ORNL-4191 - the Molten Salt Energy Technologies Web Site

Carbon Corporation, Poco Graphite, Inc., Stackpole
Carbon Company, and Speer Carbon Company.
The first grades of graphite that have been re-
ceived or will be received in larger quantities are
listed in Table 17.2. Grade 1425-64 is being used
in irradiation studies and graphite-to-metal joint
investigations. Grades 13-337 and BY12 will be
used in graphite-to-metal and graphite-lo-graphite
joint studies and in an engineering test loop. This
loop will be used to determine the operating char-
acteristics of the proposed MSHR fuel cell, with
special emphasis on gas permeability studies to
aid in evaluation of the fission-gas behavior in the
MS BR .
17.2 GRAPHITE SURFACE SEALING
WITH METALS
W. C. Robinson, Jr.
Chemical vapor deposition is one of the methods
being investigated to decrease the gas permeability
of the graphite. The two metals presently being
considered for a sealant are molybdenum and ni-
obium. The initial objective will be to obtain a
helium permeability of 10W7 cm2/sec or less with
a minimum thickness of metal deposit. The depo-
sition parameters will be varied in order to deter-
mine the conditions which produce an optimum
coat in g.
Run Number
MoF
211
Table 17.3. Molybdenum Coatings on R-DO25 Graphite
Gas Flow Rates
The basic technique involves the deposition of
metal on a heated substrate by hydrogen reduction
of the metal halide. In this particular case a
halide-hydrogen gas mixture is passed over
graphite that is contained in a sealed furnace
chamber. The metal halides being used are MoF',
and NbCl,.
The initial studies are being carried out on a
neatly isotropic grade of graphite, designated
K-0025, which has a helium permeability of ap-
proximately lo-' cm2/sec and an accessible pore
spectrum with maxima at 0.7 and 4 /p. Molybdenum
is deposited via the reaction
MoF, -t 3I3, 3 MO t fjHF .
Nine runs have been completed using the experimental
parameters given in Table 17.3.
An assembly was built for estimating the helium
permeability of the coated samples. This assembly,
which attaches to a Veeco leak detector,
is shown in Fig. 17.2. Two coated and one uncoated
graphite sample are shown. Qualitative
measurements of the helium permeability have been
performed to demonstrate the utility of the assembly.
The leak detector will be calibrated with
know? leak sources to make quatitit.ative measurements
possible. The present qualitative evidence
indicates that the helium permeability OE these
samples can be made less than with 0.05 mil
or less of molybdenum.
Temperature Pressure
(c m /m in) . . . . . .- PO (torrs)
H2
M-Mo-1 50 800 700 5
M-Mn-2 50 8 00 700 5
bf-Mo-3 50 800 700 10
M-Mo-4 50 800 700 5
M-Mo-5 50 8 00 700 10
M-M+6 50 8 00 8 00 5
M-Mo-7 50 800 800 5
M-Mo-5 so 800 800 10
M-Mo-9 50 800 800 10
Time
(min)
5
10
5
5
10
5
10
5
10