JPS63271193A - Production of nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To prevent destruction of pellets by thermal stresses and to decrease volumetric expansion and release of corrosive fission product gases by using two kinds of raw material powders having the same compsn. and different bulk densities. CONSTITUTION:A specified amt. is taken from UO2 powder (powder A) of a specified grain size passed through a sieve and sintered powder (power B) of about 50mum average particle size subjected to resintering and pulverizing is prepd. The specified amt. of the powder B is added to the specified amt. of the powder B and the mixture is agitated and mixed. The mixing is so executed at this time that the bulk density of the powder A and the powder B attains, for example, 2g/cc and 6g/cc and that the weight ratio attains 1:1. The mixture is thereafter sintered. Porous structure having fine gaps or cracks around the high-density secondary particles is thereby obtd. and, therefore, the destruction by thermal impact is prevented and the release of the corrosive corrosive fission product gases and the volumetric expansion of the gases are decreased.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to nuclear fuel pellets made of nuclear fuel material such as uranium oxide or plutonium oxide, and nuclear fuel material to which a substance other than nuclear fuel material such as gadolinium or niobium is added. .

[Prior Art] Nuclear fuel pellets constituting nuclear fuel rods used in light water reactors or fast reactors operating as power reactors are generally produced by a process as shown in FIG. That is, first, the raw material powder consisting of nuclear fuel material is pulverized.

After performing powder processing such as granulation, addition of a binder or a bore former, etc., it is press-molded into pellets.

Next, this molded body (green pellet) is heated at high temperature in a reducing or slightly oxidizing atmosphere to obtain a high-density sintered body of a certain size, that is, a pellet. Nuclear fuel rods are assembled by loading these pellets into a cladding tube whose one end is sealed with an end plug, and then filling the cladding tube with constituent materials such as filler gas and springs, and then sealing the other end of the cladding tube with an end plug. When manufacturing fuel rods, a certain gap is provided between the pellets and the cladding tube. This is to prevent the pellets, which become hot during reactor operation, from causing strong interaction (pcB) with the cladding tube due to thermal expansion.Despite these considerations, during fuel rod operation, pellets Temperature gradients of 2000°C/cm or more may occur in the radial direction, and when output fluctuates, local rapid output changes of IX 10'W/C!l/h may be experienced. Due to this, the ceramic pellets undergo static thermal stress fracture or thermal shock fracture, and the broken pellet fragments protrude outward (this is called relocation) to fill the manufacturing gap and result in strong PCI. Sometimes. Irregular and localized PCI caused by pellet fragments not only causes localized stress-strain concentrations in the cladding, causing damage to the mechanical integrity of the cladding, but also causes corrosion caused by pellet release in this area. Chemical action of nuclear fission product gas (FP gas) may induce stress corrosion damage (SCC). In addition to the PCI problem caused by pellet cracking, as the burnup increases, the amount of FP that accumulates inside the pellet increases, so the gap decreases due to swelling (volume expansion due to FP accumulation) There is a problem that PCI increases. Furthermore, when pellets are exposed to high temperatures, structural changes such as growth of crystal grains may occur. Such a structural change has the effect of sweeping away the FP gas atoms that have remained within the crystal grains or at the grain boundaries, and therefore may increase the FP gas release rate of the pellet. An increase in the FP gas release rate not only causes an increase in the internal pressure of the cladding tube, but also reduces the pellet-cladding gap heat transfer coefficient, leading to cycle effects such as an increase in pellet temperature, an increase in thermal expansion, and an increase in PCI. Become. For this reason, as efforts are made to improve the performance and longevity of nuclear fuel, solving the problems of pellet cracking, swelling, and FP gas release becomes an important issue. Conventional techniques for solving these problems include, for example, JP-A-52-98897 and JP-A-53-161 for cracking pellets.
As described in No. 98, a fibrous substance such as SiC is dispersed on the outer surface or inside the pellet to prevent catastrophic damage to the pellet due to thermal stress.
) There are those that prevent destruction, and those that suppress relocation by forming a metal film or resin film on the outer surface of the pellet as described in JP-A-54-25397 and JP-A-56-49988. Regarding FP gas release, for example, Journal of Nuclear Materials, 9
8 (1981), pp. 21, 6 to 220 (Jur.
nal of Nuclear Materials,
98) (1981) P216-220), when producing pellets, metal oxides such as Nb, O, etc. are added as sintering accelerators to the raw material nuclear fuel powder to facilitate pellet sintering. Examples include those that coarsen the crystal grains of the body and reduce the release rate of FP gas.

