Collection of Ceramics for Nuclear Application(202412)
Silicon carbide fiber reinforced silicon carbide (SiCf/SiC) composites have become the preferred candidate for structural applications in advanced nuclear energy systems, because of their low neutron toxicity, neutron irradiation tolerance and high-temperature oxidation resistance. In recent years, both academia and industry either domestic or abroad have carried out a lot of researches on SiCf/SiC composites for nuclear application, and numerous important achievements have been made. This paper summarized and analysed some critical directions of SiCf/SiC composites for nuclear applications, including nuclear-grade SiC fibers, fibre/matrix interfaces, composite processing, modeling and simulation, corrosion behavior and surface protection, joining technology, as well as radiation damage. The key issues and potential solutions of SiCf/SiC composites for nuclear applications have been pointed out in account to the requirements, anticipating to be beneficial to promoting further researches and final applications.
Fully ceramic micro-encapsulated (FCM) fuel has become the focus of nuclear energy research because of its good inherent safety. In order to overcome the difficulty of SiC matrix densification, this study focused on the sintering behavior of simulating core FCM fuel via hot oscillatory pressing (HOP), taking the advantages of HOP to accelerate mass transfer and reduce sintering temperature. The influence of oscillatory sintering temperature, oscillatory time and oscillatory pressure on the matrix densification behavior was studied, and the results were compared with those of hot pressing (HP). The results indicate that the oscillatory sintering temperature, holding time, and median pressure have important effects on matrix densification, while the amplitude of oscillatory pressure has little effect. Compared with HP, the density of the samples is increased by HOP, and the density of sample sintered at 1850 ℃ via HOP is 99.99%. The grain size of the samples via HOP is smaller, and the grain size of the sample sintered via HOP at 1850 ℃ is (284±4) nm, which is ~27% less than that of the sample sintered via HP at the same temperature. The hardness of the samples sintered via HOP is higher, and the hardness of the sample sintered via HOP at 1850 ℃ is (26.7±0.4) GPa. When the density of sample is 90%, the stress exponent n=1 and activation energy Q=430 kJ/mol are obtained by using the modified constitutive equation of HP. The dominant mechanism of densification is grain boundary sliding, which is accommodated by the grain boundary diffusion.
Neutron detection technology is widely used in homeland security, nuclear material security detection, and high energy physics, etc. Due to the shortage of 3He resources, it is urgent to develop a novel scintillator that can discriminate neutron and gamma. The Cs2LaLiBr6:Ce (CLLB:Ce) crystal has good neutron/gamma discrimination capacity, excellent energy resolution and high light yield, but its neutron/gamma discrimination performance needs further improvement. Here, the CLLB:Ce crystals co-doped with Zr4+ were grown successfully by the vertical Bridgman method. The results of different characterization methods prove that the Zr4+ was successfully doped into the matrix and did not effect on the structure of host. Meanwhile, no new luminescence center was generated after Zr4+ doping. The UV decay time is about 27 ns, presenting a fast fluorescence decay. Figure of merit (FOM) of CLLB:Ce crystal is enhanced from 1.2 to 1.5 by co-doping Zr4+, which means that the neutron/gamma discrimination performance of CLLB:Ce crystals is improved. Combined with the thermal stability and scintillation decay time, relationship between decay time and FOM was also analyzed. The co-doping of Zr4+ can inhibit shallow electron trap and VK centers, reduce electron trapping-detrapping process, and greatly increase the probability of Ce3+ direct capturing electron, which results in a shorter decay time. Data from this study indicate that the CLLB:Ce crystals exhibit a huge application prospect in the field of neutron/gamma detection.
Molten salt electrolysis is the key technology for dry reprocessing of spent fuel in the nuclear energy industry. High-temperature molten salt can cause severe corrosion to crucible materials used for spent fuel, so the selection of the crucible material with good resistance to high temperature and corrosion is crucial for the development of the dry reprocessing method. Si3N4 is considered as a promising candidate for the crucible used in dry reprocessing, primarily due to its excellent high-temperature thermal and mechanical properties. However, its resistance to high-temperature molten salts and water vapor has not been fully investigated. In this work, the corrosion behavior of Si3N4 in LiCl-KCl and NaCl-2CsCl molten salt under Ar atmosphere and water vapor (5%H2O-10%O2-85%Ar) was investigated. The results show that in argon atmosphere, Si3N4 undergoes slight grain boundary corrosion in LiCl-KCl molten salt, while NaCl-2CsCl molten salt presents weak corrosion on Si3N4. In 5%H2O-10%O2-85%Ar water vapor environment, LiCl-KCl molten salt prefers to attack the grain boundary phase. Si3N4 shows serious corrosion degradation in the NaCl-2CsCl molten salt compared with the corrosion level in argon atmosphere. The water vapor environment significantly promotes the corrosion of Si3N4 in the molten salt environment, while the grain boundary phase is the most susceptible site for the corrosion of Si3N4. In addition, no direct correlation is found between the wettability and corrosion resistance of LiCl-KCl and NaCl-2CsCl molten salts. Results of this work elucidate the mechanism of high-temperature molten salt-water vapor-induced degradation of Si3N4, offering guidelines for the selection of crucibles in the dry reprocessing of spent fuel.
