infinite multiplication factors (hereinafter referred to as kOo characteristics) with . In FIG. 3, the characteristic f is the koo characteristic of the MOX fuel assembly 25, and the characteristic g is the koo characteristic of the fuel assembly 21.
However, it is an ω characteristic.

This kco is an index representing the reactivity of the fuel assembly,
As the combustion progresses, the amount of fission isotopes decreases, so it shows a monotonous decreasing tendency. However, in the early stages of combustion, gadolinia, which is a burnable poison, is mixed into the fuel assembly in order to suppress excessive reactivity, and the initial koO is kept small.

Comparing the ω characteristics of both, the rate of change in kω due to combustion is smaller in the MOX fuel assembly 25 than in the fuel assembly 21. In general, fuel assemblies with a high plutonium content are
The rate of change in oo becomes gradual. At the beginning of combustion, MO
The koo of the X fuel assembly 25 is larger than that of the fuel assembly 21.

This increases the output of the MOX fuel assembly 25 at the initial stage of combustion.

Next, a fuel loading method for obtaining the reactor core of this embodiment using the fuel assembly 21 and the MOX fuel assembly 25 will be explained. 1st
The figure shows that 80 of the 156 replacement fuel units are MO
X fuel assembly 25, and the remaining 76 fuel assemblies 2
The case where it is 1 is shown. Area A shown in FIG. 1 is
This is the second layer from the outermost layer (the first layer from the outermost layer) adjacent to the outermost layer, and is comprised of the area outside about 9710 core radius from the core center, excluding the outermost layer. Further, region B is a region surrounding and adjacent to control cell C, and 12 fuel assemblies 25 are arranged therein. The fuel assembly marked with an O in the figure is the MOX fuel assembly 25. Fuel assemblies 21 are arranged at the positions of numbers without O marks. In this example,
As shown in FIG. 1, MOX fuel assemblies 25 are arranged in regions A and B, and fuel assemblies 21 are arranged in the central region and the outermost layer. In FIG. 1, the numbers in the squares indicate the number of cycles experienced by the corresponding fuel assembly, similar to FIG. 6. At the outermost periphery, there is k of the fuel assembly 2]. The fuel assembly 21 of the fourth cycle with the smallest value is arranged.

In the second layer region from the outermost layer adjacent to the outermost layer, MOX
A fuel assembly is placed. The control cell C, into which the control rods 24 for power control are inserted during the reactor operation, is composed of four fuel assemblies 21 in the third cycle. In region B, the MOX fuel assembly 25 is arranged. A fuel assembly 21 is arranged in a central region inside region A and excluding region B. The fuel assembly 21 arranged in this central area is 1
This is the fuel assembly in the ~3rd cycle. The fuel assemblies 25 arranged in areas A and B are also fuel assemblies in the first to third cycles. The control rods 24 disposed at positions other than the control cell C are reactor shutdown control rods that are in a fully withdrawn state during reactor operation. 1 to 13 shown in the 1 direction and the J direction in FIG. 1 are codes indicating addresses in the reactor core, respectively.

Next, the operation of this embodiment will be explained.

In this embodiment, since the fuel assembly 21 and the MOX fuel assembly 25 are arranged in the reactor core as described above, the thermal margin is reduced due to the distortion of the power distribution in the radial direction of the reactor core, as explained below. This can be prevented.

The reason why this embodiment can suppress the decrease in thermal margin is closely related to the neutron invotance distribution in the radial direction of the core.

Generally, the neutron inbortance at a certain position in the reactor core is an index representing the magnitude of the output when a fuel assembly is loaded at that position. In other words, k is placed at a position where the invotance is large. : The higher the fuel assembly is loaded, the higher the output will be at that position, and conversely, the higher the invotance is, the higher the output will be at that position. If a fuel assembly with a high value of 0 is placed,
The output at that position will not increase. The radial distribution of neutron inbortance in a boiling water reactor is high at the center of the core and becomes smaller toward the periphery of the core.

