While actinides have some established daily-life applications, such as in smoke detectors (americium) and gas mantles (thorium), they are mostly used in nuclear weapons and use as a fuel in nuclear reactors.
The last two areas exploit the property of actinides to release enormous energy in nuclear reactions, which under certain conditions may become self-sustaining chain reaction.

The most important isotope for nuclear power applications is uranium-235.
It is used in the thermal reactor, and its concentration in natural uranium does not exceed 0.72%.
This isotope strongly absorbs thermal neutrons releasing much energy.
One fission act of 1 gram of ²³⁵U converts into about 1 MW·day.
Of importance, is that ²³⁵
₉₂U
 emits more neutrons than it absorbs; upon reaching the critical mass, ²³⁵
₉₂U
 enters into a self-sustaining chain reaction.
Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example:
²³⁵
₉₂U
 + ¹
₀n
 ⟶ ¹¹⁵
₄₅Rh
 + ¹¹⁸
₄₇Ag
 + 3¹
₀n
Other promising actinide isotopes for nuclear power are thorium-232 and its product from the thorium fuel cycle, uranium-233.

Emission of neutrons during the fission of uranium is important not only for maintaining the nuclear chain reaction, but also for the synthesis of the heavier actinides.
Uranium-239 converts via β-decay into plutonium-239, which, like uranium-235, is capable of spontaneous fission.
The world's first nuclear reactors were built not for energy, but for producing plutonium-239 for nuclear weapons.

About half of the produced thorium is used as the light-emitting material of gas mantles.
Thorium is also added into multicomponent alloys of magnesium and zinc.
So the Mg-Th alloys are light and strong, but also have high melting point and ductility and thus are widely used in the aviation industry and in the production of missiles.
Thorium also has good electron emission properties, with long lifetime and low potential barrier for the emission.
The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars (see radiometric dating).

The major application of plutonium has been in nuclear weapons, where the isotope plutonium-239 was a key component due to its ease of fission and availability.
Plutonium-based designs allow reducing the critical mass to about a third of that for uranium-235.
The "Fat Man"-type plutonium bombs produced during the Manhattan Project used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction and increase efficiency.
Thus only 6.2 kg of plutonium was needed for an explosive yield equivalent to 20 kilotons of TNT. (
See also Nuclear weapon design.)
Hypothetically, as little as 4 kg of plutonium—and maybe even less—could be used to make a single atomic bomb using very sophisticated assembly designs.

Plutonium-238 is potentially more efficient isotope for nuclear reactors, since it has smaller critical mass than uranium-235, but it continues to release much thermal energy (0.56 W/g) by decay even when the fission chain reaction is stopped by control rods.
 Its application is limited by the high price (about US$1000/g).
This isotope has been used in thermopiles and water distillation systems of some space satellites and stations.
So Galileo and Apollo spacecraft (e.g. Apollo 14) had heaters powered by kilogram quantities of plutonium-238 oxide; this heat is also transformed into electricity with thermopiles.
The decay of plutonium-238 produces relatively harmless alpha particles and is not accompanied by gamma-irradiation.
Therefore, this isotope (~160 mg) is used as the energy source in heart pacemakers where it lasts about 5 times longer than conventional batteries.

Actinium-227 is used as a neutron source.
Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use.
²²⁸Ac is used as an indicator of radioactiv
While actinides have some established daily-life applications, such as in smoke detectors (americium) and gas mantles (thorium), they are mostly used in nuclear weapons and use as a fuel in nuclear reactors. The last two areas exploit the property of actinides to release enormous energy in nuclear reactions, which under certain conditions may become self-sustaining chain reaction.
The most important isotope for nuclear power applications is uranium-235. It is used in the thermal reactor, and its concentration in natural uranium does not exceed 0.72%. This isotope strongly absorbs thermal neutrons releasing much energy. One fission act of 1 gram of ²³⁵U converts into about 1 MW·day. Of importance, is that ²³⁵
₉₂U
 emits more neutrons than it absorbs; upon reaching the critical mass, ²³⁵
₉₂U
 enters into a self-sustaining chain reaction. Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example:
²³⁵
₉₂U
 + ¹
₀n
 ⟶ ¹¹⁵
₄₅Rh
 + ¹¹⁸
₄₇Ag
 + 3¹
₀n
Other promising actinide isotopes for nuclear power are thorium-232 and its product from the thorium fuel cycle, uranium-233.
Emission of neutrons during the fission of uranium is important not only for maintaining the nuclear chain reaction, but also for the synthesis of the heavier actinides. Uranium-239 converts via β-decay into plutonium-239, which, like uranium-235, is capable of spontaneous fission. The world's first nuclear reactors were built not for energy, but for producing plutonium-239 for nuclear weapons.
About half of the produced thorium is used as the light-emitting material of gas mantles. Thorium is also added into multicomponent alloys of magnesium and zinc. So the Mg-Th alloys are light and strong, but also have high melting point and ductility and thus are widely used in the aviation industry and in the production of missiles. Thorium also has good electron emission properties, with long lifetime and low potential barrier for the emission. The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars (see radiometric dating).
The major application of plutonium has been in nuclear weapons, where the isotope plutonium-239 was a key component due to its ease of fission and availability. Plutonium-based designs allow reducing the critical mass to about a third of that for uranium-235. The "Fat Man"-type plutonium bombs produced during the Manhattan Project used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction and increase efficiency. Thus only 6.2 kg of plutonium was needed for an explosive yield equivalent to 20 kilotons of TNT. (See also Nuclear weapon design.) Hypothetically, as little as 4 kg of plutonium—and maybe even less—could be used to make a single atomic bomb using very sophisticated assembly designs.
Plutonium-238 is potentially more efficient isotope for nuclear reactors, since it has smaller critical mass than uranium-235, but it continues to release much thermal energy (0.56 W/g) by decay even when the fission chain reaction is stopped by control rods.  Its application is limited by the high price (about US$1000/g). This isotope has been used in thermopiles and water distillation systems of some space satellites and stations. So Galileo and Apollo spacecraft (e.g. Apollo 14) had heaters powered by kilogram quantities of plutonium-238 oxide; this heat is also transformed into electricity with thermopiles. The decay of plutonium-238 produces relatively harmless alpha particles and is not accompanied by gamma-irradiation. Therefore, this isotope (~160 mg) is used as the energy source in heart pacemakers where it lasts about 5 times longer than conventional batteries.
Actinium-227 is used as a neutron source. Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use. ²²⁸Ac is used as an indicator of radioactiv
Actinide