Computer Programs
ESTS0075 GRASS-SST.
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ESTS0075 GRASS-SST.

GRASS-SST, Fission Products Gas Release and Fuel Swelling in Steady-State and Transients

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1. NAME OR DESIGNATION OF PROGRAM:  GRASS-SST.
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2. COMPUTERS
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Program name Package id Status Status date
GRASS-SST ESTS0075/01 Arrived 10-APR-2001

Machines used:

Package ID Orig. computer Test computer
ESTS0075/01 IBM PC
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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.
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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.
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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.
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6. TYPICAL RUNNING TIME

5  minutes on an  IBM  PS/2 Model 70 with  a
math coprocessor.
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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.
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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.
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9. STATUS
Package ID Status date Status
ESTS0075/01 10-APR-2001 Masterfiled Arrived
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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.
ESTS0075/01, included references:
- 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)
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11. MACHINE REQUIREMENTS:  212 Kbytes of memory.
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12. PROGRAMMING LANGUAGE(S) USED
Package ID Computer language
ESTS0075/01 FORTRAN-77
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13. OPERATING SYSTEM UNDER WHICH PROGRAM IS EXECUTED:  DOS 3.3; DOS 4.0
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14. OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS:
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15. NAME AND ESTABLISHMENT OF AUTHORS

S. Zawadzki
Argonne National Laboratory, IL (United States)
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16. MATERIAL AVAILABLE
ESTS0075/01
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
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17. CATEGORIES
  • G. Radiological Safety, Hazard and Accident Analysis

Keywords: nuclear fuels, reactor lattices, steady-state conditions, swelling, transients, water cooled reactors.