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Table 15.5. Salt Constituents in Graphite at Given Depths
Method of Determination
Mass Spectrograph (MS-7)
Li
(mole 70)
Be
(mole %)
Zr
(mole 70)
-
U IJ
(mole 70) @g/mg)
Activation, U
(pLg/md
Loop inventory: 27.8 65.3 4.8 2.0
Graphite - top section next-to-front channel
0.2 to 0.4 mil 22 71 5.3 1.8 39 49
0.4 to 2.0.5 mils 21 7 3 3,s 2.2 2.6 3.6
2.0.5 to 4.4 mils 15 80 3.5 1.7 2.2 0.91
4.3 to 9.8 mils 16 80 4.5 a (< 0.34) 0.32
aNot determined.
The MS-7 spark mass spectrograph was used to
examine solutions of graphite shaved from a
typical fuel tube for salt constituents. Total
amounts used were at the microgram level. Kesuits
are shown in TabIe 15.5.
The salt ingredients Li, Re, Zr, and U are in
the proper molar ratio (with the proper total of
71Ai and 'Be, bu& some drift in their ratio). Furthennore,
the absolute quantity of uranium agrees
reasonably with the values from the activation
analysis by delayed neutron counting.
Consequently, the uranium entering the graphite
did so in the forin of salt. The difficult question
is whether it permeated the pores or adhered to
the surface and filled the cracks.
Three-phase contact of graphite, molten LiF-
HeF, or MSRE fuel salt, and slightly moist gas at
elevated temperatures has been shown by Kreyger,
Kirslis, and BIankenship6 to result in the forma-
tion of adherent oxide films on the graphite, pre-
sumably BeO, with wetting of this oxide (and
thereby the graphite) by molten salt. But similar
adherence was repotted not to occur under the
salt where the graphite and molten salt were in
contact before the moisture entered. Our oxygen
analyses of the fuel, reported above, do not indi-
cate any great uptake of moisture, and such as '
did enter appears to have been involved in cor-
'E'. J. Kreyger, S. S. Kjtslis, and F. F. Blankenship,
MSR Program Semiann. Progr. Rcpl. Jan. 31, 1964,
ORNL-3626, pp. 132-47.
rosion processes and is not necessarily to be
associated with adherence of salt to graphite.
-_I_
Prior to admission of any salt to the loop, the
graphite was heated at 600°C for 20 hr under
vacuum, whereas evacuation for several minutes
at 400'jC has been shown to be sufficient to
remove normal moisture from graphite. Thus,
internal moisture in the graphite is not believed
to account for adherence of the salt to graphite.
Examinations of autoradiographs indicate that
several fuel channels had one OK two cracks ex-
tending from them and that appreciable radio-
activity was located along such craclts. Uranium-
235 was detected by alpha radiography in cracks
as well as on the hole surface. Althoggh some or
all might be a polishing artifact (or from hot-cell
contamination), this does not dispute, and possi-
bly supports, the idea that salt entered cracks.
Thus, we think !.hat the presence of the uranium
in the graphite can be associated with the entry
of salt into cracks and the substantial adherence
of thin films of salt to irregularit.ies in graphite
sur faces.
The adherence to graphite by the salt, which
we will not further expound here, is believed to
have resulted in only coverage of surface (in-
cluding cracks), with no penetration of graphite
pores being indicated. Recognition of this un--
expected entry of uranium-containing salt into
cracks did assist in understanding the behavior
of certain fission products, as discussed below.