OF A GRAPHITECREEPIRMDIATION EXPERIMENT*
DESIGNAND OPERATION
R. L. Senn, W. H. Cook, J. A. Conlin, and W. P. Eatherly
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
tests has been planned
A series of irradiation
of various
to determine the creep characteristics
graphites of interest to nuclear reactor designers.
experThe graphites selected for testing in the first
i m e n t o f t h i s s e r i e s i n c l u d e d g r a d e s H - 3 2 7' H - 4 5 1 ,
and AFX-SQBG. A constant stress compressive test was
selected as the basis for our design.
Although
force was sensed by an LVDT-type load cell.
monitoring of the pressure in the bellows was the
primary neasure of the load being applied to the
specirnens, the LVDT-type load ce11 was intended to
provide backup information during the early stages
and ensure that the force was inof the irradiation
deed transrnitted through all of the specinen column.
The prinary and secondary creep coefficients
will be determined as functions of fluence, tenperaThese
ture, stress, and Youngrs noduli of elasticity.
data will be obtained with a series of irradiation
capsules that are alternately irradiated at 900, 600,
and 1250"C in fluence increments of approximately 1,
I, 2, and 4 x 1021 neutrons/cm2 until each set of
test specimens for each test te¡nperature has accunulated á fluence in excess of 8 x 102I neutrons/cmz.
The óompressive stresses at each temperature will be
Each
13.8 MPa (2000 psi) and 20.7 MPa (3000 psi).
graphite specimen will be characterized before neuand subsequently after each accumutron irradiation
Each capsule will conlated increnent of fluence.
tain a !et of unstressed control specimens that match
the stress specimens. Basically, the total creep
strains are deternined by the dinensional differences
between stressed specinens and their matching control
The prinary creep
specimens after each irradiation.
strain has been found to be recoverable under irradiation if the stress is renoved. I Therefore, selected stress-tested specinens will be irradiated again
at their test temperature to recover and thereby
determine their prinary creep strains.
The unstressed control specimens were stacked
and centered in a manner si¡nilar to the stressed
hole was prospecirnens, except that a longitudinal
vided to serve as a guide for the 3.2-run (1/8 in.)
sheathed Chronel-Alunel movable
dia stainless-steel
in each column. These
thermocouple that was installed
thermocouples could be moved through a 0.39 m (15 in.)
stroke, thus providing tenperature traverses from the
midplane of the uppernost specimen to the nidplane of
the lohrest specinen in each colu¡nn.
The capsules for these experiments are being
a flux-trapped
irradiated in the E-5 core facility,
position,
of the 30-MWOak Ridge Research Reactor
(ORR).
Capsule 0C-1, the first experiment in this
series, was designed to irradiate twenty-eight 15.24nm (0.600 in.) dia by 25.4-mn (i'00 in.) long conpressively stressed graphite test specimens at 900oC
io the first planned exposure level of 1 x 1g2l
neutrons/cmz. A conpressive stress of 13.8 MPa
(2000 psi) was applied to 20 of the specinens in the
metal bellows expanded by gas
test by precalibrated
Eight of the
pressure against the specirnen colunns.
specimens in the two colums had reduced diameters
so as to increase the stress to 20.7 MPa (3000 psi).
The capsule also included 28 unstressed control specimens nade of the same types of graphite as were the
stressed specimens.
A cross sectional view showing the tlro stressed
specinen colunns in the in-core portion of the capThe specinens have shallow
sule is shown in Fig. l.
holes in each end to acconnodate the graphite pins
and spacers which serve as centering guides for the
specimen column. The force fron each bellows at the
top of the specinen column was transrnitted through
At the end of each colunn, the
a graphite push-rod.
*Research
The capsule was designed to operate on gan¡na
heat alone approximately 100"C below design tenperature.
A gas gap was provided between the specimen
holder sleeve (see Fig. l) and the water-cooled capsule vessel so that the thermal conductance could be
controlled by gas gap size and inert gas conposition.
Design operating temperature and control were achieved by twenty 1.6-mn (I/16 in.) dia stainless-steelsheathed MgO-i-nsulated Nichrome V heaters wrapped
around the specinen holder as shown in Fig. l, and
retained there by the graphite sleeve that is slipped
over the holder.
Because of the need to naintain a precise tenperature control along the specirnen colunn, a conputer
was acqui-red to control the experinent temperatures
Algorithms
auto¡natically and to record the data.
were developed which related the thernncouple ternperatures to the specimen temperatures which the computer used to adjust the heaters to the required
operating levels.
Irradiation
of capsule OC-.1was conducted frorn
April 1 to May 22, 1976. The capsule was operated at
a no¡ninal specinen tenperature of 900'C in the ORR
E-5 co¡e position for about 34,170 MWhr with the rea c t o r o p e r a t i n g . a t 3 0 M Wf o r ^ a t o t a l p e a k f l u e n c e o f
about 1.3 x l0z I neutrons/cn' (E > 0.18 MeV).
We had difficulty
in naintaining the 900 t l0"C
uniforn tenperature along the length of the specinen
columns. Temperatures of the specinens were high and
off-design in the upper 16%of the specinen column.
In order to naintain the lower part of the specimen
colunn at or near the design ternperature, the specinens in the upper 100 nm were pernitted to operate at
up to about 930'C. Alsq the temperature profile
along the length of the specinen colunns had a small
dip approxinately one-third of the way down from the
tops of the specimen colunns throughout the irradiaThe two movable thermocouples provided
tion period.
direct monitoring of the temperatures along the length
sponsored by the U.S. Energy Research and Developrnent Administration
Carbide Corporation.
