JPH02159597A - Coated fuel particle coated with hardened over-coat and manufacturing of nuclear fuel using it - Google Patents

Abstract

PURPOSE:To prevent a coating layer by coating a surface of a coated fuel particle with a hardened over-coating layer which consists of a hardened matrix material made of a thermosetting resin containing graphite powder. CONSTITUTION:A coated fuel particle 3 can be obtained by sticking, with a ethanol spraying, a powder material (a matrix material A) which consists of 80 weight percent of a graphite powder and 20 weight percent of a phenol resin, to a coated fuel particle 1 of 900mum in diameter which is made by coating and by pyrolysis vacuum deposition of ceramics containing a carbon and a silicon cabide or a zirconium carbide, to a uranium dioxide core, to a coating thickness of 75mum, and after completing an over-coating in this way, by heating the particle with a heating furnace at 170 deg.C to harden the phenol resin and to form an over coating layer 2. To the particle 3, the same powder material as the matrix material A is over-coated to a coating thickness of 300mum. These particles are filled in a die and pressed to obtain a formed body and then the body is heated up to 1,800 deg.C in a vacuum condition and is so kept for one hour to be sintered. Through these processes, a nuclear fuel body can be obtained.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a cured overcoat coated fuel particle and a method for producing a nuclear fuel assembly using the same, and more specifically, to a coating layer of coated fuel particles in a nuclear fuel assembly. The present invention relates to a method for producing a nuclear fuel assembly that can produce a nuclear fuel assembly with a low failure rate.

[Prior Art and Problems to be Solved by the Invention] Coated fuel particles are generally used in nuclear fuel bodies used in high-temperature gas reactors and the like.

The coated fuel particles usually include a fuel core made of a nuclear fuel material such as uranium, thorium, or plutonium, and silicon carbide,
It is coated with ceramics such as zirconium carbide, carbon (e.g. pyrolytic carbon), etc.
Some have multiple coating layers, such as O type and TRl5O type.

The coating layer of the coated fuel particles has a function such as becoming a barrier layer to prevent the release of fission products generated during reactor operation, and in nuclear fuel bodies used in high-temperature gas reactors etc. It is an important component for maintaining the health of the system.

Such a nuclear fuel body is usually formed in a structure in which the coated fuel particles are dispersed in a graphite matrix containing graphite.

Conventionally, nuclear fuel bodies as described above are manufactured as follows.

For example, first, coated fuel particles are overcoated with a matrix material consisting of graphite powder and a binder such as a thermosetting resin. At this stage, the binder in the matrix material overcoating the coated fuel particles is in an uncured state.Next, the aggregate of coated fuel particles overcoated with this matrix material is pressure-molded. The thermosetting resin is filled in a molded area and subjected to pressure molding (press molding), and the thermosetting resin is cured by heating during molding. Then, the obtained molded body is heated and fired at a high temperature in an inert atmosphere.

In the above method, during pressure molding, the matrix material overcoating the coated fuel particles softens and flows to fill the gaps between the coated fuel particles.

Then, the binder in the matrix material is carbonized in the firing step, and a nuclear fuel body consisting of coated fuel particles and a graphite matrix is obtained.

However, such a method has the following problems.

In other words, as the entire matrix material softens and flows during pressure molding, the coated fuel particles may locally come into contact with each other, or the coated fuel particles may come into direct contact with the press surface, resulting in damage to the coated fuel particles themselves or This may cause damage to the coating layer. Furthermore, even if the breakage does not occur during the pressure forming, the matrix material shrinks due to carbonization of the binder in the firing process, while the coated fuel particles expand thermally, so the coated fuel particles may come into contact with each other. When the coating layer of the coated fuel particles is subjected to significant stress,
Eventually, damage to the coating layer is caused.

Therefore, the nuclear fuel assembly produced by such a method cannot fully demonstrate the function of the coating layer of the coated fuel particles, so there is a problem that it is not sufficiently effective as a nuclear fuel assembly used in high-temperature gas reactors, etc. There is a point.

The present invention has been made based on the above circumstances.

