JPH0119556B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel rod that enables the economical and effective use of plutonium or depleted uranium obtained as a result of reprocessing spent nuclear fuel, and more specifically, to Approximately spherical uranium dioxide (UO ₂ ) nuclear fuel particles made from uranium, etc., and plutonium dioxide (PuO ₂ ) powder, or gadolinium oxide (Gd ₂ O ₃ ), which is a burnable poison to plutonium dioxide, in a certain ratio. After uniformly mixing the powder with the added
The present invention relates to a plutonium-utilizing nuclear fuel rod for a thermal neutron reactor for power generation, in which a metal fuel cladding tube is filled with these nuclear fuels and both ends of the tube are sealed.

The advanced converter reactor (ATR), a heavy water reactor that Japan is currently developing, uses cylindrical pellets made of dioxide, which is a uniform mixture of plutonium and natural uranium. At this time, there are two types of plutonium mixing ratios: approximately 1% and 1.5%.

As shown in Fig. 1, in a conventional nuclear fuel rod, a large number of cylindrical sintered pellets 1 of nuclear fuel material are loaded into a fuel cladding tube 2 in layers of 3 to 4 m, and both tube ends are connected with upper end plugs. 3 and a structure sealed with a lower end plug 4 are used in power reactors. In this case, a heat insulating pellet 5 made of alumina or the like is inserted between the bottom pellet and the bottom end plug, and a coil spring 7 spot-welded to a metal holding plate 6 is inserted between the top pellet and the top end plug. It is often provided. The cylindrical sintered pellets loaded inside the metal fuel cladding tube have an hourglass shape when sintered, as shown in Figure 2A. The radius of the gap between the pellet and the inner surface of the cladding tube (gap value) is 0.06 to 0.15 mm.
The cylindrical side surface of the pellet must be polished on the outside to make it uniform within the range of .

This polishing process is called Pellet-Clad Interaction (abbreviated as PCI), which will be described later.
It is also important from the viewpoint of reducing the amount of water and cannot be omitted. This periphery grinder (centerless grinder) is used especially when producing cylindrical pellets of mixed oxide with uranium using extremely toxic plutonium, etc.
% of abrasive debris is generated, which poses a troublesome problem in terms of maintaining the cleanliness of processing facilities. The reason for this is that plutonium, an artificial element obtained through the reprocessing of spent nuclear fuel, emits alpha (α) rays and is extremely harmful if taken into the human body.

Nuclear fuel rods using such cylindrical pellets also pose problems in terms of the service life of the fuel when the frequent power fluctuation operating modes of the nuclear reactor are encountered.
When a right cylindrical pellet 1 as shown in Fig. 2B is applied with a centerless grinder and used in a nuclear reactor, due to the large temperature gradient in the radial direction within the pellet, there is a radius around the pellet's circumference. Two to 20 directional cracks occur depending on the amount of heat generated per unit length of the nuclear fuel rod, and pellet fragments are formed with these cracks as boundaries. Note that the corners of both end faces of the pellet 1 in FIGS. 1 and 2B are chamfered to reduce PCI. Now, due to the generation of fragments, the pellet, which had a right cylindrical shape at the initial stage of use, changes when the reactor enters the rated power operation state, due to the effect that the temperature at the center of the pellet is higher than that at the outer periphery. Due to the fact that the density of the oxide fuel material near the end faces of the pellet is slightly greater than that at the center of the height, the fragments 8 of the pellet 1 tend to curl outward from their central axis, as shown in FIG. transforms into

This means that the envelope of the cylindrical pellet 1 used in a nuclear reactor still has a millet glass shape. For this reason, as shown in FIG. 4, the fragments 8 of the pellets come into strong contact with the inner surface of the fuel cladding tube 2, and the inner surface of the fuel cladding tube 2 facing the crack opening 9 of the fuel pellet 1 has a
PCI occurs due to the interaction between pellets and cladding, and stress is locally concentrated. This stress concentration area is exposed to a high-temperature, intensely corrosive environment that contains fission products (hereinafter referred to as FP) such as iodine and cesium, which are produced during the combustion of nuclear fuel materials.
A crack 10 occurs. hair crack 10
There is a high possibility that the fuel cladding tube 2 will expand in the thickness direction of the fuel cladding tube during operation of the nuclear reactor, and that the fuel cladding tube 2 will eventually break before the fuel reaches its designed life.

