JP2505119B2 - Manufacturing method of nuclear fuel pellets - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a nuclear ceramic fuel, and more particularly to a method for producing a nuclear fuel pellet having a large particle size and stable against thermal shock.

[0002]

2. Description of the Related Art Uranium oxide pellets for high burnup have low FP (fission product) gas generation amount during high combustion and good PCMI (mechanical interaction between pellet and cladding), That is, it is required to reduce PCMI, and it has been attempted to increase the crystal grain size for the former and to make the pellet softer or have high thermal shock resistance for the latter.

By the way, conventionally, in the production of such pellets, the development of a production method in which various additives are added to uranium oxide fuel has been advanced for the purpose of obtaining soft pellets, and JP-A-55-27941 discloses the method. Some of them are disclosed in JP-A-55-27942, JP-B-54-8837, JP-B-63-16716 and JP-A-63-73189.

That is, in JP-A-55-27941, UO₂To
At least one type that does not easily form a solid solution and precipitates at grain boundaries
It is also possible to add certain additives, as disclosed in JP-A-55-27942.
UO₂ To Al₂O₃, BeO, CaO, MgO,
SiO₂, Na₂O, P₂O _FiveFrom 2 to more than 0.2-5
%, The UO in Japanese Patent Publication No. 54-8837
₂Aluminum silicate as an additive for immobilization of fission products,
Calcium iodide, magnesium silicate, SiO₂-Al
₂O₃-MgO, SiO₂-Al₂O₃-CaO, Chita
Aluminum oxide, silicon titanate, CaO-SiO₂-Cita
It has been shown to add phosphates and the like.

Further, in JP-B-63-16716 and JP-A-63-73189, Al ₂ O ₃ , BeO, CaO, UO ₂ is added to UO ₂ for the purpose of obtaining a pellet having a soft and large particle size.
It is described that two or more kinds of MgO, SiO ₂ , Na ₂ O and P ₂ O ₅ are added, and Nb ₂ O ₅ and TiO ₂ are further added.

On the other hand, as a method for producing soft pellets for reducing PCMI, a uranium oxide powder is compression-granulated by the applicant of the present invention in Japanese Patent Application Laid-Open No. 62-98292. After crushing, sieving, and then compression molding, and then sintering, the molding pressure is about 20 to 60% of the initial granulation pressure, and the particle size after granulation is 100 μm to 600 μm. At least 40% by weight
In addition, a method for producing fuel pellets has been proposed in which sintering is performed at 1700 ° C. to 1800 ° C. for 3 hours or more under a reducing atmosphere. This pellet has a high thermal shock resistance, and has a characteristic that it hardly causes cracks and hardly causes PCMI, while the pellet produced by the usual method causes small fragments in the fuel rod and causes PCMI.

[0007]

However, when Nb ₂ O ₅ , TiO ₂ or the like is added in the above-mentioned additive addition method, the particle size is large and the PCMI characteristic is good, but Nb ₂ O FP by addition of ₅ , TiO ₂ etc.
There is in spite of the grain size for the remaining the diffusion speed increases can not hope much improvement in FP retention flame, also on the neutron economy, not good even on a change in physical properties, further SiO _2, Al ₂ In the method of combining a grain boundary precipitation property such as O ₃ with an additive having a grain size increasing effect such as Nb ₂ O ₅ , TiO ₂ or the like, although some improvement can be seen, the same drawback is still present. Can be seen.

On the other hand, when pellets are manufactured, sintering at high temperature for a long time increases the grain size but hardens the PCM.
This will result in deterioration of the I characteristic. On the other hand, the fuel pellets are subjected to the first compression granulation and the subsequent compression molding.
In the production method by step compression molding, pellet destruction due to abrupt temperature change can be prevented, that is, thermal shock resistance can be improved and PCMI characteristics can be improved to prevent fuel damage, but pellet size is small and FP gas However, there is a problem that a large amount is released. Therefore, the present inventor copes with the above-mentioned situation and, in order to improve it, based on the above-mentioned two-stage compression molding, a method of adding an additive is combined with this, and the method of the treatment is further added to obtain a crystal. It is an object of the present invention to increase the particle size and to enable the production of soft pellets having high thermal shock resistance and to have both FP characteristics and PCMI characteristics, and further improve.

