NESC1095 FASTGRASS.

1. NAME OR DESIGNATION OF PROGRAM:  FASTGRASS.

3. DESCRIPTION OF PROGRAM OR FUNCTION

FASTGRASS is a mechanistic code that predicts atomic and bubble behavior of fission gas in U02 fuel  under steady-state and transient conditions. FASTGRASS also calculates the behavior of the volatile fission products (VFP) I, Cs, and Te as well as the alkaline earth fission products (AEFP) Ba  and Sr. The chemistry models include the reaction products Cs2Mo04,  Cs2U04, Ba0, Sr0 and BaU04. Both the condensed and vapor phases of Ba0 and Sr0 are considered. A basic premise of the FASTGRASS analysis is that the noble gases provide the major pathways for fission product release as well as the major reaction sites required for vapor phase formation of various reaction products.
Models are included for fission-product generation, atomic migration, bubble nucleation and re-solution, bubble migration and coalescence, channel formation on grain faces, the interlinking of porosity along grain edges, and microcracking on both the amount of  fission products released and on their distribution within the fuel. Mechanistic models are also included for grain growth/grain boundary sweeping, and for the behavior of fission products under liquefaction/dissolution and fuel melting conditions. FASTGRASS uses a realistic equation of state for xenon, experimentally derived steady-state bubble mobilities, and phenomenological modeling of bubble mobilities during transient nonequilibrium conditions to calculate the swelling due to retained fission-gas bubbles in the lattice, on grain faces, and along the grain edges. FASTGRASS also calculates fission-product release as a function of time for steady-state and transient thermal conditions.
Two variants of the software are included: FASTGRASS, for multinode calculations, and PARAGRASS, for single-node calculations. PARAGRASS can be used as a module in a larger program.

4. METHOD OF SOLUTION

FASTGRASS solves a set of coupled nonlinear differential equations for the intra- and inter-granular concentrations of fission product atoms and gas bubbles. The evolution of the gas bubble population in the lattice, on the faces, and on the edges is phrased in terms of the evolution of an average-sized bubble in each region. The numerical schemes for computing the average bubble size in all three regions include the effects of decreased yield strength of the fuel matrix at higher temperatures. FASTGRASS calculates grain face saturation by fission  gas be dealing directly with the calculated fission gas bubble distributions. The model for calculating the probability of long- range grain edge tunnel interconnection is based on the assumption that the long-range interconnection is a function of the grain edge  bubble swelling.

5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM

FASTGRASS uses only  one bubble-size class to characterize the fission gas bubble distribution. The behavior of only stable fission products is calculated.

6. TYPICAL RUNNING TIME

The sample problem requires 4 CPU seconds on  an IBM3033, 3 CPU minutes on an IBM4331, 5 CP seconds on a CDC CYBER170/875, and 1 minute on an IBM PS/2 Model 70 with a math coprocessor.

The test case included in this package was executed by  NEA-DB on an IBM PC/AT compatible computer with a math coprocessor in about 5 seconds.

7. UNUSUAL FEATURES OF THE PROGRAM:

8. RELATED AND AUXILIARY PROGRAMS

Although based on GRASS-SST software (NESC 1094), FASTGRASS is approximately one to two orders of magnitude quicker in execution, depending on the complexity of the problem.

10. REFERENCES

- FASTGRASS, NESC No. 1095.PC_, FASTGRASS IBM PC Version Flexible
  Disk Cartridge Description,
  National Energy Software Center Note 89-23, December 22, 1988.
J.Rest and A.W. Cronenberg
  Modeling the Behavior of Xe, I, Cs, Te, Ba and Sr in Solid and
  Liquified Fuel During Severe Accidents,
  Journal of Nuclear Materials, Vol. 150, pp. 203-225, 1987.
- J. Rest
  An Improved Model for Fission Product Behavior in Nuclear Fuel
  under Normal and Accident Conditions,
  Journal of Nuclear Materials, Vol. 120, pp. 195-217, 1984.
- Argonne National Laboratory, Light-Water-Reactor Safety Research
  Program: Quarterly Progress Report October--December 1980,
  NUREG/CR-2251 (ANL-81-42), July 1981.
- J. Rest
  Evaluation of Volatile and Gaseous Fission Product Behavior in
  Water Reactor Fuel Under Normal and Severe Core Accident
  Conditions,
  Nuclear Technology, Vol. 61, pp. 33-48, April 1983.
- J. Rest
  The Prediction of Transient Fission-Gas Release and Fuel
  Microcraking Under Severe Core-Accident Conditions,
  Nuclear Technology, Vol. 56, pp. 553-564, March 1982.

- J. Rest and S.A. Zawadzki:
  FASTGRASS-VFP/PARAGRASS-VFP Version 61030
  Interim User Guide.
- L. Eyberger:
  FASTGRASS IBM PC Version Flexible Disk Cartridge Description
  NESC Note 89-23 (December 22, 1988).

11. MACHINE REQUIREMENTS

235 Kbytes of memory are required on an IBM3033; 416 Kbytes on an IBM4331; 63,000 (octal) words on a CDC CYBER170/875; and 112 Kbytes on an IBM PS/2 Model 70.

Main storage requirements on an IBM PC/AT are about 100K bytes.

13. OPERATING SYSTEM UNDER WHICH PROGRAM IS EXECUTED

MVS (IBM3033), VM/CMS (IBM4331), NOS 2.4 (CDC CYBER170), DOS 3.1, 4.0 (IBM PC).

The program ran under MSDOS version 3.2. The compilers  Microsoft FORTRAN version 5.0 and RM/FORTRAN version 2.4 were used.

14. OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS:  The IBM PC version contains only the PARAGRASS program.

15. NAME AND ESTABLISHMENT OF AUTHORS

          J. Rest and S.A. Zawadzki
          Materials Science and Technology Division
          Argonne National Laboratory

File name	File description	Records
NESC1095_03.001	NEA DATA BANK information file	74
NESC1095_03.002	Fortran source (1/5)	740
NESC1095_03.003	Fortran source (2/5)	593
NESC1095_03.004	Fortran source (3/5)	362
NESC1095_03.005	Fortran source (4/5)	289
NESC1095_03.006	Fortran source (5/5)	423
NESC1095_03.007	Auxiliary routine Fortran source	240
NESC1095_03.008	Sample case input data file	31
NESC1095_03.009	Sample case output file	326
NESC1095_03.010	DOS file-names	9