WO2019231046A1 - Uranium dioxide pellets having excellent fission gas adsorbing property and manufacturing method therefor - Google Patents

Abstract

The present invention relates to pellets containing an oxide additive to promote the size growth of crystal grains of uranium dioxide pellets used as nuclear fuel and increase fission gas adsorbing performance, and to a manufacturing method therefor. The La₂O₃-Al₂O₃-SiO₂ sintering additive is added to uranium dioxide, so that a liquid generated upon sintering of the uranium dioxide pellets accelerates mass transfer to promote the growth of crystal grains and a low vapor pressure of the liquid results in less volatilization during sintering, and thus the additive can exert efficient performance, and as a result, the liquid enclosing crystal grain boundaries can effectively adsorb the fission gas cesium.

Description

Sintered uranium dioxide with excellent fission gas adsorption and manufacturing method

The present invention relates to a sintered body containing an oxide additive in order to promote grain size growth of the sintered uranium dioxide sintered body used as a nuclear heat fuel and to increase nuclear fission gas adsorption performance, and a method of manufacturing the same.

Uranium dioxide sintered bodies used in nuclear power plants have been actively studied from the 1970s to the present for improving the performance. First, the development of uranium dioxide sintered compacts, which started in the 1970s, focused on preventing fuel rod failures that could be caused by pellet cladding interaction (PCI) during normal and transitional operations due to grain size growth through the addition of oxides. It became. Nuclear fuel suppliers ANF, AREVA (currently Framatome), and Westinghouse Atom, respectively, have found that Al ₂ O ₃ -SiO ₂ , Cr ₂ O ₃ , Cr ₂ O ₃ -Al ₂ O ₃ The development has been completed and is currently licensed or under license for regulatory development.

The fuel rod damage caused by PCI occurs when the cladding tube and the sintered body come into contact with each other from 30 GWD / MTU or more. From this time, the sintered body exerts a mechanical deformation and causes breakage by applying an external force in the radial direction of the cladding. However, the sintered bodies having the large grain microstructure as a result of the addition oxides cause plastic deformation of the sintered body itself before the deformation of the cladding, and solve the mutual stress with the cladding resulting from the volume expansion by heat. In addition, the area of the grain boundary, which is a passage through which various kinds of fission gases generated by the nuclear reaction, can be reduced, thereby reducing the rate of fission gas out of the sintered body.

Thus, by trapping the nuclear fission gas that deteriorates the fuel rod inner surface into the sintered body, it is possible to weaken the failure behavior due to stress corrosion cracking. Thus, the role of the PCI damage reduction sintering additive is basically to make the grains of the uranium dioxide sintered body large. This is the result of the oxide additive during sintering of uranium dioxide to promote the movement of uranium cations at the sintering temperature, this developed microstructure improves safety and power plant operating margin when burning in the furnace of a nuclear power plant (furnace).

In 2011, a global study was undertaken with the goal of improving the performance of nuclear fuel, which further strengthened safety due to the radiation leakage caused by the hydrogen explosion at the Fukushima Daiichi Nuclear Power Plant. Eventually, development of accident-tolerant fuel (ATF) began with sintered bodies and cladding. Starting with the US Department of Energy's R & D funding in 2012, global ATF fuel development is currently underway to identify appropriate final candidates, up to the unit test level of the LTR (Lead Test Rod) after survey testing. Progressed. Among the ATF sinters developed so far, uranium dioxide-based candidates include silicon carbide whiskers or composite materials in which diamond particles are dispersed in the uranium dioxide sintered body, and metal microcells in which a metal network is formed in the uranium dioxide base. These are all aimed at improving the thermal conductivity. However, in terms of dimensional stability during manufacture and furnace combustion, uranium dioxide sinters with large grains added with oxides (containing PCI damage reduction additives) are considered to be the most promising final candidates for the most feasible ATF sinters. There is.

In general, performance improvement through large grain growth is observed in sintered bodies having an average grain size of 40 μm (two-dimensional measurement) or more. According to a study by C. Delafoy et al., Proceeding of the 2007 International LWR Fuel Performance Meeting (2007), as a result of the oxidation test, a sintered body having an average grain size of 20 µm has an average grain size of 40 µm or more. 5 times higher than the sintered body was reported. That is, it turns out that a large grain sintered compact has high oxidation resistance.

