JP3778438B2 - Fuel assembly for fast breeder reactor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel assembly for a fast breeder reactor loaded in a core of a fast breeder reactor.
[0002]
[Prior art]
Conventionally, in a fast breeder reactor fuel assembly 20 loaded in the core of a fast breeder reactor, as shown in FIG. 7, several hundred fuel rods 23 enclosing fuel pellets 22 are arranged in a triangular lattice pattern. Things are covered with a wrapper tube 24 which is a hexagonal stainless steel outer cylinder.
[0003]
Stainless steel is a substance that absorbs neutrons. Therefore, in order to increase the neutron utilization rate and improve the performance of the core, a concept of a fast breeder reactor fuel assembly 26 in which the wrapper tube 24 is omitted as shown in FIG. 8 is also considered.
[0004]
In the fast breeder reactor fuel assembly 26 in which the wrapper tube 24 is omitted, the fuel rods 23 bundled in a triangular lattice shape are held by a grid spacer 27.
[0005]
[Problems to be solved by the invention]
However, the fast breeder reactor fuel assembly 26 from which such a wrapper tube is omitted has the following problems.
[0006]
That is, when the fast breeder reactor fuel assembly 26 from which the wrapper tube shown in FIG. 8 is removed is loaded on the core of the fast breeder reactor, the barrier between the adjacent fast breeder reactor fuel assemblies 26 is used. Disappears.
[0007]
For this reason, in the unlikely event that the fuel pellets 22 melt, an area where the fuel pellets 22 are melted is connected between the adjacent fast breeder reactor fuel assemblies 26 to form a wide melted fuel area. However, there is a concern that the nuclear fuel material may reach recriticality.
[0008]
The present invention has been made in view of such circumstances, and even if an event that causes the fuel pellets to melt occurs, the region where the fuel pellets melted is prevented from being connected, Accordingly, an object of the present invention is to provide a fuel assembly for a fast breeder reactor in which recriticality due to molten nuclear fuel material does not occur.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention takes the following measures.
[0010]
That is, the invention of claim 1 is loaded with a plurality of long first fuel rods loaded with a blanket fuel containing at least one of natural uranium and degraded uranium, and a nuclear fuel material containing at least plutonium. In the fuel assembly for a fast breeder reactor constructed by bundling and arranging a plurality of long second fuel rods in a triangular lattice pattern, the first fuel rods are all disposed at least in the outermost peripheral layer. The outer diameter of the first fuel rod is the same as or larger than the outer diameter of the second fuel rod, and the first fuel rod further includes a neutron absorber on one end side. The fuel core is loaded so that the longitudinal direction of each fuel rod is substantially vertical, and the end side loaded with the neutron absorber is on the lower side. Each of the plurality of second fuel rods generates a blanket region loaded with a blanket fuel on both sides of a fuel region loaded with nuclear fuel material, and is generated by fission of the nuclear fuel material on the end side of both blanket regions. A third fuel rod having a plenum region for accumulating FP gas, a fourth fuel rod having a blanket region of the third fuel rod as a plenum region, and further loaded with a neutron absorber on one end side; The fuel core is loaded so that the longitudinal direction of each fuel rod is substantially vertical and the end side of the fourth fuel rod loaded with the neutron absorber is on the lower side.
[0011]
The invention of claim 2 is the fast breeder reactor fuel assembly according to claim 1, wherein when the nuclear fuel material loaded in the fuel region of the third fuel rod is melted, the melted nuclear fuel material is: When the nuclear fuel material loaded in the fuel region of the fourth fuel rod is melted by the blanket regions provided on both sides thereof, the molten nuclear fuel material is retained in the core. At the time of loading, the plenum region on the lower side of the fuel region is dropped to reach the lower side.
[0012]
In the invention of claim 3, in the fuel assembly for a fast breeder reactor according to claim 1 or claim 2, the plenum region of the third fuel rod and the plenum region of the fourth fuel rod have substantially the same volume. And
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0018]
In addition, the code | symbol in the figure used for description of each following embodiment attaches | subjects and shows the same code | symbol about the same part as FIG.7 and FIG.8.