340
under contract
with
the Union
.: :he specinen columns and provided data to deter-.e
"-.- the set-points for tenperature control , the
.:::er being based on the measured temperatures in
:-= graphite specimen holder.
Results fron a typi:i- traverse of the novable thernocouDles are shown
-. fíg. 2.
The control and data acquisitions by the corn:..:er worked very well during the entire experinent.
*-.:
average temperature measured by two fixed therno.::ples nounted in the graphite specimen holder in
::-'h zone was controlled by the computer to 11"C
::¡n the requested set-point as long as adequate
":eter power was available to the corresponding zones
:: pernit the computer to naintain the requested ten: e r a t u r e a s t h e g a m n ah e a t p r o f i l e c h a n g e d .
Operation of the two LVDTload cell transducers
. - : , sd i - s a p p o i n t i n g . A t t e m p t s t o c a l i b r a t e t h e d e v i c e s
,. situ before irradiation
were largely unsatisfacto_
-_;. They did respond to specinen loading bellows
::essure changes, but large hysteresis effects in the
-:ad cell responses were noted as the bellows pres:-ires wele changed. This was particularly
evident
.: the West specirnen column, and nay have been an in::.-ation that the bellows-specinen stack was beconing
.:dged, and a ratchetting
rnay have occurred during
::lcreases to full reactot power and design operating
:enperatures.
This bindj-ng would cause excessive
:cupressive stresses on the specinens due to thernal
.-xpansion. Postirradiation
exaninations of the test
;pecinens indicate that this probably occurred dur-ing
:he irradiation.
Resutts frorn the creep measurernents
- ; , n do t h e r p a r a n e t e r s b e i n g m o n i t o r e d i n t h i s s e r i e s
rf tests are being presented by Kennedyet aL.2 at
:his conference.
Certain design modifications havc been madc for
'lhese
future experinent-s of this series.
inclu<jc additional guide pins to prevent thc wedging and ratchetting that occurred with the first experinent and
additional clearance between the specirncn centcring
guide and the ID of the hole in the specimen holder
to prevent possible friction
forces that nay be caus_
ed by interference due to thermal expansion and graphite creep of the guides. A pneunatic electronechanjc a l d e v i c e , d e s i g n a t e d a P I I Mc e l l , h a s b e e n d e s i g n e < 1
to replace the LVDT-type load cells used in capsule
OC-f. The new load cell uses metal bellows similar
t o t h o s e u s e d t o l o a d t h e s p e c i n e n c o l u m n s . $ I h e nt h e
load is applied to the specirnens, an insulated cont a c t b u t t o n l o c a t e d i n s i d e t h e p E Mc e l l b e l l o w s i s i n
electrical contact with the top flange of the bellows.
The PIM cel1 bellows is then pressurized with helium
to a high enough pressure to óvercone the load on the
specinens and open the electricaL contact.
By measur_
ing the pressure required and detecting the resistance between the contact button and the grounded bel_
lows (infinite
when open), a deternination of the
load being applied to the specinen colunn can be nade,
thus ensuring that the load is indeed being applied
to the column. By virtue of its all-rnetal construc_
tion, except for the alumina insulator, we expect the
PEII cell to be relatively
i n s e n s i t i v e t o d a m a g ef r o m
high ternperature and radiation.
References
(1)
R. J. Price,
(2)
C . R . K e n n e d ye t a L . , t h i s
G A - A I 2 3 3 2 ( N o v e r n b e rI ,
I}TZ).
conference.
34y2
IYPICAL ÍEST
SP€CIMEN -
SPECIMEN
HOLOER
(GRAPHIIE}-----\
MEfAL
BE LLOWS
\
PUSHROD
(GRAPHIT€)
-
HEAfER
corLs
\
HEATERS
raND2 i
['-l
É,,
z
9
F
----
O
|
SLEEVE
/
(GRAPHITE
}]
?i^.
GASCAP
CORE PIECE
{ALUMIÑUM)
sro
t¿l
J
COREPIECE
(ALUMINUM)
:)
l¡J
MOVAELE
fHERMOCOUPLE
SLEEVE
(trAPHIT€)
.
TAERMOCOUPLES,
iNSTRUMENT
LEAOS
( fYPlcAL)
-
HEATERS
( T Y P t C A) L
J
rf
SPECIM€N
HOLO€R
(GRAPAIfEI
o
(J
36
É.
IfESf
SPECIMEN
( GRAPH ITE )
TESl
STRESSED
SP€CIMENS
O
l-#
O
|
2i¡,
23
!Oññ
UNSÍRESSEO
CONTROL
SPECIMENS
z
=
o
o
I
l¡J
'E
F
\
COI'ITAINMENT
(5TAIt{LESSSTEEL)
I
fL
l
--J¡
t
(L
lrl
-4
DIMENSIONS ARE IN INCHES
sEcTtoÑ
Fig. 1 (Left)
Cross-Sectional View of In-Core Portion of Graphi-te
C r e e p C a p s u l e 0 C - 1 . F i g . 2 ( R i g h t ) O R RC a p s u l e 0 C - 1 . R e s u l t s
from Typical Movable Thernocouple Traverse.
34r
850
900
950
('C)
TEMPERATURE