JPS62259087A - Fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fuel assembly, and particularly to a reactor core and a fuel assembly suitable for effective use of nuclear fuel resources.

[Prior art] From the perspective of effective use of uranium resources, nuclear reactors that improve the conversion of uranium-238 to fissile material (plutonium-239) and that use a dense lattice structure are developed by Nuclcar Technolo.
gy), 59, 212 (1982), General Features of Advanced Pressure Rized by Oldekop et al.
Water Reactors with Imbrobut
Fuel Utilization (General
features of advanced p
ressurized water reacto
rs wit, promoted fuel
utilization). The nuclear reactor in the above document is a pressurized wooden reactor technology, and in order to apply this to a boiling water reactor, it is necessary to solve various technical problems.

For example, in current boiling water reactors, cross-shaped control rods are inserted from the bottom of the reactor core, whereas in the above document, narrow-diameter control rods are inserted from the top of the core; In a nuclear reactor, it is necessary to configure the equipment inside the reactor so that the control rod can be inserted from the upper part of the reactor core.

Regarding this, Japanese Patent Laid-Open No. 59-84192 discloses a configuration in which control rods can be driven from the upper part of the reactor core by arranging a steam separator and a steam dryer in the outer periphery of the reactor pressure vessel. ing.

As described above, these known examples require extensive design changes when converting a boiling water reactor to a high conversion type reactor.

[Problem to be solved by the invention] Neutrons generated in the core of a nuclear reactor are fissile uranium 2
In addition to being absorbed by uranium-35 and causing nuclear fission, it is also absorbed by uranium-238, which makes up the majority of the uranium element. Since uranium-238 is not fissile, it does not cause nuclear fission directly, but when it absorbs neutrons, it is converted into fissile plutonium-239. A substance like this uranium-238 that absorbs neutrons and creates fissile material is called parent material, and the process of creating fissile fuel material from parent material is called conversion.

Therefore, the conversion ratio (CR) is defined as follows.

In the case of conversion, when N atoms of fuel are consumed during reactor operation, (CRX N ) new atoms of the fission nuclide will be created.

Generally, in a light water reactor, this conversion ratio is about 0.6, but a nuclear reactor with a somewhat higher conversion ratio of 0.8 to 1.0 is called a converter reactor.

Increasing the conversion ratio increases the ratio of converting uranium-238, which does not cause nuclear fission, into fissile plutonium, which makes it possible to use uranium resources more effectively and is effective in reducing fuel costs.

To increase the amount of plutonium produced in the reactor core,
Since neutron absorption of uranium-238 is caused by relatively high-energy neutrons (resonance capture absorption),
This can be achieved by shifting the neutron energy spectrum of the reactor core toward higher energies. For this purpose, it is necessary to reduce the atomic ratio (H/U ratio) between hydrogen atoms, which have a large neutron moderating effect, and uranium atoms, which are the fuel.

On the other hand, as mentioned above, it is necessary to burn off the plutonium-239 produced in the nuclear reactor as efficiently as possible. To achieve this, the absorption into the fissile material can be increased by improving the moderation of neutrons and increasing the proportion of thermal neutrons.

This is achieved by increasing the H/U ratio, contrary to the conversion case.

In addition, in the case of fuel loaded with plutonium, the conversion to the fissile element shown above also occurs through the production of plutonium-241 by neutron absorption of plutonium-240. H
/HM (fuel heavy metals).

Conventional light water reactors do not take into consideration the above-mentioned improvements in conversion (high conversion reactors) and effective combustion of fissile material (burner reactors).

An object of the present invention is to provide a light water nuclear reactor that can reduce the amount of fissile material required to generate unit energy by improving the conversion ratio and effectively burning the fissile material.

[Means for solving the problem] The above objective is achieved by arranging the fuel rods in the fuel assembly in a triangular lattice shape and making the diameter of the fuel rods at the periphery of the fuel assembly smaller than the diameter at the center of the fuel assembly. It can be achieved.

[Function] In order to make the boiling water reactor a high conversion type, the spacing between the fuel rods is narrowed and the reactors are made denser. When the fuel rods are densified, the clearance between the fuel rods is required to be a certain value or more due to limitations such as critical output.

