JPS6129678B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses oxides for nuclear reactor fuel (UO ₂ and U,
This paper relates to a method for sintering pellets of Pu (mixed oxide of Pu). As is well known, nuclear fuel for normal nuclear reactors is
Sintered uranium dioxide pellets filled in a Zircaloy or other cladding tube are used as nuclear fuel elements. In this case, the pellets are made by mixing _UO2 powder (crude product) → (granulation) → (addition of lubricant) → molding → 1600
It is manufactured into _UO2 pellets by a sintering process (steps in parentheses may be omitted) at temperatures between 1700°C and above. Nuclear fuel pellets are required to have various desirable physical and chemical properties, and among these properties, the present invention is intended to improve the sintered density and grain size of nuclear fuel pellets. The sintered density of UO ₂ pellets is usually around the theoretical value.
95% and is required to be thermally stable at this sintered density. Here, thermal stability means that the degree of densification (sintering compaction) when re-sintering the sintered pellets at 1700℃ for 24 hours is small, and usually 2.
% or less, preferably 1% or less. In order to achieve this thermally stable 95% sintered density, it was necessary to sinter at a temperature of at least 1600°C or higher, usually 1700°C or higher. Requiring high temperatures of 1600℃ to 1700℃ or higher in this way requires expensive equipment and high energy costs.
In addition, in furnaces that use refractories, the refractories are rapidly worn out and there is a lot of loss (repair costs, equipment operating rate) due to their repair. Also, economic efficiency increases. Conventionally, various studies have been conducted to improve the sintered density of UO ₂ and lower the sintering temperature, such as activating the UO ₂ powder used as a raw material, improving the sintering atmosphere, and investigating the effects of additives. However, as a result, none of these methods have been used industrially at this stage because their effects are small. In addition, when thermally stable pellets are required as mentioned above, the sintering temperature must be adjusted.
Sintering at a temperature of 1,700°C or higher is usually carried out industrially, and sintering at a low temperature results in thermally stable 90°C.
There have been few attempts to produce pellets with a sintered density of . In addition to the above-mentioned thermally stable sintered density, properties required for nuclear fuel pellets include the ability to retain fission products in order to reduce stress corrosion cracking of the fuel cladding caused by fission products. Large pellets are desired. For this purpose, attempts have been made to grow and coarsen crystal grains during sintering.
For this purpose, research is being conducted on adding Nb compounds to UO ₂ and sintering it. Under these circumstances, the inventors of the present invention attempted to add trace amounts of various metal compounds when sintering UO ₂ , but adding a trace amount of Li compounds resulted in a lower temperature than before. It was found that sintering progresses and the grain size becomes coarser. This makes it possible to obtain pellets with high density and coarse grains at a lower temperature than before, and this effect is important in terms of manufacturing technology. Furthermore, by conventionally sintering powder to which a small amount of Li compound has been added at a temperature of 1,600°C or 1,700°C or higher, the crystal grains become significantly coarser, and the ability to retain fission products is expected to increase. That is, according to the present invention, Li ₂ O or a powder of a compound that generates Li ₂ O upon heating is added to the UO ₂ powder.
