JPH02257093A - Production of nuclear fuel body and overcoated and coated fuel particle - Google Patents

Abstract

PURPOSE:To lower the failure rate of a coating layer by constituting an overcoat layer into two layers and forming the inside layer of a matrix material having the shrinkage rate smaller than the shrinkage rate of the matrix material forming the outside layer thereof at the time of calcination of this material. CONSTITUTION:The inner overcoat layer 3b is formed of the matrix material B on the surface of the coated fuel particle 2 and the outer overcoat layer 3a is formed of the matrix material A thereon to form the overcoated and coated fuel particle 1. The compounding ratio of a novolak type phenolic resin is preferably in an 85:15 to 70:30 range at the time of adopting this resin as the binder of the matrix material A. The shrinkage of the matrix material by moldability and calcination and the mechanical characteristics after the calcination are optimized by maintaining the compounding ratio within the above-mentioned range.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application 1] The present invention relates to a method for producing a nuclear fuel body and an overcoat coated fuel particle. More specifically, the present invention relates to a method for producing a nuclear fuel assembly in which the coating layer of coated fuel particles has a low failure rate, and overcoated fuel particles that can produce the nuclear fuel assembly.

[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 has a multilayer coating of three or more layers of ceramics such as zirconium carbide and pyrolytic carbon.

The nuclear fuel body usually has a structure in which such coated fuel particles are dispersed in a graphite matrix containing graphite as a main component.

The coating layer of the coated fuel particles serves as a barrier to prevent the release of fission products generated during reactor operation,
In nuclear fuel assemblies used in high-temperature gas reactors and the like, it is an important component for maintaining the integrity of the nuclear fuel.

Therefore, in nuclear fuel assemblies used in high-temperature gas reactors etc., defects in the coating layer of the coated fuel particles (
It is important to reduce the amount of damage (damage, etc.).

In addition, the graphite matrix of the nuclear fuel assembly disperses the clad fuel particles to equalize the thermal output of the nuclear fuel assembly, and the mechanical properties do not deteriorate even with the operating temperature of the high temperature gas reactor, so the dispersion state of the clad fuel particles is maintained. is held constant.

Conventionally, nuclear fuel bodies as described above are manufactured by the following method.

For example, first, coated fuel particles are overcoated with a matrix material consisting of graphite powder and a binder such as a thermosetting resin. Next, a certain amount of aggregates of coated fuel particles overcoated with this matrix material are filled into a pressure molding device, etc., and pressure molded (press molded), and the binder is hardened by heating during molding. A molded body is obtained so that the shape is maintained even during firing. Then, the obtained molded body is heated and fired at a high temperature in an inert atmosphere or under vacuum. The purpose of overcoating the coated fuel particles with a matrix material before molding is to prevent the coated fuel particles from coming into direct contact with each other as much as possible and reduce defects in the coating layer of the coated fuel particles. be.

However, in the above method, the binder in the matrix material is thermally decomposed and the matrix material contracts, while the coated fuel particles thermally expand, so significant stress is applied to the coating layer of the coated fuel particles. Finally, there is a problem in that damage such as cracks occurs in the coating layer of the coated fuel particles.

Note that a nuclear fuel assembly containing damaged coated fuel particles may not be able to contain fissile material generated during nuclear reactor operation, and may contaminate the reactor atmosphere.

Therefore, the use of nuclear fuel assemblies containing damaged clad fuel particles must be avoided, and improvements in methods of manufacturing such nuclear fuel assemblies are desired.

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

That is, an object of the present invention is to solve the above-mentioned problems and to manufacture a nuclear fuel assembly in which damage such as cracks occurs in the coating layer of the coated fuel particles during production, and the rate of damage to the coating layer of the coated fuel particles is small. An object of the present invention is to provide a method and an overcoated fuel particle by which a nuclear fuel body thereof can be manufactured.

[Means for Solving the Problems] The invention according to claim 1 for solving the above problems includes forming an overcoat layer on the surface of coated fuel particles with a matrix material made of graphite powder and a binder, After obtaining the overcoat coated fuel particles, in the method for producing a nuclear fuel assembly, the overcoat coated fuel particles are press-molded and fired. A method for producing a nuclear fuel assembly, characterized in that the inner layer is formed with a matrix material having a shrinkage rate smaller than the shrinkage rate, and the invention according to claim 2 provides a method for producing a nuclear fuel body, characterized in that the inner layer is formed with a matrix material having a shrinkage rate smaller than the shrinkage rate, In the overcoat coated fuel particles, the overcoat layer has a two-layer structure, and has a shrinkage rate smaller than that of the matrix material forming the outer layer during firing. The present invention is an overcoated fuel particle characterized in that an inner layer is formed of a matrix material.

