Load following in a Swedish nuclear power plant
Abstract: The energy system in Sweden is currently under a great transformation, where the part of weather dependent energy sources, so called intermittent sources, like solar and wind power will get greater share in the total production of electricity while the production from stable energy sources like nuclear power is decreasing. For a long time, the nuclear power have been operating at baseload and the hydropower as flexible, though, due to the change in the current power system, nuclear power plants may need to transit to flexible operation and apply load following. The aim with this master thesis has been to investigate the possibilities for the nuclear reactor "Oskarshamn 3" to apply load following for future scenarios and to see how the reactor could support the power grid when there is a potential disturbance in the frequency or the voltage. The scenarios where the load is low on the power grid are nights, weekends and during the summer months. To investigate the possibilities, the nuclear process have been simulated in the program APROS 6.07, where scenarios to decrease the power during the night have been chosen. Since the equipment in the nuclear power plant is optimized to operate at maximum allowed thermal power, a decrease will most likely lead to increased risks of wear and tear and damages in some parts of the equipment. Components that may become vulnerable with the power decreased are for instance turbines, pumps, valves and pipes. The mechanisms for the damages are for example erosion-corrosion, fatigue, vibrations and wear. In the reactor core, there are also limitations for the rate of how quickly the power decrease and increase can be performed and how low the power can be reduced before problems with xenon poisoning and PCI occur. The lowest simulated power level was at 95 %, where 129.1 % is the maximum power level. The simulations were performed at the power levels 95 %, 100 %, 109 % and 120 %. It could be concluded that the power change gave a stable decrease and increase of the electrical power from the generator. An increased usage of the pumps and valves was shown, which will give an added risk of wear and tear. Furthermore, simulations about how the power plant could cope with a rapid frequency change from 50 Hz to 49 Hz on the power grid by quickly reducing the power in the reactor with a few bar were performed, with a unit in the control structure called the PSPA-unit (Pressure Set Point Assessment). The simulations showed that the generator power increased when frequency dropped for all simulations. PSPA-unit was also used to investigate how the generator power momently could increase when a larger voltage disturbance on the power grid occurs.
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