Also, the area around control cell 0 is also smaller than the center of the core.

For this reason, the burnup is 0 GWd/l (new fuel assembly) in the center of the core, where the inbortance is large.

A fuel assembly 21 with a small invoice is loaded, and the burnup is 0 GWd/l (new fuel assembly) in the area surrounding the core (area A) and the control cell C, where the inbortance is small.
. Arranging fuel assemblies 25 with a large diameter is an effective means for making the core radial power distribution relatively flat without creating extreme distortion. In this way, areas A and B
By placing a new fuel assembly of the MOX fuel assembly 25 in the central region of the core (
Thermal margin can be increased compared to the case where the cell is placed inside the region A except for the region B and the control cell C.

For this reason, in this embodiment, the MOX fuel assembly 25 is loaded at the first layer from the outermost layer and at a position surrounding the control cell C, as described above. Thereby, distortion in the power distribution caused by the MOX fuel assembly 2S can be minimized. This is because, as described above, the neutron inbortance distribution in a boiling water reactor is small where neutron leakage is large. Therefore, neutron leakage is small in the central part of the core, which is far enough away from the radial ends of the core. Inside the core, it is almost constant except for the control cell C and the positions surrounding it. Here, if MOX fuel assemblies 25 are placed also at the outermost periphery of the core, it has the effect of suppressing the power distribution, but the leakage of neutrons is too large, and if MOX fuel assemblies 25 with high reactivity are placed, reactivity loss will increase. It is not effective because the effect is greater. In addition, since the loading amount of fuel assemblies in the area of the second layer from the outermost periphery and the area surrounding the control cell is limited, if the loading ratio of MOX fuel assemblies is large, the amount of loading of the fuel assemblies in this area for the first and second cycles is limited. The fuel assemblies 25 are loaded with priority, and MOX fuel assemblies 25 that cannot be loaded in these areas have no choice but to be loaded in the central area. In this case, the third cycle MOX fuel assembly 25 is loaded in the central region.

Initially, the reactor core is composed of only the fuel assemblies 21 as fuel assemblies. Spent fuel assembly 2 from such a core
1 was removed and replaced with a new MOX fuel assembly 25 having a burnup of ○GWd/l.

By repeating such fuel exchange, the core shown in FIG. 1 is obtained after four cycles. Fuel assembly 2 as a fuel assembly
A method of loading MOX fuel assemblies 25, which are new fuel, after the completion of a certain cycle of operation in a reactor core in which only MOX fuel assemblies 25 are loaded will be explained with reference to FIG.

This fuel loading method shows the case where MOX fuel assemblies 25 are loaded for the first time into a reactor core loaded with fuel assemblies 21 shown in FIG. Approximately 1/2 of the 140 new fuel assemblies to be loaded after removing the spent fuel assemblies 21
(7) 68 are MOX fuel assemblies 25, and the remaining 7
The two bodies are fuel assemblies 21. When the MOX fuel assembly 25 is loaded for the first time in this way, the area A of the second layer from the outermost periphery and the area B surrounding the control cell C
The MOX fuel assembly 25 is loaded into the MOX fuel assembly 25. In operation in the next fuel cycle following this fuel exchange, distortion in the power distribution in the radial direction of the core can be reduced and a reduction in thermal margin can be suppressed. After the operation of one fuel cycle is completed in the state shown in FIG. 7, the spent fuel assembly 21 is taken out from the core, and new fuel assemblies 21 and 25 are loaded into the core. The fuel assembly 21 is loaded in the central area, and the MOX fuel assembly 25 is loaded in areas A and B. Note that in the next cycle and onward, by loading new fuel assemblies of MOX fuel assemblies 25 in area A and above, an equilibrium core in which MOX fuel assemblies 25 and fuel assemblies 21 coexist as shown in FIG. 1 is created. It is possible to prevent the thermal margin from decreasing until the end. However, M.O.
During fuel exchange after two cycles of operation have passed since the first loading of the X fuel assembly 25, among the MOX fuel assemblies 25 arranged in areas A and B, the fuel assembly that has experienced two cycles of operation A part of the body 25 is moved to the position (■) in FIG. Such a transition also occurs during subsequent fuel changes. Therefore, even in the balanced core, the MOX fuel assemblies 25 with large koo, which are new fuel assemblies, can be loaded into regions A and B.

