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tivities of 99M0 and 132Te deposited in a long
exposure are a measure of deposition rate over
only the last week or so of exposure. If we
assume a constant deposition rate, the amount
deposited in time t is given by the familiar
equation:
k
p/ :=-(I - e-
At
) ,
A
wheic k is the constant deposition rate and A
is the decay constant. Using this equation a
theoretical ratio can be calculated for each iso-
tope for exposure times of 3340 and 8 hr:
These ratios turn out to be 12.6 for "Mo, 14.5
for '"Te, 157 for lo3Ru, 365 for lo6Ru, 142 for
"Nb, and 218 for "Zr. Whether fortuitous or
not, these calculated ratios agree with the ob-
served ratios for deposition on I-Iastelloy N
very well €or the ruthenium isotopes and within
a factor of 2 for molybdenum, tellurium, and
niobium. This agreement suggests that the
deposition of noble metals on Hastelloy N does
indeed proceed at approximately the same con-
stant rate for a 3340-hr exposure as for an 8-hr
exposure.
By contrast, the calculated ratios are much
higher (usually by tenfold or more) than the ob-
served ratios for deposition 011 graphite except
for "Nb arid "Zr, whose calculated ratios are
high by factors of only 1.5 and 4 respectively.
This indicates that the deposition rate of noble
metals (except 35Nb) on graphite decreases with
exposure time. Neutron economy in MSRE would
benefit if the deposition rate of noble-metal fis-
sion products on graphite decreases with espo-
sure time while that on Hastelloy N remains
constant. As the data in Table 9.10 indicate,
the 8-hr deposition rates :ire not greatly different
135
on graphite and metal, and the activities of
"hI0, I3*Te, Io3Ru, and lo6Ru were similar
€or the 7800- and 24,000-Mwhr exposures. This
suggests no change of rate between 7500 and
24,000 Mwhr for '%lo, '32Te, and Io3Ru and a
decreased rate for the long-lived 6Kri.
The discussion above gives a generally satisfactory
picture of noble-metal deposition for exposures
of 8 hr or longer. The picture is greatly
complicated by the inclusion of tesults from the
short (1 to 10 min) exposures of the stainless
steel. cable specimens in the pump bowl fuel (see
Table 9.8). For most isotopes, the depositions
were only slightly greater (a factor less than S)
for the 8-hr exposure than for 1- to 10-min exposures.
'Tellurium-132 and ""Zr deposits in 8
hr were about 100 times larger than for the short
exposures.
The calculated ratios from the formula given
above would be vecy close to the t,ime rat-ios for
these short exposures. Comparing the 8-hr run
with a 1-min run, the time ratio is 480. The
short-exposute data thus indicate that (except
for 13'Te arid 95%r) the original deposition rate
is very fast compared with the rate after 8 hr.
It may he that when fresh samples of metal are
exposed to fuel salt two deposition mechanisms
come into play. The first process rapidly deposits
noble metals on the stainless steel in
less than a minute and then stops (or reaches
equilibrium). The second (slow) process then
continues to deposit noble metals at a fairly
constant rate for exposure times up to 3340 hr.
It may be speculated that the first process is a
rapid pickup of colloidal noble-metal particles
from the fuel on the metal surface; the second
may be a slow plating of noble metals in higher
valence states present at very low concentrations
in the fuel.
It will be interesting to expose graphite
samples in the fuel for short times to see
whether a short-term mechanism also exists for
deposition on graphite.