The present invention has been made in view of the following general background in addition to the current state of the prior art as described above.

(1) The proportion of Japan's nuclear power plants in the total power installed capacity will increase, forcing a shift from the conventional base load operation method to a load following operation method in the future. thing.

(2) As the amount of spent nuclear fuel stored domestically and the amount reprocessed domestically and internationally increases, the most effective use of plutonium is to use it as nuclear fuel for peaceful nuclear purposes. .

(3) Fast breeder reactors, which are considered to be the best option for using plutonium, are still in the development stage and are expected to face numerous technical and economic difficulties. Furnace (HWR)
There is also a growing momentum to burn plutonium in thermally neutral reactors such as light water reactors (PWR and BWR).

(4) Due to severe economic, international political, and environmental constraints and difficulties in reprocessing spent nuclear fuel, nuclear fuel must be kept in a reactor for a long period of time (up to 8 years).
There has been an increasing demand for nuclear fuel that can be used to achieve high fuel burn-up.

Therefore, an object of the present invention is to maintain the integrity of the fuel even in the load-following operation mode of a nuclear reactor over a long period of time, to reduce PCI, and to streamline the equipment of a plutonium fuel processing facility. It is an object of the present invention to provide a nuclear fuel rod for use in a power generation neutron nuclear reactor that is suitable for remote automatic production, has high overall safety, is inexpensive and easy to produce.

Another object of the present invention, in addition to the above objects, is to provide a plutonium-based nuclear fuel rod capable of achieving high burnup.

The present invention will be explained in detail below. The first aspect of the present invention
The invention provides approximately spherical nuclear fuel particles with an average particle size of 0.6 mm to 1.5 mm of stable dioxide (UO ₂ ) using enriched uranium, depleted uranium, natural uranium, or depleted uranium as a nuclear fuel material; Stable plutonium dioxide (PuO ₂ ) powder is added to this, mixed uniformly, and then filled into a metal fuel cladding tube so that the macrospace density distribution is almost uniform. This is a plutonium-based nuclear fuel rod consisting of a cladding tube with both ends sealed.

Here, the enrichment of the approximately spherical uranium dioxide fuel particles of 0.6 to 1.5 mm largely determines the overall nuclear and thermal properties of the nuclear fuel rod. The same particle has a theoretical density ratio of
When the TD is 94 to 97%, it is a sintered particle that is difficult to sinter during use in a nuclear reactor. The approximately spherical shape with a grain size of 0.6 to 1.5 mm was chosen because the inner diameter of the metal fuel cladding tube is 7 to 18 mm, depending on the reactor type, so the macro space of the fuel is This is because it is advantageous to use particles with a large particle size within a range where the density can be considered to be approximately uniform.

The second invention of the present invention is to replace the plutonium dioxide powder used in the first invention with gadolinium oxide (Gd ₂ O ₃ ), which is a burnable poison that suppresses plutonium dioxide and the nuclear fission reaction in the nuclear reactor. This is a plutonium-based nuclear fuel rod that uses a powder mixture of plutonium and plutonium. Stable plutonium dioxide powder or its mixture powder with gadolinium oxide is approximately 1/5 of the diameter of the uranium dioxide particles used.
Although sintered fine particles are preferable and can fit into the voids between uranium dioxide particles, powder may also be used in view of manufacturing costs. Gadolinium oxide (referred to as gadolinia) in plutonium dioxide is 1~
0.1~ with sintered fine particles mixed in the range up to 20%
A size of 0.3mm is good. If more gadolinia is added, it will become a mixed powder of plutonium dioxide and gadolinium oxide. At that time, the plutonium dioxide powder has a particle size distribution of about 0.2 to 10 μm and an (O/Pu) ratio of 2.00 to 2.03.
It is stable even in the atmosphere and suitable for practical use.

