Code Manual for CONTAIN 2.0 - Federation of American Scientists

concentration in the cell maybe higher or lower than the local concentrations that actually control
ignition.
If deliberate ignition sources are not present, the situation is much less clear. If AC power is
available, it is customary to assume that chance sparking will still provide an ignition source
somewhere within the containment; see, e.g., Reference Gre90. However, the timing of ignition is
uncertain in part because of uncertainty in the timing of whatever provides the ignition source, and
in part because the energy required to ignite the mixture is a strong function of the gas composition.
Hence a weak spark source might fail to ignite a mixture near the lean flammability limit, but could
ignite a mixture with higher combustible gas content. In the Three-Mile Island Unit 2 (TMP2)
accident, a hydrogen bum initiated with an estimated hydrogen mole fraction of about 0.08. There
were no deliberate ignition sources in this accident but AC power was available and steam mole
fractions were low, tending to favor ignition.
If neither a deliberate ignition source nor AC power is available, initiation of combustion is
especially uncertain and may not occur at all. In the NUREG- 1150 analysis of station blackout
accidents in the Grand Gulf plant, [Bro90] for example, it was assumed that the probability of
ignition was only about 0.2 for hydrogen mole fractions of 0.04-0.08, rising to about 0.5 for mole
fractions >0.16. Large uncertainties were assigned to these estimates, and they apply only to the
specific scenarios and plant considered. For scenarios of this type, the only safe way to bound the
problem would be to assume that ignition could occur at any concentration up to the maximum that
corresponds to the total hydrogen input to the problem, and also include the case where ignition does
not occur at all unless an identifiable ignition source is provided (e.g., power recovery, or perhaps
hot core debris at the time of vessel breach).
In terms of containment loads, assuming a low ignition threshold is generally nonconservative, since
a low threshold prevents development of higher hydrogen concentrations that could result in stronger
burns. By this criterion, the CONTAIN default threshold of 7% can be nonconservative except for
scenarios with operating igniters. If prediction of loads is an important goal of the intended
application, sensitivity studies with higher ignition thresholds may be warranted. Alternatively, the
TACTIV keyword maybe used to suppress premature deflagrations; see Section 14.3.1.7 for details.
Section 9.1.1 provides additional discussion of the ignition criteria and the supporting database.
Flame SDeed. The flame speed can have a significant effect upon the maximum containment and
pressures calculated for a hydrogen burn because, when the flame speed is low, atmosphere-structure
heat transfer during the burn can mitigate the pressure and temperature rises to some degree. The
event summary file TAPE21 includes information about the time, location, and duration of all
deflagrations occurring during a calculation; by checking the burn duration, the user can estimate
whether the calculated loading is likely to be significantly reduced by a low flame speed.
The CONTAIN flame speed correlations are taken from the work of Wong, [Won88] who reports
that the correlations agree with the data to within &40%, with the error being smaller if some cases
with low flame speeds (< 2 m/s) are not included. The general tendency was for the correlation to
either predict the correct order of magnitude of the flame speed or to overpredict the flame speed.
There was some tendency for the flame speed to increase with increasing scale, an effect that is not
Rev. O 13-9 6/30/97