Applicability and limitations of miniature specimens for properties determination of fine-grained graphite

The ability to determine mechanical, physical, and thermal properties using small-volume specimens is essential for development and qualification of nuclear materials for which the neutron-irradiated properties need to be established in a timely and cost-effective manner. This is particularly the case if one considers the small irradiation volume in fission materials test reactors and the limited availability of radiological test facilities that can handle the high-radioactivity and high-contamination potentials of nuclear materials. Historically, applicability of small-volume test specimens has been limited for nuclear graphite primarily by the sizes of particles and intrinsic defects in the as-fabricated materials in relation with the critical dimensions of the test specimens, in particular, for mechanical properties. However, this is often no longer the case for the more recent superfine-grained nuclear graphite grades in which the typical maximum grain sizes range from 15 to 50 μm. In this paper, recent studies undertaken at Oak Ridge National Laboratory on the influences of specimen geometry and dimensions on several properties of the fine-grained nuclear graphite of various grades in unirradiated and non-oxidized condition are reported. The properties to be discussed include bulk density, dynamic Young's modulus, flash thermal diffusivity, uniaxial and equibiaxial flexural strength, uniaxial compressive strength, and uniaxial tensile strength. Moreover, statistical aspects of the strength are discussed. Limitations to the specimen dimensions for these properties are discussed in relation with the recommendation in the current ASTM test standards.