The results of K. Fuglesang HWR-1161, (2016) reported that the results of combustion of experimental reactors showed that the amount of fission gas measured in sintered bodies having grains of 40 µm or more was significantly lower than that of ordinary sintered bodies of 10 µm. Source term acceptance criteria that can be released outside of an accident in a reactor are described in U.S. Based on NRC 10CFR50.67 "Accidnet source term", it cannot exceed 25 rem on the basis of the Exclusion Area Boundary. The large grain sintered compact has.

In general, gaseous fissile materials have the potential to have a significant impact on external coverage in terms of atmospheric diffusion. Therefore, the positive result that can prevent atmospheric diffusion by reacting fission gas with the material inside the sintered body to form a compound, Oak ridge national Lab. Known through CNF11-19. Since cesium (Cs), a representative fission gas, combines with silicon oxide to form a Cs ₂ Si ₄ O ₃ compound and is stabilized, development of additives for producing large grain sintered bodies containing silicon oxide for nuclear fission production capture has been carried out in Japan.

Japanese Patent Nos. 263382 and 3999843 show that Al ₂ O ₃ and SiO ₂ are added at 0.01 to 0.25% by weight in various ratios to obtain large grains, thereby obtaining a high creep rate. A study on the interaction of cesium / yodod and helium oxide fuels with the 2014 doctoral dissertation from Matsunaga, Osaka University, Japan, was carried out to capture the results of the collection of cesium in aluminosilicate liquids formed by Al ₂ O ₃ and SiO ₂ additives. Reported. However, according to T. Matsuda et al., IAEA-TECDOC-1036 (1996), the liquid phase formed by Al ₂ O ₃ and SiO ₂ is formed well before the uranium dioxide sintering temperature, which reduces the weight due to volatilization during sintering due to high vapor pressure. In the case of Matsunaga's doctoral dissertation, it was confirmed that the average grains remained at 20 μm (two-dimensional measurement) in both Al ₂ O ₃ -SiO ₂ addition 0.025 wt% and 0.25 wt%. In view of this, at the sintering temperature and in a specific sintering atmosphere, it is possible to promote mass transfer to promote grain growth, and its function is caused by volatilization of SiO ₂ -containing compounds capable of capturing cesium, a fission product generated during combustion in a reactor. It turns out that this falls. That is, even if the additive content is increased to promote more active grain growth and adsorption of fissile material, the Al ₂ O ₃ -SiO ₂ additive means that the additive effect is limited due to the decrease of the liquid fraction by volatilization.

Rare earth metal oxides are generally known to be added to refractory materials to serve to raise the glass transition temperature (T _g ) or the melting point (T _m ). In other words, by forming a high field strength in the bonding of the amorphous metal-oxygen compounds present in the glass or liquid phase at a high temperature, by improving the bonding strength of the non-bridging oxygen atoms (NBO), Can suppress evaporation.

The present invention provides a uranium dioxide sintered body and a manufacturing method containing an additive capable of efficiently trapping cesium, a fissile material, by stably forming a liquid phase capable of promoting grain growth of uranium dioxide, which is a nuclear fuel used in a nuclear power plant. The purpose is to do that. By using the sintered body with improved microstructure, it is possible to secure the combustion margin in the furnace against PCI damage and nuclear power plant accidents.

In order to achieve the above object, the present invention in the uranium dioxide nuclear fuel sintered body, uranium dioxide; And it provides a uranium dioxide sintered body comprising a sintering additive consisting of La ₂ O ₃ , Al ₂ O ₃ , SiO ₂ . The sintering additive La ₂ O ₃ -Al ₂ O ₃ -SiO ₂ to move the uranium ions quickly through the liquid phase obtained through the liquid phase to promote grain growth and is applied to the grain boundary sessile capacitive gas produced during combustion in the furnace always You can.

The additive may contain 0.05 to 0.15 parts by weight based on 100 parts by weight of uranium dioxide.

The additive may be a La ₂ O ₃ : Al ₂ O ₃ : SiO ₂ mixed weight ratio of 1 to 2: 1 to 2: 7 to 8.

In addition, the present invention provides a method for producing a uranium dioxide fuel sintered body, comprising the steps of: (a) preparing an additive by mixing La ₂ O ₃ , Al ₂ O ₃ , SiO ₂ ; (b) adding the additive to uranium dioxide powder and mixing the same to prepare a mixed powder; (c) compression molding the mixed powder to produce a molded body; And (d) provides a method for producing a uranium dioxide sintered body comprising the step of sintering the molded body in the sintering furnace during the reducing atmosphere. For this reason, the present invention adds lanthanum oxide (La ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) to uranium dioxide powder, which is a fuel sintered body, and is sintered in a weakly reducing atmosphere and stable at high temperatures. By forming a liquid phase, large crystal grains can be efficiently formed.

