RSICC CODE PACKAGE PSR-489

1. NAME AND TITLE

GRASS-SST: Code System to Predict Fission-Gas Release & Fuel Swelling.

2. CONTRIBUTORS

Argonne National Lab., Illinois, through the Energy, Science and Technology Software Center, Oak Ridge, Tennessee.

3. CODING LANGUAGE AND COMPUTER

Fortran 77; CDC CYBER170; IBM 3033; IBM PC (P00489MNYCP00).

4. NATURE OF PROBLEM SOLVED

GRASS-SST is a comprehensive, mechanistic model for the prediction of fission-gas behavior in UO2-base fuels during steady-state and transient conditions. GRASS-SST treats fission-gas release and fuel swelling on an equal basis and simultaneously treats all major mechanisms that influence fission-gas behavior. Models are included for intra and inter-granular fission-gas bubble behavior as well as a mechanistic description of the role of grain-edge interlinked porosity on fission-gas release and swelling. GRASS-SST calculations include the effects of gas production from fissioning uranium atoms, bubble nucleation, a realistic equation of state for xenon, lattice bubble diffusivities based on experimental observations, bubble migration, bubble coalescence, re-solution, temperature and temperature gradients, interlinked porosity, and fission-gas interaction with structural defects (dislocations and grain boundaries) on both the distribution of fission-gas within the fuel and on the amount of fission-gas within the fuel and on the amount of fission-gas released from the fuel. GRASS-SST includes the effects of the degree of nonequilibrium in the UO2 lattice on fission-gas bubble mobility and bubble coalescence and also accounts for the observed formation of grain-surface channels. GRASS-SST also includes mechanistic models for grain-growth/grain boundary sweeping and for the behavior of fission gas during liquefaction/dissolution and fuel melting conditions.

5. METHOD OF SOLUTION

A system of coupled equations for the evolution of the fission-gas bubble-size distributions in the lattice, on dislocations, on grain faces, and grain edges is derived based on the GRASS-SST models. Given a set of operating conditions, GRASS-SST calculates the bubble radii for the size classes of bubbles under consideration using a realistic equation of state for xenon as well as a generalized capillary relation.

6. RESTRICTIONS OR LIMITATIONS

Maxima of 1 axial section and 1 radial ring. GRASS-SST assumes a cylindrical symmetry. It does not contain models for columnar grain growth.

7. TYPICAL RUNNING TIME

No study was done by RSICC on typical running times.

8. COMPUTER HARDWARE REQUIREMENTS

IBM PC, Cyber, and IBM 3033 versions of GRASS-SST are included in the distribution.

9. COMPUTER SOFTWARE REQUIREMENTS

The PC version was originally developed under DOS 3.3. This package has an executable that was created by RSICC and checked using the included sample problem. The Digital Visual Fortran v5.0 compiler was used to create the executable under Windows95 on a Pentium II. The Cyber version ran under NOS 2.4; the sample problem was executed by NESC October 1988 on a CDC CYBER 170/875. The sample problem for the MVS IBM 3033 version was executed by NESC in October 1988 on an IBM4331. The IBM 3033 and the CDC versions were screened but were not tested when they were transferred to RSICC and released in July 2001.

10. REFERENCE

J. Rest, "GRASS-SST: A Comprehensive, Mechanistic Model for the Prediction of Fission-gas Behavior in UO2-base Fuels during Steady-state and Transient Conditions," NUREG/CR-0202, ANL-78-53 (June 1978).

11. CONTENTS OF CODE PACKAGE

Included are the referenced document and one DS/HD diskette which includes the source codes, test cases and PC executable in self-extracting Windows files.

12. DATE OF ABSTRACT

July 2001.

KEYWORDS: FISSION PRODUCTS; LWR