Perovskite-structured praseodymium aluminum oxide (PrAlO3) exhibits high stability which has a site that can be doped with other rare earth ions, enabling it a promising new neutron-absorbing material matrix. However, the current research on PrAlO3 mainly focuses on the preparation methods of single crystal materials and their optical and magnetic property. Here, we firstly prepared a high-density perovskite phase PrAlO3 ceramics by solid-phase reaction synthesis using tetraethyl orthosilicate (TEOS) as a liquid phase sintering aid, and then studied its microstructure and thermal property by XRD, SEM, push-rod technique, and laser flash method. The results showed that, by pre-synthesizing PrAlO3 powder at 1200 ℃ and adding 0.4%-1.0%(in mass) TEOS as a liquid phase sintering aid, PrAlO3 ceramic with a relative density higher than 99% could be obtained at around 1500 ℃, while the relative density of the product without sintering aids was only 96%. The thermal conductivity of PrAlO3 ceramic at a subcritical temperature of 360 ℃ was 4.99 W·m-1·K-1, superior to those of Dy2TiO5 and GdAlO3 ceramics, and its linear thermal expansion coefficient from room temperature to 800 ℃ was only 10.2×10-6 K-1. Moreover, the bending strength and Vickers hardness of PrAlO3 ceramics reached 95.55 MPa and 7.95 GPa, respectively, and the fluorescence spectrum exhibited characteristic emission peaks of Pr3+. This study shows that high-density perovskite phase PrAlO3 ceramics can be prepared by a convenient method with good thermophysical property and mechanical property. They exhibit good application prospects as a rare earth-based neutron-absorbing nuclear material.
In contrast to conventional solid-phase sintering, molten salt method can provide a fast mass transfer and nucleation process at lower temperatures which has potential to synthesize the ceramic solid solution for immobilization of high-level nuclear waste (HLW). In this work, Nd-doped zircon (ZrSiO4) ceramics (Zr1-xNdxSiO4-x/2 (0≤x≤0.1)) were prepared by the molten salt synthesis(MSS) at different sintering temperatures (1100, 1200, 1300, 1400, 1500 ℃) for different sintering time (3, 6, 9, 12, and 15 h). Chemical stability of Nd-doped zircon ceramics in simulated geological disposal environment was studied by static leaching test (PCT). Zr1-xNdxSiO4-x/2 was synthesized by the molten salt method under the optimum molar ratio of molten salt to oxide at 10:1, sintering temperature at 1200 ℃ and sintering time of 6 h with the solid solution of Nd in ZrSiO4 being increased to 8% (in mol). The MSS can reduce the synthetic temperature, shorten the sintering time and save the solid solution. The immobilizing mechanism of ZrSiO4 ceramics for trivalent actinide nuclides is lattice immobilizing. Experimental results show that the normalized leaching rate (LRNd) of Nd is as low as ~10-5 g·m-2·d-1. ZrSiO4 ceramics have no phase evolution before and after leaching, suggesting good structural stability. TLeaching model consolidates that Nd leaching is due to dissolution of the ceramic surface layer. Data from this study show that MSS is a promising method to synthesize ceramics solid solution.
MAX/MAB phases are a series of non-van der Waals ternary layered ceramic materials with a hexagonal structure, rich in elemental composition and crystal structure, and embody physical properties of both ceramics and metals. They exhibit great potential for applications in extreme environments such as high temperature, strong corrosion, and irradiation. In recent years, two-dimensional (2D) materials derived from the MAX/MAB phase (MXene and MBene) have attracted enormous interest in the fields of materials physics and materials chemistry and become a new 2D van der Waals material after graphene and transition metal dichalcogenides. Therefore, structural modulation of MAX/MAB phase materials is essential for understanding the intrinsic properties of this broad class of layered ceramics and for investigating the functional properties of their derived structures. In this paper, we summarize new developments in MAX/MAB phases in recent years in terms of structural modulation, theoretical calculation, and fundamental application research and provide an outlook on the key challenges and prospects for the future development of these layered materials.