In step (a), La ₂ O ₃ , Al ₂ O ₃ , SiO ₂ may be mixed in a weight ratio of 1 to 2: 1 to 2: 7 to 8.

In the step (b), the additive may be added 0.05 to 0.15 parts by weight based on 100 parts by weight of uranium dioxide powder.

The step (d) may be performed at 1730 to 1780 ° C., and may maintain the hydrogen gas injection rate at 200 to 2,000 ml / min.

According to the present invention as described above, La ₂ O ₃ -Al ₂ O ₃ -SiO _{2 in} uranium dioxide By adding the sintering additive, the liquid phase produced during the sintering of the uranium dioxide sintered body accelerates the movement of grains and promotes grain growth, and the low vapor pressure of the liquid phase reduces the volatilization during sintering, thereby exhibiting an efficient additive performance. The liquid phase surrounding the grain boundary can effectively adsorb cesium, a fission gas.

In addition, the sintered uranium dioxide sintered body can reduce PCI damage as well as improve safety margin in an accident scenario.

BRIEF DESCRIPTION OF THE DRAWINGS The process flowchart which showed schematically about the manufacturing method of the uranium dioxide sintered compact of one embodiment of this invention.

2 is a three-component equilibrium state diagram of La ₂ O ₃ -Al ₂ O ₃ -SiO ₂ .

3 is an energy dispersive X-ray method of preparing a uranium dioxide sintered body containing 5 wt% La ₂ O ₃ -Al ₂ O ₃ -SiO _{2 according} to the sintered body manufacturing method of an embodiment of the present invention. Spectroscopy (EDS) is used to show the distribution of metal elements located in the microstructure.

4 is a photograph taken by a microstructure optical microscope of a uranium dioxide sintered body to which La ₂ O ₃ -Al ₂ O ₃ -SiO ₂ is prepared according to an embodiment of the present invention (x 1,000 magnification).

5 is a photograph taken by a microstructured optical microscope of a uranium dioxide sintered body to which Al ₂ O ₃ -SiO ₂ is prepared according to a comparative example of the present invention (x 1,000 magnification).

Figure 6 shows the results of the quantitative analysis of the mixed powder used in one embodiment and Comparative Example in the present invention, and the Si element contained in the sintered body prepared in Example and Comparative Example using an inductively coupled plasma spectrometer .

Hereinafter, the present invention will be described in detail.

The present invention is a uranium dioxide nuclear fuel sintered body, uranium dioxide; It comprises an additive consisting of lanthanum oxide (La ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ), the additive contains 0.05 to 0.15 parts by weight relative to 100 parts by weight of uranium dioxide, La _It provides a uranium dioxide sintered body characterized in that the mixed weight ratio of ₂ O ₃ : Al ₂ O ₃ : SiO ₂ is 1-2: 1-2: 7-8. The additive is composed of lanthanum oxide (La ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ), forms a liquid phase at the sintering temperature to accelerate the material movement of uranium atoms to promote grain growth Promote In addition, it forms a film on the grain interface to adsorb the fission gas generated during combustion in the furnace of a nuclear power plant.

In addition, the present invention is a method for producing a uranium dioxide fuel sintered body, (a) La ₂ O ₃ , Al ₂ O ₃ , SiO ₂ by mixing in a weight ratio of 1-2 to 1-2: 7 to 8 to prepare an additive Step (S11); (b) adding the additive to 0.05 to 0.15 parts by weight based on 100 parts by weight of the uranium dioxide powder and mixing to prepare a mixed powder (S12); (c) compression molding the mixed powder to produce a molded product (S13); And (d) provides a method for producing a uranium dioxide sintered body comprising the step (S14) of heating the molded body at 1730 to 1780 ℃ in the sintering furnace in the reducing atmosphere.

1 is a process chart showing a method for manufacturing a uranium dioxide sintered body including the additive according to the present invention. Referring to FIG. 1, step S11 is a step of mixing an additive used as uranium dioxide oxide, and the additive composition includes La ₂ O ₃ , Al ₂ O ₃ , and SiO ₃ . The ratio of the oxide additive in the step S11 is the composition range with the highest liquid fraction, as La ₂ O ₃ -Al ₂ O ₃ -SiO ₂ equilibrium three-component state diagram of Figure 2, La ₂ O ₃ , Al ₂ O ₃ , SiO _It adds in ₃ ratio (weight ratio). The mixing is performed homogeneously using a Turbula mixer capable of three-axis rotational mixing with a zirconia ball of 5 mm diameter. In the step S11, only the additives are mixed first. This is to maintain the composition ratio between the additive powder which is mixed with the uranium dioxide which is the parent powder because the additive system is multicomponent. The sintering additive La ₂ O ₃ -Al ₂ O ₃ -SiO ₂ to move the uranium ions quickly through the liquid phase obtained through the liquid phase to promote grain growth and is applied to the grain boundary sessile capacitive gas produced during combustion in the furnace always You can.

