Keywords

1 Introduction

The purpose of research and development of advanced reactor systems is to meet the economic, environmental and social development needs of the 21st century. Its technical goals involve four aspects: sustainability, economy, safety and reliability, nuclear non-proliferation and physical protection. Specifically, it contains eight technical goals, covering various technical directions related to the design and implementation of reactors, energy conversion systems and fuel cycle facilities.

The nuclear fuel element is the core component of the reactor, and its advanced nature and safety are the important basis for the advanced nature and safety of the reactor. ATF (Accident Tolerant Fuel) fuel that can tolerate accidents to a certain extent and has inherent safety, is an important development direction in the field of nuclear fuel in the world. As an important part of ATF, the coated particle-dispersed fuel obtained from TRISO particle-dispersed silicon carbide matrix is ​​aimed at the weakness of the traditional UO2-Zr alloy fuel system, and combines the mature TRISO (Tri-Structural Isotropic) particles with the tolerance of fission products. As well as the advantages of good thermal performance and thermal stability of SiC matrix, it is a more promising development direction in advanced accident-resistant nuclear fuel.

The coated particle-dispersed fuel is based on a fuel pellet with three-dimensionally isotropic TRISO-coated particles embedded in a SiC matrix. The structure of the pellets and the internal TRISO particles is shown in Fig. 1. Such pellets feature high thermal conductivity, multiple layers of protection against cracking products, and high burnup, which facilitates normal and transient operability of the reactor.

Fig. 1.
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Schematic diagram of TRISO particle and coated particle dispersion fuel

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After the concept of coated particle dispersion fuel in which TRISO particles are dispersed in a silicon carbide matrix was proposed, researchers from the United States, South Korea and other countries have conducted in-depth research in this direction, and prepared products with good thermal conductivity. All-ceramic microencapsulated die with uniform phase distribution.

In this paper, the effect of binder on the dressing of TRISO granules and the effect of pre-compression molding pressure on the green density were studied through mixing, molding and hot-pressing sintering experiments. After the SiC matrix and TRISO particles were uniformly mixed and molded, the effects of different sintering aid contents, different hot-pressing holding times, different hot-pressing sintering temperatures, different hot-pressing sintering pressures, and different powder particle sizes on the properties of pellets were studied.

2 Materials and Methods

2.1 Materials

The TRISO particles used in the study are fuel particles with spherical UO2 as the core. The outer layer of the core is sequentially coated with loose pyrolytic carbon, inner dense pyrolytic carbon, silicon carbide and outer dense pyrolytic carbon. The SiC powder used is β phase and its purity is greater than 99.9%. The particle size distribution of the raw material powder is relatively uniform, but the deviation between the median value and the average value is small, and the powder particle size as a whole presents a normal distribution centered on the median value.

Due to the poor sintering performance of SiC powder, in the process of preparing by NITE liquid phase sintering method, adding a small amount of sintering aid for co-sintering can improve the sintering performance of the material to a certain extent. The sintering aids used in the test are Al2O3 and Y2O3 powders with a purity > 99.9%.

2.2 Methods

Put SiC powder with a particle size of 10nm and 3wt% sintering aid (the mass ratio of nano-scale Al2O3 and Y2O3 is 7:3) into a ball mill jar, with anhydrous ethanol as the dispersant, stainless steel balls as the grinding balls, The ratio is 3:1, and the ball milling time is 4 h. After ball milling, the powder is dried, crushed and sieved; the sieved mixed powder is evenly wrapped on the surface of the TRISO particles; the TRISO particles after uniformly wrapping the base powder and the base powder are mixed uniformly according to a certain volume ratio, and then pre-compressed and formed; finally Vacuum hot pressing sintering, heating rate 10 ℃·min-1, sintering temperature 1650 ~ 1750 ℃, 1 ~ 2.5 h holding time, 65 ~ 90 MPa.

3 Results and Discussion

3.1 Influence of Binder on Dressing Effect of TRISO Particles

In order to improve the compatibility of SiC and TRISO particles during the green forming process of the coated particle-dispersed fuel pellets, and at the same time prevent the TRISO particles from contacting each other, it is necessary to coat a layer of SiC on the surface of the TRISO particles.

Due to the large difference in particle size between TRISO particles and SiC powder, it is difficult for SiC to directly coat the surface of TRISO particles without binder, as shown in Fig. 2(a). After investigation and test, use glycerol as binder, absolute ethanol as diluent, configure 10% glycerol-absolute ethanol as binder, and use glue tip dropper to evenly wrap the surface of TRISO particles with a layer of adhesive. Then, it is dispersed into the matrix powder (SiC + sintering aid) to realize the bonding of a layer of matrix powder on the surface of the TRISO particles. As shown in Fig. 2(b), the surface of the TRISO particles is completely covered and the dressing effect is good.

