JP4674312B2 - Nuclear fuel pellet manufacturing method and nuclear fuel pellet - Google Patents

Description

The present invention relates to a method for producing nuclear fuel pellets and fuel pellets used as fuel for fast breeder reactors and light water reactors, and more particularly, minor actinide oxides, uranium oxides (UO ₂ ), plutonium oxides (PuO ₂ ). Relates to a method for producing nuclear fuel pellets comprising a fuel phase comprising a nuclear fuel material oxide such as non-radioactive nuclear / physically / chemically inert molybdenum (Mo) and a nuclear fuel pellet. .

  From the nuclear fuel used for the combustion of the nuclear reactor, that is, spent nuclear fuel, minor half-actinoids (hereinafter abbreviated as MA) such as neptunium (Np), americium (Am), and curium (Cm) are generated. As an effective processing method of this MA, conversion to a nuclear stable element is performed by irradiating neutrons in a nuclear reactor to cause nuclear conversion or the like.

The above-mentioned form of processing of MA into nuclear fuel is performed by adding a small amount of MA to a standard nuclear fuel such as uranium oxide (UO ₂ ) or uranium / plutonium mixed oxide (MOX) and mixing it. In general, a method of forming an MA-containing nuclear fuel pellet is generally used.
However, the thermal properties such as thermal conductivity and melting point of UO ₂ and MOX fuels with MA added are reduced due to being lower than the thermal properties of UO ₂ and MOX fuels mainly due to the influence of MA oxides. Therefore, it is necessary to limit the amount of MA added in order to establish the core thermal / nuclear design. This indicates that depending on the prior art, rapid combustion treatment of MA currently accumulated in large quantities may become difficult due to long-term operation of the light water reactor.

  Therefore, in order to increase the MA content in the nuclear fuel, a nuclear fuel different from the conventional nuclear fuel has been proposed. This nuclear fuel is composed of a sintered body composed of a fuel phase containing MA and a non-radioactive nuclear / physical / chemically inert matrix phase. Because it has an excellent adjustment effect of the oxygen potential, it not only shows excellent characteristics as a fuel form for burning MA, but also as a high-performance nuclear fuel by using U, Pu, etc. as the fuel phase As shown in Non-Patent Document 1, there is metal molybdenum (Mo) as a promising inert matrix phase, and a large amount of spent nuclear fuel It has also been proposed to use the Mo fission product produced as a base material for the inert matrix.

  Since the nuclear fuel using Mo as an inert matrix phase (hereinafter also referred to as Mo cermet fuel) has a high thermal conductivity and an excellent adjustment effect of oxygen potential during combustion, Therefore, there is a demand for providing a Mo cermet fuel in which the Mo phase content is further improved, that is, the Mo content is large, the fuel phase and the Mo inert matrix phase are uniformly dispersed and processed at high density.

However, when trying to obtain high-density pellets using the conventional method for producing Mo cermet fuel pellets, a high sintering temperature of 1,800 ° C. or higher is required due to Mo being a refractory metal. It becomes.
Furthermore, when MA is contained in the fuel phase, it has been pointed out that Am has a high vapor pressure and may evaporate and scatter during sintering. This also results in higher doses in the fuel production facility due to adhesion of evaporated and scattered Am to the facility, resulting in an increase in the maintenance cost of the facility, etc., as well as a decrease in the quality of the produced pellets and the resulting production. There is a problem of increasing the cost.
Furthermore, because of the high temperature sintering, there is a problem that the life of the refractory material and the heat insulating material and the heater constituting the sintering furnace is shortened, resulting in an increase in production cost.
For this reason, it is required to sinter Mo cermet fuel pellets containing MA at as low a temperature as possible.

  Further, the mixing step and the molding step before the above-described sintering step have conventionally required complicated processes such as mixing by a planetary ball mill and pressure molding by an isostatic press. Such complicated powder processing leads to an increase in the dose of equipment due to adhesion of highly radioactive substances such as MA to the equipment, resulting in deterioration of equipment maintenance, an increase in maintenance costs, and an increase in radioactive waste. For this reason, the manufacturing process is required to be as simple as possible from the viewpoint of handling highly radioactive substances.