[0019]
(First embodiment)
A first embodiment of the present invention will be described with reference to FIGS.
[0020]
FIG. 1 is a 1/2 perspective cross-sectional view showing an example of fuel rod arrangement in a fuel assembly for a fast breeder reactor according to the first embodiment.
[0021]
FIG. 2 is a conceptual diagram showing a configuration example in the longitudinal direction of each fuel rod of the fuel assembly for a fast breeder reactor according to the first embodiment.
[0022]
That is, the fast breeder reactor fuel assembly 1 according to the present embodiment has a regular hexagonal cross section structure in which the fuel rods 23 bundled in a triangular lattice shape are held by the grid spacers 27 as shown in FIG. It is said.
[0023]
As shown in FIG. 1, tie rods 2 fixed to the grid spacers 27 by welding or the like are disposed instead of the fuel rods 23 at locations corresponding to the respective vertex positions of the regular hexagonal cross section.
[0024]
The fuel rod 23 is composed of three types of fuel rods: a blanket fuel rod 3, a type 1 core fuel rod 4 (# 1), and a type 2 core fuel rod 4 (# 2). As shown in FIG. 2, each of these fuel rods is plugged with an upper end plug TS at its upper end and a lower end plug BS at its lower end. The outer diameter of each fuel rod is the same for the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2), but for the blanket fuel rod 3, the type 1 core fuel is the same. The diameter is the same as that of the rod 4 (# 1) and the type 2 core fuel rod 4 (# 2), or larger than that.
[0025]
The blanket fuel rod 3 is a fuel rod loaded with a blanket fuel made of natural uranium, and has an axial configuration as shown in FIG. 2 (a).
[0026]
That is, the blanket fuel rod 3 includes a blanket fuel region N, which is a region in which fuel pellets 22 made of blanket fuel are loaded, at the center in the axial direction. The lower end portion of the blanket fuel region N is positioned by caulking D or an intermediate end plug having a through hole in the center, and the fuel pellet 22 is prevented from dropping by the caulking D or the intermediate end plug having a through hole in the center. On the other hand, if the fuel pellets 22 are melted, the fuel pellets 22 easily fall from the blanket fuel region N to the lower plenum region LP and the neutron absorber region A side.
[0027]
The upper and lower sides of the blanket fuel region N respectively absorb an FP gas such as Xe or Kr generated by nuclear fission of the blanket fuel, and an upper plenum region UP that is a plenum region that alleviates an increase in the internal pressure of the blanket fuel rod 3, And a plenum region LP.
[0028]
In the upper plenum region UP, a plenum spring SP that presses the fuel pellets 22 loaded in the blanket fuel region N from the upper side to the lower side is disposed. Furthermore, a neutron absorber region A, which is a region loaded with a neutron absorber, which is a substance that absorbs neutrons such as boron carbide (B4C), is disposed below the lower plenum region LP. The blanket fuel loaded in the blanket fuel region N may be loaded with not only natural uranium but also fuel pellets 22 made of deteriorated uranium.
[0029]
The type 1 core fuel rod 4 (# 1) is a fuel rod loaded with a nuclear fuel material containing at least plutonium, and has an axial configuration as shown in FIG. 2B.
[0030]
That is, the type 1 core fuel rod 4 (# 1) is provided with a core fuel region F, which is a region in which a fuel pellet 22 made of nuclear fuel material containing at least plutonium is loaded at the center in the axial direction. As the nuclear fuel material, for example, a mixed oxide containing about 20% by weight of fissile uranium and plutonium is used. The lower end portion of the core fuel region F is also positioned by caulking D or an intermediate end plug having a through hole in the center, and the fuel pellet 22 is prevented from falling by the caulking D or the intermediate end plug having a through hole in the center. ing.
