Zirconium alloys are currently used in all commercially operated light water and heavy water nuclear reactors. Devices in nuclear reactors have to serve for a very long time, quite faultlessly and without maintenance, and their environmental parameters can become very extreme. Zirconium alloys are affected by surrounding setting even during normal nuclear reactor operation, but at temperatures above 800 ° C dangerous high temperature corrosion occurs. This corrosion is strongly exothermic and high kinetic reaction, during which water molecules are dissociated, zirconium is strongly oxidized and explosive hydrogen is formed. Released heat further complicates the core cooling and strengthens further course of high-temperature oxidation of zirconium alloys. During several nuclear accidents, intense heat and pressure triggered a reaction between the nuclear fuel Zirconium alloys cladding and the surrounding water steam, producing explosive hydrogen gas, and several hydrogen-air chemical explosions occurred e.g. at Fukushima nuclear plant. A solution to the problem, namely how to fully utilize advantages of the bulk of the zirconium-based material and to reinforce its surface chemical resistance, is to cover the surface with a thin film of a protective substance. Scientists from the Group of Materials and Nanosystems for Biointerfaces, Department of Functional Materials designed and successfully applied protection of zirconium alloy surface using a layer of chemically vapor deposited polycrystalline diamond.
Left: Zirconium alloy samples coated by polycrystalline diamond (PCD) layer. Right: Samples were placed on the holder for the entire surface diamond film growth.
Diamond withstands very high temperatures, it has excellent thermal conductivity and low chemical reactivity, it does not degrade over time and (important for the nuclear fuel cladding) being pure carbon, it has perfect neutron cross-section properties.
Under normal operating conditions, polycrystalline diamond layer (PDL) is involved in heat dissipation and protects the surface from unwanted chemical reactions. Polycrystalline diamond layer consisting of crystalline and amorphous carbon is able to monitor volume changes of substrate so that there is no violation of the integrity of the protective PDL. After ion implantation simulating interaction with elementary particles in a nuclear reactor a partial graphitization of PDL was observed, but diamond crystalline phase was still present. After PDL protected Zr sample high temperature (1200 °C) heating in a steam environment original PDL was transformed into a mixture composed mostly of graphite and amorphous carbon. Elemental analysis showed that the transformed layer absorbed the released atoms from the surroundings and was able to separate the zirconium alloy from the environment so that the thermally affected Zr alloy under the protective layer differed from the original Zr alloy material composition very slightly. Desirable original composition of Zr alloys was effectively protected by polycrystalline diamond layer even at emergency situation in a nuclear reactor.
In collaboration with the Faculty of Mechanical Engineering of the Czech Technical University, we have applied for the Invention and Utility Model (Layer protecting the surface of zirconium alloys used in nuclear reactors) registrations under numbers PV 2013-727 and PUV 2013-28502 (team of authors - I. Kratochvílová, A. Taylor, F. Fendrych, M. Nesládek, R. Škoda and J. Škarohlíd).
Contact: Irena Kratochvílová
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