10.2. The Earlier SSCOMP Model and Current Capability

Assuming chemical equilibrium at annular zone boundaries and mass conservation of fuel constituents, a model was developed earlier [10-4] to directly calculate zone formation at the end of steady-state irradiation. This model was also calibrated using zone formation data obtained for the cross section at the mid-height of a U-Pu-Zr alloy fuel pin in an EBR-II steady-state irradiation experiment [10-3]. This earlier SSCOMP model calculates the alloy compositions of the zones, the boundary radii of the zones, and the resulting in-pin power radial profile. The earlier model does not calculate the time-development of the fuel zones from the fabricated condition to the end-of-steady-state irradiation condition. This is so because the study of fuel constituent radial migration leading to zone formation was not at that time developed to a state where an appropriate dynamic model could be developed.

The zone formation calculation method of the earlier SSCOMP model is not incorporated into SAS4A/SASSYS‑1 Version 3.x codes due to its limitations. Some of its limitations are:

  1. The earlier SSCOMP model directly calculates the fuel compositions and boundary radii of the zones based on chemical equilibrium under end-of-steady-state reactor operating conditions, and does not calculate the migration of fuel constituents with time during steady-state irradiation. Reactor operating conditions usually change during the steady-state irradiation of a fuel pin, and therefore, chemical equilibrium under end-of-steady-state operating conditions may not have been attained if the operating conditions changed during a (fuel constituent) migration time constant before the end of steady-state irradiation.

  2. The earlier SSCOMP does not model other important pre-transient phenomena such as irradiation-induced fuel swelling, fission gas generation, retention and release to pin plenum, etc. These phenomena are important for characterizing a fuel pin at the beginning of a transient as noted in Section 10.1.

  3. The earlier SSCOMP model is not well validated by analysis of experimental data for zone formation.

Therefore, after evaluating the earlier SSCOMP in light of new data on zone formation and current understanding of phenomena causing it (zone formation), the earlier model or another more advanced model will be incorporated into SAS4A/SASSYS‑1 codes, and this will be done in the framework of models for other important pre-transient phenomena that characterize the fuel pin at the beginning of transient calculation. At present, the zone formation data (such as zonal U-Pu-Zr alloy compositions, boundary radii of the zones, etc.) are supplied to the code as input data, and used in pin transient temperature calculation. This multiple radial fuel zone option is controlled by the input parameter IFUELC.