How is neptunium-237 produced in nuclear reactors?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transuranium element Modern version of the periodic table of the elements (printable).

Neptunium-237 is produced in nuclear reactors as a by-product of the large-scale production of plutonium. It is synthesized from the reactor fuel uranium-235 and uranium-238 through a process of successive neutron capture and radioactive decay.12

In breeder reactors, for example, approximately one part of neptunium-237 is generated for every 1,000 parts of plutonium produced. Once formed in kilogram quantities, this isotope can be separated from used reactor fuel for scientific study.12