ORNL-4191 - the Molten Salt Energy Technologies Web Site

The losses indicated by these samples are ap-
preciable. The mean of four samples shows nearly
30 pg/sample or about 3.5 pg per cubic centimeter
of helium. This would correspond to somewhat
less than 20 g/day of 'U or nearly 60 g/day of
the uranium in the fuel. (The figure remains at
nearly one-half this level if the highest figure is
rejected.) It seems absolutely certain that the
reactivity balances on the reactor through 250
days of full-power operation would have speedily
disclosed losses of far smaller magnitude than
these. It is difficult to conceive of a mechanism
other than volatile UF6 to account for losses of
uranium to the vapor. It is, however, extremely
difficult to see how losses of this sort could be
reconciled with the nuclear behavior (and the
chemical analyses) of MSRE. Further samples
of the gas system will be made to help resolve
this apparent dilemma.
The effect of short shutdown and pump stoppage
(FP11-42) on noble-metal volatilization was in-
significant. This suggests that the volatilization
process does not depend on fuel nor bubble cir-
culation nor on the fissioning process itself, but
is probably a local phenomenon taking place at the
fuel-salt-gas interface. Beryllium additions to the
fuel had no discernible effect on the volatilization
of noble-metal fission products. This fact is rein-
forced in some detail (see subsequent sections of
this chapter) by data obtained by insertion of
getter wires into the pump bowl vapor space.
The effect of reactor drain and long shutdown
(FP12-7) was consistently to lower observed
noble-metal volatilization (observed activities
were, of course, calculated back to the time of
previous shutdown). This observation is con-
sistent with all previous data on the effect of
long shutdowns. Presumably an appreciable
fraction of the noble metals is deposited on the
walls of the drain tank and reactor system during
a long shutdown. The minor effect of short shut-
down and the incomplete deposition during long
shutdown seem to indicate a slow deposition
process.
The pressure release experiment (FPll-53) re-
sulted in an appreciable increase of volatilization
of most fission products but not of "Mo or 132Te.
Niobium-95 showed the most distinct increase. It
is particularly puzzling that the '' 'Te concentra-
tion increased markedly while the ' 32 Te concen-
tration fell slightly. Clearly, further experiments
119
of this kind would be required to permit definite
interpretations.
It cannot be claimed that the mechanism for
volatilization of many of thcse materials is known.
As has been shown elsewhere' f2 the volatile flu-
orides of many of these materials (notably Mo, Ru,
Te, and, probably, Nb) are too unstable to exist
as equilibrium components of a fuel system con-
taining appreciable quantities of UF3. It is true
that, for example, "Mo formed at about 5 x 10"
atoms/day probably exceeds greatly the solubility
limit for this material in the salt. Since it is born
in the salt in an atomic dispersion, it may, in ef-
fect, behave as a supersaturated solution; that is,
the activity of molybdenum may, in fact, be mark-
edly greater than unity. It seems most unlikely
that it can have activities greater than 10- lo,
which would be required if MoF, were to be stable.
Such Eindings as the presence of l1 Ag, for which
it is difficult to imagine volatile fluorides, seem
to render the volatile fluoride mechanism more un-
likely. It still seems more likely that an ex-
planation based upon the finely divided metallic
state will prove iruc?, though such explanations
also have their drawbacks.
9.2 FISSiON PRODUCTS IN MSRE FUEL
The program of sampling and analysis of samples
of MSRE fuel for fission product species has been
continued by use of apparatus and techniques de-
scribed previously. '-' Table 9.2 shows data for
16 fission product isotopes and for 23 'Np obtained
from a series of four samples from a long stable
run of MSRE, from a sample obtained shortly after
shutdown in this run, a sample after the salt had
been cooled for 42.5 days, and a sample from a
relatively early stage in the subsequent power run.
Captions in this table show the operating history
of MSRE and the special conditions prevailing in
the reactor at the time the sample was taken.
Table 9.2 also includes estimates of the fraction
of the isotope represented by these analyses in
the 4850 kg of circulating fuel.
'5. S. Kirslis and F. F. Blankerrship, Reactor Cham.
Div. Ann. Progr. Rept. Dec. 31, 1966, ORNL-4076, p.
48.
3S. S. Kiislis arid F. F. Blankenship, MSR Program
Serninnn. Progr. Rept. Feh. 28, .1967, ORNL-4119, p.
124.