ORIGEN2: A Versatile Computer Code for Calculating the Nuclide Compositions and Characteristics of Nuclear Materials. ORIGEN2 is a versatile point-depletion and radioactive-decay computer code for use in simulating nuclear fuel cycles and calculating the nuclide compositions and characteristics of materials contained therein. It represents a revision and update of the original ORIGEN computer code, which was developed at the Oak Ridge National Laboratory (ORNL) and distributed worldwide beginning in the early 1970s. Included in ORIGEN2 are provisions for incorporating data generated by more sophisticated reactor physics codes, a free-format input, and a highly flexible and controllable output; with these features, ORIGEN2 has the capability for simulating a wide variety of fuel cycle flow sheets. The decay, cross-section, fission product yield, and photon emission data bases employed by ORIGEN2 have been extensively updated, and the list of reactors that can be simulated includes pressurized water reactors, boiling water reactors, liquid-metal fast breeder reactors, and Canada deuterium uranium reactors. A number of verification activities have been undertaken, including (a) comparison of ORIGEN2 decay heat results with both calculated and experimental values, and (b) comparison of predicted spent fuel compositions with measured values. The agreement between ORIGEN2 and the comparison bases is generally very good. Future work concerning ORIGEN2 will involve continued maintenance and user support along with additional verification studies and limited modifications to enhance its flexibility and usability. ORIGEN2 can be obtained, free of charge, from the ORNL Radiation Shielding Information Center.