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Coolant-Boiling in Rod Arrays-Two Fluids (COBRA-TF) is a thermal/hydraulic (T/H) simulation code designed for light water reactor (LWR) vessel analysis. It uses a two-fluid, three-field (i.e. fluid film, fluid drops, and vapor) modeling approach. Both sub-channel and 3D Cartesian forms of 9 conservation equations are available for LWR modeling. The code was originally developed by Pacific Northwest Laboratory in 1980 and had been used and modified by several institutions over the last few decades.COBRA-TF also found use at the Pennsylvania State University (PSU) by the Reactor Dynamics and Fuel Modeling Group (RDFMG) and has been improved, updated, and subsequently re-branded as CTF. As part of the improvement process, it was necessary to generate su cient documentation for the open-source code which had lacked such material upon being adopted by RDFMG. This document serves mainly as a theory manual for CTF, detailing the many two-phase heat transfer, drag, and important accident scenario models contained in the code as well as the numerical solution process utilized. Coding of the models is also discussed, all with consideration for updates that have been made when transitioning from COBRA-TF to CTF. Further documentation outside of this manual is also available at RDFMG which focus on code input deck generation and source code global variable and module listings.

For a parallel run, a separate input file is required for each solution domain in the model. Each input deck should include the channels, rods, and gaps solved by that domain plus any additional ghost channels, rods, and gaps that are needed for sharing information with other solution domains. A single master input file is also required to summarize the total number of rods, channels, and gaps in the model, the global CTF mesh, as well as the total number of solution domains in the model. Creating parallel input files by hand can be confusing and error prone and, therefore, it is recommended that the CTF preprocessor, "xml2ctf", be used to automate this process. The default naming convention for the parallel input file is "pdeck.〈nproc〉.〈rank〉.inp" and "pmaster.inp" for the master input file. The term, "〈nproc〉" is the number of solution domains in the model and "〈rank〉" is the index of the solution domain described by the input file (using 1-based indexing). The custom naming convention for the parallel input file is "〈name〉.〈nproc〉.〈rank〉.inp" and "〈name〉.master.inp" for the master input file. The parallel simulation is run for default file names as:

The initial mass flow rate is set to 0.0 by default. Therefore, a time-dependent ramp for the inlet mass flow rate should be applied (see description in CARD GROUP 13). Note 2: Variables PREF and HIN are used to determine the initial properties of the fluid in computational domain. Note 3: Because the mass conservation equation for non-condensable gases is not actively solved in this version of CTF, the volume fraction of vapor in the vapor-gas mixture should not be less than 0.9999! Note 4: Variables BRIN and RDIF are only read if the boron tracking/precipitation model is activated (IBTM = 0). RDIF is only used when IBTM = 2. May 26, 2016 Reactor Dynamics and Fuel Management Group www.mne.psu.edu/rdfmg pg. 14 of 178 0-Constant mesh cell length (default) >0-Read IVARDX pairs in variable ∆X table (see Card 4.3) Integer Required Card 4.3 is only read if IVARDX >0. Up to 5 sets of (JLEV, VARDX) may be entered. If IVARDX is greater than 5, repeat CARD 4.3 until IVARDX pairs has been entered. If IVARDX is less than 5 enter (0; 0.0) until 5 sets of (JLEV, VARDX) have been entered.

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