In recent years, human beings have become increasingly dependent on energy. However, non-renewable resources such as coal, oil and natural gas are not inexhaustible and inexhaustible. Is it possible to solve the dilemma of human energy shortage once and for all? As nuclear technology matures, the controllable nuclear fusion reactor, known as the “artificial sun” and the “ultimate energy for mankind”, may provide a steady stream of clean energy for the benefit of future generations. The main principle of this technology is that deuterium and tritium produce nuclear fusion reactions under high temperature and high pressure conditions, and generate a large amount of heat energy for power generation. Recently, the team of Professor Chen Zhangwei and Lao Changshi from the Institute of Additive Manufacturing of Shenzhen University, in cooperation with the Southwest Institute of Physics of the Nuclear Industry of China National Nuclear Corporation (hereinafter referred to as the Southwest Institute of Physics), for the first time proposed and realized the free design and shaping based on the integration of 3D Printing. Lithium orthosilicate ceramic parts with complex porous structure are expected to replace the traditional microsphere bed structure and become a new generation of tritium-producing devices, showing important application prospects. The results have been published in the “Additive Manufacturing” magazine.

The tritium production unit is like the heart of a nuclear fusion reactorSince the discovery of nuclear reactions, people have been continuously exploring the effective use of nuclear energy. At present, more and more scientists and energy experts are beginning to focus on nuclear fusion. The raw materials for nuclear fusion are mainly hydrogen isotopes-deuterium and tritium. Deuterium can be obtained in sea water, and each liter of water contains about 30 milligrams of deuterium. A 1,000-megawatt nuclear fusion power station consumes only 304 kilograms of deuterium per year. According to this calculation, the deuterium in the global seawater is enough for humans to use for tens of billions of years. However, tritium is almost non-existent in nature, and it needs to be produced by the continuous catalytic reaction of helium and lithium ceramics. As an important component of the magnetic confinement fusion reactor, the solid-state tritium-producing cladding is one of the core problems that need to be solved before the commercial application of fusion energy. At present, the preferred tritium multiplier material for scientists in various countries is lithium orthosilicate (Li4SiO4). The prevailing method is to react lithium orthosilicate ceramics with helium to produce tritium. Scientists call the ceramic components that achieve this function as a tritium production unit. Traditional lithium ceramic tritium production units generally make lithium orthosilicate into microspheres with a diameter of about 1 mm, and stack them to form a pebble bed structure. The gaps between the microspheres can be filled with helium. However, the filling rate of such tritium-producing units is limited and cannot be freely controlled. In addition, the stress concentration generated by the accumulation of microspheres is likely to cause damage such as structural deformation and cracking of the tritium-producing unit, which becomes a constraint on the uniformity and stability of the pebble bed structure and performance. Once the tritium production unit fails, it will directly cause the fusion reactor to fail to operate smoothly. Therefore, scientists have been trying to optimize the structure of the tritium production unit.

Another path can greatly increase the efficiency of tritium productionIn response to the above problems, in 2018, Chen Zhangwei, Lao Changshi and others, together with the Southwest Institute of Physics, proposed a method of 3D Printing lithium orthosilicate ceramic units to develop a new structure of tritium-producing units. However, the first problem facing 3D printing is that lithium orthosilicate is particularly sensitive to the environment, and can easily react with water and carbon dioxide, causing phase damage and becoming lithium metasilicate. “For this reason, we have strictly restricted and controlled environmental variables from the storage of lithium orthosilicate powder, the preparation of printable powder slurry, the realization of the printing process to the heat treatment, etc., for example. The process of preparing powder slurry needs to be carried out in a glove box filled with inert gas, and all kinds of additives are organic solvent materials that do not contain water and cannot react with lithium orthosilicate. The preparation of the slurry is carried out in such an environment And 3D printing can ensure the phase stability of lithium orthosilicate.” Professor Chen Zhangwei told a reporter from Science and Technology Daily. In order for the lithium orthosilicate powder slurry to be cured quickly after 3D printing, a suitable curing method must be selected.

