JPS6373192A - Boiling water type reactor - Google Patents

Abstract

(57) [Abstract] 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 boiling water nuclear reactor with an improved conversion ratio for effective utilization of nuclear fuel resources.

[Conventional technology]

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 to fissile pul) = ram-239. Substances such as uranium-238 that absorb neutrons and create fissile material are called parent materials, and the process of creating multi-fissile fuel materials from the 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, CR-N new atoms of fissile material will be created. Generally, in a light water reactor, this conversion ratio is about 0.6, but the conversion ratio is slightly higher than this.
．． 8 to 1.0 reactors are called converter reactors.

Increasing the conversion ratio is the key! Since the ratio of converting uranium-238, which previously did not cause nuclear fission, to fissile plutonium increases, uranium resources can be used more effectively and fuel costs can be reduced.

In order to increase the conversion ratio in order to increase the amount of ram produced (pull in the reactor core), the neutron energy in the reactor must be This can be achieved by shifting the spectrum towards higher energies. To achieve this rounding, in light water reactors, it is necessary to reduce the atomic ratio (VU ratio) in the core of hydrogen atoms, which have a large neutron moderating effect, and fuel.

One way to reduce the fuel ratio in the core is to densely arrange fuel rods in bundles (fuel assemblies).
One possible method is to reduce the ratio within the bundle. From the perspective of effective use of uranium resources, nuclear reactors that use a dense lattice structure to improve the conversion of uranium-238 to plutonium-239, a fissile material, are Nucl, T@
ehno1.59t212 (1982) 01dko
General f@atur@sof
advance@d pre@5uriz@d
wat@r r@actarm with Impr
This is shown in a paper titled ``Oved Fuel Utll Zatlon''.

[Problem that the invention seeks to solve]

The reactor in the above paper is a pressurized water reactor technology,
In order to apply this to boiling water reactors, there are various technical issues that need to be solved. In other words, each fuel rod in a bundle must be spaced apart from each other by a certain distance to prevent contact due to vibration, etc., and it is necessary to create a bundle with the same horizontal cross-sectional area as that used in current boiling water reactors. There are restrictions on how it can be used. In addition, it is possible to increase the horizontal cross-sectional area of the bundle in order to reduce the power ratio in the reactor core, but if the bundle is made larger, it will not be possible to safely shut down the reactor from the standpoint of reactor control. In order to achieve this, it is necessary to avoid reducing the number of control rods.

An object of the present invention is to effectively reduce the H7U ratio in the reactor core in a boiling water reactor without increasing the horizontal cross-sectional area of the reactor core or reducing the number of control rods. There is a particular thing.

[Means for solving problems]

In the boiling water reactor of the present invention, a control rod having a cross-shaped cross section is located directly below a pair of diagonal positions of one lattice of one lattice-shaped upper core support member, and one lattice of the upper core support member has a cross-section. A single fuel assembly with a square cross section is loaded in the interior, the cross-sectional wings of the control rod extend along the sides of the fuel assembly, and the upper half of the control rod is made of a material with a small neutron moderation effect. The lower half of the control rod forms a neutron absorbing part containing a neutron absorbing material, and both parts have a length equal to or 5/6 or more of the effective length of the fuel. It is something to do.

[Effect]

According to the above configuration of the present invention, the horizontal cross-sectional area of each bundle (fuel assembly) can be increased without increasing the horizontal cross-sectional area of the core, and the number of control rods can be kept the same as the current one. Moreover, by configuring the control rod as described above, the light water in the area outside the bundle (leakage area), which is normally filled with light water, is replaced by the follower part that has a small neutron moderation effect. The diameter ratio in the peripheral portion of the bundle can also be reduced, and therefore the φ ratio can be uniformly reduced throughout the core.

〔Example〕

FIG. 2 is a plan view showing a part of the core of a conventional boiling water reactor; in the figure, 101 is an upper grid plate;
105 is a control rod having a cross-shaped cross section, and 105 is a bundle (fuel assembly) having a square cross-section and four pieces are arranged in a unit grid of the grid plate 101.

