[Why&Next] Nuclear 'Output Restrictions' Surge 12.5 Times in 5 Years... Extraordinary Becomes Routine

Expansion of Renewable Energy Is the Main Cause
Korea Hydro & Nuclear Power Aims for Practical Application by 2032
Developing Technology for 50% Output Adjustment
Economic Viability, Equipment Fatigue, and Compensation System
Key Challenges Still Remain

With the expansion of renewable energy, output reductions (restrictions) at nuclear power plants, which have traditionally served as "inflexible power sources" responsible for base load (the minimum level of power demand that must be constantly maintained), are increasing significantly. In preparation for more frequent output restrictions, Korea Hydro & Nuclear Power is also developing technology that can instantaneously reduce nuclear plant output by up to 50%. However, there are concerns that if the capacity factor drops, the main advantage of nuclear power-its economic efficiency-will deteriorate, and supplementary measures must be put in place.

According to the Korea Power Exchange and Korea Hydro & Nuclear Power on September 5, the number of nuclear power output restrictions was only 2 in 2020, but increased to 3 in 2021, 4 in 2022, 7 in 2023, and 7 in 2024, reaching 25 cases in the first half of this year alone (January to June). This represents a 12.5-fold increase over five years. Including the second half of this year, the growth rate is expected to accelerate even further. There are predictions that nuclear output restrictions will increase again as the autumn season approaches, when electricity demand typically declines.

9 Nuclear Reactors Reduced Output in 3 Hours

The increase is even more pronounced when measured by the number of reactors subject to output restrictions. In the first half of this year alone, the Korea Power Exchange issued dispatch instructions to reduce output for a cumulative total of 115 reactor units. This is more than 11 times higher than the total for last year (10 reactor units). This means that multiple reactors were subjected to output restrictions simultaneously during the same time period.

For example, on March 9, from 11 a.m. to 2 p.m.-about three hours-nine reactors, including Hanbit Units 1, 4, and 6; Shin Kori Unit 2; Kori Unit 4; Saeul Unit 2; Shin Wolsong Unit 2; and Hanul Units 2 and 3, underwent output control.

The reduction in electricity production from nuclear plants due to output restrictions reached 18,434 megawatts (MW) in the first half of this year alone. This figure is about six times higher than the total output control generation of 3,203 MW last year.

As recently as 2023, just two years ago, the Ministry of Trade, Industry and Energy-the main government agency in charge-maintained that "output reductions at nuclear power plants are only considered in extremely exceptional circumstances." However, the situation has changed.

Output control refers to artificially lowering output when electricity supply exceeds demand. When an imbalance in power supply and demand is anticipated, the Korea Power Exchange issues dispatch instructions to power generators to reduce output. Output restriction measures are mainly implemented on weekends in the spring and autumn, when renewable energy production increases while electricity demand drops to its annual low.

In the past, output restrictions were mainly applied to thermal power plants, but as renewable energy supply has increased, nuclear plants are now also affected. In the first half of this year, half (51.3%) of the nuclear plants subject to output restrictions were Hanbit Nuclear Power Plant in Yeonggwang, South Jeolla Province. This means that output restrictions were most frequent at nuclear plants in the Honam region, where solar power generation is abundant. On March 29, Hanbit Units 1, 4, and 6 reduced output for 27 hours.

Korea Hydro & Nuclear Power Developing Technology for 50% Output Adjustment

The problem is that while demand for nuclear output restrictions is rising rapidly due to the surge in renewable energy, domestic nuclear plants are fundamentally "inflexible" power sources, making output adjustments difficult.

Suk Kwanghoon, a senior researcher at the Energy Transition Forum, stated at the "New Government Nuclear Power Policy Direction and Issues Forum" held in the National Assembly last month, "Even in France, which is interconnected with five neighboring countries via transmission lines, the excessively high share of nuclear power and the rapid growth of renewables resulted in 9.1% of nuclear generation and 7.2% of solar generation being curtailed in the first half of this year. In Korea, with an isolated power grid, building new inflexible nuclear plants is a shortcut to massive waste of public funds and a contraction in solar investment."

However, experts explain that domestic nuclear plants can also adjust output flexibly. Output control at nuclear plants is achieved by inserting control rods, injecting boric acid water, or adjusting coolant temperature in the primary system, or by regulating steam flow in the secondary system.

Currently, Korea only reduces output by up to 80%, but in Germany and other European countries, output can be controlled down to 20%. In France, where nuclear accounts for more than 60% of total power generation, flexible operation technology has been developed since the 1970s, and now load-following (adjusting generator output according to power demand) is possible 24 hours a day, 365 days a year.

Korea Hydro & Nuclear Power is accelerating the development of flexible operation technology for "coexistence with renewable energy." At the end of July, the company launched the "Development of Flexible Nuclear Operation Technology" project, led by the Korea Institute of Energy Technology Evaluation and Planning, which will run until 2028. In the short term, Korea Hydro & Nuclear Power plans to develop technology to reduce output by up to 70% for about 100 days per year, and in the medium to long term, to reduce output by up to 50%. The technology is expected to be applied to actual nuclear plants starting with Shin Hanul Unit 2 in October 2032.

Safety Regulations and Economic Viability Must Be Addressed

Even if output adjustment at nuclear plants is technically feasible, challenges remain. The nuclear industry is concerned that frequent output adjustments could increase fatigue on key equipment, potentially leading to malfunctions.

Jung Dongwook, a professor at Chung-Ang University's School of Energy Systems Engineering, explained, "Current operating nuclear plants are designed to withstand extremely harsh conditions, so expanding the range of output control does not pose significant risk of malfunction. If experience is accumulated gradually, output reduction is fully achievable."

The nuclear industry is calling for government support to ease regulations and address the deterioration in economic viability caused by lower capacity factors. If output reductions become frequent, nuclear power's advantage as the cheapest power source will be diminished. According to the Korea Power Exchange, as of July, the settlement price per kilowatt-hour (kWh) of nuclear power was 83 won, while that of renewable energy was 133 won.

Shin Hocheol, director of the Central Research Institute at Korea Hydro & Nuclear Power, emphasized, "The cumulative time limit on control rod insertion should be lifted, and a compensation system for output adjustment should also be established."