The radioisotope generator usesRadioactive materialThe heat released by decay is used to generate electricity.Radioactive materials are usually used in the form of one or more plutonium-238 particles.Other isotopes can also be used, but for space applications, only elements with low radiation penetration ability can be used and can be blocked by shielding materials to protect sensitive equipment.In particular, the European Space Agency is studying americium-241 as a potential isotope for European space power systems[1]。
The development of radioisotope generator has been paid attention to because of space application.This work was first carried out in the Monde Laboratory (the experiment was carried out for the Atomic Energy Commission by the Monsanto Research Company in the early 1950s).Later (1960), Westinghouse completed the development of NAP-100, a 100 watt nuclear auxiliary generator.NAP-100 is composed of 140 pairs of thermocouples (PbTe Ge, Bi Te) thermally insulated by glass and mica, with a maximum power of 131 W and a total efficiency of 5.16%.
Since then, with the promotion of the meteorological station, SNAP-3 (3-watt strontiumninetyNuclear auxiliary generator) and five SNAP-7 generators. The structure of SNAP-7 generators is shown in Figure 1, which is the first batch of radioisotope generators for practical use.It has been used in the power supply of marine buoys, navigation lights, sonar beacons and other equipment.In order to carry out the lunar earthquake experiment, the astronauts of Apollo 11 once put an isotope heater on the moon surface to operate. The only power supply for the instruments left on the moon by the astronauts of Apollo 12 is SNAP-27, which has withstood the extremely high temperature difference test on the moon surface, with a power output of 70 watts.NASA's Pioneer and Pirate and other unmanned interplanetary exploration rockets all use radioisotope power[2]。
Main structure
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Figure 2 Radioisotope Generator Profile
If we peel off the shell of a radioisotope generator, as shown in Figure 2, we will find that it has four main parts.The first part is the shell itself, which is a thin metal tank to prevent internal pollution, and is usually used as a radiator.The second part is a large amount of radiation shielding materials, although these are sometimes unnecessary for the fuel emitting alpha particles.The third part is the thermoelectric element or thermionic converter arranged in order to convert part of the heat released during isotope decay into current.Finally, the central part of the generator is the source of the whole energy, namely the fuel jacket.Most radioisotope generators are cylindrical in shape[3]。
Power generation principle
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Radioactive materials release high-energy radiation, which has been transformed into heat energy by wrapping radioisotopes in appropriate containers in the first step.The application of shielding to control radiation is not only related to manned flight missions, but also to prevent interference with scientific instruments.
The decay of radioisotopes provides a constant degree of heat. The radioisotope generator can directly convert the thermoelectric conversion system into electrical energy, or for example, the Stirling engine can convert mechanical energy through media.The former provides a strong robust system without wear of moving parts or engines, while the latter provides higher thermal electrical conversion efficiency, but improves the complexity of the overall system.[1]
application
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Radioisotope generators have been successfully used in ground and space applications for more than a decade.The ground system is mainly used for remote power generation, because it is impractical to supply fuel generators.At the same time, radioisotope generators have also been widely used in space, especially in space missions outside the solar system.Radioisotope generators can be used for interplanetary space missions and other space missions that are separated from the earth's gravitational field.If the generator is stored in high orbit after the space mission is completed, it can also be used in Earth orbit.In any case, the final treatment must be made.There are also mobile systems with relatively low power consumption, such as Mars surface rovers that cannot use solar photovoltaic panels due to size[1]。
The radioisotope generator shall be protected by a closed system when it is applied. The design and construction of the system shall ensure that it can withstand thermal and aerodynamic forces when entering the upper atmosphere under foreseeable track conditions. The track operation conditions include high elliptical track or hyperbolic track when relevant.In case of impact, the physical form of the closed system and isotopes shall ensure that no radioactive substances are scattered into the environment, so that the radioactivity in the impact area can be completely removed through a recovery operation[4]。
Correlation rule
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Since radioisotope generators are mainly used in outer space, relevant rules must be followed, mainly the Principles on the Use of Nuclear Power Sources in Outer Space.In 1992, the General Assembly of the United Nations adopted the Principles Governing the Use of Nuclear Power Sources in Outer Space. All countries should abide by the following rules when using nuclear power sources in outer space:
1. Activities involving the use of nuclear power sources in outer space should be carried out in accordance with international law, in particular the Charter of the United Nations and the Outer Space Treaty.
2. The use of nuclear power sources in outer space should be limited to space missions that cannot be reasonably performed with non nuclear power sources.
3. The design of space objects carrying nuclear power sources should include redundant equipment, fault correction systems, containment and isolation, and the independence of components.To prevent or minimize public exposure to radiation.
4. The nuclear reactor shall be designed so that it will not enter the critical state before reaching the operating orbit, and it will be stored in the orbit of sufficient height after use, so as to ensure that the radioactive material decays to a safe level before the system enters the atmosphere;The radioisotope generator shall be designed to withstand reentry into the atmosphere and ground impact without scattering radioactive materials into the environment.
5. The launching country of a nuclear power source shall publicly evaluate the safety of the system. In case of a failure of the system leading to re-entry, the launching country shall notify the relevant country and help eliminate any harmful effects.
6. All countries should comply with the Outer Space Treaty and the Liability Convention.Take responsibility for its outer space activities designed to use nuclear power sources.In terms of compensation for damages, in addition to determining compensation according to the principle of fairness and reasonableness, it should also include compensation for the costs of search, recovery and clean-up work with appropriate and sufficient basis, including the cost of assistance provided by a third party[5]。
