Solar Power Expansion Leads to Nuclear Power Output Cut Every 5 Days This Spring

This spring, power authorities issued output control orders to domestic nuclear power plants approximately once every five days. This phenomenon is attributed to the surge in solar power generation as daylight hours increase while power demand approaches its annual low in spring. Notably, Hanbit Nuclear Power Plant in South Jeolla Province, an area with high solar power plant density, was identified as the primary target for output control.

According to the spring nuclear power plant output control status obtained through the office of Rep. Kang Seung-kyu of the People Power Party on July 14, the Korea Power Exchange (KPX) implemented “output reduction” measures for nuclear plants on 19 days out of the 93 days during the spring off-peak period (March 1 to June 1, 2025). While there were no instances of output reduction last year, forced measures have sharply increased this year. An official from the Ministry of Trade, Industry and Energy pointed out, “It is most efficient for nuclear plants to operate at rated capacity 24 hours a day,” adding, “Frequent control measures also put a strain on equipment.”

Experts worry that limiting nuclear power output could become routine every spring and fall when power demand is low. As the proportion of distributed power sources like solar power plants increases in the overall power generation facilities, it becomes more difficult to respond flexibly to power demand. The insufficient transmission network compared to the rapid expansion of renewable energy is also analyzed as a factor promoting nuclear power reduction. There are calls for urgent investment in energy storage systems (ESS) to distribute solar power generation to other time periods and to establish infrastructure that can transmit renewable energy generation concentrated in specific areas to other local governments.

According to the KPX, power authorities implemented nuclear power reduction measures virtually every weekend during the spring off-peak countermeasure period (March 1 to June 1, 2025). While it is desirable to operate nuclear power plants at maximum output 24 hours a day if possible, output had to be reduced on spring weekends when daily maximum power demand falls below 40GW.

The total reduction in output directed by the KPX to each nuclear plant from March to May amounted to 60,045 MWh. This resulted from limiting output for a total of 346.22 hours, ranging from 20-30 MW to 300 MW per reactor at a time. This is equivalent to forcibly reducing the power that a large 1.4GW nuclear power plant would produce in about 43 hours. The number of reactors subject to reduction was 12 in total, with 2-3 from each headquarters. This means that nearly half of the currently operating reactors (26 units) suffered from frequent output reductions.

Due to power demand falling below expected levels, there were also 8 days when a standby state for additional reduction was maintained for two or more consecutive days. Output reduction measures were also confirmed to have been implemented on weekdays, not just holidays, on four occasions: March 28, April 1, April 3, and April 28. The actual output reduction performance of nuclear power plants nationwide is likely to be even higher, as this does not account for output control due to nationwide wildfires that raged in late March or transmission equipment abnormalities.

An industry insider explained, “Until now, nuclear power reductions have been concentrated during extended holidays like Lunar New Year and Chuseok, which last nearly a week,” adding, “It is quite unusual to limit nuclear power generation on non-holiday weekdays, even during off-peak periods.” Indeed, according to the KPX, the number of nuclear power reductions in spring (March-May) from 2020 to 2024 was only once in 2020 and five times in 2023. Nuclear power reduction during off-peak periods, which was considered exceptional until last year, has suddenly started to become routine this year. As a result, the control rate, which represents the amount of reduction compared to the total possible power generation, increased 3.1 times in two years from 0.0388% in 2023 to 0.1201% this year.

Experts note that nuclear power reduction occurs significantly in the South Jeolla Province area, where the density of solar power plants is high. The story is that while the supply rate of solar power generation is relatively high, the transmission network to send it to other regions is insufficient, ultimately necessitating a reduction in nuclear power output. Analysis of KPX data shows that during this year’s spring off-peak period, the output control volume of Hanbit Nuclear Power Plant in Yeonggwang-gun, South Jeolla Province, was 22,098 MWh, accounting for 36.8% of the total control volume. Hanbit Units 1 and 6 were also subject to reduction measures on Feb. 22-23, before the start of the spring off-peak countermeasure period.

Prof. Yoo Seung-hoon of the Department of Energy Policy at Seoul National University of Science and Technology said, “In the South Jeolla region, there have long been talks that solar power plants cannot operate in spring and fall unless Hanbit Nuclear Power Plant reduces its output,” adding, “The power grid is likely being operated with reductions to levels that should not be lowered any further.”

The problem is that frequent implementation of such reduction measures puts a strain on nuclear plant facilities. Professor. Yoo expressed concern that continuous nuclear power reduction could lead to safety issues, stating, “Korean nuclear power plants were not designed with large-scale output reduction in consideration at the design stage.”

Meanwhile, concerns have also been raised that it will be difficult to meet zero-carbon electricity demand even with steady increases in renewable energy. According to the Korea Enterprises Federation, even if solar and wind power plants are expanded as per government policy, renewable energy plants will only meet 93% of the electricity demand for the four major energy-intensive industries — steel, petrochemicals, semiconductors, and data centers — by 2042. The additional zero-carbon electricity needed for full satisfaction is about 21.4 TWh, equivalent to half of Seoul’s electricity consumption last year. The federation stated, “Global companies are strongly required to use zero-carbon power sources,” adding, “Adding nuclear power to the power sources that can be procured through power purchase agreements (PPA) would help solve the excess demand.”