[Problems to be Solved by the Invention] The above-mentioned prior art has the following problems. In other words, although the fibrous substance-added pellets cited as the first example of the prior art are effective against thermal stress fracture of the pellets, in reality the added fibrous substances do not have good coexistence with the fuel material matrix. At present, it is difficult to put it into practical use because the material must meet the following conditions: be able to withstand high temperatures, and not cause significant loss in terms of neutron economy. The idea of covering pellets with a metal film or resin film, which was described as the second example of the conventional technology, in addition to the conditions listed in the first example above, especially the problem of coexistence with the cladding tube and the manufacturing technology. There are manufacturing problems, such as the need to form a cladding without significantly reducing the amount of nuclear fuel material per pellet and without reducing the gap between the pellet and the cladding tube. Furthermore, metal coatings or resin coatings must maintain ductility that can sufficiently follow the thermal expansion-contraction cycle of the pellets for long periods of time under high radiation conditions, so this also has not yet been put into practical use using conventional technology. I gave this as a third example. In the case of so-called large-grain pellets that coarsen the crystal grains, a different substance is added and mixed with the fuel material as a sintering accelerator, which may lower the thermal conductivity and melting point of the pellets.
In addition, the solid solution of foreign substances into the fuel matrix means that crystal defects are formed within the matrix crystal, and the diffusion of FP gas atoms within the matrix generally occurs through these crystal defects. Considering this, a problem arises in that the addition of a sintering accelerator may lead to an increase in FP gas release.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a fuel pellet that can prevent catastrophic thermal stress fracture of the pellet without causing the above-mentioned problems, and at the same time reduce swelling and EP gas release.

[Means for solving the problem] The above purpose is to produce a mixed powder in which an appropriate amount of a second powder having the same composition as the raw powder but differing only in bulk density is added to the raw powder (first powder) in the powder adjustment process of pellet production. This can be achieved by manufacturing pellets using raw material powder.

[Working] The raw material powder for the nuclear fuel pellets had a plurality of secondary particles agglomerated. They form so-called secondary particles.

The degree of aggregation of secondary particles can be understood as the bulk density of the powder. Therefore, if a powder with the same composition as the main raw material powder but different bulk density is used as an auxiliary raw material powder and a mixed powder added to the main raw material powder is used to sinter the pellets, the powder with a different secondary particle density will be produced during the sintering process. Since there is a difference in shrinkage rate (degree of compaction) between the two, the structure of the fired pellet becomes a so-called porous structure with fine voids or cracks around high-density secondary particles. Pellets with such a porous structure have high fracture toughness and excellent thermal stress damage resistance, as expressed by equation (1).

・・・・・・・・・・・・・・・・・・・・・・・・(
1) Here, Sa: Residual strength of ceramics after thermal shock P: Porosity of ceramics N: Initial crack density G: Fracture energy E: Young's modulus Eo: Young's modulus when porosity is 0 S: Poisson ratio
α: Coefficient of thermal expansion ΔTc: Critical thermal shock temperature difference B:
Bio number source, M, OGUMA: Integrity
Dagradation of UO, Pe1le
tsSubjected to Thermal 5h
ock; J, Nucl, Mater, 127 (198
5) For this reason, even if the pellets are subjected to thermal stress such as thermal shock when the output increases, destructive cracks that cause relocation will not occur in the pellets, and the porous structure is This means that there is a lot of space to accommodate. Therefore, it is possible to reduce not only FP gas release but also solid and gas swelling. Furthermore, in a pellet having such a microstructure, when an external load is applied, the voids around the crystal grains absorb the strain in the matrix, resulting in a pellet with excellent creep deformability and plastic deformability, reducing PCI. or isolated grains surrounded by voids or cracks;
Alternatively, if the secondary particles are present dispersed in the entire matrix, the growth of other crystal grains is hindered by the secondary particles, so that such pellets generally become pellets that are difficult to grow. Therefore, it is possible to reduce the release of FP gas due to grain growth when the output of the fuel rod increases. Although the bulk density of the raw material powder can be selected arbitrarily, it is preferable that one is 1 to 5 g/cc and the other is 2 to Log/cc.

[Example] Hereinafter, specific examples of the present invention and their effects will be explained in a light water reactor (
Let's take the UO□ bullet used in LWR as an example.

FIG. 2 shows the pellet manufacturing process including the powder processing process of the present invention. First, a certain amount of UO powder was weighed (a) and then cold pressed (b) to produce green pellets. The green pellets are crushed (c) in an agate pot until they become granular, and then passed through a sieve to produce UO with a constant particle size.

A powder was obtained. (This powder is referred to as powder A.) A certain amount (d) was collected from this powder and cold pressed (e) to produce green pellets. The green pellets were heated in an electric furnace under a reducing atmosphere of 100% hydrogen for 1,700 yen.
Sintering was performed at ℃ for 4 hours. The sintered density of the pellets was approximately 96.5% in terms of theoretical density ratio. The sintered pellets are crushed (g) and sieved (ch) again to have an average particle size of approximately 50.
Next, a certain amount of this B powder was weighed, added to a certain amount of A powder, and thoroughly stirred and mixed with a V-type blender. , and A
The bulk densities of powder and B powder are approximately 2 g/cc and 6 g/cc, respectively.
The ccp weight ratio was 1:1.

