Carbon deposition on transition metal- and uranium-oxides

Hallam, Keith Richard (1991). Carbon deposition on transition metal- and uranium-oxides. PhD thesis The Open University.

Abstract

Advanced gas cooled nuclear reactors (AGRs) contain a range of alloys, selected for their physical and chemical performance in the conditions present. Carbon deposition on boiler and fuel pin heat transfer surfaces affects a reactor's efficiency and may necessitate downrating to maintain safety margins. This is believed to arise from decomposition of some of the coolant gas constituents. Deposition minimisation, while maintaining the structural integrity of the reactor, is technologically and economically important. This study has looked at deposition on a range of transition metal spinels, manganese oxides, uranium oxides and single crystal magnetite samples with a view to furthering knowledge of catalytic reactions that may occur within an AGR. In particular, the effect of mixed valency on deposition rates was studied.

The spinels were successfully prepared by solid state reactions between the relevant oxides, oxalates and I or carbonates. A range of elemental and chemical analytical techniques were used to characterise the samples both before and after exposure under controlled gas and radiological conditions. Deposition was induced, to varying extents, on all the samples exposed. No filamentary deposits were observed. The spinels gave quantities of deposition in the order:

Manganese spinels gave increasing deposition with increasing manganese content at 650°C, but decreasing deposition at 550°C. Iron-cobalt spinels showed no consistent increase or decrease in carbon deposition with changing composition. Nickel rich spinels were unstable in the reaction gas mixture and generated metallic nickel during exposure. At both temperatures, this gave levels of carbon deposition which increased with increasing nickel content of the original oxide. NiF~04 exposed at 550°C fragmented as it catalysed carbon formation.

Manganese oxides converted to MnO during exposure, MnO proving also to be a most effective catalyst. Mn304' an Mn2+ I Mn3+ compound where the manganese ions do not form an electron exchanging octahedrally coordinated pair, did not yield large quantities of deposit. The uranium oxides examined converted to the interacting mixed valence U409' which gave copious carbon deposition. The U4+ I U6+ non interacting mixed valence Ot-U30 S gave the least deposition. Magnetite slices gave laminar carbon deposits, but no filamentary growth. Structured deposit was seen in two cases, including on one face oriented approximately parallel to the [111] plane, the plane previously expected to catalyse deposition most effectively.

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