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on final salt samples. 'This value was used in
calculating total activities. of the various iso-
topes produced in the experiment.
15.12 ISOTOPE ACTIVITY BALANCE
The calculations above provide an estimate of
the amount of isotope to be accounted for. We can
estimate how much was actually found in a kind of
"isotope activity balance" by accounting for all
regions of the loop in terms of the measured
activity of samples.
For the various samples of metal, graphite, and
salt obtained from the loop, activity determinations
for the 15 isotopes shown in 'Table 15.4 were
requested. The concentration of ' 35U was also
determined. Because of the short half-life of such
isotopes as "Mo and 'Te, these were run as
promptly as possible, and others were deteimined
later. Results for some isotopes are not yet
complete. The available data will be examined
in temis of activity balances for the respective
isotopes and the penetration profiles of these
isotopes in graphite.
The ratio of the area of each loop (or fuel
channel) region to the sample representing the
region was determined so that the total loop area
was accounted for in terms of samples. The
cumulative activity of all shavings from a graphite
channel section was used as the sample from that
channel. The sample activities multiplied by the
proper ratios have been totaled for each isotope
under the categories of graphite, loop metal, salt
sample lines, gas lines, and salt. These values,
plus values for salt based on the final sample
activities, are shown in Table 15.4. Estimated
total activities from the calculations based on
irradiation and inventory history are also shown.
From Table 15.4 it may be seen that over half
(and generally less than all) the expected activity
appeared to be accounted for in the cases of
99Mo,7 132're, 95Nb, 95Zr, *'SI, 137Cs, 141Ce,
'44Ce, and 147Nd. A substantial propohion,
although less than half, was accounted for in the
cases of 14'Ba and I3'I. Inasmuch as iodine
readily volatilizes from all samples, without
doubt especially from the powdered graphite, it
is to be expected that iodine determinations shall
be low. Determinations are not yet complete in
the cases of lo3Ru, '06Ru, ''''re, "Y, and
137cs.
190
Molybdenum, tellurium, and ruthenium are almost
entirely departed from the salt, along with sub-
stantial proportions of 89Sr, 95Nb, 14'Ba, and
most probably 13'I. Except for 89Sr and possibly
40Ba, which favor graphite, these elements show
no strong preference for graphite or metal but
seem to deposit on whatever surface is available.
The alkali-metal and rare-earth isotopes, includ-
ing "Y, 137Cs, 141Ce, 144Ce, and 147Nd, and
also "Zr, remain aImost completely in the salt,
the amounts found in graphite generally being
ascribed to salt contained in the samples, as
discussed later
15.13 URANIUM-235 0
GRAPHITE SAMPLES
Uranium-235 on the various graphite and metal
samples was also determined. An activation
technique was used in which delayed neutrons
were counted; it is sensitive to less than 1 pg
of 235U. The determinations served the dual
purpose of rneasuting small quantities of salt
which could have adhered to suiface samples and
of determining the penetration of uranium in one
form or another into the graphite.
In seven of the eight graphite channels from
which samples were taken, the quantity of 2351J
ranged from 1.6 to 2.8 mg (per 8.3 to 9.5 cm '> with
over half being found within the first mil and over
80% generally within the first 3 mils. However,
some uranium was detected even in the 35- to
45-mil cuts. The first sample from the remaining
channel weighed 290 mg and contained 18.9 mg of
235U, equivalent to 190 mg of fuel salt. 'The
samples from deeper cuts contained uranium at
levels only moderately higher than those from
other channels. Thus, it appears that this sample
probably contained a small piece of fuel salt
which had remained on the surface.
X-ray diffraction patterns obtained from the
surface of a specimen of graphite cut from an
exit fuel channel surface showed patterns of
Li 'Bel?, and Li $rF6, with no indication of
oxides or uranium compounds. Such a pattern is
characteristic of normally frozen fuel salt, so
these observations indicated that fuel salt had
adhered to the graphite surface even though not
directly visible under ordinary hot-cell viewing
conditions.