Relationship between crystal structure and properties and irradiation behavior of reactor graphites

Abstract

Nineteen graphites manufactured with a wide variety of raw materials and processes were characterized and correlations sought between structure and properties. The raw materials of the graphites ranged from highly isotropic cokes to anisotropic needle cokes and processing from conventional molding and extrusion to proprietary processes that produce high-strength isotropic materials. Strength, thermal expansivity, and the degree of anisotropy were found to correlate with crystallite size. The materials with relatively large mean apparent crystallite sizes L ̄ c were anisotropic and weak and had low thermal expansivity values, whereas those with smaller L ̄ c values were isotropic and strong and had large thermal expansivities. The graphites were irradiated at 625°–1625°C to fluences of up to 2 × 10 22 n/ cm 2. The only strong correlation found was between preirradiation strength and dimensional and volumetric changes. High-strength isotropic graphites with intermediate L ̄ c values were most stable with lowest initial contraction rates and lowest expansion rates after turnaround. Low-strength isotropic graphites were in the same range of stability with low-strength anisotropic graphites, except distortion was minimized due to their isotropic dimensional changes