That is, one object of the present invention is to solve the above-mentioned problems, and to provide a nuclear fuel assembly in which damage such as cracks does not easily occur in the coating layer of coated fuel particles during manufacturing, and the rate of damage to the coating layer of coated fuel particles in the nuclear fuel assembly is low. An object of the present invention is to provide a cured overcoat coated fuel particle that can be produced in a manner similar to that of the present invention, and a method for producing a nuclear fuel assembly using the cured overcoat coated fuel particle.

[Means for Solving the Problems] In order to achieve the above problems, as a result of intensive studies, coated fuel particles having a specific structure are adopted, and when the coated fuel particles are used, the coated fuel particles are It is difficult for damage such as cracks to occur in the coating layer or damage to the coated fuel particles themselves.

It has been discovered that it is possible to produce a nuclear fuel assembly in which the coating layer of coated fuel particles in the nuclear fuel assembly has a low failure rate,
We have arrived at the present invention.

The present invention for solving the above problems is characterized in that the surface of coated fuel particles is coated with a cured overcoat layer containing graphite powder and made by curing a matrix material made of a thermosetting resin. (Claim 1)

In a method for producing a nuclear fuel assembly by press-molding coated fuel particles and a matrix material consisting of graphite powder and a binder, the coated fuel particles contain graphite powder and are thermoset on the surface of the coated fuel particles. 2. A method for producing a nuclear fuel assembly, characterized in that the method uses hardened overcoat coated fuel particles which are coated with a hardened overcoat layer formed by hardening a matrix material made of a synthetic resin (claim 2), the coated fuel particles In the method for producing a nuclear fuel assembly by pressure molding a matrix material comprising graphite powder and a binder, the coated fuel particles contain graphite powder and are made of a thermosetting resin. Two-layer coat-covered fuel particles are used in which a graphite matrix layer made of graphite powder and a binder is formed on the surface of the cured overcoat-covered fuel particles, which are coated with a cured overcoat layer made by curing a matrix material. (Claim 3) A method for producing a nuclear fuel assembly by press-molding coated fuel particles, wherein the coated fuel particles have a surface coated with graphite. A graphite matrix layer made of graphite powder and a binder is formed on the surface of cured overcoat-coated fuel particles containing powder and coated with a cured overcoat layer made by curing a matrix material made of a thermosetting resin. (Claim 4)
.

The cured overcoat coated fuel particles of the present invention will be explained.

One of the important points in the present invention regarding the cured overcoat is that the surface of the coated fuel particles is overcoated with a matrix material made of graphite powder and a thermosetting resin, and the matrix material (hereinafter referred to as matrix material A) is ) is thermally cured to form a cured overcoat layer on the coated fuel particles.

The coated fuel particles used are usually.

Known coated fuel particles used in nuclear fuel bodies used in high-temperature gas reactors and the like can be mentioned.

The coated fuel particles include a fuel core made of nuclear fuel material, ceramics such as silicon carbide and zirconium carbide, and carbon (
A coating layer is formed by coating with pyrolytic carbon (pyrolytic carbon), etc., and some have a multilayer coating layer, such as the BISO type and the TRl5O type.

The fuel core is made of nuclear fuel material containing uranium, thorium, plutonium, etc. 1 Usually has an average particle size of 400 to 6
Examples include oxide particles such as uranium, thorium, and plutonium with a diameter of 00 μm.

The average particle size of the coated fuel particles is usually 800 to 900.
It is le m.

The thickness of the coating layer is usually 150 to 250 m.

The coating layer of the coated fuel particles has a function such as serving as a barrier layer for preventing the release of fission products generated during nuclear reactor operation.

Therefore, in nuclear fuel bodies used in high-temperature gas reactors and the like, the coating layer is important for maintaining the integrity of the nuclear fuel.

The matrix material A used contains graphite powder and thermosetting resin.

As the graphite powder, any known graphite powder can be used, and there are no particular limitations.

The average particle size of the graphite powder is usually 20 to 30 ILm.

