Code Manual for CONTAIN 2.0 - Federation of American Scientists

Any attempt to model the effects of this water would be subject to uncertainties comparable to those
involved in co-dispersed cavity water and co-ejected RPV water. In the absence of any significant ~
evidence that basement water is important to DCH, no attempt at defining a standard prescription
for including this water has been made.
Gas co mbustion. Although DCH experiments have been performed with hydrogen initially present
in the atmosphere, in no case were the compositions within the flammable range. Hence the
deflagration model never played an important role in the experimental analyses. However,
flammable atmospheres may exist at vessel breach in some NPP scenarios. In this event, hot debris
particles flying through the atmosphere are expected to provide multiple ignition sources that can
substantially shorten bum times in scenarios for which the CONTAIN default flame speed
correlation would normally predict a slow bum. Hence the flame speed (FLAM) is specified to be
at least 10 m/s in the standard prescription. No other nondefault deflagration parameters are
specified in the standard prescription. Note, however, that hot debris might affect the combustion
completeness, and sensitivity calculations may be desirable in cases in which incomplete
deflagrations are predicted to occur.
Most of the combustion of DCH-produced hydrogen that occurs in a CONTAIN calculation usually
occurs in the DFB model. The standard prescription sets MSFCB and MFOCB equal to 0.95 and
0.01, respectively. These settings virtually eliminate inerting against combustion that might result
from high steam concentrations or low oxygen concentrations in the receiving cell. One reason for
these settings is that, with less conservative values, the composition of the receiving cell might
initially permit combustion, but steam and oxygen concentrations would rise and fall, respectively,
as the event proceeds, possibly terminating combustion if the default values were used. In reality, =
temperatures would rise rapidly in the receiving cell, which would tend to compensate for the
changes in atmospheric composition, but the temperature dependence of the flammability limits is
not modeled in CONTAIN. Experimentally, combustion has been observed to occur during DCH
events when the default flammability limits (55% steam and 590oxygen) are not satisfied. In the
SNLllET-9 experiment, for example, even the initial steam concentration was about 67% and the
find steam and oxygen compositions were estimated to be -80% and -2%, respectively, yet most
of the DCH-produced hydrogen burned. ~la94] A CONTAIN analysis of this experiment assuming
default DFB parameters would therefore yield nonconservative results. Note also that the likelihood
of jet combustion is expected to increase somewhat with increasing facility scale, other things being
equal.
In both the CONTAIN model and in the experiments, DFB initiation tends to be a threshold effect,
although the model appears to exaggerate the sharpness of the threshold. Given that the threshold
is exceeded in the actual event, the standard prescription appears to do a good job of predicting the
extent of DCH-produced hydrogen combustion and its impact upon DCH loads. However, the
standard prescription clearly can be overly conservative in predicting the threshold: in the SNIJtET-5
experiment, there was an initial C02 concentration of about 76~0and oxygen concentration of 4.4%,
and combustion of DCH-produced hydrogen was quite limited while the standard prescription
predicted efilcient combustion. The initial containment atmosphere temperature in this experiment
was low (302 K) and rose to only 400 to 450 Kin the dome during the event; hence heating would
not have had a large effect upon the flammability limits.
Rev O 13-46 6/30/97