How are transuranium actinoid elements made in nuclear reactors?

Transuranium actinoid elements are synthesized in nuclear reactors primarily through a process of successive neutron capture.1

In this method, an actinoid target—typically uranium—is subjected to continuous, intensive irradiation by slow (low-energy) neutrons. As the target nuclei capture these neutrons, they may undergo negative beta-particle decay, which increases their atomic number and results in the formation of successively heavier elements.1

For example, plutonium-239 is produced in large quantities in reactors through a specific sequence of neutron capture and decay starting from uranium. Similarly, neptunium-237 is formed as a by-product of this large-scale plutonium production.1

This process of building heavier elements is limited by the stability of the resulting isotopes; the production chain typically terminates at fermium-257 because the subsequent isotope, fermium-258, has a half-life too short to allow for the accumulation of heavier elements via this method. While reactors are effective for producing these elements, even heavier synthetic elements often require the use of particle accelerators to bombard targets with heavy ions.12