An activation product is a material that has been made radioactive by the process of neutron activation.
Fission products and actinides produced by neutron absorption of nuclear fuel itself are normally referred to by those specific names, and activation product reserved for products of neutron capture by other materials, such as structural components of the nuclear reactor or nuclear bomb, the reactor coolant, control rods or other neutron poisons, or materials in the environment. In these cases their production is undesired and they need to be handled as radioactive waste. Some nuclides can be produced both as activation products or as fission products. For example molybdenum-99 which is an important nuclide in "molybdenum cows" used in medical diagnostics can be produced either by fissioning <sup>235</sup>U or by neutron irradiation of <sup>98</sup>Mo.
However, neutron activation (usually in a dedicated research reactor, but sometimes also in power reactors like the CANDUs at Bruce nuclear generating station) is also used deliberately to produce desired radioisotopes for uses in food irradiation, nuclear medicine and to sterilize equipment via gamma radiation emitted from isotopes such as Cobalt-60.
Activation products in a reactor's primary coolant loop are a main reason reactors use a chain of two or even three coolant loops linked by heat exchangers.
Fusion reactors will not produce radioactive waste from the fusion product nuclei themselves, which are normally just helium-4, but generate high neutron fluxes, so activation products are a particular concern.
Activation product radionuclides include:
[1] Branching fractions from LNHB database.
[2] Branching fractions renormalised to sum to 1.0..