ORNL-4191 - the Molten Salt Energy Technologies Web Site

186
Fig. 15.6. Inner Surfoce of Cold-Leg Tubing from In-Pile Loop 2 (Hastelloy N). 250x.
to have been intergranular and without any indication
of ductility. Such behavior at temperatures
in excess of 650°C and at a thermal neutron dose
of 5 x lo1’ neutrons/cm2 is consistent with recent
ORNL studies3 of the effect of irradiation on
elevated-temperature properties of Hastelloy N, as
discussed in Sect. 15.8 in connection with the
break in the core outlet pipe.
Some evidence of attack on the inner surface of
the core outlet may be noted in Figs. 15.4 and
15.5. The upper inner surface shows evidence
of corrosion, even though part of the 1-mil, more
finely grained layer had been removed hy reaming
prior to the assembly of the loop. The lower
surface does not show corrosive pitting but does
have a darkened bank almost 1 mil deep that
could be carburization.
The bottom surface (not shown) at the core end
of the gas separation tank showed no evidence
of either corrosion or carburization.
The cold leg of the loop showed substantial
corrosive attack - probably largely intergranular
in the 1-mil, more finely grained inner layer as
shown in Fig. 15.6. An unexposed piece of the
same tubing is shown for comparison in Fig. 15.7.
This tubing was also used for fabrication of the
core outlet pipe.
The depth of the attack on the cold-leg pipe is
of the same magnitude as would be anticipated
from chromium and oxygen analyses of the salt
reported above. The tubing used to fabricate the
cold leg and core outlet appears to be more sensitive
to corrosion than materials used in other
parts of the system.
Carburization to a depth of about 1 mil appears
to have occurred in the inner surface of the core
shell wall in contact with the graphite core, as
shown in Fig. 15.8. Similar carburization was
also noted on core top and bottom pieces (not
shown). Hardness tests were taken at various
depths below the surface of the metal, the nearest
about 1 mil. The test nearest the surface showed
definitely greater hardness, as would be expected
from carburization.