Proactive maintenance and structural modifications of VVER-1000 steam generator blowdown systems to ensure long-term safety

Steam generators of VVER-1000 pressurized water reactors play a crucial role in separating the primary and secondary circuits and safely removing heat from the reactor core. This paper systematically evaluates the degradation mechanisms on the secondary side of the steam generator, focusing on the continuous blowdown and sludge removal systems in the context of ensuring long-term safety functions. Based on operational data and experience from a reference nuclear power plant, a transition from reactive to proactive preventive maintenance is proposed. This approach combines continuous diagnostics, water chemistry management, technical modifications to piping layouts, and risk-based planning. The study identifies dominant damage mechanisms (pitting corrosion, stress corrosion cracking, sludge accumulation in inspection pockets, and foreign material ingress), quantifies their impact on the integrity of heat transfer tubes, and proposes practical measures to maintain the barrier function of the steam generator. The benefits include a reduced probability of failures, stabilization of the water chemistry regime, shortened outage durations, and improved long-term reliability of Safety Class 1 components.