JPH0731267B2 - Manufacturing method of nuclear fuel pellets - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing nuclear ceramics, and more particularly to a method for producing nuclear fuel pellets having a large crystal grain size and a high creep rate.

(Prior Art) UO ₂ pellets for high burnup have good FP (fission product) retention and good PCMI (mechanical interaction between pellet and cladding), that is, PCMI can be reduced. It is sought to increase the grain size of the former,
For the latter, attempts have been made to make soft pellets.

By the way, conventionally, in the production of such pellets, various additives have been added to uranium dioxide (UO ₂ ) fuel to aim at the production of pellets as described above. (1) JP-A-55-27
941 gazette, (2) JP-A-55-27942, (3) JP-B-54-8837, (4) JP-B-63-16716, and (5) JP-A-63-73189. Some of them are disclosed in.

That is, (1) in JP-A-55-27941, at least one kind of additive that does not easily form a solid solution with UO ₂ but precipitates at grain boundaries is added. In the 55-27942 publication, UO ₂ is converted into Al ₂ O ₃ , BeO, CaO, MgO, SiO ₂ , Na ₂ O, P ₂ O ₅ to 2
Addition of 0.2 to 5% of seeds or more is (3) Japanese Patent Publication No. 5
In U.S. Pat. No. 4,88,37, aluminum silicate, calcium silicate, magnesium silicate, SiO ₂ —Al ₂ O ₃ —MgO, SiO ₂ —Al ₂ O ₃ —CaO, titanium are added to UO ₂ as fission product immobilization additives. acid aluminum, silicon titanate, CaO-SiO ₂ - be added titanates like are shown respectively.

Further, (4) JP-B 63-16716 discloses and (5) to the UO ₂ in JP 63-73189 discloses _{Al 2 O 3, BeO, CaO} , MgO,
It is stated that two or more kinds of SiO ₂ , Na ₂ O and P ₂ O ₅ are added, and Nb ₂ O ₅ and TiO ₂ are further added.

(Problems to be Solved by the Invention) However, in the method of adding additives as described above, Nb
_The ones to which ₂ O ₅ , TiO ₂ etc. are added have a large particle size and good PCMI characteristics, but due to the residual Nb ₂ O ₅ , TiO ₂ etc., FP retention is not improved relative to the particle size. There are difficulties that I can not hope for, and it is not good in terms of neutron economy and changes in physical properties,
Furthermore SiO _2, Al ₂ some of the grain boundary precipitation characteristics of O _3, and the Nb ₂ O
₅ , a method of combining additives such as TiO _{2 having} a particle size increasing effect is complicated and not preferable.

On the other hand, in the production of pellets, performing high temperature and long time sintering increases the grain size but makes it harder, resulting in a decrease in PCMI characteristics.

It should be noted that there are many examples in which Al ₂ O ₃ or SiO ₂ was simply added to the uranium dioxide fuel to improve the characteristics, but none of them have produced the desired results, and the crystal grain size was increased. There is almost nothing that mentions.

In particular, when the above-mentioned various additives are added, it is usually necessary to add 0.1% or more except for a part thereof, and in many cases, the amount of impurities currently specified is exceeded. Al ₂ O ₃ above
And 0.1 wt% also for the addition of SiO ₂ and is SiO _2> Al ₂ O ₃ is the limit.

Therefore, the present inventor has dealt with the actual situation as described above, and has conducted repeated studies to improve it. As a result, Al ₂ O ₃ and Si with respect to UO ₂
The inventors have found that the amount of O ₂ added is limited to a specific range, and further proposed it. (Japanese Patent Application No. 1-64868) The present invention goes one step further than the invention of the above-mentioned proposal, and adds Al ₂ O ₃ and SiO ₂
The purpose of the present invention is to increase the crystal grain size and enable the production of soft pellets by taking into consideration the size of the powder and the treatment method, and further improve the FP retention and PCMI characteristics. .

(Means for Solving the Problems) That is, the feature of the present invention which meets the above object is that Al ₂ O ₃ and SiO are added to a dioxide of at least one element of uranium and plutonium or a nuclear fuel substance obtained by adding gadolinia to the substance. _When adding ₂ to produce a nuclear fuel pellet, the powder of Al ₂ O ₃ and SiO ₂ is made into an ultrafine powder with a particle size of 1 μm or less, and its addition amount is 0.005% to 0.05% and Al ₂ O ₃ .
The weight ratio of SiO ₂ is in the range of 1: 1 to 30: 1, and the powders are mixed to form a powder, and thereafter, in a reducing atmosphere containing hydrogen, saturation of 40 to 70 ° C with respect to the main body gas flow rate is performed. This is to add a carrier gas containing water vapor in an amount of 0.1 to 10% by volume and sinter at 1600 to 1750 ° C. for 2 to 6 hours.