FIG. 1 is a plan view showing a part of the core of a boiling water reactor according to an embodiment of the present invention, and is a plan view of an upper lattice plate 1 located at the top of the core, viewed from above the core. be. The upper grid plate 1 is installed in a reactor pressure vessel (not shown) of a boiling water reactor. In FIG. 1, the upper lattice plate 1 in this embodiment is located at the opposing intersections among the four intersections in the unit lattice of the upper lattice plate 101 (indicated by dotted lines in FIG. 1) in the current core structure shown in FIG. It has a shape in which the two are tied together, and is rotated 45 degrees relative to the current core grid. Control rod 103 (
The position of the central axis of ) shown by the dotted line in FIG. 1 coincides with the intersection point 2 of the upper grid plate 1 of this embodiment. The control rods 3 will be rotated 45 inches without changing their position in the current core, and the cross-shaped blades will be completely located directly below the upper grid plate 1 when viewed from above the core. The length of one side of the lattice is 77 times that of the upper lattice plate 101 of the current core, and the intersection of the sb upper lattice plate 1 is at the true position of the center axis of the control rod 3. Intersection 2 located above and intersection 4 not located at the control rod position (current core upper grid plate 10
1) are arranged alternately.

The control rods 3 are respectively arranged at a pair of opposing diagonal positions 2 of one lattice of the upper lattice plate 1 and are inserted between adjacent fuel assemblies (bundles) 5 directly below the upper lattice plate 1. . One fuel assembly 5 with a square cross section is attached to one of the upper grid plates 1.
inserted into one grid. When the control rods 3 are fully inserted into the reactor core, the fuel assemblies 5 whose upper ends are inserted into this one grid are directly connected to the control rods 3 arranged at opposite corners of the grid. The control rods 3 are sandwiched between adjacent (opposed) control rods 3.

The technical significance of the above core configuration is as follows.

As mentioned above, uranium-238 to grotonium-239
In order to improve the conversion to neutrons, it is necessary to reduce the ratio of hydrogen atoms, which have a large neutron moderating effect, to uranium atoms, which serve as fuel, in the reactor core. Inside the reactor core, there are an area inside the bundle (fuel assembly) and an area outside the bundle (
leakage area). The leakage region is a region filled with light water containing hydrogen, which has a large neutron moderating effect.

Therefore, it is possible to reduce the leakage ratio of the entire core (by reducing the proportion of the leakage area in the core and increasing the proportion of the area inside the bundle).

For this purpose, it is sufficient to use large bundles in which the horizontal cross-sectional area of each bundle is increased without changing the horizontal cross-sectional area of the core. However, from the perspective of safely shutting down the reactor, it is impossible to reduce the number of control rods compared to the current level.
Therefore, the core configuration shown in Figure 1 has made it possible to maintain the current number of control rods and increase the size of the bundle without major modification of the current boiling water reactor. be. In the configuration shown in Figure 1, the number and installation of control rods is the same as the current one, but by rotating the direction of the control rod wings by 45 degrees, a grid surrounded by two opposing control rods can be created. Since the length of each side of each grid is approximately twice the length of one side of the current bundle, the length of the − side can be reduced to 7
A large bundle doubled in size can be equipped within each grid. That is, the number and position of the control rods are the same as the current model, but the direction of the cross tool is rotated by 45 degrees. Furthermore, the upper core support member is changed so that the control rods are located directly below the diagonal corner of one grid of the upper core support member, and approximately 100% of the current fuel assembly is placed within one grid of the upper core support member. It is equipped with a fuel assembly with a side length twice that of the previous one. By increasing the side length of the bundle (fuel assembly) to v/2 times the current length, the horizontal cross-sectional area of the bundle will be approximately twice the current length. When all control rods are inserted, the control rod wings are adjacent to all four sides of each bundle.

In this way, according to the above configuration, the bundle can be increased in size without reducing the number of control rods, thereby reducing the power ratio of the reactor core as a whole.

However, with the above configuration as it is, the following problems arise. In other words, during normal operation, most control rods are fully withdrawn, so the fuel rods around the bundle are exposed to leakage areas filled with light water containing hydrogen atoms, similar to current boiling water reactors. I will come into contact with you. Therefore, when looking at each bundle, the ratio (2) is small at the center of the bundle, but cannot be made small at the periphery of the bundle. Therefore, in the periphery of the bundle where many fuel rods are present, the neutron spectrum shifts to the lower energy side, which suppresses plutonium production in the fuel rods in the periphery of the bundle. Therefore, in order to achieve a high conversion ratio,
It is necessary to take measures to reduce the fuel rod ratio around the bundle so that the fuel rods around the bundle also contribute to high conversion.

