Scientists of Russia’s Tomsk Polytechnic University (TPU) are investigating the behaviour of welded joints made of E110 zirconium alloy, obtained by means of laser welding with the aim of developing accident tolerant fuel.
Zirconium products used in the manufacture of fuel assemblies (FA) for water-moderated power reactors (VVER) were coated with protective chromium coatings, which protect them from oxidation.
“Uranium fuel in VVER reactors is located in long tubes made of zirconium alloy. They make up the shell of the fuel element. When the fuel is irradiated with a neutron flux, a fission chain reaction begins, accompanied by the release of thermal energy, which is then transferred to the coolant, explains Dmitry Sidelev at the Laboratory for Plasma Hybrid Systems of the Weinberg Research Centre, part of TPU’s School of Nuclear Science & Engineering.
“If the operating mode of the reactor is violated and emergency situations occur, the risk a self-sustaining steam-zirconium reaction leading to intensive oxidation of fuel assemblies, heat release and hydrogen generation 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 spacer grids made of zirconium alloy. Chrome coatings are a thin film 10 micrometres thick. As a material for creating protective coatings, chromium was chosen because it is a heat-resistant metal that is also resistant to oxidation and corrosion.
Studies were carried out first on test samples, then on industrial samples of welded joints produced for the Tomsk scientists by the Novosibirsk plant of chemical concentrates plant (part of Rosatom’s fuel company TVEL). The coatings were applied using a vacuum ion-plasma installation developed by the staff at the Weinberg laboratory.
“A lot of work has been done to optimise the structure and composition of chrome coatings. Empirically, we have revealed an acceptable mode for depositing coatings in order to achieve their maximum dense structure. This is necessary so that oxygen does not penetrate the alloy and initiate the oxidation process. We also found out how the thickness of the coatings affects the oxidation process. As a result, the required range of coatings thicknesses (10-15 micrometres) was established," Yegor Kashkarov, head of the Advanced Materials Laboratory. The use of protective chromium coatings increases the time for reactor operators to make decisions in emergency situations.
At the moment, TPU scientists are studying the possibilities of optimising the elemental composition and structure of protective coatings to improve their protective properties. The work is carried out with a grant from the Russian Science Foundation.
Equipment for producing chromium coatings (Photo: TPU)