Code Manual for CONTAIN 2.0 - Federation of American Scientists

Ti, total gas mass W, and free volume Vi, respectively, in each cell i. On the first timestep, if the
relative rates of change satisfj
lVi-l dVi/dt I s ~
the cell i is replaced in subsequent flow timesteps within the same system timestep by fixed-flowrate
boundary conditions, with the flow rates and composition taken to be those calculated in the fnst
flow timestep. In addition, the atmospheric conditions in the cell related to the noncondensable
gases and the condensable material will be held constant for those timesteps. Note that while the
atmosphere thermodynamic and flow conditions will be held constant, aerosol and fission product
inventories will not be held constant but will be calculated as evolving according to the assumed
flow rates and thermodynamic conditions.
4.4.9.2 Gu idelines for Applvimz the FIX-FLO W ODtion. In applying the FIX-FLOW option, the
user should be aware of the fact that even for relatively small tolerances, mass and energy
conservation problems may result and the calculated cell conditions for the cell atmospheres replaced
by fixed-flow boundary conditions may deviate significantly from the correct ones. These effects
in general scale with the user-specified rate tolerance ~. The degree to which these effects occur
depends on the detailed nature of the processes occurring within a cell. The purpose of this section
is to supply guidelines to the user as to when FIX-FLOW should be invoked.
Mass and energy will in general not be conserved in a cell replaced by fixed-flow boundary
conditions, since these quantities should in general be changing, albeit slowly. Under the quasisteady
conditions likely to be present when the fixed-flow boundary conditions are invoked, one can
estimate errors by assuming linear rates of change of mass and energy over the system timestep.
Estimates for the relative error per system timestep on the atmosphere temperature and total
atmospheric mass in this case can be expressed, respectively, as:
(4-29)
where At, is the system timestep and s is the ratio A? /At, of the fwst flow timestep to the system
timestep. The degree to which such errors accumulate and can be tolerated depends on the situation.
In some cases, such as an adiabatic closed cell with a very low source rate extending over a long
period of time, the cumulative errors may not be acceptable even for q = 10-6. However, as
discussed below, for a cell that rapidly relaxes to a quasi-steady state and is comected to a very large
reservoir (i.e., the environment), the cumulative errors may be negligible.
Rev. O 4-42 6/30/97