Next, an embodiment of the present invention is shown in FIG. In this embodiment, the above-mentioned stable uranium dioxide (UO ₂ ) nuclear fuel particles 11 and plutonium dioxide (PuO ₂ ) are contained in a long fuel cladding tube 2 to which a lower end plug 4 is circumferentially welded. Powder or mixed powder of plutonium dioxide and gadolinia (PuO ₂ +Gd ₂ O ₃ ) (hereinafter collectively referred to as plutonium-containing powder) 12 is uniformly mixed and filled to form the effective heat generating part of the nuclear fuel rod. However, the upper end of the fuel cladding tube 2 is sealed with an upper end plug 3. Lower end plug 4 and approximately spherical uranium dioxide nuclear fuel particles 1
A lower end partitioning layer 13 of graphite or metal wool is provided between the lowermost part of the homogeneous/mixed fuel filling bed with plutonium-containing powder 12 and plutonium-containing powder 12. In addition to fine stainless steel wire, the metal wool is preferably nickel-plated zirconium wire or strip material that absorbs little thermal neutrons.

This graphite wool or metal wool lower end partitioning layer 13 is subject to relative displacement based on the difference in thermal expansion in the length direction of the uniform/mixed fuel filling layer and the metal fuel cladding tube 2 during operation and shutdown of the reactor. , and acts to prevent excessive stress from being applied to the fuel cladding tube 2 in the longitudinal direction. Further, in this embodiment, an upper end partitioning layer 14 of graphite wool or metal wool is also provided at the top of the uniform/mixed fuel filling bed. This function involves filling the fuel cladding tube with homogeneous and mixed fuel, then evacuating the fuel cladding tube, replacing it with inert gas helium, and then circumferentially welding the upper end plug 3 to the cladding tube 2. , during this vacuum evacuation, the powdered nuclear fuel material is
The purpose is to act as a filter so as not to contaminate the vacuum equipment system.

A heat insulating pellet (upper part) 15 is inserted in contact with the upper end partition layer 14, and a coil spring 7 is inserted in contact with this pellet between it and the upper end plug 3. The coil spring 7 compresses and holds the uniformly mixed fuel filling layer within the fuel cladding tube 2, thereby ensuring a uniform and mixed fuel filling layer.
This prevents the generation of voids in the mixed fuel packed bed due to changes in volume due to the operation and shutdown of the nuclear fuel and reactor, thereby helping to maintain the integrity of the nuclear fuel rods in use in the reactor.

Here, the function of the burnable poison for suppressing the nuclear fission reaction in the core of a nuclear reactor according to the second invention of the nuclear fuel rod of the present invention will be described in detail with reference to the control ability of the nuclear reactor.

In order for a nuclear reactor to be able to operate continuously for a certain period of time without replenishing or replacing nuclear fuel, the reactor core must have a surplus reactivity greater than the minimum value that maintains the fission reaction. . On the other hand, if the value of the surplus reactivity is not less than the reactor shutdown margin, the reactor output will run out of control and reach a dangerous state. In other words, the excess reactivity of a nuclear reactor must be within a range that can be compensated for by solid control rods or liquid neutron absorbers in the reactor equipment. In this way, the surplus reactivity must always be maintained within the specified value during the operating period of the reactor, so the value of the surplus reactivity must have a certain upper limit in relation to the output control system of each reactor. The value is determined.