Fig. 2.
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Mixture macro effect diagram:(a) The effect of dressing without binder.(b) The effect of dressing without binder

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3.2 Influence of Pre-press Forming Pressure on Green Density

The weight of the TRISO particles and the SiC powder after adding the sintering aid are calculated and weighed according to the volume fraction, and the ingredients are mixed, and then loaded into the pre-compression mold, and the floating female mold is pre-compressed using a micro-controlled pressure testing machine to determine the molding pressure and raw material. The relationship between blank density and TRISO particle integrity is shown in Fig. 3. It can be seen from Fig. 3 that the higher the pressing pressure, the higher the green density of the pellets. The higher the green density, the better the density of pellets after sintering, so the molding pressure should be raised as high as possible. It was observed in the experiment that when the pressure was lower than 2.0 KN, the TRISO particles were not damaged, and when the pressure reached 2.5 KN, the surface layer of some TRISO particles appeared to fall off, and there was abnormal noise during the pressing process.

Fig. 3.
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Variation curve of green density with pressure

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3.3 Effects of Different Hot Pressing Sintering Processes on the Properties of Pellets

Table 1 shows the density changes of pellet fuel pellets covered by pressureless sintering under different additions of sintering aids. When the addition of sintering aids is less than 3wt%, the density of pellets increases with the addition of sintering aids., when the addition amount of sintering aid is greater than 3wt%, the increase in the density of pellets is no longer obvious.

Table 1. Effect of Sintering Aid Additives on the Density of Pellets
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Table 2 shows the change results of the density and phase of the pellets at different hot pressing sintering temperatures. It can be seen from the results that as the sintering temperature increases, the density of the pellets increases, and the β-SiC in the pellets will be partially converted into α -SiC, when the sintering temperature is higher than 1710 ℃, obvious α-SiC diffraction peaks appear in the pellet phase.

Table 2. Influence of sintering temperature on the density and phase of pellets
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Table 3 shows the change results of the density of pellets under different hot pressing and holding time. With the prolongation of holding time, the diffraction peak of α-SiC phase in the pellets gradually increased; when the holding time was 1h, the inner and outer layers of the pellets appeared Phenomenon, the pellets are not burned through, and after the holding time exceeds 2h, the density of the pellets does not increase significantly.

Table 3. The effect of holding time on the density of pellets
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Table 4 is divided into the change results of the density of pellets under different hot-pressing pressures. The analysis results show that with the increase of hot-pressing pressure, the density of pellets gradually increases. When the hot-pressing pressure is in the range of 75 ~ 80 MPa, the The increase in density decreases. When the hot pressing pressure reaches 90 MPa, the punch on the die breaks.

Table 4. The effect of sintering pressure on the density of pellets
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Table 5 shows the experimental results of SiC powder with different particle sizes. It can be seen from the results that with the decrease of powder particle size, the density of pellets gradually increases.

Table 5. Effect of Powder Particle Size on Pellets Density
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3.4 Performance Characterization of Coated Particle-Dispersed Fuel Pellets

Using SiC matrix powder with a particle size of 10nm, vacuum hot-pressing sintering at 3wt% sintering aid addition, 1690 ℃ hot-pressing sintering temperature, 1.5 h holding time, and 80 MPa hot-pressing sintering pressure, the obtained all-ceramic micro-encapsulated dispersed fuel pellets were prepared. The real thing is shown in Fig. 4.

Fig. 4.
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Coated particle dispersion fuel pellets

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The SEM of the SiC matrix is ​​shown in Fig. 5, the second phase composed of sintering aids exists inside the SiC matrix, and the distribution is uniform. The metallographic photos of TRISO particles can clearly see that the outer layer of the particles has 4 layers of cladding layers, the particle structure is complete, and there is no damage during the molding and sintering process. After core sintering, the thickness and element distribution of each layer of TRISO particles are still comparable to those of the original particles, and the thickness of the outer dense pyrolytic carbon layer is 19 μm.

Fig. 5.
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SEM image of coated particle-dispersed fuel pellets

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It can be seen from Fig. 6(a) that the thermal diffusivity of the coated particle-dispersed fuel core gradually decreases with the increase of temperature, and the thermal diffusivity at each temperature point is lower than that of the base SiC. The addition of TRISO particles reduces the core to a certain extent. The thermal diffusivity is still significantly higher than that of the traditional UO2 fuel core. It can be seen from Fig. 6(b)that the thermal expansion coefficient of the fuel core increases gradually with the increase of temperature, and the change trend is linear. As the content of TRISO particles increases, the thermal expansion coefficient of the core decreases slightly. In the range of 0 –1000 ℃, the thermal expansion coefficient of the coated particle-dispersed fuel core is significantly lower than that of the traditional UO2 core.

Fig. 6.
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Thermophysical properties vary with temperature:(a) Thermal diffusivity. (b) Thermal expansion coefficient

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4 Conclusions

  1. (1)

    Through the research on the dressing process of TRISO granules, TRISO granules with complete surface coating and good dressing effect were prepared.

  2. (2)

    The hot-pressing sintering process of all-ceramic micro-encapsulated pellets was established, and the coated particle-dispersed fuel pellets with a relative density of 96% T.D. were prepared, and the SiC matrix phase was β-SiC, and the TRISO particles inside the pellets were evenly distributed.

  3. (3)

    The thermal diffusivity of the coated particle-dispersed fuel pellets obtained under the optimal process is higher than that of the conventional UO2 pellets, and the thermal expansion coefficient is lower than that of the conventional UO2 pellets.