JPH01253694A - Manufacture of nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To enhance a prevention effect against a pellet collapse caused by thermal stress at power upsurge of fuel rods, by mixing the same kind as an original one or a different kind of metal nuclear fuel material with a powder of oxidized nuclear fuel material. CONSTITUTION:An UO2 mixed with threads of metallic uranium, for example, is agitated and mixed with a mortar then with a ball mill. This mixed powder is weighed as to be a certain amount, is mounted by compaction and green pellets are produced. These green pellets are sintered in a vacuum atmosphere. The pellets produced in the above process have a structure that has a matrix made of crystalline grains of UO2 1 and porosity holes 2 scattered with metallic fibers of uranium 3 and fine cracks 4 can be observed at the vicinity of the fibers as shown in the figure. This pellets with metallic fibers not only have a very high initial collapse strength but also show an excellent failure resistibility against thermal shock.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention 4.1 relates to a method for producing nuclear fuel pellets used in nuclear power generation.

[Prior Art] Nuclear fuel rods 1 to 10 constituting nuclear fuel rods used in light water reactors or fast reactors that are in operation as power reactors are generally
It is manufactured through the steps shown in the figure. That is, first, a predetermined amount of raw material powder made of nuclear fuel material is weighed out, and after being subjected to powder processing such as pressing, pulverization, and addition of a binder or a bore atomer, it is press-molded into a pellet shape. Next, this molded body (green pellet) is subjected to a degreasing process and then 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 with combustible particles.

Nuclear fuel rods are made by loading these pellets into a cladding tube sealed at one end with a plug, and then filling it with gas, springs, and other components (Δ
After filling, the other end of the cladding tube is fitted with an end plug and assembled.

When producing nuclear fuel rods, a certain gap (gear) is placed between the pellets and the cladding tube. This is to prevent PCI) from occurring.Despite these considerations, vanes 1 to 1 must be
Temperature gradients of 2000°C/cm or more may occur in the radial direction, and local temperature gradients of 1
xlob W/cm/h Due to the experience of sudden power fluctuations, the ceramic pellets undergo static thermal stress fracture or thermal shock fracture, and the broken pellets protrude outward ( This is called relocation) and fills in gaps during manufacturing, which can result in strong PCI.

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 release of pellets at this location. When corrosive fission product gas (FP scum) acts chemically, stress corrosion failure (SCC) of the cladding occurs.
There is a fear that it may attract

In addition, due to the PCI problem caused by such cracking of the bellet 1~, the burnup is high/according to the increase in the amount of [P] accumulated inside the bellet 1~, swelling (volume due to accumulation of FP) occurs. PCI from cap reduction due to expansion)
There is a problem with the increase in

When the temperature is further increased, structural changes such as crystal grain growth may occur.3 Such structural changes [J
: Since it has the effect of pulling the P gas atoms accumulated at the grain boundaries, there is a risk that the P gas release rate of the pellets will increase. (1) The cladding gap reduces the thermal conductivity, leading to cycle effects such as increased pellet temperature, increased thermal expansion, and increased PCI.
Ru.

It is also believed that the 07M ratio of fuel pellets will gradually increase as combustion progresses. This (J, - per nuclear fission, uranium oxide is required (J1 pull 1 ~ from nium oxide y'fi R11L/2 oxygen atoms are not affected by neutron irradiation, and the atoms that bond all these oxygen atoms are This is because no skewer]2F) atoms are generated.

Excess oxygen atoms dissolve in the fuel 71 to increase the oxygen potential. An increase in the O/M ratio and an increase in the oxygen potency have various effects on the physical properties and characteristics of the fuel N1 pellets. For example, as the O/M ratio increases, the thermal conductivity decreases, and the P-nosing rate increases.

As mentioned above, the more we try to improve the performance of nuclear fuel 1 and make it more efficient, the more we will be able to reduce pellet cracking, swelling, 07M ratio, F.
Solving problems such as P sludge release will become an important issue.

Conventional techniques for solving these problems include the following. For pellets, JP-A-52-988
As described in No. 97 and JP-A No. 53-16198, a fibrous material such as S+C is dispersed inside Beret 1.
Something that prevents catastrophic failure of pellets due to thermal stress, (
Japanese Patent Publication No. 4-2 J 397 and Japanese Patent Application Publication No. 56-499
As described in No. 88, there are methods that prevent relocation by forming a metal film or a resin film on the outer surface of the pellet.