[0031]
On the upper and lower sides of the core fuel region F, the blanket fuel regions N as described above are arranged. Thus, the height of the upper end of the blanket fuel region N disposed on the upper side of the core fuel region F is equal to the height of the upper end of the blanket fuel region N of the blanket fuel rod 3 as shown in FIG. ing. Further, the height of the lower end portion of the blanket fuel region N disposed on the lower side of the core fuel region F is made equal to the height of the lower end portion of the blanket fuel region N of the blanket fuel rod 3 as shown in FIG. Yes.
[0032]
Further, on the upper and lower sides of both blanket fuel regions N, nuclear fuel loaded in the core fuel region F, and FP gases such as Xe and Kr generated by nuclear fission of the blanket fuel loaded in both blanket fuel regions N And an upper plenum region UP and a lower plenum region LP, which are plenum regions that relieve the increase in internal pressure of the type 1 core fuel rod 4 (# 1).
[0033]
Similarly to the case of the blanket fuel rod 3, a plenum spring SP that presses the fuel pellet 22 from the upper side to the lower side is disposed in the upper plenum region UP.
[0034]
Since the type 1 core fuel rod 4 (# 1) contains more fissile material than the blanket fuel rod 3 and generates more FP gas, the volume of the lower plenum region LP is The volume of the lower plenum region LP of the blanket fuel rod 3 is larger.
[0035]
The type 2 core fuel rod 4 (# 2) has a configuration in which both blanket fuel regions N are omitted from the type 1 core fuel rod 4 (# 1) as shown in the axial configuration example in FIG. Yes. Then, the lower end portion of the core fuel region F is positioned by caulking D or an intermediate end plug having a through hole in the center, and the fuel pellet 22 is prevented from dropping by the caulking D or the intermediate end plug having a through hole in the center. Yes.
[0036]
In the unlikely event that the fuel pellet 22 is melted, the fuel pellet 22 is likely to fall from the core fuel region F toward the lower plenum region LP.
[0037]
In the fast breeder reactor fuel assembly 1 according to the present embodiment, at least the blanket fuel rods 3 are arranged in the outermost peripheral layer. Further, the number of blanket fuel rods 3 is larger than the number of fuel rod arrangement positions in the outermost circumferential layer, and the blanket fuel rods 3 that cannot be arranged in the outermost circumferential layer are arranged preferentially from the outermost circumferential side. To do. The figure shown in FIG. 1 is an example in which blanket fuel rods 3 are arranged in the two outermost layers.
[0038]
In the place where the blanket fuel rod 3 is not disposed, any type of the core fuel rod 4 of the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2) is disposed. A type 2 core fuel rod 4 (# 2) is arranged on the center side of the body.
[0039]
Next, the operation of the fast breeder reactor fuel assembly according to the present embodiment configured as described above will be described.
[0040]
FIG. 3 is a cross-sectional view showing an example of a core configured by loading the fuel assembly for a fast breeder reactor according to the present embodiment as described above.
[0041]
That is, the core 6 is configured by densely loading a plurality of fast breeder reactor fuel assemblies 1 having a regular hexagonal cross section in a two-dimensional manner.
[0042]
In the fast breeder reactor fuel assembly 1 according to the present embodiment, since the blanket fuel rods 3 are disposed at least in the outermost peripheral layer, the fast breeder reactor fuel assembly 1 is densely loaded. In the resulting core 6, as shown in FIG. 3, a wide blanket fuel region 7 is formed in which the regions where the blanket fuel rods 3 are arranged are connected to each other between the fast breeder reactor fuel assemblies 1.
[0043]
By forming the blanket fuel region 7 as described above, the region where the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2) are arranged in each fast breeder reactor fuel assembly 1. The core fuel region 8 is not coupled. That is, even between the adjacent fast breeder reactor fuel assemblies 1, each core fuel region 8 is arranged spatially separated.
[0044]
In the core 6 configured in this way, a case will be described in which an event occurs in which the fuel pellet 22 is melted.
