ORNL-4191 - the Molten Salt Energy Technologies Web Site

As in previous analyses of the reactor fuel, it
is clear that a very high fraction of isotopes (such
as *40Ba, 89Sr, 141Ce, 14'Ce, and "Sr) that form
very stable and soluble fluorides is present in the
circulating fuel. As judged by the behavior of
'I, the isotopes of iodine also seem to be present
in a high fraction.
On the other hand the more noble elements, such
as Mo, Nb, Te, and Ru, are generally present in
much lower fractions. It is worthy of note that
these materials seem to show a much wider scatter
than do the "salt seekers"; this may indicate that
they are not present in true solution. It must also
be noted that these noble metals are present in
considerable quantity in the gas stream and that
they adhere tenaciously to metal spe-' Limens exposed
to them. It is possible, therefore, that a
considerable fraction of the material found in the
"salt" is due to the fact that the sampler is lowered
and then raised through this gas phase. It is
possible that the relatively high values obtained
for g9Mo, lo9Ru, and 132Te in FPll-45, where
no gas was used, may be due to this phenomenon.
This possibility will be verified, if possible, experimentally
in the near future.
9.3 EXAMINATION OF MSRE SURVEILLANCE
SPECIMENS AFTER 24,000 Mwhr
A second set of long-term surveillance speci-
mens of MSRE graphite and Hastelloy N was ex-
amined after exposure to circulating molten-salt
fuel in an axial core position of the MSRE for
24,000 Mwhr of power operation. During the last
92 days of this exposure, the MSRE operated at a
steady power of 7.2 Mw essentially without in-
terruption, whereas power operation was frequently
interrupted during the previous 7800-Mwhr expo-
sure. Examinations of the specimens gave results
very much like those previously reported after a
7800-Mwhr exposure.' -'
Examination of Graphite
As before, three rectangular graphite bars were
available for examination: a 9.5 x 0.47 x 0.66 in.
bar from the middle of the core, and 4.5 x 0.47 x
0.66 in. bars from the bottom (inlet) and top (out-
let) of the cote. Visually, the graphite appeared
undamaged except for occasional bruises incurred
during the dismantling. The two 4.5-in.-long bars
121
gained 0.002 k 0.002 in. in length and 13 mg (0.03%)
in weight. Metallographic examination showed no
radiation or chemical alteration of the graphite
structure and no evidence of surface films.
X radiography of thin transverse slices showed
occasional salt penetration into previously exist-
ing cracks that happened to extend to the surface
of the sample. Similar penetration was observed
in control samples that were exposed to molten
salt but were not irradiated. X-ray diffraction by
th.e graphite surface exposed to fuel gave the
normal graphite lines except for a very slightly
expanded lattice spacing. A few weak foreign
lines that were probably due to fuel salt were
observed. Autoradiography of the samples showed
a high concentration of activity within 10 mils of
the surface, with diffuse irregular penetration to
the center of the cross section. These observa-
tions confirmed previous demonstration of the
satisfactory compatibility of graphite with fis-
sioning molten salt as far as damage and per-
meation by fuel are concerned.
concentration profiles for fission products in
the graphite were determined by milling off layers,
which were dissolved and analyzed radiochemically.
Near the surface, layers as thin as 1 mil were ob-
tained; farther in, layers as thick as 10 mils were
collected until a total depth of about 50 mils was
reached. The predominant activities found were
molybdenum, tellurium, ruthenium, and niobium.
These elements can be classed as noble ele-
ments since their fluorides (which are generally
volatile) are relatively imstable. The practical
concern, because of long-term neutron economy,
is with g5Mo, 97Mo, ggTc, and "'Ru, but these
particular isotopes are either stable or long-lived
and thus were not amenable to direct analysis.
However, we considered as sound the assumption
that their deposition behavior was indicated by
either that of a radioactive isotope of the same
element or that of a radioactive noble-metal pre-
cursor with a suitable half-life.
Over 99% of the noble-metal activities were en-
countered within the first 2 or 3 mils of the
graphite surface; the same was true for 95Zr. This
is illustrated in Figs. 9.1-9.1, The values for
blanks shown in the figures were obtained from
samples of about 0.2-g (1 mil) size that were
milled from fresh unexposed graphite in the hot
cell and with the equipment used for the irradiated
specimens. The blanks show that the apparent
tails on the curves for "Mo, "Nb, and '0611zu are