Coated Fuel Particles The coated fuel particles used in the present invention usually include:
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., and usually has an average particle size of 400 to E.
Examples include oxide particles of i00gm of uranium, thorium, brutonium, etc.

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

The thickness of the coating layer is usually 150 to 250 p.
It is Lm.

Overcoat Layer In the present invention, a two-layer overcoat layer consisting of an inner overcoat layer (inner layer) and an outer overcoat layer (outer layer) is formed on the surface of the coated fuel particles.

Both the inner overcoat layer and the outer overcoat layer are formed of a matrix material consisting of graphite powder and a binder.

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 3 Jtm.

The graphite powder in the matrix material forming the outer overcoat layer (hereinafter sometimes referred to as matrix material A) and the matrix material forming the inner overcoat layer (hereinafter sometimes referred to as matrix material B).

) is preferably the same material and average particle size as the graphite powder.

The binder may be used for molding various ceramics, etc.
Known resins used as binders can be used, and thermosetting resins are usually used in consideration of dimensional stability during firing.

Examples of the thermosetting resin include phenol resin, urea resin, melamine resin, xylene resin, polyester resin, epoxy resin, urethane resin, silicone resin, etc. Phenol resin is usually used, and novolak resin is particularly used. A type phenolic resin is suitably employed.

In matrix material A, the blending ratio (weight ratio, graphite powder:binder) of graphite powder and binder is usually 90:1.
It is within the range of 0 to 60:40.

When a novolac type phenolic resin is employed as the binder for the matrix material A, the blending ratio (weight ratio, graphite powder:binder) is preferably within the range of 85:15 to 70:30. When the blending ratio is within the above range, moldability, shrinkage of the matrix material upon firing, and mechanical properties after firing can be made appropriate.

One of the important points in the present invention is the shrinkage rate of matrix material B during firing [(volume before firing - volume after firing)/(
volume before firing)] is smaller than the shrinkage rate of matrix material A during firing.

The shrinkage rate of the matrix material B during firing is usually 2.5.
% or less, preferably 1.5% or less.

In order to make the shrinkage rate of matrix material B smaller than the shrinkage rate of matrix material A during firing, for example,
This can be done in such a manner that the amount of binder blended in matrix material B is smaller than the blended amount of binder in matrix material A.

Note that since the graphite powder in the matrix material does not substantially shrink even when fired, the shrinkage rate of the matrix material during firing can be reduced by reducing the amount of binder blended in the matrix material.

In the above embodiment, for example, when a novolac type phenolic resin is used as the binder, (1) the amount of the binder in the matrix material B is usually less than 15% by weight in order to reduce the shrinkage rate during firing; More preferably, the amount is 10% by weight or less, and (2) the amount of the binder in matrix material A is usually 15% by weight or more.

The blending amount of the binder in matrix material A is 15% by weight.
In this case, the matrix material A has sufficient fluidity during press molding, and a good molded product can be obtained by press molding.

In addition, if the blending amount of the binder in the matrix material A is too small, the fluidity during press molding will be insufficient, and the gaps between the overcoat coated fuel particles before press molding cannot be sufficiently filled. The molded product after press molding may have many openings and be of poor quality.

In the present invention, as the binder in the matrix material B, it is preferable to select a resin that exhibits small shrinkage during sintering.

Resins with low shrinkage during firing are preferably those with a molecular structure that does not cause partial melting of decomposed products during firing, and those containing inorganic components such as resins with aromatic rings or heterocycles and silicone resins. Examples include resin. Among these, for example, a compound of a bismaleimide thermosetting resin can be suitably used. In addition, in matrix material B, when a novolak type phenolic resin with an amine type curing agent as a curing agent is used as a binder as a resin with small shrinkage during firing, when hexamethylenetetramine is used as a curing agent of a novolac type phenol resin. In order to reduce the amount of formaldehyde generated during firing, it is preferable that the amount of hexamethylenetetramine added to the novolac type phenol resin is less than 6% by weight.

On the other hand, in matrix material A, when a novolak type phenolic resin is used as a binder, it is preferable to use an amine curing agent as a curing agent for the novolak type phenolic resin, but hexamethylenetetramine is used as a curing agent for the novolak type phenolic resin. When used, the amount of hexamethylenetetramine added is less than 6% by weight,
Since the heat curing rate during press molding may become extremely slow and impractical, the amount of hexamethylenetetramine added is usually 6% by weight or more.

