Neutron activation is a nuclear reaction where an incident neutron latches on to a target nucleus to make an isotope of the original element.
Usually the resulting compound nucleus is radioactive and decays to a different isotope which itself may decay further. The resulting series of decays emits radiation of various types over a period of time. This process causes materials in nuclear reactors to become radioactive.
The process occurs in both fission and fusion reactors.
Further details on neutron activation are give on the page: Essential of Nuclear Physics