ESTS0075 GRASS-SST.

1. NAME OR DESIGNATION OF PROGRAM:  GRASS-SST.

3. DESCRIPTION OF PROGRAM OR FUNCTION

GRASS-SST is a comprehensive, mechanistic model for the prediction of fission-gas behaviour 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 behaviour. Models are included for intra- and inter-granular fission-gas bubble behaviour 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 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 behaviour of fission gas during liquefaction/dissolution and fuel melting conditions.

4. 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 generalised capillary  relation.

5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM

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

6. TYPICAL RUNNING TIME

5  minutes on an  IBM  PS/2 Model 70 with  a
math coprocessor.

7. UNUSUAL FEATURES OF THE PROGRAM

The steady-state analysis has undergone verification for end-of-life fission-gas release, fuel swelling, and intragranular bubble-size distributions.

8. RELATED AND AUXILIARY PROGRAMS

GRASS-SST is based on the GRASS code originally developed for the prediction of fission-gas behaviour in Liquid Metal Fast Breeder Reactor fuel during steady-power irradiations. FASTGRASS (NESC 1095), which is based on  GRASS-SST, is approximately one to two orders of magnitude faster in execution than GRASS-SST.

10. REFERENCES

- GRASS-SST, NESC No. 1094.PC, GRASS-SST IBM PC Version Flexible
  Disk Cartridge Description, National Energy Software Center Note
  91-95, August 30, 1991.
- J. Rest and A.W. Cronenberg:
  Modelling the Behaviour 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 Behaviour in Nuclear Fuel
under Normal and Accident Conditions, Journal of Nuclear Materials    Vol. 120, pp. 195-217, 1984.
- J. Rest:
  The Prediction of Transient Fission-Gas Release and Fuel
  Microcracking Under Severe Core-Accident Conditions, Nuclear
  Technology, Vol. 56, pp. 553-564, March 1982.

- J. Rest:
  GRASS-SSET: 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. MACHINE REQUIREMENTS:  212 Kbytes of memory.

13. OPERATING SYSTEM UNDER WHICH PROGRAM IS EXECUTED:  DOS 3.3; DOS 4.0

14. OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS:

15. NAME AND ESTABLISHMENT OF AUTHORS

S. Zawadzki
Argonne National Laboratory, IL (United States)

source program   mag tapeGRASS1.FOR Main routine Fortran source     SRCTP
source program   mag tapeGRASS2.FOR Subroutines ZRDWR, FRCLC, HYB   SRCTP
source program   mag tapeGRASS3.FOR Subroutines GRASS & RONK        SRCTP
source program   mag tapeGRASS4.FOR Subroutines GRASS1 & GRASS2     SRCTP
source program   mag tapeGRASS5.FOR Subroutines GRASS3 & VEXP       SRCTP
source program   mag tapeDERF.FOR Subroutines DERF, CALERF & DERFC  SRCTP
test-case data   mag tapeGRASS.DAT Sample problem input             DATTP
test-case output mag tapeGRASS.OUT Sample problem output            OUTTP
report                   NUREG/CR-0202 (June 1978)                  REPPT