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2.5. COMPATIBILITY OF MATERIALS AT HIGH TEMPERATURES
PENETRATION OF GRAPHITE
BY MOLTEN FLUORIDES
H. J. Buttram G. F. Schenck'
It appears very likely that graphite is the only
reasonably effective reactor moderator material that
will remain chemically stable upon direct exposure
to molten fluoride fuel mixtures. The use of graph-
ite to contain such fuel mixtures has been known
fora long time to be practical for simple laboratory-
scale experimental equipment. The graphite most
commonly used for this purpose has the commercial
designation C-18. However, little information is
available concerning the rate of penetration of
reactor-grade graphite by molten fluoride fuel mix-
tures, and accordingly an investigation of this phe-
nomenon has been initiated.
Specimens of APC' graphite 1/4 x $ x \ in. were
soaked at 6OOoc for various periods in sealed cap-
sules of lnconel containing molten NaF-ZrF, or
NaF-ZrF,-UF, mixtures under helium atmospheres.
The specimens were then sectioned and examined
under the petrographic microscope. Prel iminary
observations indicate that the APC graphite was
completely penetrated by NaF-ZrF, (53-47 mole ?4)
in 1 hr at 6OOOC but that NaF-ZrF,-UF, (53.5-40.0-
6.5 mole 72) had not detectably penetrated the
graphite in 10 hr at 60Ooc. The rapid penetration
of the graphite by the NaF-ZrF, melt is considered
to be surprising, since such penetration is in con-
trast to the stability of C-18 graphite in this and
similar fluoride mixtures. Other graphite5 of higher
densitywjll be examined in these and other fluoride
mixtures. If necessary, attempts will be made to
prevent penetration of the graphite by prior impreg-
nation with high-melting-point materials such as
NaF or CaF,.
EFFECT OF ATMOSPHERE ON ANODIC
DISSOLUTION OF NICKEL IN
MOLTEN NaOH
F. A. Knox
An attempt has been made to study the efficiency
of anodic dissolution of nickel in NaOH under
various atmospheres and various conditions of cur-
'On l assi nment from Pratt & Whitney Aircraft.
Nationas Carbon Co. code designation.
F. Kertesz
PERIOD ENDING JUNE 10, 1956
rent density and temperature. The apparatus con-
sisted of a crucible of Morganite (recrystallized
alumina) containing molten sodium hydroxide into
which a nickel anode and a cathode, as well as a
nickel thermocouple well, were inserted. This
electrolytic cell and its furnace were placed in a
large chamber which could be evacuated or filled
with the desired atmosphere. A lead storage bat-
tery served as the power supply; the circuit also
contained a voltmeter, ammeter, and ampere-hour
meter, as well as rheostats to control the current
density applied.
When the assembled electrolytic cell was main-
tainedat the desired temperature without an applied
voltage, attack on the electrodes was negligible.
Passage of current resulted in erosion of the anode
and subsequent deposition of crystalline nickel on
the cathode. The weight gain of the cathode did
not correspond precisely to the weight loss of the
anode, since the crystalline deposit was not per-
fectly adherent; in addition some sodium alumi-
nate, formed by the reaction of NaOH with the
AI'O, of the crucible, apparently contaminated the
cathode deposit. Loss in 'weight of the anode was
accordingly used to evaluate the efficiency of the
cell.
The data shown in Table 2.5.1 were obtained at
a current density of 2 amp/cm2. At this current
density the cell efficiency is very poor under all
conditions. The dissolution efficiency is decreased
slightly by substituting hydrogen for helium as the
static cover gas and is reduced considerably by
bubbling hydrogen through the molten electrolyte.
TABLE 25.1. EFFICIENCY OF ANODIC
- SOLUTIONS OF NICKEL IN NaOH
Current density: 2 amp/cm
600 0.65 0.42
800 2.77 2.12 0.66
125