In addition, regarding the release of "P)
- Bu Nuclear Materials, 98 (1981) pp. 216-220 (Journal of
Nuclear Haterals, 98) (
1981) P, 216-220), when producing pellets 1~, IJ[
There are some methods that coarsen the crystal grains of the sintered body and reduce the release rate of P gas.

To deal with the increase in the 0/M ratio and the increase in the oxidation potential due to combustion, in JP-A No. 58-1/I 7678, a mixture of GO2/Co is filled during the production of the IN1 rod.
When one rod is in operation, the gas balance between the two produces a combustion of 1!31
It is a method of lowering the oxygen potential in the pellet, and in Japanese Patent Application Laid-Open No. 5L+-151291, Ta and Nb are added to uranium dioxide - 1~nium fuel pellets 1~.

A method has been proposed for suppressing the increase in oxygen potential within the combustion chamber by adding oxides of CI-, Mo, and W to bind oxygen liberated during irradiation to these substances.

[Problem to be solved by the invention] However, the above-mentioned prior art had the following problems.

Fibrous substance-added pellets cited as a first example of conventional technology
〜 is effective against thermal stress fracture of pellets, but in reality, it is important to have good coexistence with the added fiber material or fuel material 71〜 wax, to withstand high temperatures, and to achieve neutron economy. Currently, it is difficult to put it into practical use because the material must be free of bacteria (such as significant loss or no loss).

The idea of covering pellets with a metal film or resin film, which was described as the second example of the prior art, has the following problems in addition to the conditions listed in the first example above, especially issues of coexistence with the cladding tube and manufacturing technology. For example, it is difficult to form a coating without significantly reducing the amount of nuclear fuel material per pellet.
There is a problem with good. Furthermore, there is a requirement that metal or resin coatings maintain ductility that can sufficiently follow the thermal expansion/contraction cycle of pellets for long periods of time under high radiation exposure, which is also a requirement for practical use. It will lead to this.

In the third example of the prior art, the so-called large-grain pellets 1~ that coarsen the crystal grains, a different substance is added and mixed with the fuel material as a sintering accelerator, which reduces the thermal conductivity and melting point of the pellet. The possibility of a decrease in F F and the solid solution of foreign substances into the fuel helix 71 means that crystal defects within the helix 71 are formed.
) Considering that the diffusion of gas atoms within the 71 ~ lix generally occurs through these crystal defects, the problem arises that the addition of a sintering accelerator may lead to an increase in FP sludge emission.

The fourth example of conventional technology is CO during fuel rod manufacturing.
The method of filling 2/Co gas into the combustion slope requires handling CO gas, which is dangerous to the human body, compared to d3 and helium gas. - Cladding tube gap] There is a problem with 15° which causes a decrease in inductance. In addition, fuel pellets have a
The method of adding an oxide such as W may cause problems such as deterioration of neutron economy and thermal performance (reduction in thermal conductivity and melting point).

The purpose of the present invention is to prevent thermal shock destruction of pellets 1 to 1 without causing the above-mentioned problems, and at the same time, suppress an increase in oxygen potential to pellets 1, that is, an increase in 07M, and prevent swelling and FP gas. An object of the present invention is to provide a fuel bellet 1 which can reduce emissions.

[Means for Solving the Problems] The above purpose is to produce nuclear fuel pellets 1 in a raw material powder preparation step, that is, in a step of blending oxide powder of nuclear fuel materials such as uranium and plutonium. This is achieved by mixing raw ore oxide powder with a metal nuclear fuel material of the same or different type as the ore oxide powder.

However, the present invention provides a method for producing nuclear fuel pellets in which oxide powder of a nuclear fuel material is compacted and then sintered. The present invention relates to a method for producing nuclear fuel pellets, which comprises mixing.

It is effective to use the metal nuclear fuel material in the form of whiskers, wires, fine wires, granules, or the like.

[Function] In the present invention, by mixing a nuclear fuel oxide powder with a metal nuclear fuel material of the same type or a different type, a fuel oxide in which the metal nuclear fuel material is dispersed in a network is obtained. This pellet I~ has the following characteristics.