Therefore, the clearance must be secured above a certain value, and H/
In order to reduce the HM ratio, a triangular lattice arrangement is advantageous, and in the present invention, the fuel rods in the fuel assembly are arranged in a triangular lattice arrangement.

In order to increase the conversion ratio, it is desirable to remove the water gap between fuel assemblies in conventional boiling water reactors and further reduce the H/HM ratio. This will be a major remodeling of the structure. In addition, in order to effectively reduce the H/HM ratio, it is desirable to use a hexagonal lattice arrangement for the fuel assembly lattice, but this would also lead to extensive modification of the reactor internals, similar to the control rods. .

In the present invention, in order to avoid major modifications to these reactor internal structures, the fuel assemblies are made into a square lattice as in the past, and the control rods are cross-shaped and inserted into the water gap between the fuel assemblies. The configuration is as follows.

In addition, in order to effectively utilize the water gap between fuel assemblies, in the first half of fuel combustion (also in the first half of the operating cycle), a substance with low neutron absorption capacity is placed in the water gap between fuel assemblies. After inserting a control rod follower configured with Pull out the stick follower and press H
By increasing the 7HM ratio and improving the moderation of neutrons, it is possible to increase the reactivity of the reactor core and achieve effective combustion of fuel.

The presence of a water gap between fuel assemblies tends to increase the power output of fuel rods located at the outer periphery of the fuel assembly and increase local power peaking. It is necessary to arrange that the degree of oxidation) is low at the outer periphery of the aggregate and high at the center. It is also effective to arrange the fuel rods so that their diameters are narrower at the outer periphery of the fuel assembly and wider at the center. Due to these considerations, the fuel rods in the fuel assembly have a central region with a high enrichment (or Pu enrichment) and a large fuel rod diameter, and a central region with a low enrichment (or Pu enrichment) and a narrow fuel rod diameter. It has a two-area configuration consisting of peripheral areas.

Conversion takes place primarily in the central region (referred to as the conversion region), and effective combustion of the fissile material takes place primarily in the peripheral region (referred to as the burner region). The ratio of the number of fuel rods yielding to the surrounding area to the total number of fuel rods must be 2 to make the local power peaking equivalent to that of a conventional boiling water reactor.
0 to 40% is appropriate.

[Example] Hereinafter, an example of the fuel assembly of the present invention applied to a boiling water reactor will be described with reference to FIGS. 1 and 2.

FIG. 1 shows a horizontal cross section of the fuel assembly of this embodiment. A unit square lattice cell constructed by loading this fuel assembly 8 into the core of a boiling water type cavity converter reactor is a fuel assembly 8 consisting of a large number of fuel rods 1 and a channel box 2.
and a water gap 7 between adjacent fuel assemblies 8. A follower-equipped control rod 3 is inserted into the water gap portion 7. In the fuel assembly 8 of this embodiment, the channel box 2 has a square shape, and the fuel rods 1 within the channel box 2 are arranged in a regular triangular lattice, forming a dense lattice. Further, the cross-shaped control rods 3 with followers are moved in and out of the water gap 7 between the fuel assemblies 8 at the same pitch as the unit square lattice cells (this is called a square lattice).

If the fuel enrichment or enrichment of each fuel rod 1 is made uniform, the output of the fuel rods in the periphery of the fuel assembly will increase, and the local power peaking coefficient will increase. The fuel rods in the central area are divided into IA and fuel rods IB in the peripheral area, and the fuel rods IA in the central area are large diameter and highly enriched or highly enriched fuel rods, and the fuel rods IB in the peripheral area are small diameter and low enrichment or high enrichment rods. The length of the side of the channel box 2 of the fuel assembly 8 that uses low enrichment fuel rods is five times the length of one side of the channel box of a conventional fuel assembly. The distance between the bodies has grown that much.

Regarding the dimensions of the unit cell cell, it is desirable to increase the size of the fuel assembly 8 in order to increase the conversion ratio, but it is important to not change the pitch of the control rods of conventional boiling water reactors because it is necessary to modify the reactor internal structure. This is desirable from the perspective of minimizing the Therefore, the arrangement pitch P of the control rods shown in FIG. The angle of the blade has been rotated by 45'' compared to the conventional model.