Here, the atmosphere after molding is a reducing atmosphere containing hydrogen in the above case, but it may be a slightly oxidizing atmosphere having an oxygen concentration of 100 ppm or less, for example, a CO ₂ atmosphere containing a small amount of oxygen or a CO ₂ / CO atmosphere. However, the method according to claim 2 is such a case.

The invention according to claim 3 relates to the mode of adding Al ₂ O ₃ and SiO ₂ in each of the above methods, and is carried out by adding Al ₂ O ₃ to aluminum silicate or aluminum silicate. The invention of 1) is further characterized in that in that case, ultrafine powder of 1 μm or less is used as the aluminum silicate powder.

The method described in claims 5 and 6 relates to a preferred embodiment applied when carrying out the invention, wherein the former is Al ₂ O.
_When mixing ₃ , SiO _{2 and} aluminum silicate, it is necessary to perform mixing including pulverization of powder such as mixing with a cutter mill or a ball mill. The latter is for facilitating density adjustment and molding without affecting particle size. It is characterized by adding a granulating agent such as PVA, a lubricant such as stearic acid, and a pore-forming agent such as starch, U ₃ O ₈ and palmitic acid.

(Operation) When manufacturing nuclear fuel pellets by the above-mentioned method, it is a specification of the present UO ₂ pellets, though it is a simple method without adding various additives.

The Al content is 100ppm or less and the Si content is 250ppm or less, and the average grain size by the intercept method is 30-60μ, while the grain size by the usual method is ~ 10μm.
This makes it possible to obtain pellets having a large particle size of m and having a creep rate of about 5 × 10 ² to 10 ³ times as large as that without correcting the particle size.

Hereinafter, specific examples of the present invention will be described.

(Example 1) With an Al content of 53% in Al ₂ O _{3 and} a Si content of 47% in SiO ₂ ,
From this, when the specifications of Al and Si of UO ₂ are Al 100 ppm and Si 250 ppm, the maximum addition amount of Al ₂ O ₃ is 0.02% and the maximum addition amount of SiO ₂ is 0.05%.

Therefore, the total amount of Al ₂ O ₃ and SiO ₂ added to UO ₂ powder should be 0.01%.
Each experiment was conducted under the following mixing ratio and sintering conditions.

(A) Mixing rate (B) Sintering conditions 1750 ° C. × 4 Hr, H ₂ / N ₂ atmosphere, but the amount of water (steam) added was 0.1% for case (1) and 10% for case (2) in terms of carrier gas ratio.

This is the steam that comes out of the hot water of 40-70 ℃, let the argon carrier gas pass through it, and let the gas that contains it join the main body atmosphere gas, but the volume ratio at this time is 0.
Argon gas flow rate was set so that it was 1 to 10%.

In the above experiment, argon gas was used as the carrier gas, but of course, this may be N ₂ or N ₂ / H ₂ .

As a result of the above experiment, the change in particle size by the intercept method was as follows.

Fig. 1 shows the results of observing the crystal grain size in the case of C in case (1) according to the table above, and Fig. 1 shows the results of the crystal grain size in the case of case (1) in the case of UO ₂ only. Each is shown in FIG. By observing both of these figures, it is found that the particle size in FIG. 1 is larger than that in FIG.

Further, when 0.1% of palmitic acid as a pore-forming agent was added to the mixed powder of C in the above case (1) and the same experiment was attempted, no influence on the particle size due to the incorporation of palmitic acid was observed. (See FIG. 3) (Example 2) UO ₂ powder Gadoria (Gd ₂ O ₃₎ and was added 8%, Al ₂ O where it was subjected to the same experiment as in Example 1 as a starting material
Whereas the average particle size was 3 μm (see Fig. 5) when ₃ / SiO _{2 was} not added, powders with a mixing ratio of Al ₂ O ₃ / SiO ₂ of 5 were mixed with Al ₂ O ₃ and SiO _{2. As} in the above case, the mixed powder added at 0.01% in the total addition amount of ₂ was sintered in the 1750 ° C. × 4Hr, H ₂ / N ₂ atmosphere, and the particle size increased to 10 to 20 μm (see FIG. 4). Was recognized.

Further, similar experiments were conducted at the mixing ratios of Example 1, respectively, and it was confirmed that the addition of any of them increased the particle size to 10 to 20 μm.

In this case, the effect of water addition by adding steam was not clear, but it appeared as a reduction of fine particles.

Granular agents such as PVA (polyvinyl alcohol), lubricants such as stearic acid, and pore-forming agents such as starch, U ₃ O ₈ and palmitic acid were used as appropriate, but these can be used without affecting the particle size. It was possible.

In addition, the sintered body obtained in the above example was used as a sample for 1400
° C., was subjected to a high-temperature creep test of 45 MPa, as compared with additive-free UO ₂ without performing the particle diameter compensation either case 5 × 1
It had a creep rate of 0 ² to 10 ³ times and was an excellent ceramic fuel.

