Our partners at the department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (Semin Joo, Seok Ho Song, Seokjun Oh and Jeong Ik Lee), recently completed an academic study titled: ‘Evaluation of Coal Repowering option with Small Modular Reactor in South Korea’. This is the first comprehensive analysis considering repowering possibilities for South Korea. The following text is a simplified version of that report for wider outreach and accessibility.  

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Greenhouse gas emissions, particularly from coal power, must be reduced in alignment with the Paris Agreement, to limit the increase in global average temperature to well below 2 °C above pre-industrial levels. South Korea is aiming to reduce its greenhouse gas emissions by 40% by 2030, under the Paris Agreement, and so plans to retire CFPPS in the coming decades. As electricity demands increase, it is important to find ways to reduce the proportion of coal-fired energy, without reducing overall generation capacity.  

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One approach is repowering coal-fired power plants (CFPPs) with small modular reactors (SMRs). The study investigates repowering CFPPs with two different types of nuclear Small Modular Reactors (SMRS): High-Temperature Gas Reactors (HTGRS) and Pressurised Water Reactors (PWRs). SMRs are an advanced class of nuclear reactors with an electricity generation capacity of up to 300 MWe per unit, which are physically smaller than large-scale reactors, and can be constructed in a factory with a modular construction.

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In selecting a low-carbon energy source for the repowering of CFPPs, various aspects of power generation must be considered, including cost, capacity, grid connectivity and carbon emissions. South Korea is a densely populated country, and new transmission lines must compete with other land uses, such as agriculture or conservation. Expanding transmission lines for variable renewable energy (VRE) – solar photovoltaic and wind energy – is estimated to cost around USD 41 billion, which would affect consumer energy prices. The capacity factor indicates how efficiently each power source can operate throughout the year. The capacity factor of VRE is significantly lower than nuclear (0.13 – 0.2% vs. approximately 67.3 %).  Increasing the country’s energy capacity with VREs to meet the anticipated electricity demands would put a strain on the current power transmission systems, and would require extensive land availability, in a country which is already crowded.  

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CFPPs are known for having a ‘flexible operation’ – they can adjust the amount of coal being supplied to the combustion chamber. Their power generation varies on an hourly basis to satisfy the energy demand. The study concludes that SMRs would be able to accommodate for this variation in demand with load-following capabilities, and that the installation of SMRs would be more economical compared to renewable energy sources, with lower unit electricity prices overall. Solar PV and wind energy also require a great number of Energy Storage Systems. These store energy in the short term and can be used to smooth out energy transmission due to the seasonable variability of VRE over the year. In the Winter for example, energy demand is higher, and there are less hours of sunlight in the day. The ESS capacity for a full transition to VRE would need to be 182 times larger than the current market size, which would also increase electricity prices. The study suggests that future research should explore energy management methods or hybrid systems that can meet energy demand with realistic ESS capacity during low-generation periods. If SMRs are used to repower CFPPs, the costs associated with transmission network construction and ESS installation required for VRE integration could be avoided.  

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To maximise carbon emissions, it is more efficient to replace aging CFPPs than newer models which emit relatively less carbon, and so the age of the plants formed part of the classification of assessment. 91 thermal and combined heat and power (CHP) plants with capacities of 30 MWe or greater located on the mainland of South Korea were examined. Of these the 40 plants due to retire before 2040 were given the focus. 30 of these units fall within the 500 MW class, so these were identified as the most promising targets for repowering, considering their remaining lifetime and capacity distribution.

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A CFPP can be repowered by switching the fuel which produces heat and reusing the existing steam turbine and its associated equipment, including the heat sink components. Reusing power transmission infrastructure, cooling systems and civil structures can increase the cost savings of a construction project, compared to a greenfield development. An extensive reuse of a site, including its steam cycle components, is economically sound and environmentally attractive, as it reduces waste and makes the best use of available resources. To reuse the existing steam turbine, it is crucial to maintain the corresponding high pressure, high temperature and volumetric flow rate of the steam that flows in, to ensure compatibility with the preexisting system. A thermodynamic analysis was performed in the study.

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Approximately 6.1 – 6.3 HTGR units would be required to match the required volumetric flow rate of the reference thermal power plant.  2.1 – 4.77 PWR units would be required, with the addition of an external heat source to heat the steam to a high enough temperature to meet the correct inlet conditions (temperature, pressure and volume flow) of an Intermediate Pressure (IP) Turbine. HTGRs have a higher temperature output, which would eliminate the need for additional external thermal power in repowering applications. They are thermally efficient but would require more units. Further analysis is required to determine the effective distribution and management of the steam flow throughout the system, especially when dealing with multiple reactor units, or incorporating external thermal power sources.  

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The study then assesses the regulatory issues around repowering CFPPs with SMRs in South Korea. Due to the risk of severe accidents in nuclear power plants, a strict regulatory framework governs the entire life cycle of nuclear plant operations, including licensing, construction, operation, and decommissioning. In South Korea, nuclear safety regulations are managed by two major regulatory bodies: Nuclear Safety and Security Commission (NSSC) and Korea Institute of Nuclear Safety (KINS). Regulatory challenges include the Emergency Planning Zone (EPZ) – areas designated to prepare for uncontrolled releases of radioactive material. Regulatory and legislative moderations will be necessary for repowering, such as adjusting EPZ boundaries. The presence of restricted facilities near CFPP sites, such as ports, expressways or railways used for coal transportation may also be a limiting factor in site selection. Repowering CFPPs with nuclear would also require an increase in the national-level security requirements of the facilities.  

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Further research will be required to address potential challenges and optimise the overall system design, including analysis of branching flows of steam. Modifications will be required in South Korea’s existing regulatory and legislative frameworks to enable to the effective and economical repowering of CFPPs with SMRs, thereby reducing greenhouse gas emissions, and keeping electricity prices economical.

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Read the Full Study here

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