Plates 2 and 3] Specim ens of u ran iu m dioxide h av e been q uenched in a n arc fu rnace from te m p e ra tu re s ju s t below th e m eltin g p o in t, c u t in to slices a n d chem ically th in n e d for ex am in atio n by transm ission electron m icroscopy. D efects in th e form o f stack in g fau lts a n d te tra h e d ra have been observed. Com plete c o n tra st ex p erim en ts h av e been successfully carried o u t on som e of these stack in g fau lts a n d reveal a ^[121] displacem ent

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... v ecto r lying in th e fa u lt plane. (Ill) a n d (201) stackin g fa u lt planes h av e been identified a n d it is th o u g h t th a t m a n y m ore stack in g fa u lt planes ex ist h av in g th e d isplacem ent v ecto r as zone axis. A co m bination of dark-field an d bright-field m icroscopy shows th a t th e fau lts are intrinsic. T he te tra h e d ra are th o u g h t to be stack in g fau lts lying on all four {111} planes a n d are a ttrib u te d to d is sociation of F ra n k sessile loops form ed b y v acan cy co n d en satio n on {111} planes.