ORNL-4191 - the Molten Salt Energy Technologies Web Site

likely cause of failure is the rapid stress reversal
(+10,000 to -17,500 psi) calculated for the thermal
shock caused by the rapid loss of fission heat
during a reactor setback. Approximately half a
dozen such cycles were encountered during in-pile
operation. In particular, one such cycle occurred
on March 3 after a dose accumulation of 4 x 10
nvt, and it was on March 11 that evidence of fis-
sion product leakage from the loop was first ob-
served. Thus the stress reversals resulting from
such cycles are very likely to have contributed to
failure of the loop, since the rupture occurred at
the point where the stress was a maxiinurn, the
temperature was 135OoF, and a radiation dose suf-
ficient to affect the strength and ductility of
Hastelloy N had been accumulated.
It is evident that for future in-pile loops with
similar configuration and temperatures, a material
superior to the present Hastelloy N in high-
temperature strength under irradiation is required.
184
Fig. 15.5. Inner Surface and Crack in Core Outlet Pipe of In-Pile Loop 2. Bottom side, neor core. 250~.
15.9 CUTUP OF LOOP AND PREPARATION
OF SAMPLES
Promptness in the examination of the loop was
essential. Major irradiation of the loop had
ceased on March 17, 1967, when the loop was
retracted following the identification of the fission
gas leak. Since such short-lived isotopes as
66-hr "Mo and 78-hr '?'e were of interest, it
was necessary that all such samples should be
counted within less than about eight weeks after
this time.
On April 5, 1967, following the ORR shutdown
on April 4, the loop package was removed from the
beam hole and transferred to the segmenting
facility. The sample and addition system was
stored, and inlet ("cold") and outlet (''hot") gas
tubing sections werc obtained from the external
equipment chamber region and submitted for radio-
chemical analysis.