[0009]

That is, the feature of the present invention which meets the above-mentioned object is that a powder of uranium oxide is compressed and granulated at a pressure P ₁ and then the granulated fine particles are crushed and sieved. After that, a second compression molding is performed at a pressure P ₂ and then sintering is performed to produce a ceramic nuclear fuel pellet. In the uranium oxide powder, Nb ₂ O ₅ , TiO ₂ ,
MgO, CaO, SiO ₂ , Al ₂ O ₃ or compounds that change into these oxides during sintering are added alone or in combination, and the initial granulation pressure P ₁ and the subsequent molding pressure P ₂ are The relationship is P ₂ = 0.2P _{1 to} 0.6P ₁ and the particle size after granulation is 100 μm to 600 μm at least 40% by weight, and after molding the molded body, the temperature is 1600 ° C. to 1800 ° C. in a reducing atmosphere. It is to sinter for 3 hours or more. The invention according to claim 2 is characterized in that, in the above-mentioned manufacturing method, the sintering is performed at 1600 ° C to 1800 ° C in a reducing atmosphere.
Standing, volume ratio 0.01 to 1 in the temperature range of 900 ℃ or more
It is characterized in that the crystal grain size is further increased by adding 0% of steam to create a humidified reducing atmosphere. Furthermore, the invention according to claim 3 is one in which the sintering conditions in the above method are partially changed, and after forming a molded body, CO ₂ /
The oxygen concentration is set to 0.01 ppm to 400 ppm under the condition that the presence of oxygen such as CO atmosphere is allowed, and 1400 to 400 ppm is set.
After heating at 1800 ° C for 1 hour or more, 10 for O / U adjustment
It is characterized in that heating is performed for 1 hour or more under a reducing condition of 00 ° C. or higher or a humidifying reducing condition. Next, the invention according to claim 4 is U ₃ which takes a more specific embodiment of uranium oxide into consideration.
Regarding the mode of addition of O ₈ , O / U <as uranium oxide
A UO ₂ powder of 2.25 has an average O / U of 2.25.
Using a powder obtained by adding U ₃ O ₈ to obtain a mixture of ˜2.40 and molding the mixture into a molded body, the molded body is molded at 1100 to 1400 ° C. in an atmosphere with an oxygen concentration of 0.1 ppm to 100 ppm.
After heating for 0 minutes to 4 hours, heating is further performed under reducing conditions of 1000 to 1400 ° C. or humidifying reducing conditions.

[0010]

When the nuclear fuel pellets are manufactured by the above method, it is possible to obtain fuel pellets having a large particle size and softness, and further improved thermal shock resistance.
It is possible to significantly improve the characteristics and the PCMI characteristics.

Further, in the method of claim 2, in addition to the above-mentioned action, it is possible to further increase the particle size of the pellet, and in the method of claim 3, the large particle size pellet can be obtained efficiently. At the same time, the oxygen concentration can be controlled to promote the sintering. Furthermore, in the method of the fourth aspect, the density and particle size of the fuel pellets can be controlled in addition to the above by the amount of U ₃ O ₈ added.

[0012]

The present invention will be described in detail below with reference to the details of the present invention. First, the method of the present invention uses the method of two-stage compression molding as described above. That is, in producing a ceramic nuclear fuel pellet using uranium oxide as a raw material, the uranium oxide powder is compression granulated at a pressure P ₁ , and then the granulated fine particles are pulverized and sieved, and then the pressure P ₂ in a second time of compression molding, basic and directed to a method of the later sintered ceramic nuclear fuel pellets. In this case, 1
P is the relationship between the second compression pressure P ₁ and the second compression pressure P _2.
2 = 0.2 P _{1 to} 0.6 P ₁ and the particle size after granulation is 100 μm
It is important that the size of 600 μm is at least 40% by weight.

