With the government publicly saying it is preparing to convert nuclear power into a "flexible power source," another change in the power supply structure is on the horizon. A flexible power source is an energy source that can easily and quickly change its generation (output) as needed in response to changes in power supply and demand.
Behind this policy are the phaseout of coal-fired power and the expansion of renewable energy. The problem is electricity rates. If power that is relatively expensive to generate, such as renewables, is purchased first and cheaper nuclear-generated power is used as a flexible source, electricity rates will likely have to rise. Naturally, this would also hurt the revenue of Korea Electric Power Corporation.
According to the government on the 8th, Minister Kim Sung-hwan of the Ministry of Climate, Energy and Environment said at a press briefing on the 1st, "A time is coming soon when power demand can be met only with renewables and nuclear in the spring and fall, when power demand is low," adding, "We are conducting research and development (R&D) to minimize the rigidity of nuclear plants and increase their flexibility."
A rigid power source is one whose generation output is difficult to adjust freely for technical reasons, or that is hard to switch on and off at will for economic reasons. Nuclear power plants are expensive to build but have low fuel costs and can generate at large scale, making continuous operation economical. As such, nuclear is currently classified as a rigid power source. Coal-fired power is also a rigid source, but not as rigid as nuclear, and is specifically called a medium-rigidity source.
Renewables such as solar and wind are also rigid power sources because the weather, not humans, controls their output. By contrast, sources that can quickly ramp output up and down according to supply and demand are flexible power sources. Liquefied natural gas (LNG) power, pumped storage, and energy storage systems (ESS) are flexible sources.
◇ Improving nuclear output ramp rate likely to be used to curb renewable intermittency
Technically, it is possible to control nuclear output and ramp rate. Currently, KHNP reduces output at a rate of 3 percentage points (P) per hour when requested by the Korea Power Exchange, then performs output control to set reactor operation at 80%.
If a request comes in to "operate at only 80% from 2 p.m. on the 4th for three hours," the plant would, starting at 7 a.m., reduce output by 3%P over six to seven hours to reach 80%, and then run at 80% from 2 p.m. Because of the nature of nuclear plants, output cannot be suddenly reduced or increased.
KHNP is conducting research to speed up the ramping down and up of output. KHNP launched a "development of flexible operation technology for nuclear power plants" project at the end of July. In the short term, it plans to develop technology to reduce output to 70% for about 100 days a year, and in the mid to long term, to reduce output to 50% for more than 100 days a year. In the long term, the goal is to enable reducing nuclear output by 10%P per hour in the future, up from the current 3%P per hour.
The background to improving nuclear output technologies to enhance output control responsiveness is the expansion of renewables. For solar alone, generation increases in spring and fall because of abundant sunlight. But in spring and fall, there is little demand for heating or cooling, so power demand is low. As a result, other power sources must cut generation as solar increases. Conversely, after sunset, solar generation is not possible, so the shortfall must be made up.
Other power sources have to adjust generation according to solar output because grid stability is maintained only when supply (generation) and demand (usage) match. If power supply and demand do not match, a blackout (large-scale outage) occurs. A blackout can happen when power demand spikes in summer and winter and electricity is in short supply, and also in spring and fall when demand is insufficient relative to power produced.
◇ Nuclear output control 27 times through November this year, more than triple last year
Until now, nuclear power, together with coal-fired power, handled baseload generation. That means it supplied the "baseload," the minimum power demand that continues 24 hours a day year-round. This was due to the relatively low and stable prices of uranium, the fuel for nuclear, and coal, the fuel for coal-fired power, compared with other sources, as well as their ability to generate at large scale.
According to the "2024 energy supply and demand trends" the Ministry of Trade, Industry and Energy released in May, nuclear generation last year was 188.8 terawatt-hours (TWh), accounting for 31.7% of total domestic generation. Coal, previously the largest source, generated 167.2 TWh last year, or 28.1% of the total. Nuclear and coal together accounted for 59.8% of total generation.
LNG power ranked third with 28.1% (167.2 TWh) of total generation. The share of new and renewable energy, which includes new sources such as hydrogen and fuel cells along with renewables like solar and wind, reached 10.6%, topping the 10% range for the first time. Generation was 63.2 TWh.
If nuclear plays the role of a flexible power source to supplement renewables as planned by the government, it may become difficult to serve as baseload as output control increases and generation becomes more erratic. The number of nuclear output curtailments has already surged this year.
According to KHNP, the number of output curtailments, which was only two in 2020, was three in 2021, four in 2022, seven in 2023, and seven in 2024, but rose to 27 between January and November this year (simultaneous output control at multiple plants counted as one). Nuclear power generation reduced by output control was 1.2 gigawatts (GW) in 2020, but has surged to 19.5 GW this year, 5.7 times last year's 3.4 GW.
The surge in nuclear output control this year is the result of renewable expansion. The plant with the most curtailments between January and November was the Hanbit nuclear plant in Yeonggwang, South Jeolla (26 times). The Hanul plant in Uljin, North Gyeongsang had 11, the Kori and Saeul plants in Busan and Ulsan had 13 each, and the Wolseong plant in Gyeongju, North Gyeongsang had 10.
An industry official in the nuclear sector said, "The fact that output control was most frequent at the Hanbit plant reflects the characteristics of the Honam region, where there is a lot of solar generation."
◇ KEPCO must buy high-cost renewables first… without a rate hike, financial soundness is at risk
If nuclear output adjustments are frequent, the advantage of nuclear as the cheapest power source disappears. According to the Korea Power Exchange, as of October the generation cost of nuclear power (the amount paid to generators divided by power generated) was 62.5 won per kilowatt-hour (kWh), the lowest. Coal's generation cost was 118.5 won per kWh, the next cheapest after nuclear. Solar was 131.6 won per kWh and wind was 121.1 won per kWh, more expensive than nuclear and coal.
Even so, the government argues that expanding renewables will not lead to higher electricity rates. The Minister said, "International oil prices have had the biggest impact on electricity rates," adding, "It is also a task to reduce generation costs as quickly as other countries have for wind and solar. We will increase renewable volumes to the level where we can do so without raising electricity rates and bring prices down."
If electricity rates are not raised while renewable generation, which has a high generation cost, increases, KEPCO's burden could grow. As the country's only electricity retailer, KEPCO is required by the Electric Utility Act to purchase power generated from new and renewable energy first. Regardless of generation cost, electricity produced by solar and wind is connected to the grid first, and the remaining power is filled by nuclear, coal, and LNG. An energy industry official said, "If KEPCO is told to increase purchases of high-cost renewables without raising electricity rates, it will inevitably be in a bind."
Lee Ki-bok, president of the Korean Nuclear Society, said, "Korea had no reason to control nuclear output until renewables expanded, because nuclear accounted for more than 30% of power and was used as baseload," adding, "We should prioritize using nuclear, which has low generation costs, and reducing nuclear output for the sake of renewables does not align with economic logic."
Chung Bum-jin, a professor at Kyunghee University, also said, "While it is technically possible to convert nuclear into a flexible power source, it is not the desirable direction considering nuclear safety and economic issues," adding, "Reducing the share of baseload power, including closing coal plants by 2040, could become a problem in the future."