.. _section-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 |SAS| 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 :numref:`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 |SAS| 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.