Further, as the sintering, it is preferable that after molding the molded body, the molded body is heated and sintered at 1600 to 1800 ° C. for 3 hours or more in a reducing atmosphere. Therefore, in the present invention, when producing nuclear fuel pellets by using the above-described two-stage compression molding method, a method for obtaining larger particle diameter pellets is added. The method for obtaining the large-sized pellets is basically the method previously proposed by the applicant (Japanese Patent Application No. 1-285536).
No.), but specifically speaking, Nb ₂ O ₅ , TiO ₂ , MgO, CaO, S is added to the powder of uranium oxide as a raw material.
A single compound selected from the compounds of iO ₂ and Al ₂ O ₃ or 2
The above heating and sintering is performed by adding a compound in which one or more kinds are combined, or by adding a compound such as Nb or Ti that changes into the above-mentioned oxide compound during sintering. It should be noted that water vapor (H ₂ O) is added to the reducing atmosphere containing hydrogen.
Addition of oxygen, an equilibrium state of 2H ₂ O = O ₂ + 2H ₂ is formed, and the presence of oxygen is allowed. Add 0.01 to 10% by volume of water vapor in the temperature range of 900 ℃ or more at the time of setting. In addition, in order to obtain the above-mentioned large-sized pellets, after molding the molded body in the above-mentioned method, it is 1400 ° C. to 1800 ° C. at an oxygen concentration of 0.01 ppm to 400 ppm under conditions where the presence of oxygen is allowed such as a CO ₂ / CO atmosphere. After heating for 1 hour or more, heating for 1 hour or more in a reducing condition of 1000 ° C. or more for O / U adjustment is also effective in promoting sintering. Further U ₃ O ₈
It is also preferable to add U ₃ O ₈ to the uranium oxide based on the finding that the density and particle size can be appropriately controlled by the addition amount of, in this case, for example, O / U <2.25 as the uranium oxide. To the UO ₂ powder of
After using the powder obtained by adding and mixing U ₃ O ₈ so that / U becomes 2.25 to 2.40, and molding at 1100 ° C. to 14
In an atmosphere with oxygen concentration of 0.1 ppm to 100 ppm at 00 ° C
It is heated for 30 minutes to 4 hours, and further heated and sintered under reducing conditions of 1000 to 1400 ° C.

Next, specific examples of the method of the present invention will be described. In pressure press of Example 1 specific surface area of 2.6 m ² / g of uranium dioxide (UO ₂₎ Al ₂ O ₃ and SiO ₂ were added each 500ppm in the powder, first, 4t / cm ² ~7t / cm ² Primary molding was performed. This is then crushed and sieved to a particle size of 12
5 ~ 250μm, the other one is 125μm or less 30%, 125 ~
The thickness of 600 μm was 45%, and that of 600 to 1000 μm was 25%. 20-60% of the primary molding pressure for fine particles with the above particle size
After the secondary molding was performed at the set pressure set to, the molded body was molded and then sintered at 1750 ° C for 4 hours in a reducing atmosphere containing hydrogen. 96-98
% T and D and average grain size by intercept method
Large particle size pellets of 30-40 μm could be obtained. This metallurgical photograph is shown in FIG.

Next, similarly to the above, at 1750 ° C. for 4 hours,
Water vapor with a volume ratio of 0.3% was added in the temperature range above 900 ° C. This was carried out by passing a carrier gas such as argon through water vapor emitted from hot water at 40 ° C to 70 ° C and joining it with the main atmosphere gas. The fuel pellets thus obtained had almost the same particle size as in the case of no addition of water vapor, but a decrease in fine particles was observed, and the particle size was more average. The state of this particle size is shown in FIG. Further, after forming a molded body by the above method, after heating at 1750 ° C. for 4 hours under a CO ₂ atmosphere in which 100 ppm of O ₂ is added, it is subjected to a reducing condition of 1100 ° C. for O / U adjustment. And heated for 2 hours for sintering. As a result, pellets having the same large particle size as described above were obtained. And for each of the above pellets ΔT = 800 ℃
When subjected to a thermal shock test at up to 2000 ° C, good thermal shock resistance was recognized for all of them as compared with ordinary pellets.

Example 2 Following the steps of molding, sintering, and reduction described in Example 1,
In producing nuclear fuel pellets from uranium oxide, raw material powder added with U ₃ O ₈ 35% as uranium oxide was used. That is, x = 35% of U ₃ O ₈ roasted at 400 ° C. was added to the raw material powder of O / U = 2.12, and 1100 ° C. × 3.5 Hr
After sintering in the slightly oxidizing atmosphere (oxygen concentration 100 ppm),
Heated at 1100 ℃ in reducing atmosphere, O / U = 2.00, sintered density 9
5.3% T.I. A UO ₂ sintered body of D was obtained. When the grain size of the above-mentioned sintered body was observed, it was a sintered body having an average crystal grain size of 50 to 100 μm. This money picture is shown in Figure 3.
Fig. 4 shows the particle size. In addition, as a result of performing a thermal shock test on each of the sintered pellets as in Example 1,
It had better thermal shock resistance than ordinary pellets.