First, it has a grade 11 which has excellent thermal shock resistance. (1) Thin wires of fuel material mixed into pellets (hereinafter, the case of thin wires will be explained, but the same applies to cases of other shapes), which obstructs the progress of cracks, is caused by thermal stress. (2) A sintered body with fine wires has a fine structure containing fine cranks around the fine wires due to the external field of thermal expansion coefficient between the fine metal wires and the oxide powder during sintering. As a result, cracks that occur due to thermal stress are propagated or captured by minute cranks around the thin wire (this phenomenon is called a crackarrest).
Furthermore, (3) The thermal conductivity of metals [J] is higher than that of oxides, but sintered bodies containing fine metal wires have improved thermal conductivity compared to sintered bodies containing only oxides, and the heat generated during thermal shock is This is due to reasons such as lower stress.

Second, the increase in the 0/M ratio of pellets 1 to 1 as combustion progresses is small. Does this mean that metal substances have a higher oxygen potential A than oxides? This is because the metal substance acts as an oxygen getter due to the low oxygen content. For example, 0/
When U increases and becomes UO2,1 in a normal LJO2 pellet, approximately 1 mo in a fi N'l pellet is added.
Beret with fine metal wire added with +e% metallic uranium 1~
So, UOl, 89 or UO2,. .

It becomes Pelletz 1~, which has a two-phase region in which uranium is mixed with
Even in such a case, 0/, M of Perez i~ will not exceed stoichiometry. Previous) O/M of Peretz 1~ as if it were bogus
An increase in the ratio is a factor that causes a decrease in thermal conductivity and an increase in the FP gas release rate, so suppressing the increase in the 0/M ratio is particularly important to reduce
It greatly contributes to improving fuel performance in terms of CI.

Thirdly, the swelling of the beret is small. This is due to the fact that in a sintered body with fine wires, many fine cracks are fused into a matrix (helix), which causes the cracks to diffuse from within the crystal grains to the grain boundaries.
This is because the P gas atoms are quickly released from the -C pellet through the nearby cracks, and grain boundary bubbles that cause swelling are not generated.

Fourth, nuclear economy is high. This is because materials other than nuclear fuel 1'3+ materials are not used. The nuclear enrichment of fine metal wires made of nuclear fuel material can be adjusted as desired, just like oxide powder, and the density of pellets can be adjusted by adjusting the amount, diameter, and length of the fine wires. By the way.

As described above, the fuel pellets of the present invention can be used for PCI or FP.
This solves problems such as gas release or deterioration of thermal performance due to O/M changes, especially at high burn-up.

[Example] An example of the present invention will be described using uranium dioxide (UO2) pellets used in a light water reactor (+WR>) as an example.

Weigh out a certain amount of highly active UO2 powder produced by the AtJC method, and add a certain amount of metallic uranium wire (0.5w) to it.
t%) were weighed and mixed. In this example, the metal uranium fine wire was cut from a metal uranium rod by a lathe 7Jn into a notched thin wire with a diameter of about 100 μ711 and a length of 2 to 30 mm. Metallic uranium fine wires may also be created using other methods such as uranium metal.However, since metal uranium has high activity and easily oxidizes in the air, handling such as processing and mixing must be done in an inert atmosphere. .

UO2 powder mixed with metallic uranium wire was stirred and mixed in an agate bowl and then in a ball mill. The average length of the uranium metal wire after stirring and mixing was about 2 mm. A certain amount of this mixed powder was weighed and molded into soup powder using a press at a pressure of about 1.5 t/cm 2 to produce green pellets. The green pellets I~ were sintered in vacuum at about 1100°C for 12 hours. The melting point of metallic uranium is approximately 1130°C, so sintering is preferably performed at a lower temperature. Further, since metallic uranium becomes a hydride in a reducing atmosphere and is easily powdered during temperature rise, the sintering atmosphere is preferably a vacuum or an inert gas atmosphere.

The density of pellet 1 after sintering was about 94%. Of the approximately 6% porosity of the pellet, approximately 3%, which is equivalent to approximately 50%, is open porosity, which is approximately 1% or less than the open porosity of conventional sintered pellets 1. It can be seen that the metal uranium fine wire-containing pellets 1~ are pellets 1~ with a high open porosity.

A typical microstructure of the vertically cut surface of the pellet (-) produced as described above is not schematically shown in Figure 1. ua+), pores 2 (average diameter of about 60 pa), uranium metal thin wires 3 are dispersed in the matrix, and fine cracks 4 are formed around the thin wires.

