CARBON DEPOSITION ON M(X)CO(1-X)FE(2)O(4) (M=MN, FE, NI) SPINELS

The deposition of carbon on power reactor steels results in a reductio n in heat-transfer efficiency. Methane and carbon monoxide are added t o the gaseous coolant to reduce the radiolytic oxidation of the graphi te moderator and this is known to increase the rate of carbon depositi on. However, the composition of oxides formed on steel surfaces within the reactor may also influence deposition. Previous studies have indi cated that carbon is deposited on spinel-type oxides containing mangan ese, iron, nickel and chromium. Such deposits are often associated wit h particles containing iron, nicker or manganese. In this investigatio n carefully characterised cobalt-containing spinel-type oxides of vary ing composition have been subjected to gamma-irradiation under conditi ons of temperature, pressure and atmosphere similar to those experienc ed in a reactor with the aim of furthering our understanding of the ca talytic processes involved in deposit initiation and growth. It has be en shown that the availability of rapid electron exchange (between oct ahedrally coordinated metal ions of mixed valency) greatly enhances th e role of carbon deposition in these systems.