A method for producing UO ₂ nuclear fuel pellets is provided, which comprises mixing 0.005% to 1.0% by weight of Li ₂ O into a green compact by a known method and sintering the green compact in a non-oxidizing atmosphere. The nuclear fuel oxide used in the present invention is mainly _UO2 , but as mentioned above, it may also be a mixed oxide with Pu called MOX, which is theoretically recognized by those skilled in the art. Let's be. Although the UO ₂ powder may deviate from the stoichiometric relationship, it is not necessary to deviate from the stoichiometric relationship and may be obtained by a conventional method. The Li compounds added are Li ₂ O, LiOH, Li ₂ CO ₃ ,
Anything containing Li, such as a halide of LiNO ₃ Li, can be used. Cheap and preferable
Li ₂ O, LiOH, etc., and the amount added is 0.005 to 1.0% by weight as Li ₂ O based on UO ₂ . 0.005%
If it is less than 1.0%, it will be difficult to achieve the purpose of the present invention, and even if it is added in an amount of 1.0% or more, the effect of addition will be saturated, and it is expected that the concentration of Li remaining in the pellet will increase, resulting in unfavorable results as a nuclear fuel. In the present invention, the sintering atmosphere is usually H ₂ gas, but there is no noticeable difference in the effect even if it is an H ₂ /N ₂ mixed gas or a vacuum atmosphere.
Good results can be obtained in any atmosphere. There are no particular restrictions on the molding conditions for the compact (green pellet), and the method is to fill a mold with uranium dioxide powder mixed with Li and mold it under a predetermined pressure. Regarding the sintering temperature, as already mentioned, industrially it is at least 1600°C or higher, usually 1700°C or higher. In the method of the present invention, the sintering temperature for achieving a specific sintered density varies depending on the amount of Li added, and the temperature decreases as the amount of Li added increases. In addition, the crystal grain size observed in the sintered body (sintered pellet) is similar to the sintered density, and the sintering temperature to obtain a specific grain size varies depending on the amount of Li added. Its temperature decreases. However, the effect on sintered density and grain size
The influence of the amount of Li added is noticeable when the amount added is small, and tends to become saturated as the amount added increases. In the present invention, by adding Li to uranium dioxide powder in this way, it is possible to increase the sintered density at a lower temperature than before, and at the same time coarsen the grain size. Increasing the Li content and lowering the sintering temperature result in an increase in the Li content remaining in the sintered body. An increase in the amount of Li remaining in the UO ₂ sintered body is qualitatively unfavorable from the viewpoint of simply increasing the amount of impurities, and at this stage, where quantitative numerical limits cannot be set, it is necessary to reduce the amount of remaining Li as much as possible. Preferably, the amount is small. Note that the sintering time is not particularly limited and can be easily determined empirically or experimentally.
Under normal temperature conditions, it takes about 2 hours. Next, the present invention will be specifically described with reference to the drawings. In the following description, all percentages are by weight. Figure 1 shows an average particle size of approximately 1μ (according to the sedimentation method).
To the UO ₂ powder, 0.2% zinc stearate was added as a lubricant, and further 0%, 0.01%,
The powder containing 0.05% and 0.1% Li ₂ O was made into green pellets (green pellets) with a diameter of 10 mm and a height of 15 mm using a mold at a pressure of 3 t/cm ² (weighing approximately 7 mm).
UO ₂ g) sintered at various temperatures in a hydrogen stream
The relationship between pellet sintering temperature and sintered density is shown. FIG. 2 shows the relationship between the sintering temperature and average grain size of the above UO ₂ . The relationship between the amount of Li ₂ O added, the sintering temperature, and the amount of Li ₂ O remaining in the sintered pellets is summarized in the following table and FIG. 3.

