Code Manual for CONTAIN 2.0 - Federation of American Scientists

13.2.4 Aerosol Modeling
~t. The aerosol
module assumes that all particles within a given size class have the same composition. To do
otherwise would require the code to maintain a three-dimensional mesh of compositionhize
information, in place of the current two-dimensional mesh, with significant increases in complexity
and computational cost. However, under certain conditions involving condensation/evaporation,
significant error can result in the aerosol calculation. Suppose, for example, that gas containing
water aerosols flows into a cell that contains solid aerosols, and suppose further that the cell
atmosphere is superheated so that the water aerosols quickly evaporate before they agglomerate
significantly with the solids. In reality, the evaporation of the water would have no effect upon the
solids. In CONTAIN, if the solid and water aerosol size distributions overlap, all particles in the
overlapping size region will be assumed to have the average composition for that size and thus will
be assumed to be part water and part solid. When the water is evaporated, the solid residue will
therefore be shifted down in size. In one extreme case of this kind, an aerosol component with a
mass median diameter of 10 pm was found to be shifted down to the one-pm size range.
A somewhat sirn.h problem can arise when a large amount of water first condenses upon solid
aerosols, and subsequently re-evaporates. Since condensation acts to make the smaller particles
grow more (in a relative sense) than the large ones, it acts to collapse the size spectrum into a smaller
number of size classes. The resulting loss of size resolution is irreversible, and the initial size
distribution is not fully recovered when the water evaporates. In some cases, some of the aerosol
particles will be calculated to be smaller than the smallest allowed size after evaporation and will
be lost from the mesh unless the TRAPUNFL keyword is specified in the global AEROSOL block
(see Sections 7.1,14.2.5, and 14.3.1.12).
No Model for Turbulent Ener~ Dissipation Rate. The aerosol agglomeration model includes terms
for both the inertial and shear components of turbulent agglomeration. The agglomeration rates
predicted depend upon the turbulent energy dissipation rate, ~, in Equation (7-9). However, the code
has no model for calculating q. The default value of this parameter is 0.001 m2/s3. The user can
specify a different value through the input parameter TURBDS in the global AEROSOL block. This
value will then be used in all cells and at all times during the calculation. There is not much
guidance available for specifying this parameter. The default value is thought to correspond to rather
quiescent conditions. Appendix E of Reference Lip85 describes a model for estimating an upper
limit to e, assuming that only natural convection driven by temperature differences is generating
turbulence. An upper limit of about 0.02 m2/s3was estimated for conditions typical of station
blackout accidents over long time periods. Much higher values could exist for short times during
and immediately following energetic events such as blowdown of the primary system, hydrogen
burns, and DCH events.
Aerosol-Atmos~here Thermal Eauilibnum. The aerosols ae assumed to be at the same temperature
as the atmosphere. Because the mass of the suspended solid aerosols is assumed to be small
compared with the total atmosphere mass, such aerosols are assumed not to contribute to the total
atmosphere heat capacity in the model. This simplifying assumption is expected to be valid for most
applications, but in extreme cases it could lead to energy conservation errors.
Rev. O 13-5 6/30/97