In FIG. 1, it is also possible to arrange the third cycle MOX fuel assembly 25 in the control cell C. In this case, the fuel assembly 21 of the third cycle is loaded at the position of the MOX fuel assembly 25 marked (■) in the central region.

FIG. 4 shows the core of a boiling water reactor that is another embodiment of the present invention. In this example, approximately 1 out of 156 replacement bodies was used.
52 pieces of /3 are MOX fuel assemblies 25, and the remaining 1
04 shows the core constructed when the fuel assembly 21 is the fuel assembly 21. Even in this embodiment in which the loading ratio of the MOX fuel assemblies 25 is small, a large number of MOX fuel assemblies 25 are loaded in the region A of the second layer from the outermost periphery and the region B surrounding the control cell C. Needless to say, even in this case, there is an effect of preventing a decrease in thermal margin.

〔Effect of the invention〕

As described above, according to the present invention, since the new fuel assembly of the MOX fuel assembly is arranged in a region of small importance,
The output of the MOX fuel assembly can be suppressed, the thermal margin can be increased, and good reactor operation can be performed for a predetermined period.

[Brief explanation of drawings]

FIG. 1 is a partial plan view of the core of a boiling water reactor that is a preferred embodiment of the present invention, FIG. 2 is a cross-sectional view of the MOX fuel assembly loaded in the core of FIG. 1, and FIG. The figure is a comparison diagram of the koo characteristics of uranium fuel assemblies and MOX fuel assemblies.
The figure is a partial plan view of a reactor core that is another embodiment of the present invention.
The figure is a cross-sectional view of a uranium fuel assembly that contains uranium and does not contain MOX, Figure 6 is a partial plan view of a reactor core composed only of uranium fuel assemblies 21, and Figure 7 is a MOX in the core of Figure 6.
FIG. 2 is a partial plan view of a reactor core configured by loading fuel assemblies for the first time. 21・Fuel assembly, 22.26・Fuel rod, 23゜2
7 channel box, 24 control rod, 25M0X fuel assembly, C... control cell.

Claims (1)

[Claims] 1. A nuclear reactor core including a plurality of fuel assemblies and a plurality of control cells including four fuel assemblies and control rods for adjusting reactor power, including plutonium. The new fuel assemblies are arranged in a first region where the fuel assemblies in the second layer from the outermost periphery of the core are arranged and a second region where the fuel assemblies are arranged adjacent to the control cells. A nuclear reactor core featuring: 2. In the core of a nuclear reactor that has a plurality of fuel assemblies and is equipped with a plurality of control cells including four fuel assemblies and control rods for adjusting reactor power, 2.
A fuel assembly containing plutonium in a new fuel assembly state in a first region where the fuel assembly of the layer is arranged and a second region where the fuel assembly is arranged adjacent to the control cell. A nuclear reactor core characterized by a large proportion. 3. The reactor core according to claim 1 or 2, wherein in areas other than the control cell, the first and second areas, a large proportion of fuel assemblies containing uranium but not plutonium in the state of new fuel assemblies is present. . 4. A method for loading fuel into the core of a nuclear reactor having a plurality of fuel assemblies and a plurality of control cells including four of the fuel assemblies and a control rod for adjusting reactor power, in which plutonium is loaded into the core of a nuclear reactor. A new fuel assembly including the new fuel assembly is loaded into a first area where the fuel assembly is placed in the second layer from the outermost periphery of the core and a second area where the fuel assembly is placed adjacent to the control cell. A fuel loading method characterized by: