Which isotopes mostly make up the plutonium generated in civil power reactors?

The isotopes that predominantly make up the plutonium produced in civil power reactors are commonly identified as plutonium-239 and plutonium-240. Plutonium-239 is formed through the neutron capture of uranium-238 followed by two beta decays, and it is the primary isotope utilized in nuclear fuel because of its fissile properties.

Plutonium-240 is also generated as a byproduct when uranium-238 captures additional neutrons. Its presence in the fuel can affect reactor performance and criticality safety due to its relatively high likelihood of absorbing neutrons without fissioning.

The nuclear fission process in civil power reactors typically leads to a mixture of different isotopes, but plutonium-239 and plutonium-240 are the most significant in terms of quantity and relevance to reactor operations. The emphasis on these isotopes reflects their dual role: one as a fuel and the other as a byproduct influencing reactor dynamics.

In contrast, other isotopes mentioned in the choices, such as plutonium-238 (which has a short half-life and is typically associated with radioisotope thermoelectric generators rather than civil power reactors) or plutonium-241 (which decays to americium-241), do not dominate