The request to increase the stay period of nuclear fuel in a reactor from 6 to 8 years to increase the burn-up of the fuel and increase the total amount of fission energy generated per nuclear fuel rod is based on the need to recycle spent nuclear fuel. Processing operations are becoming increasingly urgent due to the constraints and difficulties of economic and international political or local environmental factors.

A conventional nuclear fuel assembly loaded into the core of a power reactor consists of a large number of nuclear fuel rods, mainly made of cylindrical pellets of enriched uranium oxide. ) books, and in a pressurized light water cooled reactor (PWR), (17 x 17) books are assembled into a single unit with a grid-like cross section using spacers, upper and lower tie plates, etc.
The amount of nuclear fuel material contained in a nuclear fuel rod, i.e.
The content of fissile material or the degree of enrichment of uranium is set to different values depending on the position of the above-mentioned lattice arrangement in which the nuclear fuel rods are installed in the nuclear fuel assembly, and the local heat generation peak of the nuclear fuel rods is suppressed. There are many cases.

However, it is not possible to economically achieve the expected high fuel burn-up simply by employing different values of uranium enrichment for the fuel pellets of nuclear fuel rods. This is because the surplus reactivity of a normal nuclear reactor/core decreases in a monotonically decreasing manner as core combustion progresses, so if the initial uranium enrichment is increased in an attempt to improve fuel burnup, It will also be necessary to increase the margin for shutdown of the reactor and core.
This means that the number of solid control rods or the concentration of liquid neutron absorber must be increased, thus increasing uranium enrichment to improve fuel burnup and shutting down the reactor/core. The relationship with affordability becomes what is called a ``catching battle.''

By the way, if fuel pellets to which burnable poison, which suppresses the fission reaction of nuclear fuel material, is appropriately added are used in nuclear fuel rods, the burnable poison absorbs neutrons, so that the atomic nuclei of the burnable poison material do not absorb neutrons. Since it is converted into the nucleus of another substance, the initial surplus reactivity can be kept within the specified value specific to the reactor, and as the fuel burns, the concentration of burnable poison gradually decreases, and the decrease is However, the nuclear fission reaction of the fuel will proceed even more actively.

Due to the equilibrium relationship between the amount of burnable poison that disappears and the relative moss of nuclear fuel material, the curve with fuel burnup on the horizontal axis and surplus reactivity on the vertical axis shows the initial core surplus. The mixing ratio of burnable poison and nuclear fuel material should be set so that the reactivity is suppressed to a certain value or less, and after a certain burnup level, it becomes "flat" or even slightly "convex upward" in the center. Therefore, it is possible to improve the burn-up of the fuel.

BWR fuel pellets are made of uranium dioxide, or of uranium dioxide and gadolinium added uniformly throughout the pellet at various mixing ratios.

When compared with the prior art, the features of the present invention are clear.According to the second aspect of the present invention, substantially spherical uranium dioxide particles are all produced with the same enrichment and diameter, and mixed with the same concentration and diameter. By finely blending the formulation of gadolinium in a relatively small amount of plutonium-containing powder, it is possible to easily provide a nuclear fuel rod with a controlled content ratio of nuclear fuel material and burnable poison.

FIG. 6 is a partially enlarged cross-sectional view of the nuclear fuel rod according to the present invention as shown in FIG. As is clear from comparing this Figure 6 with the above-mentioned Figure 4, since particulate fuel is used in the first and second aspects of the present invention, conventional cylindrical pellets are not used on the inner surface of the cladding tube. PCI as seen when using this method does not occur at all.

