Carbon deposition on nickel ferrites and nickel-magnetite surfaces

Two types of carbon deposit have been identified on oxides formed in nuclear reactors: a thin but dense layer and low-density columnar deposits with filamentary structure which are often associated with particles containing iron, nickel or manganese. In order to identify the role of nickel in the deposition process, surfaces composed of nickel-iron spinels or metallic nickel-magnetite mixtures have been exposed to γ radiation in a gas environment simulating that in a nuclear reactor. Examination of these surfaces by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have shown that whereas metallic nickel (Ni0) catalyses the formation of filamentary low-density carbon deposits, the presence of divalent nickel (Ni11) sites in spinel-type oxides is only associated with dense deposits. (author)