ORNL-4191 - the Molten Salt Energy Technologies Web Site

The appearance in appreciable concentrations of
several fission product species in the gas space of
the MSRE pump bowl remains the biggest chemical
surprise in MSKE operation. This hchavior does
not, of itself, seem to pose serious problems for the
PASRE or for subsequent large molten-fluoride reactors,
but it is clearly important to establish the
mechanism by which these diverse species, notably
99Mo, 95Nb, lo3Ku, loSRu, I3'Te, and even "'Ag,
get into the gas stream.
It is possible that at least some of the gas-borne
species listed above have volatilized from t.he melt
as fluorides. We have, accorchngly, initiated studies
of the (less well known) lower fluorides of these
rriaterials both as pure compounds and as their solutions
in molten LiF-BeF, preparations. The preliminary
studies performed to date have been devoted
to the fluorides of molybdenum, hut it is
anticipated that, should the results warrant, the
study will be broadened to include other elements
on the list.
C. F. Weaver H. A. Friedman
Molybdenum is a constituent of Hastelloy N, the
MSRE structural metal, as well as an important
high-yield fission product. Consequently, and
especially since molybdenum is one of the elements
appearing in the MSKE exit gas stream, the be-
havior of molybdenum and its fluorides in molten
fluoride fuels in contact with graphite and Hastel-
loy N is of considerable interest. '-'
A search of the literature on the fluorides and
oxyfluorides has been completed and reported.
The fluorides MoF,, MoF4, Mo,F9, MoF,, and
MoF, have been shown to exist, but of these only
MoF, is commercially available. It has been neces--
sary, therefore, to refine methods for preparation,
and to prepare gram quantities, of the lower-
valence compounds €or study.
Preparation of MoF, by the reaction
has been described by Edwards, Peacock, and
Small, wlio have also established the crystal
structure of MoF,. We have successfully employed
this method in which MoF6 is iefluxed over metallic
molybdenum at temperatures in the interval 35 to
100cC in glass apparatus.
Molybdenum pentafluoride is a yellow, hygroscopic
material which melts at about 65OC to a
yellow liquid of high viscosity. Its vapor pressure
at the melting point is about 2 mm,' and it boils at
about 212OC. ' s 6
Disproportionation behavior of MoF appears to
be very complex. We have observed that heating of
the material to 100°C while maintaining a good
vacuum leaves the MoF, unchanged. However, WE
have shown that if MoF, is heated at 20OOC and if
the volatile product is removed by pimping, the
reaction is
3MoF,y 2MoF, t MoF, .
'W. R. Grimes, Chemrca1 Research and Development
for Molrcn Salt Breeder Reactors, ORNL-TM-1853, pp.
61 -81 (June 6, 1967).
'5. S. Kirslis, F. F. Blankenship, and C. F. Baes, Jr.,
Reactor Chem DJV Anti Progr Rept Jan 31 1967,
ORNL-4076, pp. 48--53.
3S. S. Kirslls and F. F. Blankenship, MSR Program
Senizann Progr Rept Feb 28, 1967 ORNL-4119, pp.
124-43.
4C. F. Wcaver dnd 13. A. Friedman, A Literature Sur
vey of the Fluorides and Oxyfluondcs of Molybdenui-n,
ORNL-TM-1976 (October 1967).
'A. J. Edwards, K. 1). Peacock, and K. W. H. Small,
J Chem SOC. 4486-91 (1962).
,D. E. L.aValle et at.. J. Am. Chem. SOC. 82, 2433-34
(1 960).