ORNL-4191 - the Molten Salt Energy Technologies Web Site

L-
a
w
r
94
ORNL-OWG 67-(1815
io‘ 1 10 100 io00 10.000
TIME AFTER REAClOR SHUTDOWN (mln)
Fig. 7.3. Afterheat in MSBR [556 Mw (thermal)] Graphite from Noble Gases and Their Daughters After 10 Years
of Power Operation.
bubble rises, its interface is continually being
replaced by fresh fluid (penetration theory). Both
of these cases are for a bubble rising at its terminal
velocity in a stagnant fluid. There is very little
information in the literature concerning the effect
of fluid turbulence on the bubble mass transfer
coefficient. Nevertheless, from turbulence theory
it has been estimated that mass transfer coef-
ficients as high as 6 ft/hr could be realized under
MSRK conditions. The analyses that lead to this
number are generally optimistic in their assump-
tions.
The target I3’Xe poison fraction for the MSBR
is 0.5%. From Fig. 7.2 it can be seen that this
goal will be easy to attain if the mass transfer
coefficient is over 4.0 ft/hr. It is still attainable
if the mass transfer coefficient is between 2.0
and 4.0, but with more difficulty. From this figure
it is apparent that a small amount of recirculating
bubbles is as effective as a large amount of once-
through bubbles. One reason is that the contact
time for recirculating bubbles is about four times
that for the once-through bubbles.
Another variable that will strongly affect tbe
poison fraction is the graphite surface area in the
core. Calculations indicate that if the graphite
surface area is doubled, all other parameters re-
maining constant, the poison fraction will increase
by 50 to 70%.
This model has also been used to compute the
noble gas contribution to afterheat of the unclad
graphite. Xenon and krypton are involved in over
30 fission product decay chains. The model was
used to compute the flux of each xenon and krypton
isotope into the graphite, assuming that this flux
is constant for the entire time the reactor is at
power. From this we computed the concentration
of each noble gas and all its daughters in the
graphite as a function of time that the reactor is
maintained at power, Results of calculations for
the reactor after ten years at full power are shown
in Fig. 7.3. The reactor parameters are the same
as used in the 13’Xe poisoning calculations. Two
curves are shown in the figure. Rather than listing
all the circulating bubble parameters involved
(e.g., void fraction, mass transfer coefficient, etc.),
it is sufficient to list the equivalent 13’Xe poison
fraction. The afterheat is proportional to this
value,
Work is under way in two areas. First, we are
considering ways to introduce circulating bubbles
of uniform size and about 0.020 in. in diameter.
A small model of a mechanical bubhle generator
that operates somewhat like a mixer has been built
for testing with air and water. No quantitative
results are yet available. Second, a closer look
is being taken at the bubble mass transfer coefficients.
An experiment is being consideied that
will yield a measured value to this parameter.