La ₂ O ₃ , Al ₂ O ₃ , SiO ₃ added in step S12 In adding the powder, the additive content is limited to 0.05 to 0.15 parts by weight relative to 100 parts by weight of uranium dioxide powder. The addition of 0.05 wt% or more in the step S12 is to form a sufficient liquid phase. On the other hand, the reason for not exceeding 0.15% by weight is to minimize the neutron economy deterioration due to the addition of elements having a high thermal neutron absorption cross-sectional area. Therefore, the amount of the additive is limited to 0.05 to 0.15 parts by weight based on 100 parts by weight of the uranium dioxide powder in order to apply the liquid phase containing SiO ₂ to the crystal grains for growth up to 40 μm or more and high fission material adsorption. The mixing is carried out for three hours using a Turbula mixer with a zirconia ball of diameter 5 mm for three hours of rotating homogeneous mixing.

In step S13, as a step of compacting the mixed additive powder and uranium dioxide powder, a mixed powder is added to a molding mold and a molded body is manufactured at a pressure of 2.5 ton / cm ² .

In the step S14, as a step of sintering the manufactured molded body, 100% hydrogen gas is injected at 200 ml / min to 2,000 ml / min at a temperature in the range of 1700 to 1780 ° C, and the sintering is performed for 3 to 5 hours. In order to maintain a reducing sintering atmosphere by injecting 100% hydrogen gas at a rate of 200 ml / min or more in step S14 by inhibiting the increase of oxygen potential by trace oxygen released from uranium dioxide or refractory materials. On the other hand, the reason for not exceeding 2,000 ml / min is to lower the pressure loaded inside by the clogging phenomenon of condensed water generated in the gas outlet by a small amount of steam discharged during sintering. Through the above-described process, a large grain sintered body having an average grain size of 40 µm or more can be produced.

La ₂ O ₃ -Al ₂ O ₃ -SiO ₂ in FIG. As can be seen from the equilibrium three-component state diagram, the liquid phase can be formed in the present sintering process. As an example, referring to FIG. 3, the elements constituting the secondary phase in the liquid phase state obtained by containing the additive were identified. To this end, the lanthanum surrounding the uranium dioxide grains in the sintered compact containing 5 parts by weight of the La ₂ O ₃ -Al ₂ O ₃ -SiO ₂ additive in an amount of 5 parts by weight relative to 100 parts by weight of the uranium dioxide powder The signal from the area | region consisting of La), aluminum (Al), and silicon (Si) element was confirmed. From the signal of the region consisting of lanthanum (La), aluminum (Al), and silicon (Si), it can be seen that the actual amorphous liquid phase is formed. Through this, it can be clearly seen that the liquid phase obtained by containing the additive surrounds the uranium dioxide grain boundary, which can be confirmed to contribute to the formation of large grains of uranium dioxide.

Through the particle growth promotion and the application of the nuclear fission material adsorption component grain boundary according to the formation of an efficient liquid phase prepared by the additive and the process of the present invention, it is possible to manufacture a sintered compact having excellent PCI damage reduction and accident safety.

Hereinafter, the present invention will be described in more detail with reference to Examples. These examples are only for illustrating the present invention, it will be apparent to those skilled in the art that the scope of the present invention is not to be construed as being limited by these examples.

Example.

La ₂ O ₃ , Al ₂ O ₃ , SiO ₂ additives were mixed in three dimensions homogeneously for 4 hours using a Turbula mexer with a 5 mm zirconia ball diameter (Φ) at a weight ratio of 1: 1: 8. 0.1 parts by weight of the mixed additive was added to 100 parts by weight of uranium dioxide powder, mixed for 4 hours using a Turbula mixer, and then pressed at 3.5 ton / cm ² to prepare a molded product. The molded body was heated to 1730 ° C. at a rate of 5 ° C./min, and sintered at 1730 ° C. for 4 hours. The sintering atmosphere was controlled by injecting 100% hydrogen gas at a rate of 250 ml / min.

