Russian scientists develop chrome coatings to protect fuel assemblies

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Scientists at Tomsk Polytechnic University (TPU) in Russia are studying the behavior of welded joints made of zirconium alloy E110, obtained by laser welding, with the aim of developing an accident-tolerant fuel.

Zirconium products used in the fabrication of fuel assemblies (FA) for water moderated power reactors (VVER) have been coated with protective chromium coatings, which protect them from oxidation.

“Uranium fuel in VVER reactors is located in long zirconium alloy tubes. They constitute the envelope of the fuel element. When the fuel is irradiated with a flux of neutrons, a fission chain reaction begins, accompanied by the release of thermal energy, which is then transferred to the coolant, explains Dmitry Sidelev of the Laboratory of Plasma Hybrid Systems at the Center for Weinberg Research, part of TPU’s School of Nuclear Science and Engineering.

“If the mode of operation of the reactor is violated and emergency situations occur, the risk of a self-sustaining steam-zirconium reaction resulting in intensive oxidation of the fuel assemblies, heat release and generation of hydrogen increases. As a result, the integrity of the fuel assembly may be violated.”

The TPU team is working to prevent the destruction of fuel element cladding and zirconium alloy spacer grids. Chrome coatings are a thin film 10 micrometers thick. As a material to create protective coatings, chromium was chosen because it is a heat resistant metal that also resists oxidation and corrosion.

Studies were first carried out on test samples, then on industrial samples of welded joints produced for Tomsk scientists by the Novosibirsk Chemical Concentrates Plant (part of Rosatom’s fuel company, TVEL ). The coatings were applied using a vacuum ion plasma setup developed by Weinberg lab staff.

“A lot of work has been done to optimize the structure and composition of the chrome coatings. Empirically, we have demonstrated an acceptable mode of deposition of the coatings in order to achieve their maximum dense structure. This is necessary so that oxygen does not enter the alloy and initiate the oxidation process. We also discovered how the thickness of the coatings affects the oxidation process. As a result, the required range of coating thicknesses (10-15 micrometers) was established,” said Yegor Kashkarov, director of the Advanced Materials Laboratory. The use of protective chrome coatings increases the time reactor operators have to make decisions in emergency situations.

Currently, TPU scientists are investigating the possibilities of optimizing the elemental composition and structure of protective coatings to improve their protective properties. The work is carried out thanks to a grant from the Russian Scientific Foundation.


Equipment to produce chrome coatings (Photo: TPU)

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