[Table] As is clear from Figure 1, the sintered density increases as the amount of Li ₂ O added increases, but the rate of increase is remarkable at relatively low sintering temperatures.
For example, in sintering at 1150°C, the sintered density can be increased from 63% to 91% by adding 0.05% Li ₂ O, and the effect of Li ₂ O on promoting sintering is extremely excellent. However, even if the amount of Li ₂ O added was increased from 0.05% to 0.1% and 0.2%, the sintered density remained 91%, 93%, and 93.5%, respectively.
It is confirmed that the promoting effect of adding Li ₂ O is saturated at around 0.1%. (The case of 0.2% Li ₂ O is not shown in Figure 1.) As shown in Figure 2, the average grain size observed in the sintered pellets is The addition of Li ₂ O causes coarsening, and the rate of coarsening is remarkable when the sintering temperature is high. for example
By adding 0.05% Li ₂ O during sintering at 1650℃, the average grain size becomes approximately 120μm.
Approximately 10μm compared to 11μm without adding Li ₂ O
Showing a fold increase. Note that the amount of Li ₂ O added starts from 0.05%.
Even if it increases to 0.1%, the average grain size will still be 120 μm.
It showed only a relatively small increase of about 135 μm,
It can be said that the tendency of crystal grain coarsening due to the addition of Li ₂ O is saturated at around 0.1%. As shown in FIG. 3 and Table 1, the amount of Li remaining in the UO ₂ sintered body increases as the amount of Li ₂ O added increases and as the sintering temperature decreases. As mentioned earlier, it can be said that the smaller the residual amount of Li, the better. From this point of view, the expected effects of adding Li ₂ O are to promote sintered density, coarsen grains, or both. The amount of Li ₂ O added and the sintering temperature should be selected accordingly. Judging from the results shown in these figures, Li
It can be said that even if the amount of the compound added is about 0.005% as Li ₂ O, it is significant. As is clear from these figures, Li ₂ O at 0.05%
When added, a sintered density of 94% of the theoretical value and an average crystal grain size of about 40 μm were obtained at 1350° C., and the residual amount of Li at that time was 70 ppm. If sintered at 1650℃, the average grain size will reach 120μm, and the remaining amount of Li will be
It is 30ppm. When 0.1% Li ₂ O was added, a sintered density of 95.5% of the theoretical value and an average crystal grain size of about 45 μm were obtained at 1350° C., and the remaining amount of Li at that time was 160 ppm.
If sintered at 1650℃, the average grain size is 135μ
m, and the remaining amount of Li is 45 ppm. Next, examples will be shown in which the conditions were changed. Example 1 UO ₂ powder with an average particle size of about 1 μm was added as a lubricant.
Added 0.2% zinc stearate and further 0.01
% of Li ₂ O was added to 3t/cm ² using a mold.
A green compact with a diameter of 10 mm and a height of 15 mm was sintered at 1,650°C in a hydrogen stream for 2 hours at a pressure of . obtained
The sintered density of the UO ₂ pellets was 95.1%, the average grain size was about 70 μm, and the residual Li content was 15 ppm. The sintered density and average grain size when Li ₂ O was not added were 94.5% and 20 μm, respectively. Example 2 LiOH was used at 0.1% instead of Li ₂ O, and sintering was carried out at 1350° C. and 1650° C. under the above conditions. Those sintered at 1350℃ have a sintered density of 94% and an average grain size of 35μ.
m, and the residual Li amount was 95 ppm. Those sintered at 1650℃ have a sintered density of 96%, an average grain size of 105μm,
The remaining amount of Li was 25 ppm. Example 3 LiF was used at 0.1% instead of Li ₂ O, and sintering was carried out at 1350°C and 1650°C under the above conditions. 1350℃
The sintered one has a sintered density of 94% and an average grain size of 40
μm, and the residual Li amount was 80 ppm. Those sintered at 1650℃ have a sintered density of 96% and an average grain size of 100μ.
m, and the residual amount of Li was 20 ppm. Example 4 Using 0.2% lithium acetate instead of Li ₂ O, sintering was carried out at 1350°C and 1650°C under the above conditions.
The material sintered at 1350°C had a sintered density of 92.5%, an average grain size of 25 μm, and a residual Li content of 60 ppm. 1650
Those sintered at °C have a sintered density of 95% and an average grain size.
The diameter was 75 μm, and the residual amount of Li was 10 ppm. As described above, according to the present invention, the three benefits of lowering the sintering temperature, increasing the sintered density, and increasing the average grain size can be achieved by adding a small amount of Li compound. Therefore, it can be said that the effects of the present invention are extremely large.

[Brief explanation of the drawing]

Figure 1 shows the relationship between sintering temperature and sintered density when Li ₂ O is added to UO ₂ powder and sintered. FIG. 2 shows the relationship between sintering temperature and average grain size in the same case as FIG. 1. FIG. 3 shows the relationship between the sintering temperature and the amount of residual Li in the same case as FIGS. 1 and 2.

Claims (1)

[Claims] 1 UO ₂ powder with Li ₂ O or powder of a compound that generates Li ₂ O by heating as Li ₂ O from 0.005% to 1.0
UO ₂ consisting of mixing % by weight to form a green compact by a known method and sintering this in a non-oxidizing atmosphere.
Method for producing nuclear fuel pellets. 2. The method according to claim 1, comprising:
A method characterized by sintering at a temperature of 1350°C or higher. 3. The method according to claim 2, comprising:
A method characterized by mixing 0.1% by weight or more of a powder of a compound that generates Li ₂ O.