Since the present invention is a nuclear fuel rod configured as described above, it can produce many excellent effects as described below. First, since there is no centerless grinding process for cylindrical plutonium-uranium mixed oxide pellets in the prior art, there is no generation of highly toxic plutonium grinding debris, and the roughly spherical uranium dioxide particles and plutonium-containing powder are Both can be regarded as semi-fluid, therefore, it is possible to manufacture and inspect fuel particles and fill them into metal fuel cladding tubes in a semi-automated continuous process.
Radiation exposure for fuel processing workers will be significantly reduced, and processing and inspection costs will be reduced. In addition, since the fuel in the present invention is in the form of particles, there is no PIC as seen in the case of conventional cylindrical pellets, making it suitable for load following operation, and it is easy to adjust the content of nuclear fuel material, especially fissile material. It is. Furthermore, when depleted uranium is used as the uranium dioxide used in the present invention, the depleted uranium obtained by reprocessing spent nuclear fuel can be easily mixed with plutonium powder to form desired nuclear fuel rods without re-enriching. Therefore, it is possible to significantly reduce costs. Normal plutonium obtained through reprocessing is mostly fissile material, but this type of plutonium also contains non-fissile isotopes, such as U-235.
Since plutonium dioxide corresponds to 60-80% highly enriched uranium, it is possible to adjust the nuclear and thermal performance of nuclear fuel rods with a relatively small amount of plutonium dioxide. In the second invention of the present invention, which uses a mixed powder of burnable poison and plutonium dioxide, in addition to the above-mentioned effects, the formulation of gadolinia in a relatively small amount of plutonium-containing powder mixed with uranium dioxide particles is This makes it possible to finely blend the nuclear fuel material and burnable poison, thereby providing a nuclear fuel rod with a high burnup and a controlled content ratio of nuclear fuel material and burnable poison. Also, when mixing burnable poison with plutonium dioxide,
Unlike boiling water reactor fuel, it is not mixed uniformly into pellets, making production line rearrangement and quality control easier.

As described above, the present invention provides an ideal plutonium-based nuclear fuel rod for use in a thermal neutron reactor for power generation, which is highly safe, inexpensive, and easily manufactured.

[Brief explanation of drawings]

Figure 1 is a diagram of a conventional nuclear fuel rod, Figure 2A is a diagram showing that the as-sintered cylindrical pellet has an hourglass shape, and Figure 2B is an illustration of an hourglass-shaped pellet. Figure 3 shows a model of a conventional right cylindrical pellet being deformed during use in a nuclear reactor. is an explanatory diagram of pellet-cladding interaction (PCI) observed on the inner surface of the cladding tube of a conventional nuclear fuel rod, and FIG. 5 is an explanatory diagram showing an embodiment of the nuclear fuel rod according to the present invention.
FIG. 6 is a partially enlarged cross-sectional view. 2... Metal fuel cladding tube, 3... Upper end plug, 4
... lower end plug, 11 ... nuclear fuel particles, 12 ... plutonium-containing powder.

Claims (1)

[Claims] 1. The average particle size of stable uranium dioxide (UO ₂ ) using enriched uranium, depleted uranium, natural uranium, or depleted uranium as a nuclear fuel material is 0.6 mm.
Approximately spherical nuclear fuel particles of 1.5 mm to 1.5 mm are uniformly mixed with stable plutonium dioxide (PuO ₂ ) powder, and then mixed in a metal fuel cladding tube so that the macroscopic spatial density distribution is almost uniform. A nuclear fuel rod utilizing plutonium, which is filled with plutonium and sealed at both ends of the metal fuel cladding tube. 2 The average particle size of stable uranium dioxide (UO ₂ ) using enriched uranium, depleted uranium, natural uranium, or depleted uranium as nuclear fuel material is 0.6 mm.
Approximately spherical nuclear fuel particles of 1.5 mm to 1.5 mm, stable plutonium dioxide (PuO ₂ ), and gadolinium oxide (Gd ₂ O ₃ ), a burnable poison that suppresses nuclear fission reactions in nuclear reactors, are mixed from 1% to 70%. After uniformly mixing the mixed powder added in the range of
A nuclear fuel rod utilizing plutonium, which is formed by filling a metal fuel cladding tube so that the macroscopic spatial density distribution is almost uniform, and sealing both ends of the metal fuel cladding tube.