Radioisotope heat source is used to provide heat source for thermal insulation of thermoelectric conversion devices or electronic equipment by using the decay heat of radioisotopes.
Radioisotopes will decay spontaneously. According to the different particles released during the decay, they can be divided intoαdecay，βDecay,γThere are three kinds of decay.αThe decay produces a helium nucleus with a positive charge of 2 units;βDecay produces electrons,γDecay generates high-energy electromagnetic waves, namelyγRay.[1]
The particles produced in the decay of radioisotopes collide with atoms, making the kinetic energy of the particles transform into the heat energy of the system.Some isotopes have very high heatpower density , you can use it as a heat source.These radioisotopes suitable for use as heat sources are packaged in an appropriate manner to become radioisotope heat sources.Such heat source has irreplaceable advantages in space exploration, especially in the field of deep space exploration.
characteristic
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Characteristics of radioisotope heat source:
1. Long life
Pu-238 is generally used as space isotope heat source. Its half-life is 87.1 years. That is to say, after 87.1 years, its thermal power rate will be reduced to half of the initial value, so it can provide heat source for other equipment for years or even decades.
2. High power density, small mass and small volume
Due to the high thermal power density of radioactive isotope decay, such astwo hundred and thirty-eightPuOtwoThe thermal power density is 0.406 W/g, and the compact shielding layer and source box can be used to make heat sources with high density, small mass and small volume, which are particularly beneficial to space exploration.The first for space technologytwo hundred and thirty-eightPuOtwoThe isotope heat source thermoelectric generator (SNAP-3B) has a diameter of about 12.5cm, a height of 13.7cm, and a weight of 2. Ikg, but its electric energy is equivalent to that provided by a 300kg nickel chromium battery.
3. Stable thermal power and reliable performance
Radioisotope heat sources for space use generally use isotopes with appropriate half-life, so that the change of thermal power is not very obvious during the whole working period.For example, the change of thermal power of Pu-238 has only decreased by 4% in five years.In addition, the isotope decay is determined by the nature of the isotope itself and is not affected by the outside world, so it can generate continuous and stable heat and therefore has high reliability.
4. High operating temperature
The operating temperature of isotope heat source generally refers to the operating temperature that will not lead to radioactive leakage or source box damage.Use appropriateIsotope chemistryThe situation and source box materials can make the isotope heat source operate in a relatively high temperature range.two hundred and thirty-eightPuOtwoIt is a highly chemically inert fuel at high temperatures and has a very high melting point, so its operating temperature can also be as high as 1500K, so the quality of heat generated is relatively high.
Common radioisotopes
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There are more than 1300 radioisotopes that can be used as heat sources, but they must be screened in order to meet the requirements of isotope heat sources.First of all, those with half-life less than 100 days and more than 100 years should be excluded, so there are only more than 100 kinds of available isotopes left.Secondly, it should be ruled out that the power density is lower than 0.1W/g or the shielding is difficult and the radioactive hazard is too large (such asγRadioactive source), so there are only more than 30 kinds of radioactive isotope heat sources.Then, according to the cost and physical and chemical properties, the most feasible isotopes are determined.
The following table lists the eight most viable isotopes.
8 practical radioisotopes
isotope
Half life (year)
Specific power (W/g)
Melting point (℃)
Main radiation types
Strontium-90
twenty-eight
zero point nine three
seven hundred and seventy
β、a fewγ
Cesium-137
thirty
zero point two six
twenty-eight
β、γ
Cerium-144
zero point seven eight
twenty-five
eight hundred
β、a fewγ
147pm
two point five
zero point three six
one thousand and three hundred
β、a fewγ
210po
zero point three eight
one hundred and forty-one
two hundred and fifty-four
α
Plutonium-238
eighty-seven point one
zero point five five
six hundred and forty
α
Curium-242
zero point four five
one hundred and twenty-one
nine hundred and fifty
α
Curium-244
eighteen
two point eight
nine hundred and fifty
α
Among the eight feasible isotopes, plutonium-238 (Pu-238) and strontium-90 (Sr-90) are the most widely used.SNAP series space use researched by US Department of EnergyRadioisotope thermoelectric generatorPu-238 is basically used as the isotope heat source, while Sr-90 is widely used as the isotope heat source for ground use.[2]
Basic structure
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The design of isotope heat source requires that it has enough safety in harsh environment (i.e. no damage and radioactive material leakage), and its quality, volume and cost must be reasonable and feasible.The forces that isotope heat sources may bear can be divided into three categories: thermal shock,Mechanical shockForces and chemical forces.The design of isotope fuel must first satisfy that its fuel source box can withstand these possible forces without causing damage to the source box.
Practice has proved that the application of high-strength special alloy as sandwich and shell of a three layer cylindrical container can ensure the absolute sealing of fuel.For the Pu-238 source, the particles generated by its decay are condensed into helium in the fuel. Therefore, when packaging the fuel, the internal pressure of the fuel box should be considered, and enough space should be left to store helium.In addition, the compatibility of fuel and packaging materials must be considered.
According to the above design requirements, the US SNAP program has designed various types of isotope heat sources.There are two types of these heat sources, namely, two layer and three layer heat sources.The outer layer of a two-layer heat source is not only a strength component, but also requires good environmental adaptability.The heat source of three-layer structure separates the functions of each layer;The inner layer has good fuel compatibility, the middle layer has high mechanical strength, and the outer layer has good environmental compatibility.[2]
application
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The radioisotope heat source can further transform heat energy into electric energy through the thermoelectric conversion device, which is called "radioisotope battery".Radioisotope batteries are mainly used in space flight and space exploration, marine engineering, biomedicine and other fields.Although the cost of radioisotope battery is very expensive, it is considered as a good space power source, and it is often in a state of competition with solar cells.The United States has repeatedly used radioisotope batteries in space development such as artificial satellites, spaceships, lunar surface scientific experiment stations and planetary probes.In ocean engineering, several lily radioisotope batteries have been put into use all over the world. They have been used in sea lighthouses, sea buoys, underwater sonar generators and submarine microwave relay stations, and can also be used as power supplies for swimming weather stations and diving suit heaters.It is more reliable than using solar cells, is not affected by waves and climate, and has a long service life.[2]
Research progress
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Radioisotope heat source not only provides heat source for thermoelectric generator, but also serves as heat source for detector electronic equipment in space technology to ensure normal operation of electronic equipment.With the development of a new round of space exploration in recent years, radioisotope heat sources have also made great progress.In order to increase the operating temperature of radioisotope heat sources to meet new operating requirements, the current radioisotope heat sources gradually use coated particles, and then use graphite matrix to heat press into fuel blocks.The core temperature of such fuel assembly can reach 1500-2400K.It can meet a variety of newly developedThermoelectric generatorProvide power for the detector.
In addition, in order to meet different power requirements of thermoelectric generators, radioisotope heat sources have also developed into modular units.For example, the general modular isotope heat source developed by the U.S. Department of Energy for NASA is such a model.This heat source can combine modules according to different tasks, so as to obtain heat sources that meet power requirements.
Radioisotope heat source has a broad application prospect in the field of deep space exploration, but the research in China is still basically blank. It is of great significance to carry out basic and applied research in this area for the development of China's aerospace industry.With the development of interstellar flight exploration in China, nuclear energy will become an increasingly important power source, and isotope thermoelectric generators will be greatly developed.[2]