[0045]
Fission occurring in the core 6 occurs mostly in the core fuel region 8 of the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2), which contains a large amount of fissile material. Yes. On the other hand, in the blanket fuel region 7 composed of the blanket fuel rod 3 containing almost no fissile material, the occurrence of fission is small.
[0046]
Therefore, the temperature of the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2) is higher than the temperature of the blanket fuel rod 3. That is, when an event that the fuel pellet 22 melts occurs, first, the fuel pellets loaded in the core fuel region F of the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2). 22 melting occurs.
[0047]
When the fuel pellets 22 loaded in the core fuel region F are melted, in the case of the type 2 core fuel rod 4 (# 2), the melted fuel pellets 22 are moved to the lower plenum region LP side as shown in FIG. Fall. On the other hand, in the case of the type 1 core fuel rod 4 (# 1), a blanket fuel region N is provided below the core fuel region F. In the blanket fuel region N, there is little fission and the temperature is low, so the possibility of melting is low. Therefore, in the case of the type 1 core fuel rod 4 (# 1), when the fuel pellet 22 loaded in the core fuel region F is melted, the melted fuel pellet 22 is used as it is in the core fuel region F or the lower blanket fuel region N. Is likely to be held in the area.
[0048]
As described above, when the fuel pellet 22 is melted, in the case of the type 1 core fuel rod 4 (# 1), the melted fuel pellet 22 is held in the core fuel region F or the lower blanket fuel region N. On the other hand, in the case of the type 2 core fuel rod 4 (# 2), the molten fuel pellet 22 falls to the lower plenum region LP side. As a result, the fissile material is dispersed in the axial direction in the core 6 and the effective multiplication factor of the core 6 is reduced.
[0049]
Furthermore, in the type 2 core fuel rod 4 (# 2), when the molten fuel pellet 22 falls to the lower plenum region LP side, the neutrons loaded in the neutron absorber region A provided on the lower side of the blanket fuel rod 3 Since the neutrons emitted from the fuel pellets 22 falling into the lower plenum region LP are efficiently absorbed by the absorber, the effective multiplication factor of the core 6 is further reduced.
[0050]
On the other hand, with respect to the radial direction of the core 6, the core fuel, which is an area where the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2) are arranged in each fast breeder reactor fuel assembly 1. Region 8 is spatially separated by blanket fuel region 7, as shown in FIG. In the blanket fuel loaded in the blanket fuel region 7, there is little fission and the temperature is low, so the possibility that the fuel pellets 22 melt is low.
[0051]
Therefore, even if the fuel pellet 22 containing a large amount of fissile material in the core fuel region 8 is melted, the blanket fuel region 7 becomes a boundary, so that adjacent fuel assemblies for the fast breeder reactor It is not combined with one core fuel region 8. In other words, the adjacent fuel assemblies for fast breeder reactors 1 are not connected to each other in a region where the fuel pellets 22 are melted to form a wide molten fuel region. This effect is further enhanced by making the outer diameter of the blanket fuel rod 3 larger than the outer diameter of the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2).
[0052]
As described above, in the fuel assembly for a fast breeder reactor according to the present embodiment, even if an event that the fuel pellet 22 is melted occurs due to the above-described action, When the molten pellet in the core fuel region F of the type 2 core fuel rod 4 (# 2) falls to the lower plenum region LP side, the fissile material is dispersed in the axial direction, and the effective multiplication factor of the core 6 is reduced. Can do.
[0053]
Furthermore, the neutrons emitted from the fuel pellets 22 that have fallen into the lower plenum region LP are efficiently absorbed by the neutron absorbers loaded in the neutron absorber region A, thereby further reducing the effective multiplication factor of the core 6. Can do.
[0054]
Further, in the fast breeder reactor fuel assembly 1 according to the present embodiment, since the blanket fuel rod 3 is disposed at least on the outermost periphery, such a fast breeder reactor fuel assembly 1 is densely loaded. In the core 6 to be formed, a wide blanket fuel region 7 can be formed in which the regions where the blanket fuel rods 3 are arranged are joined between the adjacent fast breeder reactor fuel assemblies 1. Since the blanket fuel region 7 becomes the boundary, the core fuel region 8 can be isolated for each fast breeder reactor fuel assembly 1. If the fuel pellets 22 in the core fuel region 8 are melted, Even if it exists, it does not physically combine with the fuel pellet 22 melted in the adjacent core fuel region 8.