Examples of the thermosetting resin include phenol resins, urea resins, melamine resins, xylene resins, polyester resins, epoxy resins, urethane resins, etc. 1 Usually, phenolic resins are employed, and in particular, novolac type phenolic resins. will be adopted.

In the present invention, the thermosetting resin preferably has a gel residue of 70% by weight or more when heated to acetone boiling. There is little deformation over time, and sufficient effects can be obtained. however.

Even if the gel residue after boiling and heating acetone is less than 70% by weight, the object of the present invention can be achieved.

The blending ratio of the graphite powder and the thermosetting resin (graphite powder:thermosetting resin) in the matrix material A to be used is usually 9:1 to 6:4, preferably 8.5.
: 1.5 to 7:3.

Note that the matrix material A may contain additives in addition to the graphite powder and the thermosetting resin.

Examples of the additive include lubricants commonly used in powder molding, such as stearic acid and zinc stearate.

To overcoat the surface of the coated fuel particles with the matrix material A, the coated fuel particles are generally transferred over the matrix material A.

It is possible to adopt a method of overcoating by spraying a solvent or applying heat to impart adhesion between the coated fuel particles and the matrix material A.

The thickness of the layer made of the matrix material A formed by overcoating the matrix material A on the surface of the coated fuel particles is normal.

50 to 200 m, preferably 75 to 100 m.

In order to heat cure the matrix material A overcoated on the surface of the coated fuel particles, it is sufficient to heat the thermosetting resin to sufficiently cure the matrix material A. Heating conditions vary depending on the type and amount of the thermosetting resin used, but usually:
The heating temperature is 130 to 220°C and the heating time is 5 minutes to 1 hour, preferably the heating temperature is 130 to 170°C and the heating time is 20 minutes to 40 hours.

In this way, it is possible to obtain, for example, cured overcoat-covered fuel particles 3 in which a cured overcoat layer 2 is formed on the surface of coated fuel particles 1 as shown in FIG. 1.

Next, a method for manufacturing a nuclear fuel body will be explained.

Tosei Kou 1 One of the important points in the present invention is a matrix material (hereinafter sometimes referred to as matrix material B) consisting of the cured overcoat-covered fuel particles, graphite powder, and a binder.

) to manufacture nuclear fuel bodies.

The matrix material B used consists of graphite powder and a binder.

The average particle size of the graphite powder contained in matrix material B is usually 20 to 30 gm, and preferably the same average particle size as the graphite powder contained in matrix material A described above.

As the binder, a thermosetting resin, a thermosoftening resin, etc. can be used, but it is preferable to use a thermosetting resin in order to prevent deformation of the molded body during the firing step.

As the thermosetting resin that can be contained in matrix material B, the same thermosetting resin as that contained in matrix material A can be mentioned.

In the present invention, the thermosetting resin contained in the matrix material B is preferably the same thermosetting resin as the thermosetting resin contained in the matrix material A.

The blending ratio of the graphite powder and the binder in the matrix material B to be used (graphite powder: binder) is usually 1,
9:1 to 6:4, preferably 8.5:1.5 to 7
:3.

In the present invention, it is preferable that the matrix material B has the same M1 composition as the matrix material A.

When the matrix material B and the matrix material A have the same composition, after the molded body is fired, the graphite matrix formed by carbonizing the binder of the matrix material B and the thermosetting resin of the matrix material A are carbonized. Since the graphite matrix in the resulting nuclear fuel body becomes uniform, the graphite matrix in the obtained nuclear fuel body can be made uniform. Furthermore, when the matrix material B and the matrix material A have the same composition, the preparation of the matrix material can be simplified.

When manufacturing a nuclear fuel assembly, for example, as shown in FIG. As shown in the figure, a graphite matrix layer 5 made of graphite powder and a binder is further formed on the surface of the cured overcoat coated fuel particles 3,
A method of press-molding the resulting aggregate of the two-layer coated fuel particles 6, and (1) a method of forming the two-layer coated fuel particles 6 with a matrix material 4 made of graphite and a binder, as shown in FIG. Here, the matrix material 4 is a known matrix material (matrix material B) made of graphite and a binder, and the graphite in the two-layer coated fuel particles The matrix layer can be formed using the matrix material 4 (the matrix material B). However, the thickness of the graphite matrix layer is usually 150 to 350 gm, preferably 250 to 300 gm.

