The new generation of nuclear energy system requires materials with better mechanical properties, thermophysical properties, radiation resistance, corrosion resistance and thermal shock resistance, etc. Carbide ceramic materials are the focus of research. Some key nuclear carbide ceramic materials have made important progress and are going to be applied.

The research group of Associate Professor Liu Rongzheng of Tsinghua University published a review article "Application Status of carbide ceramic materials in the field of nuclear reactors" in Scientific Bulletin, which introduced the application status, basic properties, preparation methods and irradiation properties of typical carbide ceramic materials in the field of nuclear energy. The application prospect and development direction of carbide ceramic materials in the new generation of nuclear energy systems are also prospected.

Selection principles and performance requirements of nuclear materials

Microstructure: The atoms of carbide ceramics are mainly combined by covalent and ionic bonds, and the bond energy is large, so it generally has high hardness and stable chemical properties.

Mechanical properties: carbide ceramic materials generally have high hardness, elastic modulus and compressive strength, and the coefficient of thermal expansion is small. However, due to the inherent brittleness of carbide material, toughening it is also the only way to its application.

Antioxidant properties: The antioxidant properties of different carbide materials vary greatly. Although most carbide materials will oxidize at very high temperatures, some materials will form a dense oxide protective film after oxidation, showing excellent oxidation resistance.

Neutron absorption performance: The neutron absorption cross sections of different carbide materials vary greatly, and materials with large neutron absorption cross sections can be used for core neutron absorption materials, while materials with relatively low neutron absorption cross sections can be used for nuclear fuel or structural materials.

Irradiation performance: most carbide materials show good radiation resistance. For example, the radiation swelling of continuous SiC fiber reinforced SiC ceramic matrix composites (SiCf/SiC) is only about 0.1%~0.2%.

The good comprehensive properties of carbide ceramic materials make them have important applications in many fields such as nuclear fuel, structural materials and neutron absorbing materials.

Common nuclear carbide materials

Uranium Carbide: UC ceramic fuel is an important candidate fuel for advanced reactors, space power reactors and nuclear powered rockets, and can also be used as an ideal target material for the production of radioactive ion beams. Compared with UO2, UC fuel has higher thermal conductivity and higher uranium density, which can effectively increase the load of fissile nuclide and reduce the refuelling frequency, and has a good application prospect in high temperature non-aqueous media, such as all-ceramic microcapsule fuel (FCM).

Silicon carbide: The covalent bond of SiC material is very strong, can still maintain high bonding strength at high temperatures, good chemical stability and thermal stability, high temperature deformation is small, low thermal expansion coefficient, very suitable for high temperature environment. SiC is widely used in nuclear energy systems, mainly has four applications: one is as a coating layer to cover fuel particles. The second is the development of SiCf/SiC composite cladding, instead of zirconium alloy cladding. Third, it is used as matrix material in gas cooled fast reactor. Fourth, it is used as structural material in molten salt reactor. At present, the research on the improvement of antioxidant properties of SiC is also actively carried out.

Zirconium carbide: Zirconium carbide (ZrC) is a refractory metal compound with extremely high bond energy. Compared with SiC, ZrC has higher melting point, smaller thermal neutron absorption cross section, and better high temperature mechanical properties and radiation resistance than SiC. At present, there are more and more researches on ZrC, and an important research direction is to use it as a new type of fission product barrier coating fuel particles.

Boron carbide: B4C is an important neutron absorbing material, control rod material and shielding material in nuclear energy system, with low density, high melting point and hardness, and stable chemical properties. The main neutron absorption nuclide in B4C is 10B, and 10B has a large thermal neutron absorption cross section. In different reactors, B4C has different forms of use.

MAX phase ternary carbide: MAX phase material is A new type of ternary ceramic material, where M is a transition group metal element, A main group element, and X is carbon or nitrogen. MAX phase material is a layered material with low friction coefficient and good self-lubricity, and has high temperature self-healing ability. In the high temperature environment, the cracks and nicks on the surface of MAX ternary layered ceramics will be filled by the oxide of the material, which can reduce the harm of the material cracks to its performance. MAX phase materials have good chemical compatibility with coolants such as molten lead and molten sodium, and can be used as corrosion resistant cladding candidates for liquid metal cooled fast reactors.

