After a four-hour bus ride from Yongsan, Seoul, on the 14th, high-voltage transmission towers stood along the ridgelines of Buk-myeon in Uljin County, North Gyeongsang. These towers carry electricity produced at four nuclear power plants inside the Hanul Nuclear Power Headquarters in Deokcheon-ri and Gomo-ri, Buk-myeon.
On the 15th, the next day, transmission towers also loomed on a mountain near Imha Lake between Imha-myeon and Imdong-myeon in Andong, North Gyeongsang. These towers are used to move electricity generated by the floating solar panels on Imha Dam across the surface of Imha Lake to the North Gyeongsang region. A 345-kilovolt (kV) transmission tower also stood out near the Yecheon Pumped Storage Power Plant on the slopes of Mount Yongmun in Yecheon County, North Gyeongsang, visited the same day.
The Hanul Nuclear Power Headquarters is a plant operated by Korea Hydro & Nuclear Power Co. (KHNP), Korea's only nuclear power construction and operation company, and supplies about 10–11% of the nation's total electricity. KHNP is known as a nuclear plant operator, but it also generates power using renewable energy, including solar. KHNP is producing renewable energy by installing floating solar panels on Imha Dam, which is operated by the Korea Water Resources Corporation.
KHNP also operates pumped storage power plants that can prevent large-scale blackouts. Because a pumped storage plant needs only five minutes to switch from a stop to power generation, it is called the "rescue team of the power grid."
Pumped storage consumes electricity by pumping water from the lower reservoir to the upper reservoir when domestic power supply is abundant, and supplies electricity by generating power using the head when supply is short. It matches electricity supply and demand in real time, helping stabilize the grid. The Yecheon Pumped Storage Power Plant is one of seven pumped storage plants operated by KHNP nationwide.
◇ Hanul Nuclear Power Headquarters runs eight units and is building two more… set to supply 14–15% of domestic power
The dome-shaped Shin-Hanul Unit 1 is 76.66 meters tall, equivalent to a 27-story apartment building. Inside the operating Shin-Hanul Unit 1, a humming sound could be heard. Shin-Hanul Unit 1 generated about 8,821 GWh of electricity in 2024. That is about 18% of Seoul's 2024 electricity demand (50,352 GWh).
At the Hanul Nuclear Power Headquarters, a total of eight reactors are in operation: Hanul Units 1, 2, 3, 4, 5 and 6, and Shin-Hanul Units 1 and 2. The installed capacity of Hanul Units 1 and 2 is 950 megawatts (MW) each, and for Hanul Units 3, 4, 5 and 6 it is 1,000 MW each. Including Shin-Hanul Units 1 and 2 at 1,400 MW each, the Hanul Nuclear Power Headquarters has an installed capacity of 8.7 gigawatts (GW).
Nuclear power accounts for about 30–32% of total electricity generation in Korea. As more than one-third of the nation's 26 operating reactors are at the Hanul Nuclear Power Headquarters, it makes up about 33% of total power generation. Hanul alone supplies about 10–11% of Korea's electricity.
Next to Shin-Hanul Units 1 and 2, construction is underway on Shin-Hanul Units 3 and 4, each rated at 1,400 MW. Shin-Hanul Units 3 and 4 applied for construction permits in 2015, but during the Moon Jae-in administration, which pushed a "nuclear phaseout policy," the plan for new reactors was entirely scrapped in 2017, halting work when more than 30% of the basic design had been completed. Construction resumed in June 2023 after the change of administration.
As of late April, overall progress on Shin-Hanul Units 3 and 4 was 29.8%. At Shin-Hanul Unit 3, work is underway to place the steel plate for the reactor containment building. On site that day, steel plates about 6 mm thick were being prepared to form the reactor dome. At Shin-Hanul Unit 4, crews are compacting the reactor building foundation ahead of the first concrete pour on the 27th.
