Code Manual for CONTAIN 2.0 - Federation of American Scientists

Furthermore, the composition limits for the receiving cell and the incoming gas are independent of
one another. All of these simplifications are at best approximations and maybe quite inaccurate in
some instances. It is recommended that the user check the behavior of the DFB model for the
calculation of interest, and consider whether alternative behaviors are plausible for the conditions
of the calculation. If alternative behaviors are plausible, sensitivity calculations with parameters set
to obtain the alternative behavior maybe warranted. For example, if the default model predicts that
most of the hydrogen should bum as it enters the containment, but it is not entirely clear that this is
actually the correct result, then a sensitivity case might be run with the DFB parameters set to impose
more stringent conditions for combustion, or with the DFB model suppressed entirely.
13.2.6.3 Bulk Spontaneous Reaction (BSR) Model. This model is intended for use when very high
gas temperatures are expected to cause rapid reaction of combustible gas with oxygen, even though
the normal flammability criteria for the other combustion modes are not satisfied. The BSR model
is controlled by a simple user-specified threshold temperature and a user-specified fractional reaction
rate (see Section 14.3.1.7). Both values are simple constants that do not depend upon any of the
containment parameters. The model is only intended to simulate the consequences of this mode of
hydrogen combustion; it cannot provide a mechanistic prediction as to whether it will occur in any
given scenario. It is up to the user to justify the controlling parameter values chosen.
The default value of the threshold temperature, 773 K, is close to what has been observed for
autoignition of stoichiometric air-hydrogen mixtures (see Section 9.3); addition of steam and/or
substantial departures from a stoichiometric composition are expected to result in higher threshold
values. On the other hand, the threshold temperature may decline somewhat with increasing scale.
In one approach that has been used, the threshold temperature was estimated to be that temperature ~
at which energy release rates exceeded the energy loss rates resulting from atmosphere-structure heat
transfer, with the energy release rates being calculated by the chemical kinetics code SENKIN.
[Lut91] Applied to lean, steam-rich mixtures (-4% hydrogen and -50% steam), this approach
predicted threshold temperatures of -800 K, however, there maybe substantial uncertainties in the
kinetics data set employed by SENIUN when this model is applied to these particular conditions,
implying that there may be significant uncertainty in the threshold temperatures estimated by this
technique (see Appendix A of Reference Pi194b).
The user must speci@ a reaction rate in order to model BSR, since the default value of zero prevents
BSR. Too slow a reaction rate will permit the atmosphere-structure heat transfer to cool the
containment more rapidly than the reaction heats it. A chemical kinetics calculations can provide
insight as to the appropriate reaction rate for a given temperature, provided a validated set of kinetics
data is available. If the heating that initiates BSR in the cell of interest is very rapid (e.g., due to a
sudden influx of hot gas or a large, sudden DFB), the entire cell may not be heated all at once. In
this situation, the reaction rate specified may be more meaningfully defined to simulate the rate at
which the superheated zone expands through the cell volume rather than to simulate reaction rates
governed by chemical kinetics. This approach was used to estimate the reaction rate specified in the
standard input prescription for DCH calculations (see Section 13.3.2.2.2). Whatever approach is
used, it is evident that substantial uncertainties must be allowed for in estimating the reaction rate
in the BSR model.
Rev O 13 12 6/30/97