An Analysis of the Behavior of Zinc Compounds in the Primary Coolant Circuit at a Nuclear Power Station with VVER Power Reactors

Abstract

Technology of zinc metering into the coolant has already been used for more than 20 years at foreign nuclear power stations (NPS) with PWR reactors for reducing radiation fields on the equipment and suppressing corrosion cracking of nickel-rich alloys. The most likely forms in which zinc compounds can exist in the primary coolant circuit of water-cooled power reactors (such as VVER, PWR, or BWR) were assessed, and the data on their solubility were analyzed. It is demonstrated that zinc oxides and silicates feature the minimum solubility under the primary coolant circuit conditions. The conditions for crystallization of compounds on the surface of fuel rod cladding in the core in case of local boiling (i.e., subcooled liquid boiling) were analyzed. A review is presented of foreign publications with the assessment of the risk that zinc compounds are contained in zinc deposits on the surface of fuel rods when zinc is metered into the coolant of PWR power units with a thermally stressed core. The predictions are presented of the limit thickness of the deposits at which zinc oxides and silicates can precipitate into them in case of local subcooled boiling on the fuel rod surface. The predictions are presented for the effect of the concentration of boric acid, silica, and zinc in the coolant on the limit thickness of depositions at which zinc compounds can crystalize. It is demonstrated that, in the presence of boric acid, zinc can interact with borates with the formation of borate complexes, which reduce the risk of deposition of zinc silicates in the layer of deposits on the fuel rod surface. The calculated results confirm that increasing the thickness of oxide film on fuel rods increases the risk of crystallization of zinc compounds in the core.