The calculation of activation inventories is a key input to virtually all aspects of the operation, safety and environmental assessment of nuclear plants. For the licensing of such devices, regulatory authorities will require proof that the calculations of structural material, fuel inventories, and calculations to which those quantities are the inputs, are either correct or conservative. An important aspect of activation transmutation is decay heat power. In power plants, decay power arises after shutdown from the energy released in the decay of the products of neutron interaction from, and emissions. Computation of the decay power is performed by sophisticated computer codes which solve the large number of coupled differential equations governing the generation and decay chains for the many nuclides involved. They rely on a large volume of nuclear data that includes both neutron activation-transmutation cross sections and radioactive decay data. Validation of decay power computational predictions by means of direct comparison with integral data and measurement of sample structural materials under high energy relevant neutron spectra generates confidence in the values calculated. It also permits an assessment of the adequacy of the methods and nuclear data, and indicates any inaccuracy or omission that may have led to erroneous results. No experimental data on decay power existed for most fission reactor materials other than reactor fuel, or for materials under high energy irradiation conditions typical of fusion, until a series of experiments were performed using the Fusion Neutron Source FNS facility at the Japan Atomic Energy Agency JAEA. Many elements and some alloy micro-samples were irradiated in a simulated D-T neutron field for times up to 7 hours and the resulting decay power generated was measured for cooling times of up to a year or more. Using the highly sensitive Whole Energy Absorption Spectrometer (WEAS) method, both and emission decay energies were measured at selected cooling times as early as a few tens of seconds after the irradiation ended. Overall the results of this particular validation exercise indicate that the calculational methods and nuclear databases, with some notable exceptions, generally allow predictions, with quantifiable margins, of the decay power of the tested materials against cooling time. It tests the specific production pathways and at the same time the decay data associated with the nuclides that dominate the decay heat. Note that the decay characteristic of the predominant isotopes is independent of the production route: be it fission, fusion or transmutation in general.