Thursday, 13 October 2022: 11:00
Room 308 (The Hilton Atlanta)
Nuclear power plants or NPP have become one of the world’s clean energy drivers which drive 15% of the world’s energy sources. Pressurized water reactors (PWR) become the most planted type of NPP that has two main coolant loop systems which are primary cooling system (PCS) and secondary cooling system (SCS) that integrated one another into electricity generation. PCS is responsible for transferring captured heat from the fission reactor to the SCS while the SCS is responsible for transferring the exchanged heat to the steam generator (SG) which makes the SG internal components have a moisture condition and the most susceptible components to rust. Hence, SG maintenance is an annual problem in nuclear power plants which can generate fouling with improper maintenance. Certain treatments such as phosphate chemistry treatments (PCT) and all volatile treatments (AVT) do not perfectly work in handling corrosion problems in steam generator system. Corrosive species are primarily caused by high concentrations of impurities which are sourced from coolant loops, material structure, and non-volatile impurities (NVI) where non-volatile impurities are the main corrosion products and become the major cause in corrosive environment. Released non-volatile impurities can be transported, dissolved, or deposited by the water flow causing flow-accelerated corrosion (FAC) which becomes the main problem in the secondary piping system on the pressurized water reactor, especially steam generator devices. FAC is a corrosion process caused by an electrochemical process that a flowing stream of water that diffuses the protective oxide layer formed on carbon or low-alloy steel. Unsaturated dissolving species between the dissolving oxide layer and a flowing fluid led to mass transfer which breaks down the oxide layer. As the mass transfer increase and oxide layer becomes thinner which is less protective, the corrosion rate increases. However, FAC can be controlled by certain parameters to keep them outside the sensitive range. Those main parameters are temperature, pH, oxygen concentration, flow rate, and mass transfer. In the FAC risk zone, the temperature must be maintained between 120 and 180 °C, <9.2 value of pH, oxygen concentration under 5 ppb with < -0.3 Volt of ECP, and the mass transfer above the coefficient threshold. To control the FAC parameter, an advanced steam generator circuit system (ASGS) is one of the effective solutions for corrosion handling. ASGS controls each sensitive FAC parameter through automation. Flow rate affects the decrease of boundary layer thickness as the flow velocity increases so the ASGS will calculate the mass transfer in the fluid as the mass transfer controlled the FAC rate in low fluid velocity so then, the system execution will increase or decrease the flow rate base on the calculation. On other hand, the temperature affects the FAC through the solubility of Ni and Fe which at a lower temperature the Fe (OH)2 will create, and higher temperature, magnetite will be formed. The ASGS then will calculate the temperature with a temperature sensor then it will execute the system temperature through the actuator that can reduce or increase temperature through an integrated packed cooling system, a radiator-like cooling system that increases temperature decrease the temperature by the packed cooling system (a bag like a radiator that planted on the outer pipe). The next considered variable in the ASGS is pH as the pH takes a chemical role on the FAC in range (9.3 - 9.6) that proved effective in reducing long-range corrosion product transport. So, the ASGS will then determine the pH value through a pH sensor and then will execute if the pH value is too high or too low through an actuator that will inject acid or base solution that will control the pH without damaging the pipe environment. The next FAC sensitive zone is oxygen concentration which can decrease the corrosion rate but increase the ECP. on other hand, in high O2 concentrations the ECP will increase and the ASGS will inject hydrazine into the pipe and inject oxygen in lower concentrations. The ASGS will determine the concentration with the O2 lambda sensor then the calculation will be transferred to an actuator. Besides FAC, control in temperature, pH, and O2 concentration can control SCC and deposition rate as well. All electrical needs in the system are powered by the NPP generation as the low voltage electrical requirements are not big. The paper is focused on the SG corrosion control in the NPP secondary circuit through the ASGS development in engineering, technology, and economic overview. All data, system analysis, system calculation, and system assessment will be provided in the full paper.