NESC1091 COBRA-SFS.

1. NAME OR DESIGNATION OF PROGRAM:  COBRA-SFS.

3. DESCRIPTION OF PROGRAM OR FUNCTION

COBRA-SFS is used for steady- state and transient thermal-hydraulic analysis of spent fuel storage systems as well as other heat transfer and fluid flow problems. It is designed to predict flow and temperature distributions under a wide range of flow conditions, including mixed and natural convection. Two auxiliary programs, RADX1 and RADGEN, generate blackbody view factors and calculate radiation exchange factors for  unconsolidated spent fuel assemblies to be supplied as input to COBRA-SFS.

4. METHOD OF SOLUTION

The thermal-hydraulic analysis is separated into two parts - a flow field solution and an energy solution. COBRA-SFS solves a set of incompressible subchannel equations for mass and momentum conservation in the coolant and energy conservation in the fuel rods, solid structures, and coolant. The RECIRC solution method, a Newton-Raphson technique, is used to iteratively solve these equations. Radiation heat transfer between rod and slab surfaces is modeled using detailed radiation exchange factors, which are calculated from the geometry of the problem and the emissivities of the participating surfaces. The subchannel equations are fully implicit in time.

5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM

PARAMETER statements presently impose a maxima of -
    50 channel gap connections
    40 channels
    20 fuel rods
    40 slab connections
     1 fuel types
     6 assemblies
     9 fuel rods interacting with a channel
     6 fuel nodes per fuel rod
     4 axial fuel type divisions
By modifying PARAMETER statements, code dimensioning limits can be changed to fit any given problem, limited only by available storage.

6. TYPICAL RUNNING TIME

Running times are dependent on the size of the problem. The sample problem required 6.5 CP seconds on a Cray1 and 30.3 CP seconds on a CDC CYBER170/875.

NEA-DB ran the tests included in this package on a CDC CYBER 830 computer. CPU times were 295 seconds (COBRA-SFS); 21 seconds (RADGEN).

7. UNUSUAL FEATURES OF THE PROGRAM

In addition to many features of the earlier COBRA codes, COBRA-SFS has additional features specific  to spent fuel storage systems. These include a solution method that  calculates three-dimensional conduction heat transfer through a solid structure network such as a spent fuel cask basket or cask body, a detailed radiation heat transfer model that calculates radiation on a detailed rod-to-rod basis, thermal boundary conditions to model radiation and natural convection heat transfer from storage system surfaces, and a total flow boundary condition that automatically adjusts the pressure field to yield the specified total flow for a system.
The lumped finite-volume nodalization used in COBRA-SFS allows great deal of flexibility in modeling a wide variety of geometries.

8. RELATED AND AUXILIARY PROGRAMS

The equations governing mass, momentum, and energy conservation for incompressible flows are solved using a semi-implicit method similar to that used in COBRA-WC (NESC9973) that allows recirculating flows to be predicted. COBRA-SFS evolved from the earlier COBRA3C, COBRA4, and COBRA4I (NESC432) programs.

10. REFERENCES:

- D.R. Rector, C.L. Wheeler and N.J. Lombardo :
  COBRA-SFS - A Thermal-Hydraulic Analysis Computer Code
  Volume I: Mathematical Models and Solution Method
  PNL-6049 Vol. I (November 1986).
- D.R. Rector, M.J. Cuta, N.J. Lombardo et al. :
  COBRA-SFS - A Thermal-Hydraulic Analysis Computer Code
  Volume II: User's Manual
  PNL-6049 Vol. II (November 1986).
- N.J. Lombardo, J.M. Cuta, T.E. Michener, D.R. Rector et al. :
  COBRA-SFS - A Thermal-Hydraulic Analysis Computer Code
  Volume III: Validation Assessments
  PNL-6049 Vol. III (December 1986).
- D.R. Rector :
  RADGEN: A Radiation Exchange Factor Generator For Rod Bundles
  PNL-6342 (October 1987).
- D.R. Rector, T.E. Michener:
  COBRA-SFS Modifications and Cask Model Optimization
  PNL-6706 (January 1989).
- L. Reed :
  COBRA-SFS Edition D Tape Description and Implementation
  Information
  NESC Note 89-81 (August 24, 1989)

11. MACHINE REQUIREMENTS

Six units in addition to the standard input/output units are required.

The test case of COBRA-SFS requires on CDC CYBER 830 344,000 (octal) words of main storage; RADGEN requires 116,000 (octal) words.

13. OPERATING SYSTEM UNDER WHICH PROGRAM IS EXECUTED:  CTSS (Cray1), NOS 2.4 (CDC CYBER170).

NOS2.5.1 (CDC CYBER 830) with FORTRAN V compiler.

14. OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS

Although the COBRA-SFS source supplied is designed for execution on  a Cray1, a CDC-specific UPDATE correction set is provided to permit  implementation on a CDC CYBER170. COBRA-SFS is designed to perform both steady-state and transient calculations; however, the transient capability has not yet been validated. Memory must be set to zero prior to execution.

15. NAME AND ESTABLISHMENT OF AUTHORS

          D.R. Rector
          Pacific Northwest Laboratory

File name	File description	Records
NESC1091_02.001	INFORMATION FILE	125
NESC1091_02.002	COBRA-SFS UPDATE SOURCE	10633
NESC1091_02.003	UPDATE CORRECTION SET FOR CDC	46
NESC1091_02.004	COBRA-SFS FORTRAN-77 SOURCE FOR CRAY	18845
NESC1091_02.005	COBRA-SFS FORTRAN-77 SOURCE FOR CDC	18841
NESC1091_02.006	SAMPLE PROBLEM INPUT	234
NESC1091_02.007	SAMPLE PROBLEM TAPE10 INPUT	28
NESC1091_02.008	SAMPLE PROBLEM OUTPUT	1851
NESC1091_02.009	RADGEN FORTRAN SOURCE	1983
NESC1091_02.010	RADX1 FORTRAN SOURCE	640
NESC1091_02.011	RADGEN SAMPLE PROBLEM INPUT	7
NESC1091_02.012	RADGEN SAMPLE PROBLEM OUTPUT	150
NESC1091_02.013	RADGEN SAMPLE PROBLEM TAPE10 OUTPUT	55
NESC1091_02.014	CDC DAYFILES DURING TESTING	112