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Table 15.3. Comparison of Values for Meon Neutron
Flux in Salt Obtained by Various Methods
Mean Neutron Flux
Method (neutrons cm-2 sec-')
Power generation rate
Type 304 stainless steel
monitor wires
1.18 x 1013
1.15 x 1013
Activities of 137Cs, 144Ce, 0.88 1013
and "2,
loop and from metallographic examination of
samples cut from various regions of the loop.
A dissolved-chromium inventory based on analysis
of samples indicated that 13 mg of chromium
was dissolved by the flush salt, an additional
35 mg was dissolved during preirradiation operation,
20 mg more during solvent salt in-pile operation,
and 19 mg more during the ensuing fueled
operation with fissioning, for a total of 87 mg
overall. The flowing salt contacted about 110 cm2
of loop surface. A uniform 0.5-mil thickness of
metal (7% Cr) from over this area would contain
about 87 mg of chromium.
The largest increases in dissolved chromium
came during the first part of the run; this is consistent
with out-of-pile behavior reported by DeVan
and Evans. There was no indication of any aggravation
of corrosion by irradiation.
The regions of the loop contacted by flowing
salt, particularly the core outlet and cold leg,
were seen on metallographic photographs to be
attacked to depths of \,, to 1 mil. This agrees
reasonably with the chemical value, which of
necessity was calculated on an overall basis.
The gas separation tank between the core outlet
line and the cold leg showed less attack than
the tubing sections, thereby indicating varying
susceptibility of different items of metal.
Examination of metallographic photographs of
Hastelloy N from the core shell a d end pieces
showed darkened areas v2 to 1 mil deep in regions
where the metal was in contact with the
core graphite, indicative of carburization there.
2J. H. DeVan and K. B. Evans 111, "Corrosion Be-
havior of Reactor Materials in Fluoride Salt Mixtures,"
pp. 557-79 in Conference on Corrosion of Reactor
Materials, June 4-8, 1962, vol. 11, International Atomic
Energy Agency, Vienna, 1962.
18 2
Similar darkening along the core outlet tube
bottom could be either carburization or corrosion.
15.7 OXYGEN ANALYSIS
Oxygen in the salt may come from moisture (or
other oxygen compounds) absorbed by the salt,
either from the atmosphere, the graphite, or else-
where, or from the dissolution of metal oxides
previously formed. Three oxygen determinations
on salt samples were made. The original solvent
salt contained 115 ppm. Solverit salt withdrawn
from the loop after 353 hr of in-pile circulation
contained 260 ppm. The increase is equivalent to
about 25 mg of oxygen (or 80 mg of chromium oxi-
dized to Cr2+). Fueled salt withdrawn from the
loop after retraction and freezing showed 241 ppm
of oxygen. These values are well below levels
expected to cause precipitation of zirconium OK
uranium oxides. However, some or all of the
69-mg increase in chromium content of the salt
noted during the solvent-salt operation could have
been due to corrosion if the oxygen increase in
this period is attributed to moisture, all of which
reacted to dissolve chromium from the metal.
An analysis for the IJ3+/U4+ ratio in a salt
sample was attempted, but a valid determination
has not been reported.
15.8 CRACK IN THE CORE OUTLET PIPE
Following its removal from beam hole HN-1, the
loop was transferred to hot-cell facilities for
examination. Cutup of the loop is described in 3
later section. After the containment vessel was
opened, no evidence of salt leakage from the loop
was seen by visual examination. The loop was
then pressurized to cv 100 psig with helium, and
Leak-Tec solution was applied to the external
loop surfaces. By this technique a gas leak was
observed in the core outlet pipe adjacent to its
point of attachment to the core body. Subse-
quently, the loop was sectioned for metallo-
graphic examination, and a crack through the wall
of the Ilastelloy N outlet pipe (0.406 in. OD x
0.300 in. ID) was found. Figures 15.4 and 15.5
are photomicrographs of the crack, which ex-
tended almost completely around the circumference
of the pipe.
Analysis of the cause of failure in the core out-
let pipe indicates that this failure was probably