1.   NAME AND TITLE

COBRA-SFS Version 6.0: Thermal-Hydraulic Analysis of Multi-Assembly Spent Fuel Storage and Transportation Systems

2.   CONTRIBUTORS

Pacific Northwest National Laboratory, Richland, Washington, USA.

3.   CODING LANGUAGE AND COMPUTER

Fortran; Windows, Linux, MacOS (P00614MNYCP02).

4.   NATURE OF PROBLEM SOLVED

COBRA-SFS (Spent Fuel Storage), a code for thermal-hydraulic analysis of multi-assembly spent fuel storage and transportation systems. COBRA-SFS is a computer program that performs thermal-hydraulic analyses of multi-assembly spent-fuel storage and transportation systems. It 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, 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 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.

5.   METHOD OF SOLUTION

The finite difference equations for mass continuity, momentum, and energy for the fluid are solved using the RECIRC solution method, adapted from the COBRA-WC code. In this method, the set of equations is solved iteratively to obtain the flow and pressure fields. The primary advantage of the RECIRC method is that it is applicable to reverse and recirculating flows, such as those occurring in storage systems where the geometry allows natural circulation flow paths. RECIRC uses a Newton-Raphson technique similar to the one developed by Hirt to solve the conservation equations, but it has been made implicit in time, as was done in the SABRE code.

The RECIRC flow field solution is divided into two parts: a tentative flow solution and a pressure solution. The tentative flow solution is achieved by iteratively sweeping the subchannel array from inlet to exit. In each sweep, tentative axial flows and crossflows are computed for the channels by evaluating the two linearized momentum equations with current values for pressure and other independent variables. After all tentative flows and crossflows have been computed at all axial levels, the flows and pressures are adjusted to satisfy continuity by a Newton-Raphson method.

6.   RESTRICTIONS OR LIMITATIONS

None noted.

7.   TYPICAL RUNNING TIME

Run time for full cask models will be problem dependent. Large ventilated casks may take up to 3 hours on typical workstations. Well optimized models of conduction dominated systems may run in 30 minutes.

8.   COMPUTER HARDWARE REQUIREMENTS

No special requirements. The code operates single core. Memory usage is problem dependent and most workstation computers will have sufficient memory to run 3-4 models simultaneously

9.   COMPUTER SOFTWARE REQUIREMENTS

Executables for windows operating systems are distributed with the code package. Source code is also included and may be compiled for Linux, MacOS and Windows systems with a compiler that supports modern Fortran.

10.  REFERENCES

Richmond D.J., Jensen B.J., Grant C.L., Hom, B.M., COBRA-SFS A Thermal Hydraulic Analysis Code for Spent Fuel Storage and Transportation Casks, PNNL-31003, March 2021

11.  CONTENTS OF CODE PACKAGE

The package was transmitted digitally as a download link with the reference cited above, source and sample problems. Executables for windows are included

12.  DATE OF ABSTRACT

May 2021

KEYWORDS:   HEAT TRANSFER; THERMAL HYDRAULICS; SPENT FUEL; USED FUEL; STORAGE; TRANSPORTATION; WASTE MANAGEMENT