As a means for carrying out the above-mentioned pressure molding.

For example, a die may be filled with a mixture of cured overcoated fuel particles and matrix material B, an aggregate of the two-layer coated fuel particles, or a mixture of the two-layer coated fuel particles and matrix material B. , means by punching from above and below, and means by isostatic pressing such as a rubber press method, and these pressing methods may be either warm press or cold press. Furthermore, as a means for carrying out the pressure molding, an extrusion molding method can also be employed.

By means of these pressure-forming methods, it is possible to obtain a molded article in any shape, such as a solid cylinder, a hollow cylinder, a sphere, or any other shape.

In addition, the molding pressure during pressure molding is usually 20 to 50 kg.
/cs2, and is preferably set within a range of 30 to 40 t/cm2.

For example, as shown in FIG. 5, the molded body obtained in this way is composed of coated fuel particles 1 with a cured overcoat layer 2 formed thereon and matrix material 4, coated fuel particles l and coated fuel particles 1. There is at least a hardened overcoat between the fuel particles! There is a distance a corresponding to twice the layer thickness of the cured overcoat layer 2, and there is a distance a corresponding to at least the layer thickness of the cured overcoat layer 2 between the coated fuel particles 1 and the pressing surface 7.

The presence of this cured overcoat layer protects the coated fuel particles, and the cured overcoat layer acts as a buffer against stress during pressure molding. Furthermore, if heating is applied during pressure molding, this cured overcoat layer exhibits a buffering effect against thermal shock, thermal shrinkage, radiation impact, and the like.

mini 1 Next, the molded body is subjected to a sintering treatment.

The sintering temperature in the sintering process is usually 1600~
Set within the range of 1900°C.

The time required for sintering is usually 1 to 5 hours.

By performing such a sintering process, the matrix materials (matrix material A and matrix material B) in the molded body are
), the organic matter in the matrix material is carbonized and becomes a graphite matrix.

In this way, a nuclear fuel body consisting of coated fuel particles and a graphite matrix is obtained.

The nuclear fuel assembly produced by the method of the present invention can reduce the failure rate of the coating layer of coated fuel particles.

In addition, in the nuclear fuel assembly produced by the method of the present invention, since the coated fuel particles are appropriately dispersed in the nuclear fuel assembly, the density of the graphite matrix in the nuclear fuel assembly does not become too low locally, and the graphite matrix does not become too low. The notepad that the matrix has (
(Improvement in thermal conductivity, protection of mM of coated fuel particles) can be fully exhibited.

Therefore, a nuclear fuel assembly manufactured by this method can be suitably used as a nuclear fuel assembly used in, for example, a high-temperature gas reactor.

[Example] Next, Examples of the present invention will be shown to further specifically explain the present invention.

(Example 1) A core of uranium dioxide is coated with ceramics containing carbon and silicon carbide or zirconium carbide by pyrolytic vapor deposition using a conventional method. Coated fuel particles with a diameter of 900 gm are coated with 80% by weight of graphite powder and phenol resin. After applying and overcoating a powder consisting of 20% by weight to a thickness of 75 pm while spraying ethanol, 1
The coated fuel particles were heated in a heating furnace at 70° C. for 30 minutes to cure the phenol resin and form a cured overcoat layer.

Next, the same powder as the above-mentioned powder consisting of 80% by weight of graphite powder and 20% by weight of phenolic resin was added to the coated fuel particles formed by forming the cured overcoat layer in the same manner as described above. It was applied to a thick layer and overcoated. However, the amount of overcoat varies depending on the density of the coated fuel particles and graphite matrix layer in the nuclear fuel assembly determined by the specifications.

These particles (coated fuel particles formed by forming a hardened overcoat layer and further overcoated with graphite powder, etc.) were filled into a die at 18°C (1°C) and press-molded to obtain a compact. .

The temperature of this molded body was raised to 1800° C. in a vacuum, and the temperature was held for 1 hour to sinter it, thereby obtaining a nuclear fuel body.

