  The irradiation behavior of graphite is essential for its applications in
nuclear industry. However, the differences between graphite's behaviors are not
well understood because of the very limited knowledge of microstructural
differences between graphites. One typical structure, quinoline insoluble (QI)
particle, was investigated using IG-110 and NBG-18 graphite. After irradiation,
the QI particles on the polished surface were proved to become hillocks and can
be easily identified by using a scanning electron microscope (SEM). Thus a
method combining ion irradiation and SEM characterization was proposed to study
the distribution and concentration of QI particles in graphite. During
irradiation, the QI particles were found to evolve into densified spheres which
have weak bonding with the surrounding graphite structures, indicating that the
densification of QI particles does not contribute obviously to graphite
dimensional shrinkage. A much higher concentration of QI particles in NBG-18
than IG-110 was characterized and is suggested to be responsible for the
smaller maximum dimensional shrinkage of NBG-18 than IG-110 during irradiation.
