In recent years, environmental pollution and carbon emissions caused by fossil fuels have led to faster development of nuclear energy. But with the increasing demand for the safety of nuclear power systems following the Fukushima accident in Japan, a new generation of nuclear power systems has been developed on the basis of the third generation. The materials used in the new generation nuclear power system need to have better mechanical properties, thermophysical properties, strong radiation resistance, corrosion resistance, and thermal shock resistance. Therefore, it is urgent to optimize the existing material system and deeply develop new high-performance materials. Among numerous optional materials, carbide ceramic materials are currently the focus of attention.
Figure 1. Source of Nuclear Power Plant: Pixabay
1、 Overview of the Properties of Nuclear Carbide Materials
The service environment of nuclear materials is very harsh, requiring them to withstand high-temperature, high-pressure, highly corrosive, and highly radioactive particle beam bombardment, which puts forward higher requirements for material selection. The excellent characteristics of carbide ceramics provide more possibilities for the development of nuclear carbide ceramic materials.
Figure 2. Principles and Performance Requirements for Selecting Nuclear Materials
(1) In terms of microstructure, the atoms of carbide ceramics are mainly bonded by covalent bond and ionic bond, and the bond energy is large. Button type classification, carbides can be divided into interstitial carbides, covalent carbides, and ionic carbides, with the first two being more widely used in nuclear energy systems.
(2) In terms of mechanical properties, carbide ceramic materials generally have high hardness, elastic modulus and compressive strength, and coefficient of thermal expansion is also small. However, due to the inherent brittleness of carbide materials, toughening them is also a necessary path for the application of carbide ceramic materials.
(3) In terms of antioxidant performance, there are significant differences in the antioxidant performance of different carbide materials. Although most carbide materials undergo oxidation at very high temperatures, some materials, such as SiC, form a dense silica protective film after oxidation, exhibiting excellent oxidation resistance.
(4) In terms of irradiation performance, most carbide materials exhibit good radiation resistance. For example, the radiation swelling of continuous SiC fiber reinforced SiC ceramic matrix composite is only about 0.1%~0.2%.
(5) In terms of neutron absorption performance, the neutron absorption cross-sections of different carbide materials vary greatly and can be used in different scenarios. If used as a core neutron absorption material, it requires a large neutron absorption cross-section to terminate the chain reaction faster under accident conditions.
Figure 3. Summary of the Performance of Major Carbides Used in Nuclear Energy
2、 Main carbide materials for nuclear energy
(1) Uranium carbide
Uranium carbide contains UC, U2C3, and UC2. The C/U atomic ratio of UC has a narrow range at room temperature, with a carbon content of 4.80 wt%. UC2 exists in the form of a sub stoichiometric ratio, with a C/U atomic ratio of 1.86~1.96146.47. It is unstable at room temperature and appears in a tetragonal crystal system at high temperatures α- UC2 and Cubic Crystal System β- There are two types of UC2. At temperatures below 1200K, U2C3 is unstable and decomposes into UC and C. Compared with UO2, UC fuel has higher thermal conductivity, which can effectively flatten the power density and temperature gradient of the core, and has a higher uranium density, effectively increasing the loading of fissionable nuclides and reducing refueling frequency. It 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 producing radioactive ion beams.
Figure 4. Comparison of physical parameters of uranium carbide with UC and β- The Cell Structure of UC2
(2) Silicon carbide
The covalent bond of SiC material is extremely strong, and it can still maintain high bonding strength at high temperatures. It has good chemical stability and thermal stability, small high-temperature deformation, and low coefficient of thermal expansion, which is very suitable for high temperature environments. SiC is widely used in nuclear power systems, mainly as a coating layer for coating fuel particles, developing SiCf/SiC composite cladding, replacing zirconium alloy cladding, as a matrix material in gas cooled fast reactor, and as a structural material in molten salt reactor.
(3) Zirconium carbide
Zirconium carbide ZrC) is a refractory metal compound, which belongs to the typical NaCl type face centered cubic structure and has extremely high bond energy. Compared with SiC, ZrC has a higher melting point, smaller thermal neutron absorption cross section, and better high-temperature mechanical properties and radiation resistance than SiC. At present, there is increasing research on ZrC, and an important research direction is to use it as a new type of fission product barrier for encapsulating fuel particles.
Figure 6. Fuel element form and fuel particles using ZrC ceramic as matrix and/or coating layer
(4) Boron carbide
B4C belongs to rhombohedral system, which can be seen as a three-dimensional structure of a cubic primitive lattice stretched in the spatial diagonal direction, with boron regular icosahedron arranged on each top corner. B4C is an important neutron absorbing material, control rod material, and shielding material in nuclear power systems, with low density, high melting point, and hardness.
B4C has different forms of use in different reactors. In boiling water reactor, powdered B4C is encapsulated in stainless steel cladding as thermal neutron shielding material; B4C powder is also used as a neutron absorbing material in heavy water reactors, and B4C powder is loaded into stainless steel tubes to form a control rod assembly; A cylinder composed of carbon and B4C is used as a control rod in a high-temperature gas-cooled reactor; The fast neutron breeder reactor uses B4C sintered pellets placed in stainless steel cladding to make control rods, which are used as reactor core control rod materials. In addition, B4C can also be made into B4C absorption pellets, which serve as the second shutdown system for high-temperature gas-cooled reactors and can also serve as isolation blocks during spent fuel processing to avoid unexpected criticality.
Figure 7: Crystal Structure of Boron Carbide
In addition to the uranium carbide, silicon carbide, zirconium carbide and boron carbide described above, there are many other potential ultra-high temperature carbide materials, especially transition metal carbide, which is the material system with the highest melting point among known compounds. This type of carbide includes titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC).
At present, the application of carbide ceramics in nuclear energy systems has become increasingly widespread. For example, SiC as cladding material and B4C as neutron absorbing material have been put into use, while UC fuel and ZrC as cladding candidate material are both in development. Some materials have completed the irradiation test in the reactor and are about to be applied to commercial reactors.
In the future, research on nuclear carbide ceramic materials will focus on: (1) improving performance. Some carbide materials have weak antioxidant properties, and can be attempted through high-temperature pre oxidation, element doping, antioxidant coatings, and other methods; (2) The preparation process focuses on both powder synthesis and sintering, producing carbide powders with smaller particles, more uniform distribution, and better sphericity; (3) Compatibility issues, acquisition and establishment of irradiation data, scientific research, and engineering production.
1. Application status of carbide ceramic materials in nuclear reactor field Cheng Xinyu, etc
2. Preparation and Properties of Uranium Carbide and Uranium Boroide Ceramic Powders by Guo Hangxu
3. Preparation of Core Shell Structure Boron Carbide Powder and Performance Study of Composite Materials
Author: Sunny Day