[0017]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, an oxide such as Nb ₂ O ₅ , TiO ₂ , SiO ₂ , Al ₂ O ₃ or the like in the uranium oxide powder, or any of these oxides that change into these oxides at the time of sintering. A powder containing a compound or a powder obtained by adding an appropriate amount of U ₃ O ₈ powder to a raw material UO ₂ powder is used as a raw material powder, and this is molded by a two-stage compression molding method, sintered and reduced to produce a sintered body pellet. And a large particle size ceramic fuel having a crystal grain size 3 to 10 times or more than that of conventional pellets required for nuclear fuel pellets, especially high burnup UO ₂ pellets. Enabled, F
It has a remarkable effect on the improvement of the P-holding property, and has a favorable effect that it has better thermal shock resistance than ordinary pellets and prevents damage due to the PCMI phenomenon. The method according to claim 2 is effective in reducing fine particles in the nuclear fuel pellets produced by the above method and averaging the particle diameters. The invention according to claim 3 controls the oxygen concentration to promote sintering. It is effective in measuring. Furthermore, the invention according to claim 4 can control the density and particle size by adding U ₃ O ₈ , and is effective in producing a nuclear fuel pellet having a large particle size and good thermal shock resistance.

[Brief description of drawings]

FIG. 1 is a schematic diagram of Example 1 in a reducing atmosphere containing hydrogen.
It is a 50 times microscope photograph which shows the metallographic structure of what was heated at 50 degreeC for 4 hours.

FIG. 2 is a 200 × photomicrograph showing the metallographic structure of the sample shown in FIG. 1 after etching.

FIG. 3 is a 50 × photomicrograph showing the metal structure of Example 2.

FIG. 4 is a micrograph showing a metal structure of the sample shown in FIG. 3 after etching.

Claims (4)

(57) [Claims] 1. A uranium oxide powder is compression-granulated at a pressure P ₁ , and then the granulated fine particles are crushed and sieved.
In a method for producing a ceramic nuclear fuel pellet by performing a second compression molding at a pressure P ₂ and then sintering, Nb ₂ O ₅ , TiO ₂ , M is added to the uranium oxide powder.
gO, CaO, SiO ₂ , Al ₂ O ₃ or a compound that changes into these oxides during sintering is added alone or in combination, and the initial granulation pressure P ₁ and the subsequent molding pressure P ₂ The relationship is P ₂ = 0.2 P _{1 to} 0.6 P ₁ and the particle size after granulation is 100 μm to 600 μm at least 40% by weight. After molding the molded body, it is 1600 ° C. to 1800 in a reducing atmosphere.
A method for producing nuclear fuel pellets, which comprises sintering at ℃ for 3 hours or more. 2. The method according to claim 1, wherein the reduction is performed.
Before sintering at 1600 ° C to 1800 ° C in the atmosphere, 90
A method for producing a nuclear fuel pellet, comprising adding steam having a volume ratio of 0.01 to 10% in a temperature range of 0 ° C. or higher to create a humidified reducing atmosphere. 3. Uranium oxide powder is compression granulated at a pressure P ₁ , and then the granulated fine particles are crushed and sieved,
In a method for producing a ceramic nuclear fuel pellet by performing a second compression molding at a pressure P ₂ and then sintering, Nb ₂ O ₅ , TiO ₂ , M is added to the uranium oxide powder.
gO, CaO, SiO ₂ , Al ₂ O ₃ or a compound that changes into these oxides during sintering is added alone or in combination, and the initial granulation pressure P ₁ and the subsequent molding pressure P ₂ The relationship is P ₂ = 0.2 P _{1 to} 0.6 P ₁ , and the particle size after granulation is 100 μm to 600 μm at least 40% by weight, and the presence of oxygen such as CO ₂ / CO atmosphere is allowed after molding the molded body. Oxygen concentration of 0.01ppm to 40
A method for producing nuclear fuel pellets, which is characterized in that it is set to 0 ppm and heated at 1400 to 1800 ° C for 1 hour or more, and then heated under reducing conditions of 1000 ° C or more or humidification reducing conditions for O / U adjustment. . 4. Uranium oxide powder is compression granulated at a pressure P ₁ , and then the granulated fine particles are crushed and sieved,
In a method for producing a ceramic nuclear fuel pellet by performing a second compression molding under a pressure P ₂ and then sintering the same, a UO ₂ powder having O / U <2.25 as uranium oxide has an average O / U of 2.25. Using a powder to which U ₃ O ₈ was added and mixed so as to be about 2.40, the first granulation pressure P ₁ and the subsequent molding pressure P 1
The relationship between the _{2 P 2 = 0.2 P 1 ~0.6} P 1, the particle size after granulation as at least 40 wt.% That of 100 [mu] m ~ 600 .mu.m, after molding the molded body, the oxygen concentration of 0 at 1100 to 1400 ° C.. 1
A method for producing nuclear fuel pellets, which comprises heating for 30 minutes to 4 hours in an atmosphere of ppm to 100 ppm and then heating under a reducing condition of 1000 to 1400 ° C. or a humidifying reducing condition.