What kind of decay cause neptunium -238 to form from uranium -238

After 30 years, about 4.7% of the americium atoms will have decayed to neptunium. Plutonium-239 is itself the primary fissile isotope of plutonium (used in nuclear weapons), and is in turn made from uranium.

why do the isotopes of a particular element behave differently in nuclear reactions but the same in chemical reactions? chemical reactions involve changes to electrons in an atom; isotopes do not differ in electrons, and so they react the same way during chemical changes.