As a countermeasure against this, in the present invention, the lower half of the control rod inserted into the reactor core from below to above is made into a neutron absorbing part containing a neutron absorbing material, and the upper half is made into a follower made of a member with a small neutron moderating effect. The control rod is equipped with a control rod in which each part has a length t approximately equal to the effective length of the fuel. In this way, during output operation, the follower part of the control rod is inserted into the leakage part at the fuel effective length position of each bundle, so the leakage part is changed from light water, which has a high neutron moderation effect, to a member with a small neutron moderation effect. By replacing it, it is possible to reduce the power ratio near the fuel rods at the periphery of the bundle. As a result, both the peripheral and central portions inside each bundle can be made smaller and a higher conversion ratio can be achieved.

FIG. 3 is a perspective view showing an embodiment of such a control rod 3 according to the present invention. The control rod 3 has a cross-shaped cross section and four wings (blades) extending in all directions from the axis. Follower part 6 which is the upper half area of the control rod 3
is a Zircaloy plate having a small neutron moderating effect, and its length in the vertical direction is the same as or 5/6 or more of the effective fuel length of the fuel loaded in the reactor into which the control rod 3 is inserted.

In addition, the absorber section 7, which is the lower half region of the control rod 3, has a hafnium metal rod, which is a neutron absorbing material, held in a sheath made of Tsukaloy, and its length is the same as that of the 7-year-old lower section 6. It is the same as the length.

The blade length of the control rod (length l in the figure) should be as long as possible for both the follower section 6 and the neutron absorption section 7. In other words, in order to increase the conversion ratio, light water in the leakage section should be made as long as possible. It is better to eliminate as much as possible, so increasing the blade length of the follower part of the control rod will improve the effect. Additionally, the blade length of the neutron absorbing section needs to be longer than the current blade length. This is because the amount of uranium loaded in the core increases due to the densification of fuel rods and the increase in the ratio of the area within the bundle to the entire core. This is because it is necessary to make it longer (as is the case with the current one). The blade length l of the control rod is the upper core support member 1 described above.
It is preferable that the side length of one lattice is at least 172 or more.

FIG. 4 shows a structural layout diagram inside a nuclear reactor according to an embodiment of the present invention. The plan view of this core seen from above is shown in Figure 1. In FIG. 4, the control rods 3 are inserted into the lower part of the core, similar to the current boiling water reactor. In the fuel assembly 5, the effective length of the fuel section 8 containing fissile material is 1/3 to 2/3, preferably half, of the length of the fuel assembly, and the fuel section is provided in the lower part of the fuel assembly. . Further, the fuel assembly used is one in which fuel rods are densely arranged. The steam separator 9 and the steam dryer 10 are installed in the upper part of the reactor core in the current boiling water reactor.

In the reactor of this embodiment, the follower part of the control rod is placed adjacent to the effective length part of the fuel during power operation, light water in the leakage part is excluded, and the follower part is made of a material with a small neutron moderating effect. By replacing it, the power ratio in the core can be uniformly reduced within the reactor, so a core with a high conversion ratio can be realized. That is, according to this embodiment, a high conversion reactor can be realized without major modification of the current boiling water reactor.

〔Effect of the invention〕

According to the present invention, the bundle can be consecrated without reducing the number of control rods without major changes from the current boiling water reactor,
Furthermore, the φ inside the boiling water reactor, including the surrounding area of the bundle, is
Since the ratio can be uniformly reduced, a high conversion ratio can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a local plan view of the core structure of a boiling water reactor according to an embodiment of the present invention; FIG. 2 is a local plan view of a core structure of a conventional boiling water reactor; FIG. FIG. 4 is a perspective view showing an embodiment of a control rod used in a boiling water reactor of the present invention; FIG. . 1... Upper lattice plate 2... Intersection position of the upper lattice plate where the control rod is placed directly below 3... Control rod 4... Intersection point of the upper lattice plate where the control rod is not placed directly below Position 5...Fuel assembly (bundle) 6...Follower part 7...Neutron absorption part 8
...Fuel part 9...Steam water separator 10.
・・Steam generator diagram 2

Claims (1)

[Claims] A control rod with a cruciform cross section is located directly below a pair of diagonal corners of one grid of the upper core support member in the form of a grid, and one fuel assembly with a square cross section is located within one grid of the upper core support member. The wing of the control rod, which has a cruciform cross section, extends along the side of the fuel assembly, the upper half of the control rod forms a follower section made of a material with a small neutron moderating effect, and the lower half of the control rod A boiling water nuclear reactor characterized in that the part constitutes a neutron absorbing part containing a neutron absorbing material, and both parts have a length equal to or 5/6 or more of the effective length of the fuel.