[0055]
Therefore, in the fast breeder reactor fuel assembly 1 according to the present embodiment, even if an event occurs in which the fuel pellet 22 is melted, the region where the fuel pellet 22 is melted is connected. It is possible to prevent recriticality caused by molten nuclear fuel material.
[0056]
(Second Embodiment)
A second embodiment of the present invention will be described with reference to FIGS.
[0057]
FIG. 5 is a conceptual diagram showing a configuration example in the longitudinal direction of each fuel rod of the fuel assembly for a fast breeder reactor according to the second embodiment. The same parts as those in FIG. The description is omitted, and only different parts are described here.
[0058]
That is, as shown in FIG. 5, the fuel assembly for a fast breeder reactor according to the present embodiment uses a part of the lower side of the lower plenum region LP of the type 2 core fuel rod 4 (# 2) as a neutron absorber. Only the area A is different from the first embodiment. The length of the neutron absorber region A is less than the length of the blanket fuel region N provided on the upper and lower sides of the core fuel region F of the type 1 core fuel rod 4 (# 1).
[0059]
In the case of the first embodiment as shown in FIG. 2, the type 2 core fuel rod 4 (# 2) is not provided with the blanket fuel region N, so that the plenum is higher than the type 1 core fuel rod 4 (# 1). There is room in volume. This margin is allocated to the neutron absorber region A in the type 2 core fuel rod 4 (# 2) of the fast breeder reactor fuel assembly according to the present embodiment.
[0060]
That is, when the total length of both blanket fuel regions N provided on the upper and lower sides of the core fuel region F of the type 1 core fuel rod 4 (# 1) is the length of the neutron absorber region A, The plenum volumes of the type 1 core fuel rod 4 (# 1) and the type 2 core fuel rod 4 (# 2) are equal. Therefore, the length of the neutron absorber region A is the type 1 core fuel rod 4 (from the viewpoint of not reducing the ability to absorb the increase in the internal pressure due to the FP gas as compared with the type 1 core fuel rod 4 (# 1). The total length of the blanket fuel regions N respectively provided on the upper and lower sides of the core fuel region F of # 1) is not more than the total length.
[0061]
In the fuel assembly for a fast breeder reactor according to the present embodiment configured as described above, the neutron absorber region A is provided at the lower part of the type 2 core fuel rod 4 (# 2). When the event that the fuel pellet 22 melts occurs, as shown in FIG. 6, the melted fuel pellet 22 falls to the neutron absorber region A side.
[0062]
As a result, in addition to the fissile material being axially dispersed in the core 6, neutrons emitted from the dropped fuel pellets 22 are loaded into the neutron absorber region A of the blanket fuel rod 3. There is a new effect that not only the absorber but also the neutron absorber loaded in the neutron absorber region A of the type 2 core fuel rod 4 (# 2) is absorbed.
[0063]
Therefore, the core in which the fuel assembly for the fast breeder reactor according to the present embodiment is loaded is loaded with the fuel assembly for the fast breeder reactor according to the first embodiment by the operation as described above. Since the effective multiplication factor at the time of melting of the fuel pellets 22 can be further lowered than the core, it is possible to further increase the subcriticality and increase the safety.
[0064]
As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, this invention is not limited to this structure. Within the scope of the invented technical idea of the scope of claims, a person skilled in the art can conceive of various changes and modifications. The technical scope of the present invention is also applicable to these changes and modifications. It is understood that it belongs to.
[0065]
【The invention's effect】
As described above, according to the present invention, it is possible to prevent the regions where the fuel pellets are melted from being connected even if an event occurs in which the fuel pellets melt.
