Extreme heat caused by climate change is putting nuclear power plants at risk of shutdown. The continued rise in seawater temperatures, used as cooling water for nuclear reactors, could force reactors to cease operations. Korea Hydro & Nuclear Power (KHNP) projects that, starting with Shin-Wolsong Units 1 and 2 in 2030, a total of eight nuclear reactors in Korea may have to shut down within the next ten years.

According to KHNP data obtained by The Kyunghyang Shinmun through Democratic Party of Korea (DPK) lawmaker Park Ji-hye’s office on July 29, Shin-Wolsong Units 1 and 2 are expected to reach their design seawater temperature by 2030, Hanbit Units 3 and 4 by 2031, and Hanbit Units 1, 2, 5, and 6 by 2034. The design temperature refers to the maximum seawater temperature at which a nuclear plant can safely operate. If this threshold is exceeded, the reactor must be shut down within a few hours.

The eight reactors expected to reach their design seawater temperatures within their design lifespan are: Shin-Kori Units 1 and 2, Hanbit (Yeonggwang) Units 3, 4, 5, and 6, and Shin-Wolsong Units 1 and 2. Other reactors could also reach this limit if their operating lifespans are extended.

Just 0.5 degrees to shutdown… Global warming could accelerate timeline

Most Korean nuclear power plants draw in seawater to cool the heat generated in reactors, discharging the warmed water back into the ocean. Operational guidelines define reactors as "operable" only when seawater temperatures are below the design limit. If exceeded, the reactor must enter Mode 3 (hot shutdown mode) within six hours and Mode 5 (cold shutdown mode) within 36 hours. Lee Jung-yoon, head of Nuclear Safety and Future, warned that high water temperatures reduce cooling efficiency, lower power generation, and jeopardize reactor safety.

Design seawater temperatures vary by reactor: Shin-Hanul Units 1 and 2 (in Uljin) have the lowest threshold at 31 Celsius degrees, while Kori Unit 2 has the highest at 36.1 Celsius degrees.

Due to global warming, seawater temperatures near Korea are now approaching those design limits. For example, Shin-Wolsong Units 1 and 2, which have a design seawater temperature of 31.5 degrees, saw nearby seawater temperatures reach as high as 30.57 to 31 degrees last year, leaving just 0.5 degrees and 0.93 degrees of margin, respectively.

KHNP’s forecast for when reactors will hit their design temperature thresholds could prove overly optimistic. After unprecedented high coastal water temperatures last year, KHNP significantly revised its projections.

For example, Kori Unit 2 was projected in 2022 to hit its design limit by 2087, but this has now been revised to 2050, 37 years earlier. Similarly, projections for Kori Units 3 and 4 moved forward by 20 years, and Hanbit Units 3 and 4 by 9 years.

According to the Korea Meteorological Administration and the National Institute of Fisheries Science, sea surface temperatures around the Korean Peninsula have risen more than twice as fast as the global average, especially in the East Sea, where many reactors are located. In 2023, high-temperature anomalies persisted for nearly half the year (182.1 days). On July 29, the National Institute of Fisheries Science issued a Level I “Severe” high-temperature alert, the second-highest in its five-tier system.

Nuclear plants vulnerable to climate crisis… Especially in peak summer demand

In anticipation of rising temperatures, KHNP has repeatedly revised its safety standards.

So far, the design seawater temperature has been officially raised for 11 reactors. For example, in 2001, the threshold for Kori Units 3 and 4 was raised from 27.8 degrees to 33.3 degrees, an increase of 5.5 degrees. In 2005, Kori Unit 2's threshold was raised by 8.3 degrees. The most recent change was in 2022, when the design temperature for Saeul Units 1 and 2 (formerly Shin-Kori 3 and 4) was increased from 31.6 to 34.9 degrees. Internationally, permanent increases of over 3 degrees are extremely rare.

Experts have raised concerns about relaxing safety standards set during the original design phase without significant facility upgrades. Although minor improvements, such as upgrading heat exchangers, have been made, raising the design seawater temperature for Saeul Units 1 and 2 significantly reduced their operational safety margin, from 43.3 percent to just 16.1 percent.

At the time, even Korea’s nuclear safety regulator expressed concern. Commissioner Kim Ho-chul of the Nuclear Safety and Security Commission (NSSC) questioned whether KHNP was interpreting regulations to avoid necessary facility upgrades. Kim Hye-jung, former chair of the Korea Nuclear Safety Foundation and now co-director of the Center for Sustainable Development, criticized the move, saying, “They chose operation over safety.”

No follow-up measures have been submitted. When the NSSC approved the new temperature limits for Saeul Units 1 and 2 in July 2022, it also requested a comprehensive plan addressing climate change and global warming. As of July 29, three years later, KHNP has yet to formally submit this plan. The company claimed the report was “sent but not yet formally reviewed.”

Meanwhile, electricity demand continues to climb annually. The relevant ministries forecast that peak demand this summer could reach a record 97.8 gigawatts. Korea may soon face the risk of nuclear shutdowns during the hottest time of year when electricity is needed most. In Europe, where cooling water standards are stricter, nuclear reactors are frequently shut down or derated during heatwaves. “Despite being touted as a low-carbon energy solution in the age of climate crisis, nuclear power is, in fact, highly vulnerable to climate change,” said Kim Hye-jung.

KHNP responded, saying it is evaluating heat load and redesigning systems for reactors projected to hit their design seawater limits within a decade, aiming to preserve operational margins. For Wolsong Unit 1, a redesigned heat exchanger is undergoing the regulatory approval process, while for Hanbit Units 1, 2, and 3, the company is conducting a study to assess safety margins and develop appropriate improvement plans.