In order to investigate the thermal shock resistance properties of this thin wired beret 1~,
A thermal shock test using a heating/quenching method was conducted.

There were two types of test pellets: pellet 1- containing metal uranium wires and additive-free pellet 1- prepared by a conventional method for comparison. After heating the pellets in an electric furnace at a constant temperature of -C for a certain period of time, the pellets were dropped into a water tank and subjected to a thermal shock of -1/l. The degree of damage to pellet 1- due to thermal shock was evaluated from the change in fracture strength of sample pellet (-) before and after thermal shock. Figure 2 shows a representative example of the results. On the other hand, the metal wire-containing pellet i~ of the present invention not only has a high initial breaking strength (the breaking strength of pellet 1~ before the thermal shock test) but also has a high
It can be seen that the residual strength is high in all thermal shock temperature difference ranges up to 600°C, indicating excellent thermal shock damage resistance.

When combining BWR fuel and fuel, the thermal shock that occurs to the fuel pellet 1 during the initial increase in output is from 450°C to 550°C due to the temperature difference.
expected to be °C. When exposed to this degree of thermal shock temperature difference, as shown in the experiment described in Section 2, conventional combustion pellets suffer catastrophic thermal shock damage. In contrast to what is expected to occur, in the pellet 1 of the present invention, the pellet i~ of the pellet covered by cracks f, .

Therefore, pellet glue location caused by thermal shock fracture is suppressed, and therefore P○■ caused by relocation is significantly reduced.

Next, a swelling simulation experiment was conducted to investigate the swelling characteristics of the uranium metal thin wire pellets of the present invention.

This experimental method uses sintered pellets as CO2/C(]Fi:
The mixture is heated for 7 J[] to form bubbles at grain boundaries. An example of the results is shown in FIG. This result is
This shows that the uranium metal thin wire of the present invention has a considerable reduction effect on gas swelling.

In addition, in this example, since the light water reactor fuel was targeted, the UO
Pellets 1- were made by mixing metallic uranium with 2 powder, but plutonium was used as the nuclear fuel M'l material.

Fuel pellets 1 having the characteristics of the present invention can also be obtained by mixing these oxide powders with metal plumylnium, metal 1-lium, or metal uranium and sintering the fuel pellets. .

Further, in this embodiment, the shape of the metal nuclear fuel material added to the raw material powder is either a thin wire or not particularly limited to a thin wire. For example, it may be a granular metal nuclear fuel material.

[Effects of the Invention] As explained above, the pellets mixed with metal nuclear fuel produced by the present invention have a lower level of pellets (~4') at the upper limit of fuel output than conventional pellets. Great thermal stress fracture prevention effect 3° Furthermore, in the pellet 1 according to the present invention, the metal nuclear fuel material acts as an oxygen getter and has the effect of suppressing an increase in the O/M ratio to the pellet 1.

Furthermore, the pellet according to the present invention overcomes the effect of reducing swelling of the pellet at high burn-up.

[Brief explanation of the drawing]

Figure 1 is a diagram showing a typical structure of UO91 pellets produced by the fuel pellet manufacturing process of the present invention, and Figure 2 is an experimental result regarding thermal intoxication damage resistance of UO2 pellets produced by the present invention. , Figure 3 shows the experimental results regarding the bubble swelling properties of the UO2 pellets produced according to the present invention, and Figure 4 (J,
It is a figure which does not show the manufacturing process of the conventional fuel pellet. 1...Crystal grains to UO2 pellets 1...UO2 pellets] ~ Pores 3...Metal uranium fine wires in UO2 pellets 4...
Fine cracks in UO2 pellets (8733) Agent: Yoshiaki Inomata, patent attorney (and others)
1 person) Figure 1 Figure 2 Figure 3 Peshi ~ Note Figure 4

Claims (3)

[Claims] (1) A method for producing nuclear fuel pellets in which oxide powder of nuclear fuel material is compacted and then sintered, characterized by mixing the oxide powder of nuclear fuel material with a metal nuclear fuel material of the same kind or a different kind as the powder. A method for producing nuclear fuel pellets. (2) Claim 1, wherein the metal nuclear fuel material is a whisker-like or wire-like thin wire.
A method for producing nuclear fuel pellets as described in Section 1. (3) The method for producing nuclear fuel pellets according to claim 1, wherein the metal nuclear fuel material is in the form of granules.