Creation of tube shells of fuel elements using composite materials based on silicon carbide
SIC VS. ZR
The transition from the zirconium alloy fuel shell to the silicon carbide composite material (SiC/SiC) is a complex task that requires a lot of changes in the technology of reactor materials.
Zirconium alloys have a low capture cross section for thermal neutrons, satisfactory thermal conductivity, high strength under normal operating conditions. But abnormal increase in the temperature of shells of fuel elements above 600 °C leads to the beginning of the zirconium-steam reactions with the release of hydrogen from water and formation of explosive gas mixture . The use of silicon carbide as the main material for fuel element shell eliminates the possibility of formation of explosive mixture, as this material has a low oxidation kinetics and a high melting temperature.
The table presents a comparison of the main physical-chemical properties of zirconium alloy and SiC .
As it is shown in the table, the silicon carbide has a higher mechanical strength, wear resistance, hardness, thermal conductivity, and good corrosion and radiation resistance . It is stable in water vapor, very weakly react with oxygen at elevated temperatures, and, in contrast to zirconium alloys does not lead to release of hydrogen with formation of explosive gas mixture. The advantage of silicon carbide in comparison with the zirconium was confirmed by a number of studies, where it was shown that the strength of SiC/SiC composites remains stable at temperatures over 1500 °C, while zirconium completely loses its strength at a temperature of 1300 °C. In addition, the silicon carbide in extreme conditions shows the corrosion rate 100–1000 times less than that of zirconium .
CONCEPT OF WESTINGHOUSE
Westinghouse (USA) is one of the world leaders in research and creation of technology for manufacturing of tube shells made of composite material based on silicon carbide, dealing with this issue since 2004. After a series of successful experiments, the scientists from Westinghouse have chosen three options of manufacturing technology of tube shells (Fig.1), which, in their opinion, will ensure compliance with the requirements of the fuel elements .
The first conception is the formation of a composite with fibrous reinforcing frame woven of SiC-fiber at an angle of 45–52°, and SiC-matrix obtained by liquid-phase or gas-phase impregnation. Such a structure of SiC/SiC-composite eliminates the brittle fracture is and increases the strength of the finished product. The second and third conceptions involve the chemical vapor deposition (CVD) of a protective layer of SiC on an inner or outer surface of the composite manufactured according to the first conception of the technology. This layer will help to retain gaseous fission products and to increase the corrosion resistance of the product when interacting with water vapor, in particular, to increase its strength, however, may cause problems with the dimensions of the shell. It was also suggested to combine the three proposed conceptions of this technology, creating a composite with inner and outer protective layers.
The development of the described above technology is a part of ATF (Accident-tolerant fuel) project. According to recent reports, Westinghouse plans to develop the production of full-scale silicon carbide tube shells for fuel elements by 2025. Funding for the project will amount to more than 1.5 billion USD . In 2016, Westinghouse and General Electric Aviation began construction of two plants for the manufacturing of ceramic-matrix composites, which should give the first products by mid 2018. The first factory is aimed at production of SiC fiber, which is currently manufactured only in Japan, the second factory will specialize in the production of composite products with the use of SiC fiber .
Further it is planned to use the developed technologies in such areas as mechanical engineering, aviation, aerospace, defense industry, civil sector, etc.
In the period from 2011 to 2016 VNIINM’s scientists have carried out a large number of studies of different methods of creating a tube shells based on SiC, which allowed to reduce the gap with their foreign counterparts. The following methods were investigated: slip casting; isostatic pressing; vapor deposition; deposition of SiC on a SiC substrate, obtained by isostatic pressing; impregnation of the SiC fibrous frame by silicon-organic precursors. The obtained samples are shown in Fig.2.
At detailed study of each method their advantages and disadvantages were identified. For example, tubes obtained by vapor deposition have the lowest value of porosity (0.6%), high value of strength (250–275 MPa), but such samples are susceptible to brittle fracture. In turn, the method of impregnating the fibrous SiC frame by silicon-organic precursors allows to avoid brittle fracture, but the samples have high porosity and low strength.
After analyzing the results, VNIINM come to the conclusion that the compliance of tube shells to the requirements can only be achieved by combining several technologies that will allow to create multi-layered composite based on β-SiC, as shown in Fig.3.