Al ₂ O ₃ and SiO upon (Effect of the Invention) The present invention is the production of nuclear fuel pellets by adding Al ₂ O ₃ and SiO ₂ in the nuclear fuel material as above
The powder particle size of ₂ is 1 μm or less, and the total addition amount is 0.005%
0.05%, and Al ₂ O ₃ and the weight ratio of SiO ₂ 1: 1 to 30: molding a first range, a method of sintering, Al ₂ O ₃ as described above
By simply adding Al ₂ O ₃ and SiO ₂ within the impurity specification range by defining the powder particle size and amount of SiO ₂ and SiO ₂ , it is possible to easily obtain high burnup UO ₂ pellets with a smaller amount of additives. It is possible to obtain a ceramic fuel with a large grain size and a high creep rate, which has a grain size 3 to 10 times the grain size of UO ₂ pellets by the conventional manufacturing method as described above, and it is remarkably improved in FP retention and PCMI characteristics. It is expected to be applied to the industrial and economical production field of the above-mentioned high burnup UO ₂ pellets, which has various effects and is of interest in the future.

[Brief description of drawings]

FIG. 1 shows a crystal grain size metallographic observation of a nuclear fuel sintered body obtained in case (1) C of Example 1 in which 0.01% of powder having a mixing ratio of Al ₂ O ₃ / SiO ₂ = 5 was added to uranium dioxide. Micrograph of metallographic structure taken at 400x magnification in Fig. 2, Fig. 2 shows the crystal grain size of the nuclear fuel sintered body obtained by adding 0.1 parts of palmitic acid as a pore-forming agent to the sample in Fig. 1 with a metallographic microscope. A metallographic micrograph taken at a magnification of 400 times, and FIG. 3 shows a crystal grain of a nuclear fuel sintered body according to Case (1) D of Example 1 obtained only with conventional uranium dioxide at a magnification of 400 times. A metallographic micrograph taken at a magnification, Fig. 4 shows a pellet of UO ₂ powder containing 8% gadolinia added to uranium dioxide and 0.01% mixed powder of Al ₂ O ₃ / SiO ₂ = 5 and sintered. The crystal grain size was 400 as observed with a metallographic microscope as above.
A metallographic micrograph taken at a magnification of 2 ×, and FIG. 5 is a metallographic micrograph taken at a magnification of 400 × of the crystal grain size of the UO ₂ pellets containing 8% of Gadria shown in FIG.

Claims (6)

[Claims] 1. A method for producing a nuclear fuel pellet by adding Al ₂ O ₃ and SiO ₂ to a nuclear fuel material obtained by adding gadolinia to a dioxide of at least one element of uranium and plutonium, or the above-mentioned Al ₂ O ₃ and The SiO ₂ powder is made into an ultrafine powder with a particle size of 1 μm or less, and the total addition amount is 0.005%.
~ 0.05%, and the weight ratio of Al ₂ O ₃ and SiO ₂ is in the range of 1: 1 to 30: 1 to form a mixed powder, and then in a reducing atmosphere containing hydrogen against the main body gas flow rate. Carrier gas containing saturated steam at 40-70 ℃ is 0.1-10% by volume.
A method for producing nuclear fuel pellets, which comprises adding and sintering at 1600-1750 ° C. for 2-6 hours. 2. In producing a nuclear fuel pellet by adding Al ₂ O ₃ and SiO ₂ to a nuclear fuel material obtained by adding gadolinia to a dioxide of at least one element of uranium and plutonium, or the above-mentioned Al ₂ O ₃ and The SiO ₂ powder is made into an ultrafine powder with a particle size of 1 μm or less, and the total addition amount is 0.005% to
0.05% and Al ₂ O ₃ and SiO ₂ weight ratio in the range of 1: 1 to 30: 1 were mixed and molded into a powder, and the oxygen concentration after molding
A method for producing a nuclear fuel pellet, which comprises sintering at 1600 to 1750 ° C for 2 to 6 hours in a slightly oxidizing atmosphere of 10 ² ppm or less. 3. The method for producing nuclear fuel pellets according to claim 1, wherein Al ₂ O ₃ and SiO ₂ are added by aluminum silicate or a method of adding Al ₂ O ₃ to aluminum silicate. . 4. An ultrafine powder having a powder particle size of aluminum silicate to be added of 1 μm or less.
A method for producing a nuclear fuel pellet as described. 5. The nuclear fuel pellet according to claim 1, 2, 3 or 4, wherein Al ₂ O ₃ , SiO ₂ or aluminum silicate is added and then pulverized and mixed using a cutter mill or a ball mill. Manufacturing method. 6. The manufacturing method according to claim 1, wherein granules such as PVA, lubricants such as stearic acid, starch, pores such as U ₃ O ₈ are included in order to facilitate density adjustment and molding. A method for producing a nuclear fuel pellet, which further comprises adding a forming agent.