Influence of operating and water-chemistry parameters on fuel cladding corrosion and deposition of corrosion products on cladding surfaces

A description of mass transfer mechanism of corrosion products in the primary coolant circuit is a complicated problem. The deposits of crud is to be proportional to the amount of corrosion products circulating in the primary coolant circuit, therefore all models of mass transfer in the circuit include the change of corrosion products concentration and the corrosion rate in time, removing these products by filters and deposition. Decontamination of the circuit equipment and replacement work needs lead to a local change of corrosion rate which results in the increase of corrosion products concentration in the circuit and the increase of deposits on surfaces. If due to incorrect water chemistry conditions for corrosion products deposition in the core are created not only the activity of the coolant increases but the hydraulic resistance of the reactor also grows which results in the increase of the pressure drop at the reactor. The phenomenon of 'pressure drop' which takes place in NPP with VVER reactors was considered. The reasons of this phenomenon are the following: the great removal of corrosion products (CP) from steam generator surfaces after decontamination, change of CP behavior and then consequent deposit of CP on the fuel element surfaces; and, sub-cooled boiling takes place on the some of fuel element and results in the acceleration of corrosion products deposit, the increase of nuclide activation period and coolant radioactivity. A model was developed to explain pressure drop rise in-core and deposits redistribution in the core and in the primary circuit of NPP with VVER-440. The physical-chemical basis of the model is the transport corrosion products dependence on temperature, pH_T value of coolant, and correlation between rates of corrosion products (Fe) formation (after steam generators decontamination) and their removing from the circuit. The aim of our modeling is to predict the growth of pressure difference on the basis of regular data available at an NPP and correct the water chemistry so that the pressure drop across the reactor is kept at a stable level by adjusting the concentrations of KOH, H₂, and NH₃. The parameters that have been included in the model are the following: operating parameters: reactor thermal power and concentration of boric acid; standards of water chemistry; parameters determining the system redox-potential: concentrations of hydrogen and ammonia; parameters of the physicochemical model of mass transfer; and parameters characterizing the composition of corrosion products in coolant. The deposits along fuel rod bring to sub-boiling and results in acceleration of corrosion products and boron precipitation on the fuel cladding surface increase of nuclide activation period and coolant radioactivity. Activity of Co-58 is the indicator of deposits growth acceleration. (author)