COBRA4i-MIT: an updated sub-channel analysis code for sodium fast reactor design

Proper modeling of the coolant behavior in Sodium Fast Reactors (SFR) is necessary for design and safety reasons. Fuel performance, for example, can only be accurately understood by knowing the full history of local coolant conditions. Computational Fluid Dynamics (CFD) is a powerful tool for fluid modeling; however it is still too computationally expensive for parametric studies and/or transient analysis of whole assemblies. The sub-channel analysis approach is better suited for the task, trading in a luxurious level of detail for a necessary boost in speed. Most existing sub-channel analysis codes for sodium, including SABRE, SLTHEN, COBRA, and MATRA, are capable of producing reasonably accurate results, however are limited in availability or lack the most current empirical correlations. COBRA4i, which provides robust implicit and explicit solutions schemes, suffers only from the latter malady. COBRA4i produced good results when previously tested with experimental data and with its multiple solution schemes is viable for a large spectrum of operating conditions and transients. The main shortcoming of the code is its archaic nature, in its programming language (FORTRAN66) and its correlations, both of which can be remedied as described. COBRA4i was brought up to date so it could be interpreted by modern compilers. A through literature search determined the most accurate and up to date correlations for pressure drop, mixing, and heat transfer. These correlations were added to the code, which was then run parametrically to determine how different combinations of old and new correlations affected code performance. All flow (laminar, transition and turbulent) and convection (natural, mixed and forced) regimes were included in the update. A recommended set of correlations was determined. Experimental benchmarks were preformed on data from the ORNL 19-rod test assembly, Toshiba 37-rod bundle and WARD 61-rod bundle, along with a code-to-code benchmark on results from the KALIMER design (containing 271-rod assemblies). These data sets encompass a broad range of assembly sizes and flow and buoyancy combinations. The benchmarks showed good performance of both the old and new correlations. The recommended set of correlations proved to be the most successful over the widest region of operating conditions. (authors)