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DIkFU5ION DIFFUSION COLCFICILNl COEFFlClENl- OF XP Xe IN RAPI-IITE- ft2/hr
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0 i 2 3 4 5 F
MASS TRANSFEH COEFFICIENT TO ClRCUCAllNG OUBBLE (ft/hrl
Fig. 7.2. Effect of Bubble Mass Transfer Coefficient on 135Xe Poison Fraction.
The top line is )r no xenon removal through circulating
bubbles, and the poison fraction approaches
that for a solid-fueled reactor. The other lines are
for various circulating bubble parameters as indicated.
From this figure it can be seen that the
poison fraction is not a strong function of the diffusion
coefficient over the range from to
This is because the mass transfer coefficient from
salt to graphite is the controlling resistance for
migration of 13’Xe into the graphite. Since lJsXe
in the graphite is the greatest contributor to the
total poison fraction, the parameters that control
its migration will, in turn, control the poison fraction.
For permeabilities -: the resistance of
the graphite starts becoming significar. .. The
mass transfer coefficients to graphite were com-
puted from the Dittus-Boelter equation as modified
by the heat-mass transfer analogy.
Figure 7.2 shows the effect of the mass transfer
coefficient to the bubble on the poison fraction.
This mass transfer coefficient is one of the least
well known and most significant of the parameters
involved. Available information indicates its
extreme values to be 0.7 and approximately 6
ft/hr. Values of 0.7 to 0.8 ft/hr were estimated
assuming that the bubbles behave as solid spheres
and have a fluid dynamic boundary layer. Values
near 3.5 ft/lir were estimated assuming that as the