  On the other hand, in Mo cermet fuel pellets that use Mo recovered from spent nuclear fuel, contamination of metal impurities such as iron (Fe), cobalt (Co) nickel (Ni), palladium (Pd) accompanying the recovery, The influence on the sintered density due to the heat cannot be ignored. Therefore, even when using recovered Mo mixed with these metal impurities, it is necessary that the manufactured Mo cermet fuel pellets have a predetermined high density and exhibit desired characteristics.

So far, several inventions have been proposed to increase the density of nuclear fuel pellets by adding specific compounds and causing them to act as sintering aids. In Patent Documents 1, 2 and 3, a compound other than a metal is added as a sintering aid to UO ₂ fuel to improve the density. For example, Patent Document 1 is a technique in which alumina and silica are added as sintering aids, but amorphous alumina silicate grain boundary phases are precipitated in the produced pellets, and these grain boundary phases have a fuel temperature. Because it melts under high conditions, it is not applicable to fast reactor fuel. Patent Document 2 is a technique of adding a small amount of ammonium sulfate, but the density is only improved by about 2% at the maximum, and there is a problem that a fundamental increase in density cannot be expected. Further, Patent Document 3 is a technique for improving the sintered density by Li, but Li is highly corrosive, and therefore, the addition amount is said to be “as small as possible”. That is, Li is one of the metals that should be avoided most in the cladding corrosion. Therefore, there is a problem that the density cannot be further increased.

  As a technique for improving the density without adding a sintering aid, there is a method of controlling the pressing pressure and the sintering atmosphere according to the properties of the raw material powder as shown in Patent Document 4. However, there is a problem that it cannot be applied to a material powder whose composition is not constant, such as molybdenum recovered from spent nuclear fuel.

JP-A-10-293187 JP 2000-193776 A Japanese Patent Laid-Open No. 57-197496 Japanese Patent Application Laid-Open No. 2006-234753 M. Osaka, M. Koi, S. Takano, T. Misawa, Y. Yamane, "A Novel Concept for Americium-containing Target for Use in Fast Reactors," J. Nucl. Sci. Technol., 43, 367-374 (2006).

Accordingly, an object of the present invention is to provide a method for producing nuclear fuel pellets using Mo as an inert matrix phase, which has a relatively low sintering temperature, can simplify the production process, and has a high density.
Another object of the present invention is to produce nuclear fuel pellets that have a high Mo content, a fuel phase and a Mo inert matrix phase that are uniformly dispersed and processed at a high density at a relatively low sintering temperature, and a simple manufacturing process. It is to provide a manufacturing method that can be manufactured.
Another object of the present invention is to provide a method for producing nuclear fuel pellets in which, when Mo recovered from spent nuclear fuel is used as an inert matrix phase, metal impurities mixed in Mo do not hinder densification. Is to provide.

As a result of diligent experiments, the present inventors have found a sintering aid capable of lowering the sintering temperature and improving the sintering density, and have completed the present invention.
That is, the method for producing nuclear fuel pellets of the present invention comprises 80 to 20% by volume of a minor actinide, uranium or plutonium oxide as a first component, or an oxide fuel powder constituting a fuel phase composed of two or more thereof, In the method for producing nuclear fuel pellets obtained by mixing, molding and sintering 20 to 80% by volume of a molybdenum powder constituting an inert base material phase as the second component, the third component is 1 with respect to the molybdenum. 1 type or 2 or more types chosen from the group which consists of 10 mass% manganese, iron, cobalt, nickel, copper, zinc, palladium, and titanium as a sintering auxiliary agent, and said 1st, 2nd And a mixing step of mixing the third three components to obtain a mixed powder, a molding step of pressing the mixed powder to obtain a molded body, and 1,600 to the molded body By heat treatment of 1,750 ° C., characterized by comprising a sintering step to obtain a sintered body.
In addition, the method for producing nuclear fuel pellets of the present invention uses molybdenum recovered from spent nuclear fuel in place of the molybdenum powder, so that metal impurities associated with the molybdenum, manganese, iron, cobalt, nickel, copper One or two or more selected from the group consisting of zinc, palladium and titanium are made to function as the third component .
The method for producing nuclear fuel pellets of the present invention is characterized in that the minor actinide oxide contained in the first component is 50% by volume at the maximum of the fuel phase.
Moreover, the nuclear fuel pellet of the present invention is manufactured using the above-described manufacturing method.

