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Chang'e 4 mission lunar rover design and configuration. (Photo courtesy of National Defense Science and Industry Bureau)
The long moon and night, combined with the temperature difference between day and night of nearly 310 ℃, without air, it is very difficult for humans to survive on the moon. The instrument that can automatically observe for a long time has become a "clever eye" for humans to understand the moon. Undoubtedly, the energy supply of the instrument is a big deal.
According to media reports, the "Chang'e-4" isotope energy supply launched at the end of last year has achieved a new breakthrough: the use of isotope temperature difference power generation and thermoelectricity utilization combined energy supply.
What kind of energy technology is this? What is unique? A reporter from the Science and Technology Daily interviewed Cai Shanyu, an expert in isotope energy in China and a researcher at the Institute of Isotopes, China Academy of Atomic Energy.
Decay energy provides self-sustaining energy for space exploration
"Isotopic heat sources and isotope power sources are collectively referred to as isotope energy sources. This type of energy comes from the 'decay energy' generated when radioisotopes decay." Cai Shanyu told reporters that decay energy, fission energy, and fusion energy constitute three major ways of using nuclear energy.
Compared with fission energy and fusion energy, decay energy energy is much smaller, but it is unique for lunar exploration and deep space exploration: it does not need to rely on external energy sources, can provide long-term, self-sustaining, reliable power, and has a strong impact on the environment Has good adaptability.
So far, humans have discovered 118 elements, each of which has different amounts of isotopes, including 276 stable isotopes and more than 3,000 radioisotopes.
However, Cai Shanyu said that if the screening is based on the principles of longer half-life, higher power density, lighter shielding quality, less biological toxicity and lower production cost, there are no more than ten radioisotopes that can be used as energy fuel. According to the decay characteristics, isotope heat sources can be roughly divided into three categories: α, β and γ heat sources.
The biggest feature of the alpha heat source is the low quality of the shielding material required, which can greatly reduce the rocket launch cost and is most suitable for space applications. The isotope energy launched into space in the 20th century, most of the fuel used polonium-210 and plutonium-238, the latter accounted for the vast majority.
"Polonium-210 has high specific power but short half-life, which is suitable for demonstration devices or short-term space missions. Plutonium-238 has low specific power but long half-life, which can be used for long-term space missions." Cai Shanyu explained.
Isotope heat source becomes the "warm baby" of instruments on the moon
The decay energy of radioisotopes can be converted into light energy, heat energy and electric energy.
Cai Shanyu told reporters that the high-speed charged particles emitted during radioactive isotope decay interact with matter. When the kinetic energy is prevented or absorbed, the temperature of surrounding materials such as the container that encloses the radioisotope will rise, and the decay energy will be converted into heat energy.
The inside of the isotope heat source is a source core made of isotope fuel, and the outside is a fuel cartridge with a sealed source core, which can be directly applied. For example, the "Lunar Rover-1" and "Lunar Rover-2" launched successively by the Soviet Union are equipped with 800 watts of polonium-210 heat source, which is specially designed to establish a constant temperature environment for lunar surface observation instruments; Two 15-watt plutonium-238 heat sources are used for the thermal insulation of the lunar seismograph.
The "Chang'e-3" lunar probe launched by our country in 2013 is equipped with plutonium-238 in both the lander and the lunar rover to ensure that the instrument compartment is warm and springy, and the instrument is safe to spend the night. Once the sun is shining, the instrument is activated again with the help of solar cells.
Cai Shanyu told reporters that compared with the isotope heat source, the isotope power source also needs to directly or indirectly pass the thermoelectric converter (transducer) to further convert the thermal energy generated by the isotope decay into electrical energy. Because of this, the isotope battery includes a transducer in addition to the isotope heat source. At present, the most mature and practical transducer used in space is the isotope thermoelectric generator. Its advantages are no moving parts and safe and reliable power generation, but the thermoelectric conversion efficiency is only 4% -8%. As a type of transducer, dynamic conversion can improve the thermoelectric conversion efficiency, but because of the moving parts, it is difficult to manufacture.
"It can be predicted that China's increasingly abundant space activities will certainly put more demand on space nuclear power, and the research results of space nuclear power will also provide more space for the development of China's space industry." Cai Shanyu said when looking into the prospect of isotope energy.
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