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others, but all three stringers in the reactor are
bound together. The new stringers for the reactor
core specimens and their controls are all pinned at
the joints with 0.040-in.-diam Hastelloy N in order
to eliminate the pin deformation problem.
Studies of the deposition of fission products on
the graphite and the Hastelloy N are being conducted
by the Reactor Chemistry Division. For
their work, they used the graphite specimens from
the bottom, middle, and top of the reactor core
specimen stringer, plus matching controls and
selected Hastelloy N samples from the stringer and
basket. The results of their examinations are reported
in Chap. 9.
The remaining graphite samples are being measured
for dimensional changes. Tests of the types
outlined in ref. 3 will be made when the dimensional
measurements are completed.
The results of the examination of the Hastelloy N
tensile specimens from the core position and from
outside the reactor vessel are reported in detail in
Sect. 16.2.
We have begun to expose samples in the MSRE
core that are of interest for future molten-salt reactors
and thus have extended the scope of these
studies beyond surveillance of the MSRE. In the
specimens just removed, the tensile specimen rods
were made of heats of Hastelloy N modified to yield
increased resistance to radiation damage. One of
the rods had an addition of 0.52 wt % Ti, and the
other had 0.42 wt % Zr. Besides evaluating the
radiation resistance of these modified alloys, we
should also be able to obtain data on their corrosion
resistance in the MSRE environment.
The alloy modification study was continued in the
new stringer, RS3, just returned to the MSRE. One
of the tensile specimen rods had an addition of 0.5
wt % Ti and 2 wt % W; the other had an addition of
0.5 wt % Hf.
The graphite samples included the anisotropic
MSRE graphite (grade CGB), graphitized-pitch
impregnated grade CGB graphite, isotropic graphite,
pyrolytic graphite, and a joint of isotropic graphite
brazed to molybdenum with a 60 Pd-35 Ni-5 Cr
(wt %) alloy. Th.is test of the brazed joint with
radiation and in flowing salt should supplement the
data on corrosion of such joints in static salt without
radiation that are discussed later in this
section. Since the radiation dose received in the
3MSR Program Semiann. Progr. Rept. Aug. 31, 1965,
ORNL-3872, p. 89.
200
MSRE is small compared with what will be en-
countered in an MSBR, the primary purpose for in-
cluding various grades of graphite is to extend the
study of fission product behavior with these grades.
The principal objective of this program is to en-
sure the safe operation of the MSRE. We are,
therefore, continuing to retain 65% or more of the
space in the assembly for exposure of the MSRE
grades of Hastelloy N and graphite (see Table
16.1).
16.2 MECHANICAL PROPERTIES OF THE MSRE
HASTELLOY N SURVEILLANCE SPECIMENS
H. E. McCoy
The results of tensile tests run on the first group
of specimens removed from the MSRE were reported
previously. These specimens were removed after
7823 Mwhr of reactor operation, during which they
were held at 1190 k 18'F for 4800 hr and accumu-
lated a thermal dose of 1.3 x 10'' neutrons/cm2.
The creep-rupture tests on these specimens have
now been completed, and the results are summarized
in Table 16.2. The times to rupture for the surveil-
lance specimens and the controls are compared in
Fig. 16.3. The rupture life is reduced greatly as a
result of the neutron exposure. However, Fig. 16.4
shows that the minimum creep rate is not appreciably
affected. Although the reduction in the rupture life
is large, it is quite comparable with what we have
observed for specimens irradiated to comparable
doses in the ORR in a helium environment. This
point is illustrated in Fig. 16.5. The superior rup-
ture life of heat 5081 is evident. The fracture
strain is the parameter of greatest concern, since
the rupture life does not appear to be reduced
greatly by irradiation at low stress levels. The
fracture strains for the various heats of irradiated
material are compared in Fig. 16.6. The data indi-
cate a minimum ductility for a rupture life of 1 to
10 hr, with ductility increasing with increasing
rupture life. A lower ductility of heat 5085 after
irradiation in the MSRE is also indicated, although
the data scatter will not permit this as an un-
equivocal conclusion. The superior ductility of
heat 5081 and the least ductility of heat 5085 (heat
used for the top and bottom heads of the MSRE) are
clearly illustrated.
4W. H. Cook and H. E. McCoy, MSR Program Semiann.
Progr. Rept. Feb. 28, 1967, ORNL-4119, pp. 95-103.