In addition, when using the same type of novolac type phenolic resin as the binder for matrix material A and matrix material B, the shrinkage rate of matrix material B during firing can be adjusted by changing the amount of amine curing agent used as a curing agent. It can also be made smaller than the shrinkage rate of matrix material A during firing.

Production of overcoat coated fuel particles In the present invention, an inner overcoat layer (inner layer) is formed on the surface of the coated fuel particles using the matrix material B,
Outer overcoat layer (outer layer) with the matrix material A
to obtain overcoated fuel particles.

In order to form an inner overcoat layer with the matrix material B on the surface of the coated fuel particles, usually a solvent is sprayed or heat is applied while the coated fuel particles are transferred with the matrix material Bl. It is possible to employ a method of imparting adhesion between the coated fuel particles and the matrix material B to overcoat them.

In order to form an outer overcoat layer with the matrix material A on the coated fuel particles on which an inner overcoat layer is formed with the matrix material B, the surface of the coated fuel particles is overcoated with the matrix material B. You can do it in the same way as

In this way, for example, as shown in FIG. 1, an inner overcoat layer 3b is formed using matrix material B, and an outer overcoat layer 3a is formed using matrix material A, on the surface of coated fuel particles 2. Overcoat coated fuel particles l are obtained.

The thickness of the inner overcoat layer and the outer overcoat layer in the overcoat coated fuel particles vary depending on the dimensions of the nuclear fuel assembly to be manufactured, the filling amount of the coated fuel particles, and the composition of the matrix material, and are generally defined. However, it can be determined depending on each case, taking into consideration moldability and the effects of the present invention.

For example, when using a novolac-type phenolic resin as the binder, the inner overcoat layer thickness is typically 50 mm.
~200pm, preferably 75~100pm, and the total thickness of the overcoat layer consisting of the inner overcoat layer and the outer overcoat layer is usually 15
It is 0 to 350 JLm.

Press Molding Step Next, the overcoat coated fuel particles thus obtained are press molded into a desired shape using a pressure molding machine or the like.

Examples of the means for carrying out the above-mentioned press forming include a method of filling a die with a plurality of overcoated fuel particles 1 as shown in FIG. 2 and punching them from above and below, a rubber press method, etc. The isostatic pressing method may be used, and these pressing methods may be either warm pressing or cold pressing. Furthermore, as a means for performing the press molding, extrusion molding can also be employed.

By means of these press-forming methods, it is possible to obtain a molded article in a solid cylindrical shape, a hollow cylindrical shape, a spherical shape, or any other arbitrary shape.

In addition, the molding pressure during press molding is usually 20 to 50 k
g/c+m2, within the range of 30-40kg/cm2
It is preferable to set it within the range of .

As shown in FIG. 3, the state of the molded body obtained in this way is that the coated fuel particles 2 are surrounded by a material [matrix material B (■)] that has low shrinkage during firing, and the coated fuel The gaps between particles are filled with matrix material A (a)
and matrix material B (a).

- Sintering process - The sintering process usually involves removing most of the thermal decomposition products in the compact while heating the obtained compact in an inert atmosphere such as nitrogen gas, and then Finally in vacuum, 1600
A fired nuclear fuel body is obtained by heating at a temperature within the range of ~1800°C.

At this time, the matrix material B has a small shrinkage and acts as a buffer against the shrinkage stress of the matrix material A acting on the coated fuel particles, so that no damage occurs to the coating layer.

As described above, the nuclear fuel assembly produced by the method of the present invention has a low failure rate of the coating layer of the coated fuel particles, and therefore can be suitably used as a nuclear fuel assembly used in a high-temperature gas reactor.

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

(Example 1) Low-density carbon (layer thickness 60 lm, density 1
．． 1 g/c shoulder? ), high-density carbon as the second layer (layer thickness 3
3 m, density 1.82 g/c2), silicon carbide as the third layer (layer thickness 33 m, density 3.2 g/c2), and high-density carbon as the fourth layer (layer thickness 481 Lm). , density 1
．． 83g/c layer? ), with an overall diameter of 9387
Coated fuel particles of Lm were used.