[0066]
As described above, it is possible to realize a fuel assembly for a fast breeder reactor in which recriticality due to the melted nuclear fuel material does not occur even if an event that causes the fuel pellet to melt occurs.
[Brief description of the drawings]
FIG. 1 is a 1/2 perspective cross-sectional view showing an example of fuel rod arrangement in a fuel assembly for a fast breeder reactor according to a first embodiment. FIG. 2 is a fuel assembly for a fast breeder reactor according to a first embodiment. FIG. 3 is a cross-sectional view showing an example of a core constructed by loading a fuel assembly for a fast breeder reactor according to the first embodiment. FIG. 4 is a conceptual diagram showing a configuration example in the longitudinal direction of each fuel rod of the fuel assembly for a fast breeder reactor according to the first embodiment (at the time of molten pellet dropping)
FIG. 5 is a conceptual diagram showing a configuration example in the longitudinal direction of each fuel rod of a fuel assembly for a fast breeder reactor according to a second embodiment. FIG. 6 is for a fast breeder reactor according to a second embodiment. Conceptual diagram showing a configuration example of each fuel rod in the fuel assembly in the longitudinal direction (when molten pellet is dropped)
FIG. 7 is a perspective view of a fast breeder reactor fuel assembly covered with a hexagonal wrapper tube. FIG. 8 is a perspective view of a fast breeder reactor fuel assembly with the wrapper tube removed.
A ... Neutron absorber region D ... Caulking F ... Core fuel region N ... Blanket fuel region TS ... Upper end plug BS ... Lower end plug UP ... Upper plenum region LP ... Lower plenum region SP ... Plenum springs 1, 20, 26 ... High speed Breeder fuel assembly 3 ... Blanket fuel rod 4 ... Core fuel rod 4 (# 1) ... Type 1 core fuel rod 4 (# 2) ... Type 2 core fuel rod 6 ... Core 7 ... Blanket fuel region 8 ... Core fuel Region 22 ... Fuel pellet 23 ... Fuel rod 24 ... Wrapper tube 27 ... Grid spacer

Claims (3)

A plurality of long first fuel rods loaded with a blanket fuel containing at least one of natural uranium and deteriorated uranium, and a plurality of second long fuel rods loaded with a nuclear fuel material containing at least plutonium. In the fuel assembly for a fast breeder reactor constructed by arranging the fuel rods in a triangular lattice pattern,
The first fuel rods are all disposed at least in the outermost peripheral layer, and the outer diameter of the first fuel rods is the same as or larger than the outer diameter of the second fuel rods, The first fuel rod is further loaded with a neutron absorber on one end side, the longitudinal direction of each fuel rod is substantially vertical, and the end side on which the neutron absorber is loaded is a lower portion Load the core to the side,
Each of the plurality of second fuel rods has a blanket region loaded with the blanket fuel on both sides of the fuel region loaded with the nuclear fuel material, and an end side of the both blanket regions. A fourth fuel rod having a plenum region for accumulating FP gas generated by fission, a blanket region of the third fuel rod as a plenum region, and a neutron absorber is further loaded on one end side. Belonging to one of the fuel rods of
For fast breeder reactors, in which the longitudinal direction of each fuel rod is substantially vertical, and the end of the fourth fuel rod loaded with the neutron absorber is loaded on the lower side. Fuel assembly.   The fuel assembly for a fast breeder reactor according to claim 1,
  When the nuclear fuel material loaded in the fuel region of the third fuel rod is melted, the molten nuclear fuel material is held in the fuel region by blanket regions provided on both sides thereof.
  When the nuclear fuel material loaded in the fuel region of the fourth fuel rod is melted, the melted nuclear fuel material drops in the plenum region on the lower side of the fuel region when the core is loaded and moves to the lower side. Fuel assembly for fast breeder reactors that has been reached.   In the fuel assembly for a fast breeder reactor according to claim 1 or 2,
  A fuel assembly for a fast breeder reactor, wherein the plenum region of the third fuel rod and the plenum region of the fourth fuel rod have substantially the same volume.

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