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Evidence that the size of graphite samples may influence the magnitude of radiation-induced dimensional changes has recently been obtained, Although experiments designed to understand the phenomena are just getting started, the results available indicate that the contraction rate of large samples is appreciably higher than that of small samples, contraction rates with reactor distortion measurements1'* the best correlations (taking into account moderator temperatures and exposures) were obtained with large sample bars which are about the size of the bars used in the reactor construction, Moreover, in comparing these different Because of the significance of these results to investigators conducting irradiation studies on small graphite samples and to those using the data so obtained for reactor design or operations planning, a review of the results recently obtained on large samples is warranted even though the underlying causes for the effects are still speculative.Three bars,4 in. in cross section and 24 in. long, were measured after exposure 2 in a Hanford reactor of 18,650 Mwd/At (equivalent'to 1 , 8 x lo2' neutrons/cm having energies above O,18 MeV). Although the temperature in the test facility increased slowly during the irradiation period, most of the exposure was accumulated at 500 to 600° C, the average being 525O C, of nuclear graphite. However, the differences are in the type of purification Each bar represented a different grade 3 used rather than in the raw materials or formulation,

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