Thorium dioxide / uranium dioxide composite pellet and preparation method thereof

Abstract

The invention discloses a thorium dioxide / uranium dioxide composite pellet and a preparation method thereof. The preparation method comprises the following steps: step 1, mixing ThO2 powder and UO2 powder to obtain mixture powder; and 2, sintering and cooling the mixture powder obtained in the step 1 to obtain the ThO2 / UO2 composite pellet. The high-density ThO2 / UO2 composite pellet is prepared by taking ThO2 as a substrate and adding different contents of uranium dioxide through high-temperature sintering, and meanwhile, the ThO2 / UO2 composite pellet can further improve the thermal conductivity of ThO2 and improve the safety performance of the fuel.

Description

Technical Field

[0001] The invention relates to the technical field of nuclear fuel, and in particular to a thorium dioxide / uranium dioxide composite pellet and a preparation method thereof. Background Art

[0002] Uranium is generally used as nuclear fuel in ceramic fuel pellets in nuclear reactors. However, the current uranium resources are insufficient, and uranium and uranium oxides are unstable, not resistant to high temperatures, have low thermal conductivity, and produce more toxic fission gases. Therefore, it is necessary to find a substitute for uranium-based fuel as fuel pellets for reactors.

[0003] Thorium has many advantages as a nuclear fuel. Compared with uranium, the thorium resource content on the earth is three times that of uranium. The current annual output of thorium ore in the world can reach 23,000 tons, and the annual output of thorium extracted can reach 230 tons. Thorium has better neutron economy; thorium and thorium oxide have better stability, high temperature resistance, and thermal conductivity than uranium and uranium oxide, and produce less fission gas. The fission products accumulated in thorium-based reactors are less toxic than uranium-based reactors. Thorium, as an energy source, will hardly release any greenhouse gases and harmful particles during consumption, and its radioactivity is much lower than that of uranium. The nuclear radiation generated during fission is only about 0.6% of uranium. Once it leaves the high temperature or neutron-rich environment of the nuclear reaction, the nuclear reaction will stop immediately. Therefore, compared with uranium, thorium has the advantages of being safer and cleaner as a nuclear fuel, and the treatment of waste fuel is also relatively convenient.

[0004] However, thorium is a non-fissionable element, and pure thorium dioxide cannot be used alone as a fuel rod. Therefore, how to prepare a high-performance thorium dioxide nuclear fuel to replace uranium resources is an urgent problem that needs to be solved. Summary of the invention

[0005] The invention aims to solve the problem of how to prepare a high-performance thorium dioxide nuclear fuel to replace uranium-based fuel as a fuel pellet of a reactor.

[0006] In order to achieve the above technical objectives, the technical solution provided by the present invention is:

[0007] A ThO2 / UO2 composite pellet, characterized in that the mass fraction of UO2 in the pellet is 5%wt-20%wt of the total mass of the mixture of ThO2 and UO2.

[0008] A method for preparing a ThO2 / UO2 composite pellet comprises the following steps:

[0009] Step 1: Mix ThO2 powder and UO2 powder to obtain a mixture powder;

[0010] Step 2: Sinter and cool the mixed powder of step 1 to obtain ThO2 / UO2 composite pellets.

[0011] Furthermore, in the step 1, the mass fraction of UO2 is 5%wt-20%wt of the total mass of the mixture of ThO2 and UO2.

[0012] Furthermore, the ThO2 powder has a purity of 97% and a particle size of 50nm to 200μm; the UO2 has an average particle size of 50nm to 100μm and a purity greater than 99.9%.

[0013] Furthermore, the mixing is performed using a planetary ball mill for 6-8 hours.

[0014] Furthermore, before sintering, the mixed powder is loaded into a graphite grinder, and then placed in a spark plasma sintering furnace and evacuated to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10 ~ 60kPa.

[0015] Furthermore, the sintering temperature is 1500° C.-1700° C., the sintering pressure is 20-50 MPa, and the sintering time is 15-20 minutes.

[0016] Furthermore, the mixture needs to be kept warm for 0-5 minutes before cooling.

[0017] The present invention has the following beneficial effects:

[0018] 1. The present invention uses ThO2 as a substrate, adds different contents of uranium dioxide, and prepares high-density ThO2 / UO2 composite pellets through high-temperature sintering. At the same time, the ThO2 / UO2 composite pellets can further improve the thermal conductivity of ThO2 and improve the safety performance of the fuel.

[0019] 2. The rapid sintering technology used in the present invention can complete the densification sintering of the mixture of ThO2 and UO2 within 15 to 20 minutes at 1500℃-1700℃, effectively inhibiting the chemical reaction behavior of ThO2 and UO2 at high temperature, thereby ensuring the stability of the composite fuel structure and performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the microstructure morphology of ThO2-10%wtUO2 core block (a.1500℃b.1600℃c.1700℃). DETAILED DESCRIPTION

[0021] The technical solution of the present invention is described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0023] Example 1

[0024] Step 1: ThO2 powder (purity 97%, particle size 50nm~200μm), 10%wtUO2 powder

[0025] (average particle size 50nm-100μm, purity greater than 99.9%), using a planetary ball mill to mix materials such as UO2 and ThO2 for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0026] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1500℃, sintering pressure 50MPa, sintering time 15min, insulation 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0027] The density of the composite core block is 96.7%.

[0028] Example 2

[0029] Step 1: ThO2 powder (purity 97%, particle size 50nm~200μm), 10%wtUO2 powder

[0030] (average particle size 50nm-100μm, purity greater than 99.9%), using a planetary ball mill to mix materials such as UO2 and ThO2 for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0031] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1600℃, sintering pressure 50MPa, sintering time 16min, insulation 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0032] The density of the composite core block is 97.5%.

