A concerted effort over the past few years has been focused on enhancing the core model for the High Flux Isotope Reactor (HFIR), as part of a comprehensive study for HFIR conversion from high-enriched uranium to low-enriched uranium (LEU) fuel. At this time, the core model used to perform analyses in support of HFIR operation is a Monte Carlo model with the MCNP code for the beginning of Cycle 400, which was documented in detail in a 2005 technical report. An updated HFIR core depletion model that is based on current state-of-the-art methods and nuclear data was needed to serve as reference for the design of an LEU fuel for HFIR.The recent enhancements in modeling and simulations for HFIR that are discussed in the present report include: (1) revision of the 2005 MCNP model for the beginning of Cycle 400 to improve the modeling data and assumptions as necessary based on appropriate primary reference sources-HFIR drawings and reports; (2) improvement of the fuel region model, including an explicit representation for the involute fuel plate geometry that is specific to HFIR fuel; and (3) revision of the Monte Carlo-based depletion model for HFIR, in use since 2009 but never documented in detail, with the development of a new depletion model for the HFIR explicit fuel plate representation.The new HFIR models for Cycle 400 are used to determine various metrics of relevance to reactor performance and safety assessments. The calculated metrics are compared, where possible, with measurement data from preconstruction critical experiments at HFIR, data included in the current HFIR safety analysis report, and/or data from previous calculations performed with different methods or codes. The results of the analyses show that the models presented in this report provide a robust and reliable basis for HFIR analyses.

The High Flux Isotope Reactor at the Oak Ridge National Laboratory was upgraded to install a cold source in horizontal beam tube number 4. Calculations were performed and measurements were made to determine nuclear heating within the cold source and dose rates within and outside a shield tunnel surrounding the beam tube. This report briefly describes the calculations and presents comparisons of the measured and calculated results. Some calculated dose rates are in fair-to-good agreement with the measured results, while others, particularly those at the shield interfaces, differ greatly from the measured results. The calculated cold source nuclear heating is in good agreement with the measured heating. xi

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