Three-layer ceramic shell provides optimization of the properties of the internal monolith for containment of gaseous fission products, the strengthening of the fiber matrix to enhance the mechanical characteristics and outer monolithic layer for corrosion resistance. Presented schematic diagram is similar to the American concept, but there are some differences, which are associated mainly with the technology of a middle composite layer.
If the American scientists have chosen biaxial weaving (45 Ч 45°) of fibrous frame , since samples with such architecture better resist torsional loads, VNIINM considers the use of triaxial weaving or mixed-architecture weaving, which will, in particular, to resist loads in bending that will increase the strength of the finished product (Fig.4). The study of this issue is planned to finish in 2017.
Architecture of weaved frame has a significant influence on the properties of the finished products, preventing brittle fracture and ensuring the improvement of mechanical properties, but the quality of the impregnation of the braided frame is no less important. One of the key properties of the finished product is its closed porosity, which should be minimal. The porosity of the matrix depends on many factors, including the chosen method of its formation and on applied materials. In the United States the method of liquid-phase impregnation by preceramic oligomers is used that gives a fairly high porosity at the level of 6–10% .
In Russia, as in the case of selecting the architecture of the weaving, the combined technology is studied. In addition to liquid-phase impregnation by preceramic oligomers that are modified with various metals, VNIINM investigates the possibility of applying gas-phase and electrophoretic impregnation, as well as improvements of the above methods by the use of vibration and ultrasound. These options allow to increase the density and uniformity of filling in the matrix across the length of the product compared to liquid-phase impregnation.
At present, specialists of VNIINM study the deposition of interphase layer on the fibers, which compensates differences in thermal expansions of the layers and in case of operation at elevated temperatures significantly reduces the likelihood of cracking of the finished product.
SiC fiber plays an important role in the structure of the finished composite. At the moment, only USA and Japan have the technology of manufacturing stoichiometric β-SiC fibers. Such a fiber has not been supplied to Russia. However, VNIINM has a strong competencies for the manufacture of SiC-fiber using its own technology that according to preliminary calculations is much cheaper than counterparts. Its development is underway in VNIINM and is scheduled for completion in 2017–2018.
It should be noted that nanomaterials can be used in the manufacture of tube shells for fuel elements, which, because of their characteristics, will significantly improve the properties of the finished product. And they can be used at almost every stage of manufacturing. For example, the fine powders of silicon carbide of high purity can be used in the impregnation process to fill the micropores and to reduce the closed porosity. Implantation of graphene, fullerenes, or SiC nanowires on the β-SiC fibers as the auxiliary elements is possible when applying the interfacial layer to increase the strength. Work in this area is already underway, but the concrete results will be seen not earlier than in 2018.
In the coming years, it is planned to complete in VNIINM the creation of the experimental laboratory division of the production of tube shells for fuel elements made of composite material based on silicon carbide. The emphasis is on the ability to produce not only tube shells, but also other types of composite products. In the process of creating the division it is also planned to master the production of β-SiC fiber, which is needed in all areas of the composite industry, because of its advantages over carbon fiber, in particular, in heat resistance and thermal stability. In the future, the project for the creation of composite tube shells for fuel elements made of silicon carbide should be an integral part of a larger program of VNIINM, "Tolerant fuel" that is similar to the American ATF.
Currently, silicon carbide is considered as one of the most promising materials for the production of composites. It has great potential in the aerospace industry, in the manufacture of components and parts for gas turbine engines and systems. Silicon carbide and its modifications have found especially wide use in aircraft structures, operating at elevated temperatures under conditions of high load and high wear. It is caused by high antifriction properties of SiC. Today it is used in radial and lateral slide bearings, in face seals rings including gas dynamics ones. Sliding pairs with a low coefficient of friction can significantly reduce the energy consumption of mechanisms to ensure a high level of their reliability and to increase their durability [12, 13].
Composite products made of silicon carbide are widely used in the defense industry, especially in the manufacture of various armor elements. The interaction of this type of composite product with shaped charges, its combination with active protection is actively studied. Studies of various types of tank armor have shown that the use of composite ceramics based on silicon carbide gives a considerable (up to 30%) gain in weight. At the same time, the strength and bullet resistance of products increase .