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or whether it came &om the 40 lb of fuel salt
which remains in the reactor system after a fuel
drain. Some of this fuel salt is in the form of
shallow puddles in the internals of the fuel pump
where contact with the circulating helium should
he good. Another possibly pertinent fact is that
gas circulation through the reactor and pump bowl
with the pump operating in the drained reactor is
much faster than when the reactor is filled with
fuel. Thus the pump bowl specimens may contact
a larger volume of gas per unit time. 'The
fuel may also act as a scrubbing fluid to remove
gaseous or suspended activities from the pump
bowl gas, eveti while it is the source of these
activities at a different time arid place. Sorbed
gaseous activities or loosely deposited solids
from the reactor surfaces may conceivably move
to and stay in the .@s phase more readily in the
absence of fuel salt.
A few analytical results are available from test
FP14-2, which was run one day after test FP14-1
and after the reactor had been refilled with fuel
but before power operation was resumed. For the
gasphase stainless steel cable specimen, the
deposition of "Mo, 13'Te, and I3'I was lower
than for FP14-1 by an order of magnitude. The
ruthenium and niobium activities increased
slightly, and that of 95%r decreased slightly.
The amount of 235LJ deposition increased by a
factor of 4. The decreases in activity are difficult
to explain. The curious fact appears to
stand that deposition of noble metals in the
drained reactor is the same within an order of
magnitude as with fuel in th- L reactor.
Occasional radiochemical analyses were car-
ried out on the leaches of the metal specimens
for two ii dd i t iona I noble- met a 1 f is sion produ c t s,
' 'Ag and 'Pd. These behaved like the other
noble metals, with heavy deposition on the gas-
phase and submerged specimens. This behavior
is of chemical interest since no highly volatile
fluorides of silver and palladium are known to
exist. This observation favors the gaseous
metal suspension theory of noble-metal volatili-
zation.
Occasional analyses were also made for other
alkaline-earth and rare-earth species such as
"Sr, '41~e,
143Ce, '44Ce, and 147Nd. These
species remained in the fuel melt and showed
light deposition on the pump bowl specimens.
Most of the pump bowl specimens were analyzed
for 23 'W deposition by delayed-neutron counting
131
of their leaches. Unusually high values were obtairied
for the FPll-50 (8-hr exposure) specimens,
averaging 70 pg of 235U on each specimen. Another
set of high values averaging 35 pg of 235U
per specimen was obtained on run FP12-6 (42.5day
shutdown). The deposition of 95Nb was also
unusually high for this run. 'The values for tun
FP11-45 (normal) were also high, averaging about
30 1J.g of 235U per specimen. The results for
235U did not parallel those for any other fission
product, and the reasons for the wide variations
between runs are not known.
9.5 DEPOSITION OF FISSION PRODUCTS
ON GRAPMITES IN MSRE PUMP BOWL
Graphite specimens have been exposed in the
MSRE pump bowl to observe fission product depo-
sition under a known set of short-term conditions.
Such an exposure has been carried out for 8 hr
during steady 7.2-Mw operation of MSRE:. Since
this exposure, with the sampler-enricher tube
opened to its separate containment for 8 hr, posed
some problems in reactor operation, ihe test was
designed to yield a considerable amount of infor-
mation. Three graphite samples, two of CXt3 grade
and one of pyrolytic graphite, were exposed in the
gas phase, while similar specimens were exposed
to the liquid. A comparison sample of I-Iastelloy N
was exposed in each phase. Assemblies used in
this experiment consisted of a pair of the holders
shown in Fig. 9.9. These holders, for safety
reasons, enclosed the graphite and ftastelloy
specimens in perforated metal cylinders. 'Ihey
were connected by a loop of stainless steel
cable of a length such that the bottom holder was
completely immersed in the molten fuel while the
top holder was exposed only to the gas phase.
This assembly was exposed in the punip bowl for
8 hr with 200 cc/min of helium purge except for
the last 10 min. After the exposure, tiny droplets
(-*0.2 mm in diameter) of greenish white fuel salt
were seen adhering loosely to the gas-phase
graphite specimens. The mechanical disturbance
involved in removing the specimens from the holder
was sufficient to dislodge the droplets from the
graphite surfaces. Nevertheless, two of the
graphite specimens were wiped with small squares
of cloth, which were analyzed radiochemically.
The remaining graphite and metal specimens were
then leached in a neutral mixture of sodium ver-
senate, boric acid, and citric acid, which dis-