ABSTRACT The unique combination of physical properties inherent to graphite makes it an attractive material for use as a moderator in high-temperature nuclear reactors (HTR&apos;s). Hightemperature physical properties of three nuclear grade graphites manufactured by GrafTech International Holdings Inc. (GrafTech) (PCEA, PCIB-SFG, and PPEA) have been determined experimentally and are presented here. Tensile strength, Young&apos;s modulus, thermal conductivity, specific resistance, and coefficient of thermal expansion (CTE) data are collected at temperatures from 25 °C to as high as 2000 °C and are found to be consistent with classical graphite behavior

David M Kaschak

Lionel Batty

Tracy L Albers

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Downloaded From: https://proceedingHTR2008-58284 
HIGH-TEMPERATURE PROPERTIES OF NUCLEAR GRAPHITE 
Tracy L. Albers, Lionel Batty, and David M. Kaschak 
 
GrafTech International Holdings Inc. 
Parma, OH, USA 
Proceedings of the 4th International Topical Meeting on High Temperature Reactor Technology 
HTR2008 
September 28-October 1, 2008, Washington, DC USA  
ABSTRACT 
The unique combination of physical properties inherent to 
graphite makes it an attractive material for use as a moderator 
in high-temperature nuclear reactors (HTR’s). High-
temperature physical properties of three nuclear grade graphites 
manufactured by GrafTech International Holdings Inc. 
(GrafTech) (PCEA, PCIB-SFG, and PPEA) have been 
determined experimentally and are presented here. Tensile 
strength, Young’s modulus, thermal conductivity, specific 
resistance, and coefficient of thermal expansion (CTE) data are 
collected at temperatures from 25 °C to as high as 2000 °C and 
are found to be consistent with classical graphite behavior. 
 
INTRODUCTION 
Graphite was used as the moderator for the first nuclear 
reactor in 1942 because neutrons can travel a great distance in 
graphite without being captured, while having multiple 
collisions with carbon atoms that slow down the neutrons. 
Graphite’s thermal stability in inert atmospheres, ease of 
machining, relatively low cost, and high purity make it a good 
moderator for nuclear reactors [1,2]. 
GrafTech has provided nuclear graphite to the industry for 
more than sixty years and today leverages more than 125 years 
of manufacturing expertise to provide a wide product offering 
to the nuclear graphite industry. GrafTech nuclear graphite is 
specially produced for the desired strength, isotropy, and purity, 
using raw materials that are expected to be stable in their 
chemical composition and readily available in large quantities 
into the distant future. Three different nuclear graphite grades 
are presented here: PCEA, PCIB-SFG, and PPEA. These grades 
differ in both their forming methods and raw material base. 
Graphite’s unique high-temperature properties also make it 
well-suited for the future construction of highly-efficient, 
inherently-stable high-temperature reactors (HTR’s). These 
reactors will operate at temperatures of 1200 °C, and must be 
mechanically stable up to 2000 °C [3]. Thus, HTR designers 
must understand the behavior of nuclear-grade graphite at 
elevated temperatures, in addition to understanding the 
irradiation effects on graphite. s.asmedigitalcollection.asme.org on 07/02/2019 Terms of Use: This paper will highlight some of the strength, thermal, 
and electrical properties that can be achieved for these grades at 
high temperatures. These nuclear graphites are undergoing 
irradiation testing as well*; the data presented here for “as-
manufactured” graphite will later be combined with post-
irradiation measurements to give HTR designers a complete 
picture of the high-temperature performance of these three 
nuclear graphite grades.  
 
HIGH-TEMPERATURE PROPERTIES 
The focus of this paper is to present elevated temperature 
data of standard graphite samples of grades PCEA, PPEA, and 
PCIB-SFG, as determined by the modified ASTM test methods 
summarized below in Table 1 [4, 5]. 
ASTM C565, C74925-2000Tensile strength
ASTM C714 (TD)
and ASTM C781 (SH)25-900
Thermal diffusivity
and specific heat
GrafTech design: 
modified ASTM C61125-2000
Specific resistance 
(SR)
GrafTech design25-2000
GrafTech design: PATE25-100
CTE
GrafTech design:
Modified ASTM C74725-2000Young’s Modulus
Test notesTemp.
Range, (°C)
Property
TABLE 1. HIGH-TEMPERATURE TESTING 
METHODS FOR NUCLEAR GRAPHITE GRADES.
 
High-temperature tensile strength properties are presented 
in Table 2. Tensile strengths are 4-9% higher in the with-grain 
(wg) direction than against-grain (ag) at 25 °C, and 1-3% 
higher at 1600 °C. All three grades tested demonstrated 
significant improvement in tensile strengths (with-grain) at the 
higher temperature, with increases ranging from 25-63%. 
                                                          
* Controlled irradiation experiments are currently being performed at 
NRG Petten as part of the RAPHAEL programme. 
1 Copyright © 2008 by ASME 
http://www.asme.org/about-asme/terms-of-use
Download29.9 / 29.1
52.1 / 50.6
31.5 / 31.2
1600 °C
(MPa, wg/ag)
63%18.3 / 17.5PPEA
25%41.8 / 39.9PCIB
34%23.5 / 21.6PCEA
% Increase 
(wg)
25 °C
(MPa, wg/ag)
TABLE 2. HIGH-TEMPERATURE TENSILE STRENGTH.
 