Comparative example.

Al ₂ O ₃ , SiO ₂ A uranium dioxide sintered body in which 0.09 parts by weight of an additive mixed with a weight ratio of 1: 8 was added to 100 parts by weight of uranium dioxide powder was prepared in the same manner as in the above example.

Measurement example. Microstructure Observation

In order to observe the microstructure after sintering the sintered bodies prepared in Examples and Comparative Examples, the circumferential half of the cylindrical sintered body was cut and polished and thermally etched, and the microstructured image was taken using an optical microscope. 4 (Example) and FIG. 5 (Comparative Example).

Referring to FIG. 4, the linear cross-section method was used to measure the grain size, and the grain size was measured as an average size of 45.5 μm larger than about 9 μm of a typical uranium dioxide sintered body. In addition, referring to FIG. 5, the grain size was measured with an average grain size of 21.5 μm. The Example shows a grain size about twice as large as that of the sintered compact to which 0.09 parts by weight of Al ₂ O ₃ -SiO ₂ additive of the Comparative Example was added.

Measurement example. Quantitative Analysis of Metal Elements

Inductively Coupled Plasma Spectrometer (ICP) in order to analyze the quantitative determination of metal elements contained in the uranium dioxide powder mixed with the additives used in the above Examples and Comparative Examples and the sintered bodies prepared in Examples and Comparative Examples Was used.

Through this analysis, it is possible to measure the amount of volatilized silicon oxide added in the sintered body and the amount remaining in the sintered body. Metal silicon (Si) quantification was measured for quantitative analysis of silicon oxide (SiO ₂ ) added in the form of oxide.

Figure 6 shows the results of measuring the content of the silicon metal element present in the powder used in the Examples and Comparative Examples and the sintered uranium dioxide prepared in Examples and Comparative Examples. Si content in SiO ₂ added at 800 ppm was 372.8 ppm, which is 46.6%, and both Examples and Comparative Examples showed Si content close to 370 ppm. However, in the sintered body, it was measured as 320.7 and 130.7ppm, respectively, and it was found that Si was volatilized by about 50ppm and 240ppm. Therefore, the sintered compact of the Example is excellent in volatilization resistance about 4.8 times compared with the sintered compact of the comparative example.

As mentioned above, specific parts of the present invention have been described in detail, and it is apparent to those skilled in the art that such specific descriptions are merely preferred embodiments, and thus the scope of the present invention is not limited thereto. something to do. Thus, the substantial scope of the present invention will be defined by the appended claims and their equivalents.

Claims (8)

1. In the uranium dioxide fuel sintered body,

   Uranium dioxide; And

   Sintered uranium dioxide containing an additive consisting of La ₂ O ₃ , Al ₂ O ₃ , SiO ₂ .
2. The method of claim 1,

   The additive is a uranium dioxide sintered body, characterized in that it contains 0.05 to 0.15 parts by weight relative to 100 parts by weight of uranium dioxide.
3. The method of claim 2,

   The additive is a sintered uranium dioxide, characterized in that the La ₂ O ₃ : Al ₂ O ₃ : SiO ₂ mixed weight ratio of 1-2: 1-2: 7-8.
4. In the method for producing a uranium dioxide fuel sintered body,

   (a) mixing La ₂ O ₃ , Al ₂ O ₃ , SiO ₂ to prepare an additive;

   (b) adding the additive to uranium dioxide powder and mixing the same to prepare a mixed powder;

   (c) compression molding the mixed powder to produce a molded body; And

   (d) heating the sintered compact in the sintering furnace during the reducing atmosphere; and sintering the uranium dioxide sintered compact manufacturing method comprising a.
5. The method of claim 4, wherein

   In the step (a), La ₂ O ₃ , Al ₂ O ₃ , SiO _{2 The} uranium dioxide sintered body manufacturing method characterized by mixing in a weight ratio of 1-2: 1: 1-2: 7-8.
6. The method of claim 5,

   In the step (b), the uranium dioxide sintered body manufacturing method characterized in that the addition of 0.05 to 0.15 parts by weight based on 100 parts by weight of the uranium dioxide powder.
7. The method of claim 4, wherein

   Step (d) is a method for producing sintered uranium dioxide, characterized in that carried out at 1730 to 1780 ℃.
8. The method of claim 7, wherein

   Step (d), the uranium dioxide sintered body manufacturing method characterized in that the hydrogen gas injection rate is maintained at 200 to 2,000ml / min.

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