Code Manual for CONTAIN 2.0 - Federation of American Scientists

As the containment spray water drops fall through the atmosphere, their diameters can increase or
decrease as a result of condensation or evaporation. The heat transfer and mass transfer to the spray ~
as the drops fall are calculated. The model for these processes is basically the same as that for heat
and mass transfer used elsewhere in CONTAIN. However, the Nusselt number correlation used is
that of Ranz and Marshall for forced convection around a spherical droplet. [Ran52] The rate of
evaporation of, or condensation on, a spray droplet is controlled by the diffusion of water vapor
through the gas boundary layer at the surface of the drop and is driven by the difference in water
vapor pressure between the atmosphere and the droplet surface.
The spray calculation begins with determination of the number of droplets introduced in a cell
timestep. The equations for drop height, mass, and energy are solved for the entire fall of a single
droplet. The fall height is an input parameter, “sphite,” that is defaulted to the cell height. The
resulting transfers of mass and energy between the droplet and the atmosphere are then multiplied
by the number of droplets. Note that the effects on the atmosphere resulting from a given spray drop
are assumed to be instantaneous and not spread out over its fall time. This assumption is normally
of concern only if the atmosphere conditions are calculated to change rapidly over the droplet fall
time.
Spray droplets that reach the bottom of the cell contribute their mass and energy to the lower cell
pool, if present, in the cell to which the engineered system effluent is directed. The user specifies
that cell as cell “iclout” for storing residual liquid from ESFS, which by default is the cell in which
the spray is defined. If no lower cell pool is specified for that cell, the water is lost from the
problem. For most situations that call for the use of containment sprays, a liquid pool can be
expected to form as a result of spray droplets reaching the floor. Therefore, it is recommended that
the user include a lower cell model with a pool in the residual-liquid cell.
When sprays are operating, containment conditions tend to approach a quasi-steady state in which
the sprays are removing about as much steam and energy from the containment as the various
sources are supplying to the containment. (Important exceptions to this behavior arise during and
shortly after major transient inputs of steam or energy to the containment.) Likewise, the drop itself
tends to reach a quasi-steady state with respect to the containment atmosphere very early in its fall,
with little change in the drop parameters (such as temperature) after the frostfew tenths of a meter
or so of its fall.
The model assumes that (1) the gas and vapor have ideal behavior, (2) the drop is well-mixed, (3) the
drops fall at all times at their terminal velocity, and (4) the effect of the ensemble of spray drops is
obtained by scaling the effect of a single drop by the number of drops.
Water Dro~ Terminal Velocity The spray drops are assumed to fall at constant terminal velocity
speed v~throughout their entire containment fall path. The equation for v~is:
[1 pg/pg ~*
vt. — D
(12-13)