RSICC CODE PACKAGE PSR-479
1. NAME AND TITLE
FASTGRASS: Code System to Predict Fission Product Release in Ubase Fuels.
2. CONTRIBUTORS
Argonne National Laboratory, Argonne, Illinois, through the Energy Science and Technology Software Center, Oak Ridge, Tennessee.
3. CODING LANGUAGE AND COMPUTER
Fortran IV (FASTGRASS), CYBER170 and IBM3033;
Fortran 77 (PARAGRASS), IBM PS/2 and Pentium; distributed as P00479MNYCP00.
4. NATURE OF PROBLEM SOLVED
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. FASTGRASS is a mechanistic code that predicts atomic and bubble behavior of fission gas in UO2 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 Cs2MoO4, Cs2UO4, BaO, SrO and BaUO4. Both the condensed and vapor phases of BaO and SrO are considered. A basic premise of the FASTGRASS analysis is that the noble gases provide the major pathway 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.
5. 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 by 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.
6. RESTRICTIONS OR LIMITATIONS
FASTGRASS uses only one bubble-size class to characterize the fission gas bubble distribution. The behavior of only stable fission products is calculated.
7. TYPICAL RUNNING TIME
The sample problem requires 5 CP seconds on a CDC CYBER170/875. The sample problem requires 1 minute on an IBM PS/2 Model 70 with a math coprocessor. The sample problem requires 4 CPU seconds on an IBM3033.
8. COMPUTER HARDWARE REQUIREMENTS
Three versions are included in this package: CDC CYBER170/875, IBM PS/2 Model 70, and IBM 3033.
9. COMPUTER SOFTWARE REQUIREMENTS
The codes were developed at Argonne National Laboratory and first released by the National Energy Software Center in 1988. The packages were later transferred to RSICC. No changes were made to the FASTGRASS mainframe packages when released by RSICC in October 2001. A few minor changes were made to the PC version of PARAGRASS to allow it to compile with Digital Visual Fortan 5.0. The DVF executable was added to the package. Operating systems under which the code were developed are: NOS 2.4 for CDC; DOS 3.1; DOS 4.0 for PS2; MVS for IBM3033; and VM/CMS for IBM4331).
10. REFERENCES
J. Rest and S.A. Zawadzki, "FASTGRASS: A Mechanistic Model for the Prediction of Xe, I, Cs, Te, Ba, and Sr Release from Nuclear Fuel under Normal and Severe-Accident Conditions," NUREG/CR-5840 (ANL-92/3) (September 1992).
J. Rest and S.A. Zawadzki, "FASTGRASS-VFP/PARAGRASS-VFP Version 61030, Interim Users Guide" (November 1988).
11. CONTENTS OF CODE PACKAGE
Included are reference documents and the software transmitted on one 3.5" diskette which includes FASTGRASS mainframe source code, PC PARAGRASS source, sample problem input, output, and a PC executable written in self-extracting, compressed Windows files.
12. DATE OF ABSTRACT
October 2001.
KEYWORDS: FISSION PRODUCT INVENTORY; FISSION PRODUCT YIELD; LWR