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Suprem-III User´s Manual
WETO2 Log Specifies that the ambient gas consists of wet oxygen or pyrogenic steam.
(See note below.)
(unit: )
(default: false.)
(synonym: )
The DIFFUSION statement simulates impurity diffusion in the structure under a variety of oxidizing and
non-oxidizing conditions. At a minimum, only the time and temperature of a step needs to be specified. In this case
a non-oxidizing drive-in is assumed. For oxidizing ambients or gaseous predepositions additional parameters need
to be specified. The default pressures and chlorine percentages are set previously by the oxidation ambient model
cards (e.g. DRYO2, WETO2, NITROGEN), while the defaults of the other optional parameters are set in the GENII
key file S3FKY0.
The numerical solution of the diffusion equations requires that the total step time be divided into a number
of smaller time increments, dt’s, in order to insure sufficient accuracy. There are two mechanisms that control the
choice of dt’s, one is due to a restriction on interface movement such that no interface moves more than one cell
spacing during a dt. The other time step control algorithm has been chosen to give the desired accuracy without using
excessive amounts of computation time. Unless the interface control chooses a smaller dt, it will attempt to use
a dt equal to that specified by the DTMIN parameter. Subsequent dt’s are chosen in the following manner.
1. At each point in the current structure a projected
concentration is calculated from the previous two
solutions.
Cp = C’ + (C’ - C’’)*dt/dt’
Where C’ is the previous concentration, C’’ is the
concentration before that, and dt’ is the previous dt.
2. From the current solution value, C, and the absolute
and relative truncation error parameters, an ‘error’
term at each point is then calculated.
Cerr = abs(C - Cp) / (ABS.ERR + abs(C)*REL.ERR)
3. The next dt is then calculated from the following
expression.
dtnext = dt * sqrt( (1. + dt/dt’)/Cerrmax )
Where Cerrmax is the maximum value of Cerr calculated
at each point in the structure.
4. A value of dtnext is calculated for each impurity
present with the smallest value being the one that
is ultimately used.
NOTE: The effective oxidant partial pressure for pyrogenic steam reactors has been found to vary significantly from
facility to facility. It is recommended that the user set the default pressure for WetO2 to a value
that gives the best agreement with measured oxide thicknesses from his facility.