This document is an updated and reduced version of the complete documentation of the COBRA-SFS thermal analysis code (COolant Boiling in Rod Arrays – Spent Fuel Storage) (Michener et al. 1995). This manual provides the analyst with input instructions and fundamental guidelines for contrasting models using COBRA-SFS. COBRA-SFS is designed to analyze the dry storage and transportation casks that house spent or used nuclear fuel. The primary focus and application is on square light water reactor fuel assemblies, however the code also has the capability to model hexagonal assemblies and is will suited to any geometry that can be defined as an extruded 2-D map.
COBRA-SFS uses a lumped-parameter, finite-difference approach to predict flow and temperature distributions in spent fuel storage systems and fuel assemblies, under forced and natural convection heat transfer conditions, in both steady-state and transients. Derived from the COBRA family of codes (Rowe 1973; Stewart et al. 1977; George et al. 1980; Khan et al. 1981), which have been extensively evaluated against in-pile and out-of-pile data, COBRA-SFS retains all the important features of the COBRA codes for single-phase analysis,0F0F and extends the range of application to problems with two-dimensional radiative and three-dimensional conductive heat transfer. With these added capabilities, COBRA-SFS has been used to analyze various single- and multi-assembly spent fuel storage systems containing unconsolidated and consolidated fuel, with a variety of fill media (Cuta et al. 1984; Lombardo et al. 1986b; Cuta and Creer 1986; Wiles et al. 1986; Rector et al. 1986c; McKinnon et al. 1986; Wheeler et al. 1986).
Cycle 0 of COBRA-SFS was released in 1986. Subsequent applications of the code required the development of additional capabilities, leading to the release of Cycle 1 in February 1989. Since then, the code has been subjected to an independent technical review as part of a submittal to the Nuclear Regulatory Commission (NRC) for a generic license to apply the code to spent fuel storage system analysis. Minor modifications and error corrections were developed in response to the reviewers’ recommendations. In addition, new capabilities and improvements to the code had been developed, and these changes were combined to form a new release of the code, Cycle 2, in September 1995 (Michener et al. 1995). Subsequently, additional validation and verification work was done to confirm the applicability of the code to transient evaluations, leading to the release of Cycle 3 in 1998 (Rector et al. 1998).
This report constitutes the user oriented documentation of version 6.2 of COBRA-SFS. It includes a complete set of input instructions, and provides the user with guidance for model construction and “best practices” to fully utilize the capabilities of the code for evaluation of spent fuel storage and transportation. Code theory and solution approach are covered extensively in (PNNL-32245).