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Fig. 15.4. Inner Surface and Crock in Core Outlet Pipe of In-Pile Loop 2. Top side, near core. 250~.
caused by stresses resulting from differential
thermal expansion of the loop components.
Calculation of the piping stresses in the loop
has been made for two conditions' (1) for the
temperature profile around the loop at normal,
full-power in-pile operation, and (2) for the temperature
profile observed during a reactor setback
(change from full power to zero in - 1'/* min).
For both conditions (1) and (2) the piping stress
analysis indicates that the maximum stress from
thermal expansion occurs in the core outlet pipe
where the failure occurred. For the normal operating
condition the bending movement produces a
stress of cv 10,000 psi in the pipe wall (tension on
the top and compression on the bottom). For the
temperature distribution encountered during a
reactor setback, the direction of the bending movement
is reversed, causing a stress of - 17,000 psi
in the pipe wall (compression on top and tension
on the bottom).
It appears that two factors could have caused
the failure in the core outlet pipe. First, the
section of pipe where failure occurred was at a
temperature of 'v 1350OF. Stress-rupture properties
of Hastelloy N at 13s0°P; (732OC) are below those
at 1200'F (65OOC) used for design purposes, and
these properties are further reduced394 by the
accumulated irradiation dose of -'5 x 10l9 nvt.
Under these conditions (1350'F and 5 x lQ19
nvt), it is estimated that stresses of about 10,000
psi could produce rupture wjthin moderatc times,
possibly of the order of days. Further, the duc-
tility of IIastelloy N is reduced such that strains
of 1 to 3% can result in fracture. Thus the thermal
stress of -10,000 psi calculated to exist in the
outlet pipe at full power nppration may have been
sufficient to cause failure. A second and more
3H. E. McCoy, Jr., and J. R. Weir, Jr., Materznfs de
velopnient for Molten-Salt Breeder Reactors, OHNL--
TM-1854 (.June 16, 1967).
'H. E. McCoy, Jr., and J. K. Weir, Jr., In- nndEx-
Reactor Stress-ltupture Properties of Hastelloy N
Tubing, ORNL-TM-1906 (September 1967).