The isotope thermal power source stored at NASA's Kennedy Space Center is used for NASA's Mars Science Laboratory mission.
Earlier this year, Chang'e-4, with the support of the relay star "Magpie Bridge", achieved the first human lunar landing. In the dark moon back, solar cells cannot be used for power supply, and fuel cells and energy storage equipment cannot provide long-term energy. In harsh environments such as deep space, deep sea, and moonback, only nuclear energy can supply energy to instruments and equipment.
Isotope heat / power source (RTG) is a device that uses the energy released during the decay of radioisotopes to heat or power the instrument. Impact and other advantages.
At the 2019 annual meeting of the Chinese Nuclear Society held recently, Luo Zhifu, deputy chief engineer of the Chinese Academy of Atomic Energy and director of the Institute of Isotopes, reported on the current research hotspots of isotope heat / power sources and the status of their application in deep space exploration in China.
The third country to use isotope heat / power
According to reports, the application of isotope heat / power in the aerospace field began in 1961. A navigation satellite of the US Navy carried an isotope power supply containing 96 grams of plutonium 238, which has been in orbit for more than 15 years and shows isotopes. The power supply has a long life and strong reliability.
Therefore, in the next few decades, the United States will include multiple spacecraft including the Apollo 11 lunar spacecraft, the Galileo Jupiter probe, and the Cassini Saturn probe, as well as polar workstations, lighthouses, and deep seas. The facility and other facilities have developed multiple models of isotope heat / power supplies with a total thermal power of tens of thousands of watts. At most once, the Galileo carried 120 isotope heat / power supplies in one breath, each with a thermal power of 1 watt. The longest-lived isotope power source such as Voyager 1 has been in operation for more than 40 years.
Luo Zhifu said that since the Soviet Union and Russia first used the polonium 210 isotope power source as the power source for military satellite spaceborne equipment in 1965, they have also developed isotope heat / power sources for multiple lineages such as polonium 210, plutonium 238, and strontium 90. In 2018, a plutonium 238 heat source was developed for China's Chang'e 3 and 4 detectors, which ensured the survival of the detector on the moon and night, and made China the third country to use isotope heat / power after the United States and Russia.
It can be said that the isotope heat / power supply has become an indispensable "hand-warming treasure" and "recharging treasure" for space travelers traveling in space at home.
"Helium-blocking plutonium"
Luo Zhifu pointed out that the design of isotope heat / power supply should take into account aspects such as large specific power, high safety, high conversion efficiency and high reliability.
"Specific power is a key parameter of isotope heat / power supply." Luo Zhifu said that a large specific power means that the heat source has plenty of energy. Previously, the maximum specific power of the power supply for engineering applications in the United States could reach 5.23 We / kg. In order to achieve large specific power, it is necessary to select the appropriate isotope. The specific power of plutonium 238 is relatively large, the half-life is relatively long, and the acquisition cost is relatively low, which is suitable for completing tasks of several years to decades.
However, the alpha decay of plutonium 238 will produce helium, causing the source core to swell and shatter, posing a safety hazard. Therefore, the cladding should use a porous material to discharge helium, which in turn may release the extremely toxic radioactive substance plutonium 238.
Therefore, it is necessary to use integral pores for communication, and the porous materials with complex local structure and inconsistent pore shape, pore size, and trend realize "helium permeation resistance plutonium" in the cladding. In the development, it is also necessary to consider the interception of particles by the pores, which leads to the loss of ventilation function, and the technical realization is extremely challenging.
Improving conversion efficiency is another difficulty in developing isotope power supplies. Higher conversion efficiency means lower cost. Luo Zhifu introduced that the conversion efficiency of thermoelectric conversion materials using SiGe is currently only 6.6%. This aspect requires researchers to further investigate the laws of thermoelectric materials and find more efficient and radiation-resistant thermoelectric materials.
It is understood that the next-generation thermoelectric conversion device using skutterudite is expected to be 25% more efficient than before.
Can withstand fire and withstand high pressure
As a device that uses nuclear energy, safety and reliability must be the top priority in the design and manufacture of isotope thermal / power supplies. Luo Zhifu pointed out that researchers are extremely strict about the safety and reliability design of isotope heat sources.
First, the source core should choose plutonium dioxide ceramics with stable properties, high temperature resistance, corrosion resistance, non-volatile, insoluble in seawater, and does not react with the inner cladding material, even if the cladding is damaged, it can maintain its shape.
In addition, the United States and Russia have envisaged a variety of extreme accidents when testing the safety and reliability of isotope heat sources. For example, a fire broke out at the launch site, causing the rocket to "didn't die first before the division" and exploded directly. In this case, the isotope heat source is required to fall directly into the rocket fuel until the fuel is burned out without leakage. Another extreme event is that the rocket explodes at high altitude. The isotopic heat source must withstand high temperature ablation and hit hard rock at a high temperature of 1100 degrees Celsius and a high speed of 90 meters per second without leakage.
In addition, due to the high activity and extremely toxic radioactivity of plutonium 238, it is required to establish a reliable operation process and emergency plan to avoid risks to the greatest extent and ensure safety.
"The isotope heat / power source extends the application of nuclear energy from fission energy to decay energy, and promotes the development of post-processing technology." Luo Zhifu said in his summary of the research on isotope heat / power source, "The development work is a complex system Engineering, there are still a lot of scientific and technical problems that need to be solved by multi-disciplinary cooperation. "(â– Chi Han, a trainee reporter of this newspaper)
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