Add a binder to the mixed powder (this is called C powder).

After adding a predetermined amount of lubricant (e) and stirring again, green pellets were produced by cold pressing. The green pellets were first subjected to a degreasing process (Y) to remove binders, etc., and then heated to 1750°C.

Sintered in a reducing atmosphere for 4 hours (evening), the density of the obtained sintered pellets was about 95.5%. Cross-sectional microstructural examination of the pellets was performed using a scanning electron microscope. High-density particles 2 (B
The presence of semicircular voids 3 or fine cracks 3' along the grain boundaries of the sintered particles 1 of the A powder was observed around the secondary particles formed by sintering the powder. In order to investigate the thermal shock properties of the pellets produced in this way,
Thermal shock experiments were conducted by heating the sample in an electric furnace at a certain temperature for a certain period of time, and then dropping it into a water tank to rapidly cool it.

In addition to the pellets produced according to the present invention, pellets of the same density (95.5%) produced by a conventional method were also tested for comparison. The degree of damage on the pellet due to thermal shock was evaluated from the change in fracture strength of the sample before and after the test. FIG. 4 shows a representative example of the results. Pellet 4 produced by the conventional method has a lower temperature than pellet 5 according to the present invention.
However, in conventional pellets with high initial fracture strength (value of f at ΔT=O), the thermal shock temperature difference ΔT is ΔT=Δ
When Tc is reached, the fracture strength decreases rapidly.

ΔT: Most of the intensity is lost at ΔTx.

This shows that a large rack was generated inside the pellet when ΔT=ΔTc, and that the crack was sufficiently large when ΔT=ΔTx that the sample could barely hold its shape. On the other hand, although the pellets made of the present invention have low initial strength, the decrease in strength under thermal shock conditions is extremely small compared to conventional pellets, and the growth of cracks is extremely slow as ΔT increases. It shows. Even when ΔT=ΔTx, the strength of the pellet is not significantly different from that at the initial stage.As can be seen from this result, the pellet according to the present invention has a high resistance to thermal stress fracture.

Taking BWR fuel as an example, the ΔT experienced by the pellet at the time of the first increase in power is estimated to be about 450°C. Under these thermal shock conditions, conventional pellets would suffer from fractures, whereas the pellets according to the present invention were confirmed to withstand thermal shock up to a ΔT of about 600°C.

Next, in order to investigate the swelling characteristics of these pellets, an out-of-furnace swelling simulation test was conducted. In this experimental method, sintered pellets are heated in a Co/Co mixed gas to form bubbles in the grains. An example of the results is shown in FIG. From this result, it was confirmed that the multi-porous pellets of the present invention have a considerable effect of reducing gas swelling.

In this example, the mixing ratio of powder A to powder B was 5.
Although the mixing ratio was 0%, this is not limited to this embodiment, and the mixing ratio can be set in the range of 5 to 95%. This is because a porous structure can be easily obtained at a mixing ratio of 5% to 95%, whereas it is difficult to obtain a sufficient porous structure at a mixing ratio other than this range.

[Effect of the invention] The pellets of the present invention compared to conventional pellets.

It is highly effective in preventing thermal shock destruction of pellets when fuel rod output increases. Furthermore, the problem of PCI increase due to relocation and the problem of swelling can be greatly improved by using the pellets of the present invention.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the conventional fuel pellet manufacturing process;
Figure 1 shows the manufacturing process of fuel pellets according to the present invention. Figure 3 is a schematic diagram of the microstructure of UO pellets manufactured according to the present invention. Figure 4 shows the thermal shock of UO□ pellets manufactured according to the present invention. Experimental results on damage resistance. FIG. 5 shows experimental results regarding bubble swelling characteristics of UO□ pellets produced according to the present invention. Explanation of symbols 1...Sintered crystal of raw material powder, 2...Sintered crystal of powder with the same composition and different density as the raw material powder, 3...Void or crack, 4...Conventional UO , Thermal shock properties of pellets, 5... UO of the present invention, Thermal shock properties of pellets, 6.
... Bubble swelling of conventional UO and pellets, 7... Bubble swelling of UO2 pellets of the present invention.

Claims (1)

[Scope of Claims] 1. A press-molded raw material powder of nuclear fuel material is sintered, characterized in that two types of raw material powders having the same composition but different bulk densities are mixed and used. A method for producing nuclear fuel pellets. 2. The method for producing nuclear fuel pellets according to the invention of claim 1, characterized in that the mixing ratio of one of the two types of raw material powder to the other is 5 to 95%.