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(e.g., in metallic colloidal aggregiites) but deposit
individually by separate mechanisms.
In all but one of these tests, the exposure times
varied from 1 to 6 min. The amounts of noble
metals deposited during these intervals showed no
increase with exposure time (if anythj.ng a decrease)
and were not significantly different from
the results of previous 10-min exposures. Even in
the case of 132Te, which deposited heavily in the
8-hr exposure, no significant differences were observed
for exposure times between 1 and 10 min.
This and related information will be discussed in
more detail later.
The deposition behaviors of 40-day Io3Ru and
1.0-year lo6Ru were fairly closely parallel. The
incomplete data for 33-day 'Te also followed
well the variations among runs of the 77-hr '32Te
deposition data. At production-decay equilibrium,
the number of disintegrations per ininute of each
fission product should be proportional to its fission
yield. For the tellurium isotopes the ratio
of activities deposited was about 20, while the
ratio of fission yields is 13.4. The ratio of activities
of the two isotopes in the fuel. was also
about 20. The discrepancy between 20 and 13.4
may be partly due to the fact that the '*'Te may
not have reached equilibrium activity. In the case
of the ruthenium isotopes the much larger discrepancy
between the ratio of activities and the ratio
of fission yields is very probably due to the fact
that the one-year '061iu had not nearly reached
equilibrium activity.
Experiment FPll-51 was cairied out with the
pump bow1 level as low as possible, with the ohjective
of introducing more than the usual amount
of circulating helium bubbles into the fuel. Normal
deposition of noble metals was observed. Therefore,
the fraction of circulating helium bubbles has
nu effect on fission product deposition.
Experiment FPll-58 was carried out after 92.3
days of virtually uninterrupted power operation,
2.3 hr after shutdown, and 2 hr after stopping the
fuel pump. Under these conditions, the deposition
of noble metals decreased by a factor of 2 to 9,
most sharply for tellurium and least for ruthenium.
The concentrations in the salt (see Table 9.2) re-
mained nearly constant except for ruthenium, for
which the data show a fivefold increase.
The rather moderate decreases in noble-metal
deposition with the reactor shut down for 2.3 hr
and particularly with the fuel pump off are dif-
ficult to explain by the previously mentioned
130
metal colloid theory of deposition. Any suspended
species in the pump bowl cover gas should have
been swept out by the I-liter/min helium flow
through the pump bowl gas space. The 3-in.-diam
by 6-in. volume inside the mist shield was addi-
tionally swept by the 200-cc/min purge flow
prior to and during the exposure. The result is
one of the most conclusive indications that truly
gaseous species originating from the fuel surface
are responsible for the observed concentrations of
noble-metal fission products in the pump bowl gas
space.
Experiment FP12-6 was carried out after a re-
actor drain and refill after 42S days of reactor
shutdown and prior to resumption of power op-
eration. Only slightly lowei than normal depo-
sition of noble metals aYas observed. After long
shutdowns in the past, sharp decreases in depo-
sition occurred. Also contrary to past experience,
the concentrations of noble metals in the fuel
either remained constant (ruthenium) or increased
(tellurium and niobium).
An additional experiment (FP14-1, not shown in
the table) was carried out after the MSRE had
been shut down for 38.1 days, then at full power
for 2.5 days? then shut down and drained for 2.0
days. The fuel pump continued to circulate helium
in the drained reactor with the temperature of the
pump bowl top at 700°F. Specimens were exposed
for 11 min. The only specimens available for
analysis were the gas-exposed stainless steel
cable and the key.
The analyses are not complete, but the avail-
able results are remarkable. Compared with the
previous "normal" run (FP12-27), the activity
(calculated back to the end of the 2.5-day op-
eration) deposited on the cable was slightly
higher for "Mo, lo6Ru, and "Nb, distinctly
higher for I3'I and 235U, slightly lower for
'32Te and Io3Ru, and hstinctly lower for
"Zr. Most of the activities on the key dropped
by a factor of about 10 from the previous run.
These high depositions are all the more re-
markable when it is considered that there was
a two-day shutdown before sampling and that
even the short-lived fission products could not
have reached equilibrium activities in the 2.5-
day period of power operation following the 38.1-
day shutdown.
It is not clear whether the observed deposits
originated from material previously deposited on
the graphite or metal walls of the reactor system