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mechanism is available which satisfactorily
accounts for the observed behavior. A possible
cause, which is partially supported by experi-
mental data, is described as follows.
On the two occasions when the most rapid rates
of dissolution of the beryllium rods were observed,
chromium values for the next several fuel samples,
FP11-10 et seq., and FP12-16 et seq., rose tempo-
rarily above the lo level and subsequently re-
turned to normal. That the increase in chromium
levels in samples FP12-16 to -19 was temporary
indicates that the high chromium concentration
amples removed from the pump bowl was
atypical of the salt in the fuel circuit and implies
that surface-active solids were in suspension at
the salt-gas interfaces in the pump bowl.
That atypical distribution of species in this
location does indeed take place was demonstrated
earlier by the analysis of sample capsule support
wires that were (1) submerged below the pump-
bowl salt surface, (2) exposed to th
interface, and (3) exposed to the pump-bowl cover
gas. The results showed that the noble-metal
fission products, Mo, Nb, and Ru, were deposited
in abnormally high concentrations at the salt-
interface. Such behavior suggests that
chromium concentrations in the fuel spe
were caused by the occurrence of chromium in the
Fig. 8.3.
(b) FP12-57.
F
F
112
pump bowl in nonwetting, surface-active phases
in which its activity was low. A possible mecha-
nism which would cause such a phenomenon is
the reduction of Cr2+ by Beo with the concurrent
reaction of Cro with graphite present on the salt
surface to form one or more of the chromium car-
bides, for example, Cr3C, (
298'K). Such phases possess relatively low
stability and could be expected to decompose,
once dispersed in the fuel-circuit salt.
The possibility that surface-active solids were
formed as a consequence of the Beo additions was
tested late in run 12 by obtaining salt specimens
at the salt-gas interface as we11 as below the sur-
face. First, specimens were obtained in a three-
compartment sample capsule that was immersed
so that the center hole was expected to be at the
interface. Next, a beryllium metal rod was ex-
posed to the fuel salt for 8 hr with the result that
9.71 g of beryllium metal was introduced into the
fuel salt. Twelve hours later a second three-
compartment capsule was immersed in the pump
bowl. Chemical analyses of the fuel salt speci-
mens FP12-55 and -57 (Table 8.3) do not show
significant differences in chromium; however, the
salt-gas interface in FP12-57 is blackened as
as compared with FP12-55 (see Fig. 8.3).
R-39267 R-39266
Surface Appearances of Fuel Salt Specimens Taken Before and After Beryllium Addition. (a) FP12-55,