Elsevier

Carbon

Volume 109, November 2016, Pages 860-873
Carbon

Property changes of G347A graphite due to neutron irradiation

https://doi.org/10.1016/j.carbon.2016.08.042Get rights and content

Abstract

A new, fine-grain nuclear graphite, grade G347A from Tokai Carbon Co., Ltd., has been irradiated in the High Flux Isotope Reactor at Oak Ridge National Laboratory to study the materials property changes that occur when exposed to neutron irradiation at temperatures of interest for Generation-IV nuclear reactor applications. Specimen temperatures ranged from 290°C to 800 °C with a maximum neutron fluence of 40 × 1025 n/m2 [E > 0.1 MeV] (∼30dpa). Observed behaviors include: anisotropic behavior of dimensional change in an isotropic graphite, Young's modulus showing parabolic fluence dependence, electrical resistivity increasing at low fluence and additional increase at high fluence, thermal conductivity rapidly decreasing at low fluence followed by continued degradation, and a similar plateau value of the mean coefficient of thermal expansion for all irradiation temperatures.

Introduction

Graphite has been readily used in nuclear reactors since the first controlled nuclear chain reaction. Graphite has a low atomic number and low neutron cross-section, making it an ideal neutron moderator. Additionally, the high strength, high thermal stability, and high thermal conductivity make graphite an ideal structural material. But, in the presence of neutron radiation these properties are changed [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], and these changes depend upon the temperature of the graphite during exposure to neutrons.

Typical property changes include the initial densification to a maximum density, followed by swelling until the material loses all structural integrity [3], [5], [6], [7], [8], [9], [12], [16]. Typically dimensional changes are anisotropic resulting from the preferred orientation of the grains with irradiation causing grains to grow in the c-direction and shrink in the a-direction [9], [12], [13]. The elastic moduli have been shown to undergo a rapid rise, followed by a gradual increase to a peak value, followed by a decrease as the swelling becomes positive [1], [3], [6], [7], [9], [12], [13], [15]. Conversely, the electrical resistivity has been observed to rapidly rise with low fluence and holds at an elevated value for higher fluence [1], [3], [7], [12]. Strength has been shown to have a one-to-one or a square root dependence on Young's modulus change [1], [6], [7], [9], [12], [13], [15]. The thermal conductivity displays behaviors that mirror the electrical resistivity, where it rapidly decreases and then remains nearly constant [1], [3], [6], [7], [12], [15]. The thermal expansion undergoes a rapid increase, followed by a decrease to a plateau value that is lower than the preliminary value [3], [5], [7], [9], [12].

The earliest graphite grades employed in the nuclear industry were grades that were initially used as electrodes, but had undergone additional purification. These grades were manufactured with naturally occurring graphite deposits as the coke source, but the finite supply means that only a limited amount of a given grade could be manufactured. Much of the early research into the irradiation effects was primarily focused on these early grades. With these works it was noticed that the properties underwent changes with similar trends, but were not identical between grades. This lack of uniform response has resulted in every new grade of nuclear graphite requiring extensive irradiation campaigns to fully understand the property changes.

This paper presents the results of an irradiation campaign performed in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) to investigate the neutron irradiation effects on a new fine-grain, isomolded, graphite grade: G347A from Tokai Carbon Co., Ltd. The property changes investigated include: volume/dimensional, elastic and shear moduli, electrical resistivity, equibiaxial (ring-on-ring) strength, thermal conductivity, and the coefficient of thermal expansion (CTE).

Section snippets

Graphite grade

This program was primarily focused on the effects of irradiation on Tokai Carbon Co., Ltd. graphite grade G347A, but some limited investigation was also performed on another grade (G458A) from the same manufacturer. G347A has a coal coke filler while G458A has petroleum coke filler, but both G347A and G458A have a pitch binder, pitch for impregnation, and are formed via cold isostatic pressing The ORNL-measured and the manufacturer-reported bulk properties for these two grades are listed in

Irradiation-induced changes

Specimens were grouped according to the rabbit design temperature for the analysis of the irradiation-induced property changes. This decision arose from the fact that the rabbits of a specific target temperature all had the same pre-irradiation design. Therefore, it is assumed that rabbits of the same design temperature have a similar temperature history. For example, the 300 °C rabbit at 41 × 1025n/m2 [E > 0.1 MeV]) in Fig. 2 has a final average specimen temperature of ∼390 °C, but if this

Conclusions

Fine-grain graphite grade G347A (from Tokai Carbon Co., Ltd.) was irradiated in the HFIR at ORNL to a maximum neutron fluence of 40 × 1025 n/m2 [E > 0.1 MeV]. The actual specimen temperatures ranged from 290°C to 800 °C, and the specimens were grouped according to design temperature that resulted in temperature ranges of 395 °C ± 62 °C, 459 °C ± 37 °C, 535 °C ± 33 °C, 684 °C ± 41 °C, 738 °C ± 43 °C. The temperature ranges had turn-around fluences of 18, 15, 14, 10, and 10 × 1025n/m2

Acknowledgements

This program would not have been completed without the expertise and input from numerous persons throughout the laboratory. Special notice needs to be given to the technicians of the LAMDA laboratory: M.Williams, P.Tedder, S.Curlin, D.Lewis, M.McAllister, W.Comings, B.Eckhart, A.Clark, and W.D.Porter.

This research was performed at the Oak Ridge National Laboratory (ORNL) and sponsored by Tokai Carbon Co., Ltd. under the Material Science and Technology Division, Work-for-Others (WFO) Program,

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    This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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