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tenfold or more as a consequence of chemical
reprocessing. Unless either the precision of
chromium analysis is improved or low concentra-
tion of chromium is maintained in the salt which
is returned to the reactor fuel circuit, much of the
capability for immediate detection of corrosion
will have been lost.
It is anticipated that gas chromatographic
iiiethods for analysis of gas streams. will be tested
soon at the MSRE. If application of such methods
to cover gas analysis succeeds in providing a
sensitive means for the quantitative determination
of volatile fluoride, hydrogen, and oxygen-bearing
phases, a major advance toward on-line analysis
of salt purity will have been achieved.
8.2 MSRE FUEL CIRCUIT
CORROSION CHEMISTRY
Corrosion on salt-metal interfaces in the MSRE
is signaled by an increase of chromium concen-
tration in salt specimens. An increase of 10 ppm
corresponds to the removal of approximately 40 g
of chromium from the Hastelloy N surfaces. Cur-
rently, the chromium concentration of the fuel
salt is 72 z 7 ppm; this concentration repre- &nts
an increase of only 34 ppm and removal of about
170 g of chromium from the Hastelloy N container
since operation of the MSKE began in 1965. If
the total amount of chromium represented by this
increase were leached uniformly from the fuel
circuit, it would correspond to removal of chromium
from a depth of 0.22 mil. Recent evidence indi-
cates, however, that only half the chromium in-
crease observed in the fuel salt may be attiibuted
to corrosion in the fuel circuit.
On termination of run 7, graphite and metal
surveillance specimens were removed from the
core of the reactor and were replaced with speci--
mens contained in a new perforated metal basket.
Fuel specimens taken throughout the next run,
No. 8, were found to contain a chromium concen-
tration of 62 ppm, rather than 48 ppm, the average
concentration which had persisted almost from the
beginning of power operations. Since the only
known environmental alteration was the installa-
tion of the new surveillance specimen assemblage,
we speculated that the container basket and the
Hastelloy N specimens had sustained most of the
corrosion responsible for the observed increase
in chromium, and that corrosion might be evident
110
to a depth of 10 mils. However, recent inspection
of speciiiiens froim the basket (see Part 5, this
report) did not disclose that the anticipated cor-
rosion of the metal had occurred.
On several previous occasions, salt was re-
turned to the fuel circuit after storage in the drain
tanks without developing evidence o€ an increase.
Nevertheless, we are forced to conclude that the
increase of chroriiiuin in the fuel salt took place
while it was stored in the drain tank during the
ten-week interval between runs 7 and 8.
Although it is not evident how the drain tank
may have become contaminated, its surface seems
to be the source of the additional chromium in the
fuel salt. If all the chromium was leached uni-
formly, corrosion in the drain tank will have
reached a depth of 0.7 mil. If the increase of
48 to 62 pprn is attributed to the drain tank, the
total increase of chromium resulting from fuel-
circuit corrosion is only 20 ppm throughout the
entire operation of the MSRE, and corresponds to
% 100 g of chromium, or 0.13 mil of generalized
corrosion in the fuel circuit.
8.3 ADJUSTMENT OF THE U6, CONCEN-
TRATlQN OF THE FUEL SALT
The fuel salt, free of moisture and IIF, should
remove chromium from Hasielloy N only by the
equilibrium reaction
When the above corrosion equilibrium was first
established in MSRE power operations, the UF,
produced in this reaction, together with that
originally added to the fuel concentrate, should
have totaled 1500 g, with the result that as much
as 0.65% of the uranium of the system could have
been trivalent soon after the beginning of power
operation. The UF, content of the MSRE fuel
was determined after approximately 11,000 Mwhr
of operation to be no greater than 0.05%. The
fuel salt was considered to be far more oxidizing
than was necessary and certain to become more
so as additional power was produced unless ad-
justment was made in the UF, concentration. A
program was initiated early in 1967 to reduce 1 to
1.5% of the uranium inventory to the trivalent
state. The U 3t concentration has been increased