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depth profile of these isotopes (plotted semilogarithmically)
parallels that of the uranium in
the graphite. Fission product zirconium, of
course, had to remain mostly in the salt because
it necessarily mingled with the 5 mole 76 of Zr in
the salt itself.
The next group of elements (I, Ba, Sr, and Nb)
shows an appreciably greater activity in the
samples than can be accounted for by the amount
of uranium present. These elements (except
possibly 'Nb) also show concentration-depth
profiles that do not fall off nearly as rapidly as
does the uranium concentration. Such behavior
has been described by Kedl' and others (based on
MSRE surveillance specimen data obtained by
Kirslis) as being due to gaseous transport of the
isotope or a short-lived precursor. Niobium-95
could exist as a volatile pentafluoride, 89Sr has a
3-min krypton precursor, and "OB, has a 16-sec
xenon precursor. Iodine-131 is included in this
group, since iodine volatilizes readily from samples,
and the true values should doubtless be
higher than those reported.
The comparatively flat profiles (except for the
first mil or two) imply rapid transport through pores
in the graphite without very strong tendency for
adherence to graphite surfaces. Niobium-95, the
profile of which is not as flat as the others,
appeared to be adsorbed appreciably on the graphite
surface, though not as strongly as elements of
the group to follow.
'The third group contains elements Te, Mo, and
Ru. (The inclusion of mthenium is a guess, at
present, based on MSRE surveillance specimen
studies by Kirslis, since our data are incomplete
for the ruthenium isotopes.) This group is characterized
by the highest concentrations in the samples,
relative to the uranium present. The concentration-depth
profiles are even steeper than
for uranium. It appears evident that these elements
are strongly adsorbed by graphite surfaces
and are able to penetrate cracks readily, either by
rapid diffusion in the salt in the crack or by
surface diffusion. They do not tend to penetrate
bulk graphite pores to a comparable degree. The
highest concentration of 99M0 and 'Te per unit
of surface, either graphite or metal, was reported
for the Hastelloy N thermowell in the front core
'R. J. Kedl, A Model for Computing the Migrations of
Very Short-Lived Noble Gases into MSNE Graphite,
OKNL-TM-~~~~ (July 1967).
193
fuel channel. This implies that the elements in
this group tend to deposit on the first surface en-
countered, whether metal or graphite.
15.15 GAMMA IRRADIATION OF FUEL SALT
IN THE SOLID PHASE
The fuel salt in molten-salt in-pile loop 2 was
kept frozen at temperatures generally above 300'C
for a number of days before the salt was re-
moved from the loop. At temperatures below
100°C, fission product or gamma radiolysis of
fluoride salt is known to result in the generation
of fluorine gas to appreciable pressures. Fluorine
could oxidize various species (U, Wo, Ku, Nb, etc.)
to produce volatile fluorides which could then be
transported to other patts of the system. Al-
though a temperature of 3Q0°C was expected to
be more than adequate to suppress such radiolysis
completely, direct data were not available but
appeared readily obtainable. Since the phenome-
non is also of general interest in the MSR program,
the experiment described below was undertaken.
Solid MSR fuel salt (LiF-BeF ,-ZrF,-UF,,
about 65-28-5-2 mole %) from the stock used to
supply fuel for in-pile loop 2 was subjected to
very-high-intensity gamma irradiation in a spent
HFIR fuel element at a temperature of 320°C to
determine possible radiation effects on the salt
and its compatibilities with graphite and Hastel-
loy N.
For the irradiation experiment, HFIR fuel ele-
ment 9 was placed in a storage rack in the HFIR
pool, and the experiment assembly was placed in
the center of the fuel element as shown in Fig.
15.9. The capsule-type irradiation assembly
consisted of a Ilastelloy N capsule, 0.93 in.
OD x 0.78 in. ID x 3.5 in. long, containing two
CGB graphite test specimens, 3.0 x 0.87 x 0.125
in., placed back to back in the capsule. 'I'wenty-
five grams of fuel salt was added to the capsule,
melted, and then allowed to solidify. A pressure
transducer was connected to the gas space above
the salt, and the capsule assembly was welded
shut. A heater assembly using Nichrome V heater
wire surrounded the capsule, and thermocouples
were located in the capsule wall to monitor tem-
peratures. The experiment assembly was then
placed in an aluminum container to isolate it from
the pool water, as shown in Fig. 15.10.