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SIC TEMPERATURE
STAINLESS
STEEL TUBE
215
ALUMINUM
HOUSING TUBE ,
INSIDE SURFACE
COATED WITH
COLLOIDAL GRAPHITE
TUNGSTEN HEATER
IN END POSITIONS
\
RADIAL i\....-GRAPi+rE SPECIMEN
SPACER
0400 in DlAM
SIC TEMPERATURE
SENSOR
Fig. 17.4. Schematic Drawing of the Graphite Irradiation Experiment Inserted in the HFIR.
17.4 IRRADIATION OF GRAPHITE
C. R. Kennedy
Experiments to irradiate graphite in target-rod
positions in the core of the High Flux Isotope Reactor
have begun. The first two experimental containers
have been installed in HFIR for a onecycle
(three-week) exposure to determine the
accuracy of heating rate and thermal analysis calculations.
After this experiment has been removed
and analyzed and the design parameters have been
altered, the long-term graphite irradiations will be
started. It should be possible to obtain integrated
doses (E > 0.18 Mev) of 4 x 10” neutrons/cm2 in
one year.
The facility shown in Fig. 17.4 is designed to
operate by nuclear heating at 1292 to 1328°F. A
uniform axial heating rate will be maintained by
adding tungsten susceptors along the axis of the
samples to compensate for the axial falloff in
nuclear heating. The HFIR control rod design is
excellent for constancy of heating rate, and the
temperature will vary only about 2% during a reactor
cycle.
Irradiation temperatures will be determined using
beta Sic located in a center hole in each graphite
specimen. The method of temperature determination
using the dimensional expansion and annealing
characteristics of Sic is that described by
Thorne et al. This procedure has been verified
by irradiation of three Sic specimens in the ORR
GRNL-DWG 67-12716
at a controlled temperature of 1400OF. The results
indicate that temperatures can be determined
within 9OF of the operating temperature.
Past graphite irradiations to exposures greater
than 10” (refs. 7 and 8) have been limited to
graphite grades that are similar, with only slight
variations in the filler material or the coke used in
their manufacture. When irradiated at about 1200OF
all graphites seem to be characterized by an initial
shrinkage and then a very rapid expansion.
This rapid expansion corresponds closely to that
observed in the axial direction for single crystals,
so it appears that the binder has deteriorated and
the axial expansion of the individual crystals is
controlling the growth. There are, however, indications
obtained from short-term irradiations that
there may be potential modifications of the coke
materials that could extend the exposure required
to cause the binder degradation.
~~
6R. P. Thorne, V. C. H. Howard, and B. Hope, Radiation-Induced
Changes in Porous Cubic Silicon Carbide,
TRG 1024(c) (November 1965).
7
J. W. Helm, “Long Term Radiation Effects on
Graphite,” paper MI 77, Eighth Biennial Conference on
Carbon, Bnffalo, N.Y., June 1967.
8R. W. Henson, A. S. Perks, and S. H. W. Simmons,
“Lattice Parameter and Dimensional Changes in
Graphite Irradiated Between 300 and 135OoC,” paper MI
66, Eighth Biennial Conference on Carbon, Buffalo,
N.Y., June 1967.
’J. C. Bokros and R. J. Price, “Dimensional Changes
Induced in Pyrolytic Carbon by High-Temperature Fast-
Neutron Irradiation,” paper MI 68, Eighth Biennial Con-
ference on Carbon, Buffalo, N.Y., June 1967.