According to the manufacturing method of the present invention, by adding a predetermined metal as a sintering aid at a predetermined ratio, a sintering temperature lower by 50 to 200 ° C. than a sintering temperature based on the prior art can be obtained. There is an effect that fuel pellets having a high density can be manufactured.
Further, according to the manufacturing method of the present invention, the sintering density of the inert base material phase of Mo is mainly improved, and further, the particles are formed so that the inert base material phase fills the gap with the fuel phase during the sintering. Since it grows, there is an effect that high density fuel pellets can be produced even if the proportion of the Mo inert matrix phase is reduced to a minimum of 20% by volume.

  Further, according to the production method of the present invention, when Mo containing metal impurities recovered from spent nuclear fuel is used as Mo in the inert base material phase, the metal impurities include Fe, Ni, Co, Pd and the like are accompanied, but these metal impurities also function as a sintering aid. Therefore, by adjusting the metal impurities in the recovered Mo to 10% by mass or less with respect to Mo, the metal impurities mixed in the recovered Mo can function as a sintering aid without adding a separate sintering aid, and the fuel There is an effect that it is possible to contribute to high density of pellets.

  Further, since the addition of molybdenum can suppress the deterioration of fuel characteristics such as the thermal conductivity and oxygen potential of the fuel pellets, the MA content contained in the fuel phase can be reduced to a maximum of 50 volumes of the fuel phase. % Can be increased. As a result, there is an effect that it becomes possible to reprocess spent nuclear fuel and to quickly process high-level waste.

Furthermore, since the manufacturing method of the present invention and the manufacturing equipment used therefor are based on the methods and equipment conventionally used for the production of nuclear fuel pellets, a safe and simple method with sufficient results from the past. In the handling of highly radioactive substances such as MA, there are advantages from the viewpoint of maintenance of the equipment. Therefore, the manufacturing process can be simplified.

The process diagram of FIG. 1 shows an example of an embodiment of the present invention. As the oxide fuel material constituting the fuel phase of the Mo cermet fuel, UO ₂ or PuO ₂ obtained by oxalic acid precipitation method or direct denitration method, and MA such as AmO ₂ are used. Moreover, when using 2 or more types of these oxides, it is used as an oxide solid solution.

As Mo which comprises the inert base material phase of Mo cermet fuel, commercially available metal Mo or the powder of Mo collect | recovered from used nuclear fuel is used. The mixing ratio of the oxide fuel powder as the fuel phase and the Mo powder as the inert matrix phase is set to 80 to 20% by volume of the oxide fuel powder and 20 to 80% by volume of the Mo powder.
When the oxide fuel powder exceeds 80% by volume, an increase in the sintered density is hindered by oxidation of Mo, reduction of the fuel phase, or the like. If it is less than 20% by volume, the density increasing effect according to the present invention is mainly due to the densification of the inert matrix phase, and therefore the density increasing effect is reduced.

