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Code Manual for CONTAIN 2.0 - Federation of American Scientists

Code Manual for CONTAIN 2.0 - Federation of American Scientists

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Equations (6-73) and (6-74) may allow some particles to have sizes that are larger than ~= as given<br />

by Equation (6-72). Although this result is not strictly consistent with the Weber number breakup<br />

assumption as described previously, it is consistent with the size distributions observed in DCH<br />

experiments, which do not show a sharp cut<strong>of</strong>f at large particle sizes. The Weber model should be<br />

thought <strong>of</strong> as providing an estimate <strong>for</strong> ~, but it does not define the entire size distribution, since<br />

this is also governed by the user-specified value <strong>of</strong> WESIG.<br />

When the Weber number model is used, all species in the debris are distributed proportionately<br />

among the airborne debris fields. That is, the composition <strong>of</strong> the material added to each airborne<br />

field during a timestep equals the composition in the cavity non-airborne field during that timestep,<br />

and FDISTR cannot be used to distribute the individual species separately. Section 14.2.7 provides<br />

the details as to how the debris fields are to be specified when the Weber model is used.<br />

6.3 DCH Tramin2 Model<br />

The process <strong>of</strong> debris removal as a result <strong>of</strong> interaction with containment structures and/or<br />

gravitational fallout is referred to as trapping. This process is still the subject <strong>of</strong> considerable<br />

uncertainty, and plant and experiment analyses have shown that trapping can have an important<br />

effect on results. There<strong>for</strong>e, the <strong>CONTAIN</strong> model includes both a mechanistic approach to trapping<br />

as well as flexible input <strong>for</strong> per<strong>for</strong>ming sensitivity calculations.<br />

In principle, the <strong>CONTAIN</strong> DCH trapping model is a simple one, where debris trapping in a given<br />

control volume is governed by a first order linear rate equation. As implemented, however, the<br />

model has several dependencies, such as the conditions in the cell atmosphere, attributes <strong>of</strong> the<br />

debris field being de-entrained, and the debris and gas inflow rates. In addition, the trapping rate is<br />

recalculated every timestep, so that debris trapping reflects changes in the particle field, atmospheric<br />

conditions, and inflow rates as the DCH event progresses. The f~st subsection below describes the<br />

trapping rate equation and its solution. The remaining subsections describe the models and options<br />

available in <strong>CONTAIN</strong> <strong>for</strong> calculating the trapping rates used in this model.<br />

6.3.1 Rate Equations <strong>for</strong> Trapping<br />

The debris trapping process is governed by a first order linear rate equation <strong>for</strong> the time rate <strong>of</strong><br />

change <strong>of</strong> airborne debris mass in a field:<br />

dm~i,~<br />

,,,<br />

[1 dt<br />

trap<br />

= -kin m~i~~<br />

,,,<br />

where ki,nis the trapping rate <strong>for</strong> field n in cell i<br />

(6-75)<br />

Notice that each particle field in each cell is governed by its own trapping rate. There<strong>for</strong>e, these<br />

equations are solved many times throughout a calculation <strong>for</strong> each pticle field, n, and <strong>for</strong> each cell,<br />

i. The model keeps track <strong>of</strong> the mass <strong>of</strong> trapped debris by species.<br />

Rev O 6-33 6/30/97

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