Advances in Monte Carlo depletion capabilities for MCNPX - 110

Understanding the generation of fission and activation products during fuel burnup is paramount in generating proper source terms used in reactor and fuel-storage shielding calculations. The recent integration of CINDER90 into the MCNPX Monte Carlo radiation transport code provides a completely self-contained Monte Carlo burnup/depletion capability. Two advances have been made in this new MCNPX capability based user feedback: isotope tracking and multi-material burning. Isotope tracking includes multiple fission product tiers and an isotope generator algorithm. To ease user selection of proper fission products, MCNPX has preset fission product 'tiers' that contain fixed fission product sets. These fission product tiers simplify user input. Capturing every decay chain product for transport in MCNPX would be an extremely computationally expensive task. To conserve computation time, MCNPX tracks, along with all isotopes specified on the materials card, only the immediate daughter products from the burn material interactions. The isotope generator algorithm determines these isotopes at the beginning of the run and places the nuclides in the appropriate burn materials to trace the buildup and reactivity effects of these isotopes. Modeling and ascertaining important depletion information from a complex system involves understanding the varied burnup across the entire system. Because MCNPX burns a system by material and not by individual cell declaration, MCNPX must use multiple burn materials to properly represent the specific depletion witnessed in certain physical areas of the system. MCNPX now burns multiple materials and determines the individual nuclide buildup and burnup contribution from each individual burn material. The use of isotope tracking and multi-material burning are benchmarked against the MONTEBURNS Monte-Carlo-linked depletion code, and the results show promising agreement. The MCNPX depletion capability enables complete, relatively easy-to-use depletion calculations to be performed in a single Monte Carlo code. These capabilities have been enhanced by the two major new improvements described here. Further improvements are under development to enhance the usefulness of this new capability. (authors)