In the present invention, in addition to the oxide fuel powder and the Mo powder, Ni powder, Fe powder or Pd powder can be further added and mixed as a sintering aid, so that the density of the Mo cermet fuel can be increased. In addition to this, powders of transition metals such as Mn, Co, Cu, Zn, and Ti can also be used as the sintering aid. Pd can be desired to be recovered from spent nuclear fuel. Among the exemplified sintering aids, preferred sintering aids are Pd, Fe and Co. These do not cause a liquid phase during reactor operation by forming a solid solution in Mo after sintering to form an alloy. For this reason, the physical and chemical stability of the Mo cermet fuel is improved, and as a result, the safety is increased.
The addition amount of the sintering aid is in the range of 1 to 10% by mass with respect to Mo. The optimum addition amount varies somewhat depending on the type of sintering aid, powder preparation, sintering conditions, etc., but in general, if the addition amount is less than 1% by mass, a sufficient density improvement effect cannot be obtained, and about 5% by mass. The density becomes maximum at an addition amount of about%. On the other hand, when it exceeds 10 mass%, the tendency for a density to fall is recognized.

As the Mo powder, Mo mixed with metal impurities recovered from the spent nuclear fuel can also be used. Examples of the metal impurities to be mixed include Fe, Ni, Pd, and the like, but these metal impurities also function as a sintering aid. Therefore, by adjusting the metal impurity in the recovered Mo to 10% by mass or less with respect to Mo, the metal impurity mixed in the recovered Mo can be functioned as a sintering auxiliary without adding a sintering auxiliary agent separately. This can contribute to higher density of fuel pellets.
The metal which is not described in the Example mentioned later also functions as a similar sintering aid by the same sintering mechanism (liquid phase sintering). The same applies when two or more kinds of sintering aids are mixed and used.

By containing MA such as AmO _{2 in} the oxide fuel powder constituting the fuel phase, MA can be combusted together with nuclear fuel in a nuclear reactor. The MA content in this case can be up to 50% by volume of the fuel phase. If it is contained in a larger amount than 50% by volume, the thermophysical / chemical stability of the fuel phase is lowered.

When mixing the raw material powders described above, it is not necessary to use a ball mill or the like for finely pulverizing the powder, and a simple rotary mixer is sufficient for mixing, and the mixing time is from 5 minutes to 30 minutes at most. It is.
The mixed powder thus obtained is molded into a pellet shape with a pressure of about 200 MPa using a uniaxial press. Sintering of the molded body is performed at 1,600 to 1,750 ° C. for about 4 hours in an argon stream containing 4% or more of hydrogen.

Experiment No. 1 in Table 1 In Nos. 1 to 9, commercially available Mo powder and UO ₂ powder or PuO ₂ powder obtained by the oxalic acid precipitation method are weighed so that the volume ratio thereof is 50% by volume, and further commercially available as a sintering aid. Pd or Ni was added so as to have an addition amount shown in Table 1 with respect to the Mo weight, and mixed for 5 minutes in an agate mortar using acetone as a solvent.
Next, it is molded in a single-axis press at 200 MPa for 2 minutes to form a cylindrical pellet having a diameter of 6 mm and a height of 4 mm, and then fired in an electric furnace at 1,600 ° C. for 4 hours in an argon stream containing 4% hydrogen. Mo cermet fuel pellets were prepared by
In addition, although said solvent illustrated acetone, you may use ethanol which has the same property. Moreover, although the example of wet mixing was shown this time, it is not always necessary to use a solvent, and it is also possible to select dry mixing by using a powder having high fluidity and press shape retention.

Table 1 shows the results of measuring the density of the obtained Mo cermet fuel pellets. Furthermore, the graph which shows the relationship between the amount of sintering auxiliary agent addition in Table 1 and a density is shown in Drawing 2 (experiment No. 1-4). From these results, the density of pellets of Experiment Nos. 2 to 5 and 7 to 9 in which the sintering aid is added in the range of 1 to 10% by mass with respect to the Mo weight is the same as Experiment No in which no sintering aid is added It can be seen that the density is higher than those of .1 and 6.
If the sintering temperature is less than 1,600 ° C., the necessary density cannot be obtained even if a sintering aid is blended. If the sintering temperature is higher than 1,750 ° C., the sintering temperature is not changed, and there is no effect of low-temperature sintering.