“Ceramic 3D printing has two main curing and forming methods, one is light curing, and the other is powder sintering or melting.” Chen Zhangwei said that powder sintering uses high-energy lasers to directly sinter ceramic powder at high temperature, which is required for sintering. Shape, but because the temperature is relatively high, it is prone to cracking, and the accuracy and controllability are poor. The light curing not only has fewer cracking defects, higher printing accuracy, but also has a strong ability to control the details of the porous structure. Therefore, the scientific research team chose the light-curing method and developed a high-phase-purity lithium orthosilicate powder slurry for light-curing 3D printing. Chen Zhangwei said: “We have mixed optimized organic chemical additive components in the lithium orthosilicate powder slurry, as well as a small dose of photosensitive additives, which are sensitive to light of a specific wavelength, and use 405 nm UV The material can be irradiated to realize the photopolymerization and curing of the slurry.” The 3D printed structural parts are then sintered at a high temperature and fired in an environment of 1050 degrees Celsius for 8-10 hours to achieve ceramization, which can remove all parts of the cured structure. This kind of additives will no longer react with water and carbon dioxide in the environment. “These chemical additives are added in a physical way and will not cause damage to lithium orthosilicate.” Chen Zhangwei explained. The tritium production unit printed by this method is an integrated and defect-free structure. After testing, it overcomes the reliability problems caused by the limited pebble bed filling rate and stress concentration, and its stability and mechanical properties are 2 times higher than the traditional microsphere structure. .

The tritium production efficiency of the 3D-printed tritium production unit is also expected to be greatly improved. The duty cycle of the traditional microsphere structure is up to 65%, while 3D printing can be flexibly adjusted between 60% and 90% according to needs, and the specific surface area of ​​lithium orthosilicate is also greatly increased compared with the microsphere structure. International colleagues gave high praise and believed that the proposed 3D printing technology was extremely innovative in the manufacture and application of core ceramic components for nuclear fusion. The research has great prospects in the application of nuclear fusion reactors, and will provide more possibilities to replace the traditional pebble-bed ceramics to produce tritium structure and promote the commercialization of Tokamak nuclear fusion reaction technology.

Trial production of key components of nuclear fusion reactors has been completed. Although mankind is still a long way from controllable nuclear fusion, this does not prevent us from working hard towards our goals. As an emerging advanced manufacturing method, 3D printing has overturned the traditional manufacturing model. 3D printing technology can realize the integrated forming of complex structures, has the characteristics of short manufacturing cycle and high material utilization rate, and is an important innovative method for the manufacture of complex components. In nuclear fusion reactors, it has gradually shown its unique advantages. According to Professor Chen Zhangwei, the Institute of Additive Manufacturing of Shenzhen University has previously cooperated with the Southwest Institute of Physics to focus on the selective laser melting process (SLM) of the CLF-1 steel component on the first wall of the nuclear fusion reactor. A major technical approach) and its organization and performance control has carried out systematic research work. For the first time, the design ideas of heterogeneous dual/multi-mode structure are introduced to SLM forming high strength and toughness low activation martensitic steel (RAFM, ​​for future nuclear fusion reactor research and development). Based on the development of SLM process parameters and scanning strategy, SLM forming CLF-1 steel has both high strength and high plasticity, and its comprehensive strength and toughness are significantly better than the RAFM steel reported in the literature. This research provides important theoretical basis and technical guidance for the structural design of 3D printed high-strength and tough RAFM steel, and promotes the integrated molding of the key components of nuclear fusion reactors with controllable structure and performance. According to media reports, in 2018, the Hefei Institute of Material Science of the Chinese Academy of Sciences has used 3D printing technology to realize the trial production of the first wall of the cladding, a key component of the nuclear fusion reactor.

The researchers used China Low Activation Martensitic Steel (CLAM) as the raw material, and the dimensional accuracy of the printed part samples met the design requirements, and the density of the material reached 99. 7%, which is equivalent to the strength of CLAM steel prepared by traditional methods. At the same time, the study also found that the layer-by-layer melting and directional solidification characteristics of 3D printing have led to differences in the structure and performance of CLAM steel in different directions. This difference can be effectively reduced or even eliminated in the future through optimization of scanning schemes and optimization of molten pool nucleation. The research shows that 3D printing technology has good application prospects in the manufacture of complex components of advanced nuclear energy systems such as nuclear fusion reactors. The rapid development of basic science and the continuous changes and innovations of 3D printing technology have made human exploration in the field of engineering technology full of imagination. It is not impossible that all parts of the nuclear fusion reactor in the future will be manufactured by 3D printing.  

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