The obtained nuclear fuel body was roasted at 800°C, and the coated fuel particles obtained by roasting were immersed in nitric acid, and the eluted uranium was analyzed to examine the failure rate. Here, the breakage rate is the breakage rate of the silicon carbide layer (ceramic layer) in the coated fuel particle coating layer. The failure rate of the coating layer in the nuclear fuel body is
The breakage rate was 1.4 x 10-6%, which was sufficiently lower than the breakage rate of 5 x 10-4% in the conventional manufacturing method.

(Example 2) Coated fuel particles formed with a cured overcoat layer,
The same procedure as in Example 1 was carried out until obtaining coated fuel particles overcoated with graphite powder and the like.

This powder (coated fuel particles formed by forming a hardened overcoat layer and coated fuel particles further overcoated with graphite powder, etc.) is filled into 9 rubber-like shapes, and isostatically pressurized with hydrostatic pressure to form a diameter of 60 mm. After being pressure-molded into a spherical shape, the phenol resin was cured by holding it in a heating furnace at 180°C for 5 minutes to obtain a molded body.

The temperature of this molded body was raised to 1800° C. in a vacuum, and the temperature was held for 1 hour to sinter it, thereby obtaining a nuclear fuel body.

When the failure rate of the obtained nuclear fuel assembly was examined in the same manner as in Example 1, the failure rate of the coating layer of the coated fuel particles in the nuclear fuel assembly was 1.8X 10-6%, which is sufficiently low. The damage rate was

[Effect of the invention] According to the invention.

(1) Damage such as cracks is less likely to occur in the adhering layer of the coated fuel particles during manufacturing, and (2) the rate of damage to the coating layer of the coated fuel particles in the nuclear fuel assembly can be reduced.

(3) Furthermore, since it is possible to manufacture a nuclear fuel assembly in which coated fuel particles are appropriately dispersed,
It has the advantage that the density of the graphite matrix in the nuclear fuel body does not become too low, and the functions of the graphite matrix (improving thermal conductivity, protecting the coating layer of coated fuel particles) can be fully demonstrated. A cured overcoat coated fuel particle and a method for producing a nuclear fuel assembly using the same can be provided.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of coated fuel particles formed with a cured overcoat layer. FIGS. 2 to 4 are explanatory diagrams showing an example of a mode in which coated fuel particles formed with a cured overcoat layer are pressure-molded together with matrix material B. FIG. 5 is an explanatory view showing an example of the state of a molded body in the method of the present invention. l...Covered fuel particles, 2...Cured overcoat layer, 3...Cured overcoat coated fuel particles, 4...
Matrix material consisting of graphite powder and binder, 5 groups...
Matrix layer consisting of graphite powder and binder, 6...
Double-layer coat coated fuel particles, 7...Press surface. Figure 2 Figure 3 Figure 4 Figure 5

Claims (4)

[Claims] (1) A cured overcoat coated fuel particle characterized in that the surface of the coated fuel particle is coated with a cured overcoat layer containing graphite powder and made by curing a matrix material made of a thermosetting resin. . (2) In a method for producing a nuclear fuel body by press-molding coated fuel particles and a matrix material consisting of graphite powder and a binder, the cured overcoat coated fuel particles according to claim 1 are used as the coated fuel particles. A method for manufacturing a nuclear fuel body, characterized by: (3) In a method for producing a nuclear fuel assembly by press-molding coated fuel particles and a matrix material comprising graphite powder and a binder, the coated fuel particles include the cured overcoat coated fuel particles according to claim 1. 1. A method for producing a nuclear fuel assembly, comprising using two-layer coated fuel particles having a graphite matrix layer formed on the surface thereof made of graphite powder and a binder. (4) In a method for producing a nuclear fuel assembly by pressure molding coated fuel particles, the coated fuel particles include: on the surface of the cured overcoat coated fuel particles according to claim 1;
1. A method for producing a nuclear fuel body, comprising using double-layer coated fuel particles formed by forming a graphite matrix layer consisting of graphite powder and a binder.