[0033] Example 3

[0034] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 10%wtUO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0035] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1700℃, sintering pressure 50MPa, sintering time 20min, insulation 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0036] The density of the composite core block is 98.1%.

[0037] Example 4

[0038] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 5%wtUO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed for 8 hours using a planetary ball mill to obtain a uniform mixture powder of UO2 and ThO2;

[0039] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1600℃, sintering pressure 45MPa, sintering time 15min, insulation 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0040] The density of the composite core block is 96.8%.

[0041] Example 5

[0042] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 10%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0043] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1Pa, and then filled with argon to 10-60 kPa; sintering temperature 1600 ° C, sintering pressure 45 MPa, sintering time 16 minutes, insulation 5 minutes, after the insulation is completed, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0044] The density of the composite core block is 97.1%.

[0045] Example 6

[0046] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 20%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0047] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1600℃, sintering pressure 45MPa, sintering time 17min, insulation 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0048] The density of the composite core block is 99.2%.

[0049] Example 7

[0050] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 5%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0051] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1550℃, sintering pressure 25MPa, sintering time 18min, insulation 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0052] The density of the composite pellet is 94%.

[0053] Example 8

[0054] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 10%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0055] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1550℃, sintering pressure 25MPa, insulation for 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0056] The density of the composite pellet is 94%.

[0057] Example 9

[0058] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 20%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0059] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1550℃, sintering pressure 25MPa, insulation for 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0060] The density of the composite pellet is 94%.

[0061] Example 10

[0062] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 5%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0063] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1Pa, and then filled with argon to 10-60kPa; sintering temperature 1550℃, sintering pressure 45MPa, insulation for 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0064] The density of the composite core block is 94.5%.

[0065] Embodiment 11

[0066] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 10%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0067] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1550℃, sintering pressure 45MPa, insulation for 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0068] The density of the composite core block is 96%.

[0069] Example 12

[0070] Step 1: ThO2 powder (purity 97%, particle size 50nm-200μm), 20%wt UO2 powder (average particle size 50nm-100μm, purity greater than 99.9%), and UO2 and ThO2 are mixed in a planetary ball mill for 6 hours to obtain a uniform mixture powder of UO2 and ThO2;

[0071] Step 2: Load the mixed powder in step 1 into a graphite mold, place it in a spark plasma sintering furnace, and evacuate to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10-60kPa; sintering temperature 1550℃, sintering pressure 45MPa, insulation for 5min, after insulation, naturally cool to room temperature to obtain ThO2 / UO2 composite pellets.

[0072] The density of the composite core block is 97.5%.

[0073] like Figure 1The microstructure morphology of the pellet prepared by adding 10%wt UO2 powder. It can be seen from this microstructure morphology that since ThO2-10wt%UO2 can reach a relatively high density at a sintering temperature of 1500℃, the sintering temperature has a certain influence on the morphology of the pellet. At 1500℃, there are still a small number of pores dispersed in the internal structure of the pellet. As the sintering temperature increases, the number of pores gradually decreases. When the sintering temperature reaches 1700℃, there are no obvious pores inside the pellet. The added UO2 forms pellets and is evenly distributed in the ThO2 matrix.

[0074] As shown in Table 1, the thermal conductivity of ThO2 / UO2 composite pellets was studied. By adding 5%wt, 10%wt, and 20%wt uranium dioxide to thorium dioxide, the thermal conductivity of uranium dioxide was improved by preparing ThO2 / UO2 composite pellets through high-temperature sintering, thereby further improving the possibility of replacing uranium-based fuel with thorium-based fuel.

[0075] Table 1 Thermal conductivity of ThO2 / UO2 pellets

[0076]

[0077] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

[0078] The above specific implementation methods are detailed descriptions of the present invention. It cannot be determined that the specific implementation methods of the present invention are limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions and substitutions can be made without departing from the concept of the present invention, which should be regarded as belonging to the protection scope of the present invention.

Claims

1. A ThO2 / UO2 composite pellet, characterized in that: The mass fraction of UO2 in the core block is 5%wt-20%wt of the total mass of the mixture of ThO2 and UO2.

2. A method for preparing a ThO2 / UO2 composite pellet, characterized in that: The steps include: Step 1: Mix ThO2 powder and UO2 powder to obtain a mixture powder; Step 2: Sinter and cool the mixed powder of step 1 to obtain ThO2 / UO2 composite pellets.

3. The method for preparing a ThO2 / UO2 composite pellet according to claim 2, characterized in that: In the step 1, the mass fraction of UO2 is 5%wt-20%wt of the total mass of the mixture of ThO2 and UO2.

4. The method for preparing a ThO2 / UO2 composite pellet according to claim 2, characterized in that: The ThO2 powder has a purity of 97% and a particle size of 50nm to 200μm; the UO2 has an average particle size of 50nm to 100μm and a purity greater than 99.9%.

5. The method for preparing a ThO2 / UO2 composite pellet according to claim 2, characterized in that: In the step 1, the mixing time is 6-8 hours.

6. The method for preparing a ThO2 / UO2 composite pellet according to claim 2, characterized in that: Before sintering, the vacuum should be evacuated to 5×10 -2 ~5×10 -1 Pa, and then filled with argon to 10 ~ 60kPa.

7. The method for preparing a ThO2 / UO2 composite pellet according to claim 2, characterized in that: The sintering temperature is 1500° C.-1700° C., the sintering pressure is 20-50 MPa, and the sintering time is 15-20 minutes.

8. The method for preparing a ThO2 / UO2 composite pellet according to claim 2, characterized in that: Furthermore, the mixture needs to be kept warm for 0-5 minutes before cooling.

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