Shin-Hanul Unit 3 is slated for completion in Oct. 2032, and Unit 4 in Oct. 2033. When Units 3 and 4 are completed, the total installed capacity of the Hanul Nuclear Power Headquarters will expand to 11.5 GW, operating 10 reactors in total and supplying about 14–15% of the nation's electricity.
◇ Imha Dam floating solar, which shares the hydropower transmission network and avoids mountain damage
About a 25-minute drive up from Imha Dam in Andong, North Gyeongsang, a giant hibiscus appears on the calm surface of Imha Lake. It is the Imha Dam floating solar facility made of neat square solar modules. Composed of one Taegeukgi block and 15 hibiscus blocks, the Imha Dam floating solar has a total installed capacity of 472 MW. It can generate 61.67 GWh of electricity annually, enough to power about 20,000 households for a year.
The area is 521,000 square meters, about the size of 74 soccer fields combined. But as it is floating solar, it can be installed without damaging mountains or farmland. Imha Dam is a multi-purpose dam managed by the Korea Water Resources Corporation, and KHNP holds a 49% equity stake in the Imha Dam floating solar project.
A feature of the Imha Dam floating solar project is that it generates electricity without expanding the transmission network. Park Jong-am, Head of Team at KHNP, said, "The North Gyeongsang grid is saturated, so we received conditional approval from the Ministry of Trade, Industry and Resources for a cross-generation method to prevent saturation of the transmission lines."
The Imha Dam floating solar shares the transmission system of the existing Imha Dam hydropower plant. Thanks to adopting a cross-generation method, a single transmission network carries solar power by day and power generated by the hydropower plant at night.
In addition to Imha Dam, KHNP is pursuing two floating solar projects at the Cheongsong pumped storage site in North Gyeongsang with a total installed capacity of 5.2 MW. The Sancheon pumped storage floating solar project in Sancheong, South Gyeongsang, which began in 2023, has an installed capacity of 3 MW.
◇ Operating seven "rescue team of the power grid" pumped storage plants, building three more
KHNP operates seven pumped storage plants in Yecheon, Yangyang, Cheongpyeong, Cheongsong, Muju, Samnangjin and Sancheong. Their total installed capacity is 4,700 MW, about 4% of Korea's total power generation capacity.
A pumped storage plant pairs two reservoirs, upper and lower. It generates electricity by using the head of water dropped from the upper reservoir to the lower reservoir. Unusually, it also consumes electricity by pumping water from the lower reservoir to the upper reservoir.
Im Seok-chae, head of power generation at the Yecheon Pumped Storage Power Plant, said, "The purpose of a pumped storage plant is the stability of the power grid," adding, "When the grid has surplus electricity, the pumped storage plant absorbs it, and when a peak load occurs, it generates power." Large-scale blackouts can occur when supply is high or when demand is high, which is why pumped storage is used.
Pumped storage plays a big role in stabilizing power because it starts up faster than other types of plants. It takes only five minutes to ramp up output. By contrast, nuclear plants take 40 hours, coal-fired plants 14 hours, and combined-cycle plants two hours.
As the share of renewable energy has risen recently, the value of pumped storage has grown. The expansion of renewables, which have high weather-driven intermittency, is making stable grid operation harder. The utilization rate of pumped storage was only about 9% three to four years ago, but has recently risen to 14.7%.
Beyond the seven plants in operation, KHNP is building 1.8 GW of pumped storage capacity at three sites: Yeongdong in North Chungcheong, Hongcheon in Gangwon, and Pocheon in Gyeonggi. The targets for completion are 2030 for Yeongdong, 2032 for Hongcheon, and 2033 for Pocheon.
A KHNP official said, "Pumped storage can be viewed as a kind of battery because it stores water in the upper reservoir and can generate electricity immediately," adding, "It is not only larger in scale than typical energy storage systems (ESS) but also far more stable. In particular, it is an emergency rescue team that can be deployed quickly when power supply and demand conditions change abruptly."