To these coated fuel particles, 82.5% by weight of graphite powder, 7.5% by weight of phenolic resin (added amount of hexamine: 12% by weight)
An inner overcoat layer was formed by adhering a powder consisting of 807°C m in thickness while spraying ethanol, and then 80% by weight of graphite powder and 2% phenolic resin.
Form an outer overcoat layer by depositing powder consisting of 0% by weight (hexamine addition amount: 12% by weight) in the same manner until the thickness (thickness of the inner overcoat layer and the outer overcoat layer) becomes 2BOgm, Overcoat coated fuel particles were obtained.

The thus obtained overcoated fuel particles were filled into a die with an outer diameter of 2BOmm set at 130°C, and after pressurized at 30kg/cm2, the temperature was raised to 180°C and heat-cured for 10 minutes. Thereafter, it was taken out to obtain a cylindrical molded body.

The temperature of the obtained molded body was raised to 1800°C, and the temperature was increased to 1800°C, and
A nuclear fuel body was produced by holding and firing for a certain period of time.

The produced nuclear fuel assembly was roasted in air at 800°C to obtain particles (coated fuel particles). Next, the obtained particles were immersed in acid, the eluted uranium was analyzed, and the ratio of the amount of eluted uranium to the total amount of uranium was calculated as the failure rate of coated fuel particles in this nuclear fuel body. The failure rate measured in this way was 1.8 x 10-6, which was good enough to be used as a nuclear fuel body for a nuclear reactor.

(Example 2) Overcoat coated fuel particles prepared in the same manner as in Example 1 were filled into a rubber shaped material, and
The mixture was press-molded into a spherical shape with a diameter of 80 mm by isostatic pressurization with a hydrostatic pressure of 2, and then held 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 and fired to produce a nuclear fuel body.

The fracture rate of coated fuel particles in the produced nuclear fuel assembly was measured in the same manner as in Example 1, and was found to be 1.6×10 −6 , which was a sufficiently low failure rate.

(Comparative Example 1) Using the coated fuel particles used in Example 1, powder consisting of 80% by weight of graphite powder and 20% by weight of Tunor resin was coated to a thickness of 3.
The mixture was allowed to adhere until the temperature reached 40°C to form a single-layer overcoat layer, thereby obtaining overcoat-coated fuel particles.

The overcoat coated fuel particles were shaped and fired in the same manner as in Example 1 to finally produce a nuclear fuel assembly.

When the fracture rate of coated fuel particles in the produced nuclear fuel assembly was measured in the same manner as in Example 1, it was found that:
It was 4.2X 10-4.

(Evaluation) As is clear from Examples 1 and 2, the nuclear fuel assembly manufactured by the manufacturing method of the present invention has a low breakage rate of coated fuel particles in the nuclear fuel assembly. It can be confirmed that a nuclear fuel body suitable for use as a nuclear reactor fuel can be obtained without layer damage and is of great industrial significance.

[Effects of the Invention] According to the present invention, (1) Damage such as cracks is less likely to occur in the coating layer of the coated fuel particles during manufacturing, and (2) the rate of breakage of the coating layer of the coated fuel particles in the nuclear fuel assembly is reduced. It is possible to provide a method for producing a nuclear fuel body and overcord-coated fuel particles that have the following advantages.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of overcoat-coated fuel particles of the present invention. FIG. 2 is an explanatory diagram showing an example of the state of overcoat-covered fuel particles before press forming in the method of the present invention. FIG. 3 is an explanatory diagram showing an example of the state of a molded article in the method of the present invention. DESCRIPTION OF SYMBOLS 1... Overcoat coated fuel particles, 2... Coated fuel particles, Overcoat coated fuel particles, 3a... Outer overcoat layer, 3b... Inner overcoat layer, (a)... Matrix material A, (0) ...Matrix material B. 1 Figure 2 Figure 3 2 (b) j-

Claims (2)

[Claims] (1) Forming an overcoat layer on the surface of the coated fuel particles with a matrix material consisting of graphite powder and a binder,
After obtaining the overcoat coated fuel particles, in the method for producing a nuclear fuel assembly, the overcoat coated fuel particles are press-molded and fired. A method for producing a nuclear fuel assembly, comprising forming an inner layer with a matrix material having a shrinkage rate smaller than the shrinkage rate. (2) In the overcoat coated fuel particles in which an overcoat layer is formed on the surface of the coated fuel particles using a matrix material consisting of graphite powder and a binder, the overcoat layer has a two-layer structure, and the outer layer is An overcoat-covered fuel particle characterized in that an inner layer is formed of a matrix material having a shrinkage rate smaller than that of the matrix material to be formed during firing.