Although not shown in Table 1, when sintering is performed at 1,600 ° C. in a test using CeO ₂ having the same characteristics as nuclear fuel oxide, when CeO ₂ is 19% by volume, the theoretical density is 9 The density was 6.947 g / cm ³ against .649 g / cm ³ , which was only 72% of the theoretical density. Moreover, when the ratio of Mo was less than 20% by volume, the sintering phase at the same temperature decreased due to the lack of the volume of the portion where the metal was sintered due to the small amount of the metal phase.
Further, when CeO ₂ is 81% by volume after sintering at 1,600 ° C., the density is 5.496 g / cm ^{3 with} respect to the theoretical density of 7.851 g / cm ³ , which is only 70% of the theoretical density. It was.
Further, when CeO ₂ was 50% by volume, the density was 7.16 g / cm ³ when calcined at 1,750 ° C. for 4 hours.

FIG. 3 is an optical microscope gold phase photograph of a Mo cermet fuel pellet composed of a fuel phase of PuO ₂ , an inert matrix phase of Mo, and a sintering aid of Pd. The gray part is PuO ₂ and the white part is Mo and Pd are shown, respectively. As can be seen from this gold phase photograph, in the Mo cermet fuel pellet according to the present invention, the fuel phase and the Mo inert matrix phase are almost uniformly dispersed, and there is no non-uniformity of thermal conductivity in the pellet. Excellent physical and chemical stability.

In Table 1, an example in which MA is contained in the fuel phase is omitted. However, since it has the same crystal structure as UO ₂ and PuO ₂ , the addition of a sintering aid is mainly performed by Mo. Since the sintering density of the inert matrix phase was improved, the same density increase as UO ₂ and PuO ₂ was obtained. Therefore, according to the method for producing Mo cermet fuel pellets of the present invention, it is confirmed that MA can be effectively used in the combustion process of MA by containing up to 50% by volume of MA in the fuel phase. It was done.

The thermal conductivity of uranium oxide, which is a standard nuclear fuel, is 2.8 W / m · K at 1,000 ° C., whereas the value is 40 W / m for Mo cermet fuel containing 50% by volume of Mo. m · K is 14 times or more, and the effect of improving the thermal conductivity according to the present invention is clear. This is because, in addition to the high thermal conductivity of Mo, the manufacturing method with the addition of the sintering aid according to the present invention produced a sintered and connected structure. Further, by adding 20% by volume or more of Mo, it is possible to maintain a good oxygen potential adjustment ability throughout the combustion period.

It is process drawing which shows an example of embodiment of this invention. Experiment No. 1 in Table 1 It is a graph which shows the relationship between the sintering auxiliary agent addition amount and density in 1-4. It is an optical microscope gold phase photograph of Mo cermet fuel pellet by the present invention.

Claims (4)

80 to 20% by volume of minor actinides, uranium or plutonium oxides as the first component, or oxide fuel powder constituting the fuel phase composed of two or more thereof, and 20 to 80% by volume of non-oxide as the second component. In the manufacturing method of nuclear fuel pellets formed by mixing, molding and sintering the molybdenum powder constituting the active matrix phase,
As a third component, one or two or more selected from the group consisting of manganese, iron, cobalt, nickel, copper, zinc, palladium, and titanium are used as a sintering aid with respect to the molybdenum. And a mixing step of mixing the first, second and third components to obtain a mixed powder, a molding step of pressing the mixed powder to obtain a molded body, and 1 to the molded body , 600 to 1,750 ° C., and a sintering process for obtaining a sintered body. By using molybdenum recovered from spent nuclear fuel in place of the molybdenum powder, metal impurities associated with the molybdenum are selected from the group consisting of manganese, iron, cobalt, nickel, copper, zinc, palladium, and titanium. The method for producing nuclear fuel pellets according to claim 1 , wherein one or more types are allowed to function as the third component .   3. The method for producing nuclear fuel pellets according to claim 1, wherein the oxide of the minor actinide contained in the first component is a maximum of 50% by volume of the fuel phase.   A nuclear fuel pellet manufactured using the manufacturing method according to claim 1.