Microstructural changes induced by neutron irradiation of superfine grain graphite G347A (Tokai Carbon,
Japan) were examined by nitrogen adsorption at 77K and by three microscopy techniques (SEM, TEM and
FIB-SEM tomography). The specimens were irradiated at doses of up to 30 dpa, covering stages before and
after the turnaround fluence at three temperatures (300, 450, 750 °C) of their irradiation envelope. The initial
graphite densification at low fluences did not produce any detectable effect in the pore size range (<350 nm)
measured by gas adsorption. However, graphite irradiated at high fluences, after turnaround, showed severe
structural changes. At all three temperatures and high irradiation fluences, gas adsorption revealed significant
increase of the volume of narrow mesopores (50 nm) at high irradiation fluences and more structural changes on multiple
scales, from nanometers to microns. This work demonstrates the unique ability of gas adsorption techniques
to analyze open pores with sizes between sub-nanometer and sub-micron in bulk nuclear graphite, with supporting microscopy results.