Other potential ultra-high temperature carbide materials: In addition to the above-mentioned SiC, ZrC, B4C and MAX phase ternary carbides, there are many other potential ultra-high temperature carbide materials, especially transition metal carbides, are currently known compounds with the highest melting point of the material system. This class of carbides includes titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC). They can be used as fuel cladding or cladding cladding material, or as a second phase particle reinforcement of nuclear materials.

Main preparation process

Vapor deposition process: The vapor deposition process is mainly used to prepare ceramic coatings, including physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD is suitable for the preparation of coatings with complex components. CVD can be combined with fluidized bed technology (i.e. fluidized bed chemical vapor deposition, FB-CVD), which can achieve a high degree of uniform deposition in space and time, and is used for the coating of SiC layers in spherical ceramic fuel cores.

Our research group has accumulated rich experience in FB-CVD preparation of coated fuel particles. The microscale of chemical vapor deposition, the mesoscale of material nucleation growth and the macroscale of particle fluidization were correlated, and a multi-scale research framework was established. At present, 8.4 million coated particles have been successfully prepared in a single furnace, and the preparation technology of coated particles has reached the leap from laboratory to engineering. The prepared SiC coated particles have performed well in irradiation tests and have been used as nuclear fuel in the world's first modular high-temperature gas cooled reactor demonstration power station.

Powder preparation process: The preparation process of carbide powder for nuclear energy mainly includes carbothermal reduction method and hydrogenation reaction method. Carbothermal reduction method is usually used to prepare UC fuel, and can also be combined with sol-gel method to finally prepare UO2-UC(UCO) composite ceramic microspheres. The hydrogenation reaction method includes two processes of hydrogenation and carbonization. It has a short process flow and low reaction temperature, and can prepare a small particle size carbide powder, which can be used to produce UC powder.

Ceramic preparation process: The preparation process of general ceramics includes a series of processes such as powder preparation, mixing, molding, drying, sintering, post-treatment, etc. These processes are also suitable for the preparation of nuclear carbide ceramics. Recently, a new process for preparing SiCf/SiC composites has been developed, called nano impregnation and transient eutectic method (NITE). The SiCf/SiC composites prepared by this process have excellent high density, high crystallinity, high thermal conductivity and excellent radiation resistance, and have excellent thermal mechanical properties. It is an important method to prepare excellent nuclear fuel cladding.

Prospect and challenge

At present, the application of carbide ceramics in nuclear energy system has been more and more extensive. For example, SiC as a cladding material, B4C as a neutron absorption material has been put into use, and UC fuel as well as ZrC and MAX phase materials as cladding candidates are under development. Some of the materials have completed the in-reactor irradiation test and are about to be applied to commercial reactors.

Future research on nuclear carbide ceramic materials will focus on the following aspects:

Performance improvement: The oxidation resistance of some carbide materials is weak, and it can be tried to extend the effective working time of the material under accident conditions by means of high temperature pre-oxidation, element doping, anti-oxidation coating, etc., and it is necessary to explore new toughening mechanisms.

Preparation process: The development of preparation process focuses on powder synthesis and sintering. The preparation of carbide powders with smaller particles, more uniform distribution and better sphericity is a key step for the application of carbide materials, and overcoming the strong covalent bonding of materials will also be the direction of future development.

Compatibility problem: The inclusion of carbide and alloy is more complex, and how to match with the traditional nuclear material system needs a lot of verification work, and it can also be an opportunity to develop all-ceramic fuel elements.

Processing technology: carbide ceramic materials often have high hardness and poor toughness, which makes processing difficult and easy to introduce defects in processing, and the connection technology between carbides is also an important research direction in the future.

Acquisition and establishment of radiation data: At present, the radiation test of carbide ceramic materials is insufficient, and the experimental data is seriously lacking.

Scientific research to engineering production: Although a large number of laboratory studies have been conducted on carbide materials, there are many systemic problems in the transformation from basic scientific research to engineering production, which is also a major challenge to overcome carbide materials to reactor applications.