The ORIGEN computer code has been used to compute the time-dependent thermal power, photon spectrum, and neutron production rate resulting from fast- and thermal-neutron-induced fission of 235U and 239Pu fuels. Computed afterheats and photon spectra of fission products resulting from thermal fission of 235U are shown to be in good agreement with published data, and computed radioactivities and thermal power of plutonium irradiated to low exposures in both thermal- and fast-neutron spectra are found to agree well with experimentally measured properties. Radioactive decay of the actinide elements is calculated to contribute 10 to 25% of the thermal power of spent low enrichment 235U fuels at postirradiation times between one day and three years. Gamma radiation per unit mass of 30-day-cooled LMFBR core fuel is calculated to exceed that from 90-day-cooled PWR fuel by a factor of 30 in the higher energy groups, and spontaneous fission neutron production per gram of spent LMFBR core fuel is found to exceed that of PWR fuel by a factor of 3 at these times.