Details are shown in the specification of Japanese Patent Application No. 60-142465, page 6, line 7 to page 8, line 13, and FIG.

Fuel assembly 5 shown in Figure 1 of Japanese Patent Application No. 14265/1983
The structure in which the fuel assembly 8 of this embodiment is replaced with the fuel assembly 8 of this embodiment is the core structure loaded with the fuel assembly 8 of this embodiment. The lattice array is configured like this.

The above-mentioned unit square lattice cell is used. As a result, the size of the fuel assembly 8 can be increased as described above without making extensive modifications to the reactor internals, such as the arrangement pitch of the control rods.

FIG. 3 shows the operation of the control rod 3 with a cross-shaped follower. The follower control rod 3 has a neutron absorbing material region 3A and a follower portion 3B. Neutron absorber region 3A is B
The follower portion 3B is made of a material 1 that is difficult to absorb neutrons, such as a zirconium alloy. The axial length of the neutron absorbing material region 3A and the follower portion 3B is equal to the axial length of the fuel rod 1 of the fuel assembly 8. The follower control rod 3 is connected to the upper end of a control rod drive device located below the reactor core. When the reactor is shut down as shown in Figure 3, the neutron absorbing material region 3A is located adjacent to the fuel, but as shown in Figure 3(B)
During the high conversion operation in the first half of the fuel cycle, the follower section 3B is positioned so that the neutron absorbing material region 3A and the follower section 3B are located between the fuel assemblies 8 during the burner operation in the second half of the fuel cycle shown in FIG. 3(C). Let it be pulled out.

The nuclear characteristics according to this example will be shown below.

FIG. 4 shows the relationship between the average conversion ratio and the H/H~i ratio. In the example of the present invention, the H/HM atomic ratio is 0.8a degrees, which is about 0. An average conversion ratio of 8 is achieved.

The definition of average conversion ratio is as follows.

Average conversion ratio = (amount of fissile material at time of removal)/amount of fissile material at time of loading) Figure 5 shows what? This figure shows the combustion change of the infinite multiplication factor (k■) of the 8-fuel assembly 8, in which the follower part is inserted into the fuel cycle conscience part, and the follower part is pulled out from the core part in the latter half of the cycle.

Figure 6 shows the change in cycle burnup of the core reactivity based on Figure 5. At the beginning of the fuel cycle (BOC), it becomes critical due to the full insertion of the follower part 3B, and at the end of the fuel cycle (EOC), it becomes critical. It is shown that the condition becomes critical when the follower part 3B is completely withdrawn. That is, the follower portion 3B, which was fully inserted at BOC, is gradually pulled out as combustion occurs and is completely pulled out at EOC.

[Effects of the Invention] According to the present invention, a high conversion boiling water reactor with an average conversion ratio of about 0.8 can be realized without making major changes to the control rod structure and other internal structures of conventional boiling water reactors. can. Also, insert the follower part of the cross-shaped control rod with follower in the first half of the cycle,
By withdrawing the fuel in the latter half of the cycle, it is possible to effectively burn the accumulated fissile material as a burner furnace.

[Brief explanation of drawings]

FIG. 1 is a horizontal sectional view of a fuel assembly that is an embodiment of the present invention, FIG. 2 is a local plan view of a core of a boiling water type cavity converter loaded with the fuel assembly of FIG. 1, and FIG. is an explanatory diagram of the operation of a control rod with a cross-shaped follower, Figure 4 is a characteristic diagram showing the relationship between the average conversion ratio and the H/HM atomic ratio, and Figure 5 is the infinite multiplication factor of the fuel assembly in Figure 1. FIG. 6 is a characteristic diagram showing the cycle combustion change in core reactivity. IA, IB...Fuel rod, 2...Channel box. 3...Control rod with follower.

Claims (1)

[Claims] 1. In a fuel assembly having a plurality of fuel rods, the fuel rods are arranged in a triangular lattice shape, and the diameter of the fuel rods arranged at the periphery of the fuel assembly is equal to the diameter of the fuel rods arranged at the center of the fuel assembly. A fuel assembly characterized by being thinner than the diameter of the rod.