Young’s modulus was also determined for the three grades 
of nuclear graphite, over the temperature range 25-2000 °C. 
Select results are shown in Table 3. As in the case of tensile 
strength, moduli are somewhat stronger in the with-grain 
direction, increasing with temperature for each grade tested. 
Each value shown in the table is an average of a measurement 
made during heating and one made during cooling. 
12.2 / 11.9
15.1 / 13.7
13.5 / 12.8
1000 °C
(GPa, wg/ag)
16.3 / 15.610.8 / 10.4PPEA
17.3 / 15.812.8 / 11.7PCIB
16.5 / 15.911.8 / 11.3PCEA
2000 °C
(GPa, wg/ag)
25 °C
(GPa, wg/ag)
TABLE 3. HIGH-TEMPERATURE YOUNG’S MODULUS
 
Thermal conductivity values were measured for PCEA, 
PCIB, and PPEA over the temperature range of 25-900 °C. The 
data, shown in Fig. 1, illustrate that thermal conductivity 
decreases by roughly 50% over this temperature range for each 
grade tested. 
FIG. 1 HIGH-TEMPERATURE THERMAL CONDUCTIVITY.
100
110
120
130
140
160
T
he
rm
al
 C
on
du
ct
iv
ity
 (W
m
-1
K
-1
)
150
90
80
70
60
50
0 200 400 600 800 1000
Temperature (°C)
PCEA
PPEA
PCIB
with grain
against grain
T
he
rm
al
 C
on
du
ct
iv
ity
 (W
m
-1
K
-1
)
 
The specific resistance of the nuclear graphite grades was 
also measured, over the temperature range 25-2000 °C 
(Table 4). The against-grain resistances were generally higher 
than with-grain values, up to 8% greater. For all three grades, 
resistances reached a minimum between 500-1000 °C and 
increased steadily all the way up to 2000 °C. 
CTE measurements were recorded over the temperature 
range of 25-2000 °C (Fig. 2). All three grades showed a steady 
increase of with-grain CTE over the entire temperature range, 
between 65-75%. The behavior of these materials is consistent 
with calculations made from predictive models [6].  
ed From: https://proceedings.asmedigitalcollection.asme.org on 07/02/2019 Terms of Use6.8 / 7.4
7.8 / 8.2
6.6 / 6.6
1000 °C
(μΩ-m, wg/ag)
8.6 / 9.18.8 / 9.2PPEA
9.5 / 9.910.4 / 11.1PCIB
8.5 / 8.47.7 / 7.9PCEA
2000 °C
(μΩ-m, wg/ag)
25 °C
(μΩ-m, wg/ag)
TABLE 4. HIGH-TEMPERATURE SPECIFIC RESISTANCE.
 
FIG. 2 HIGH-TEMPERATURE WITH-GRAIN CTE. 
ISOTROPY RATIO IS AG/WG VALUE FOR RT-2000 °C.
Temperature (C)
3
4
5
6
7
0 500 1000 1500 2000
C
T
E
 (×
10
-6
K
-1
)
Isotropy 
ratio
Grade
1.03PPEA
0.99PCIB
1.06PCEAC
T
E
 (×
10
-6
K
-1
)
C
T
E
 (×
10
-6
K
-1
)
 
CONCLUSIONS 
High-temperature test results of these three nuclear 
graphite grades confirm classical predictions of how 
temperature affects strength, electrical, and thermal properties: 
• Mechanical properties like tensile strength and Young’s 
modulus increase with temperature. 
• Thermal conductivity decreases by approximately 50% as 
temperature is increased from RT to 900 °C.  
• Specific resistance reaches a minimum between 500-
1000 °C and increases at higher temperatures. 
• CTE increases 65-75% upon heating from RT to 900 °C, 
following established predictive models. 
This data is part of the extensive characterization effort by 
academic, government, and industrial partners needed to 
complete the design of a modern HTR. 
 
REFERENCES 
[1] Currie, L.M., Hamister, V. C., and MacPherson, H.G., 
1955, “The Production and Properties of Graphite for 
Reactors,” UN Int’l Conf. on Peaceful Uses of Atomic 
Energy, Geneva. 
[2] Nightingale, R.E., Ed., 1962, Nuclear Graphite, Academic 
Press, London. 
[3] Parme, L.L. 2001, “Regulatory Challenges for Future 
Nuclear Power Plants,” ACRS Workshop. 
[4] “2008 Annual Book of ASTM Standards,” Vol. 05.05, 
ASTM, West Conshohocken, PA, pp. 191-290. 
[5] Wagoner, G., Sprogis, G., Proctor, D.G., 1986, 
“Capacitance Bridge Measurement of Thermal 
Expansion,” Carbon Proceedings, pp. 234-236. 
[6] Tsang, D.K.L., Marsden, B.J., Fok, S.L. Hall, G., 2005, 
“Graphite Thermal Expansion Relationship for Different 
Temperature Ranges,” Carbon, 43, pp 2902-2906. 2 Copyright © 2008 by ASME 
: http://www.asme.org/about-asme/terms-of-use


HTR2008-58284 HIGH-TEMPERATURE PROPERTIES OF NUCLEAR GRAPHITE

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HTR2008-58284 HIGH-TEMPERATURE PROPERTIES OF NUCLEAR GRAPHITE

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