As climate change rapidly raises seawater temperatures, nuclear power plants on the east coast face the risk of shutdown. All domestic nuclear plants use seawater as cooling water, and when the seawater temperature reaches equipment limits, output must be halted under safety rules.
In particular, plants built when East Sea waters were colder have lower equipment limit settings, making them more vulnerable to rising seawater temperatures. Experts said this variable was not considered when building nuclear plants, so there are virtually no countermeasures against rising seawater temperatures.
According to the National Institute of Fisheries Science on the 10th, surface seawater temperatures along the east coast, where nuclear plants are concentrated, are around 30 degrees Celsius (℃). Surface temperature refers to water temperature measured within about 5 meters of the sea's top layer.
According to the National Institute of Fisheries Science's real-time marine and fisheries environment observation system, as of 2:30 p.m. on the 7th, the Onyang station in Uljin, North Gyeongsang, was at 30.1℃, Deokcheon at 30.2℃, and Nagok at 29.6℃.
Uljin hosts eight reactors in operation, including Hanul Units 1–6 and Shin-Hanul Units 1 and 2. The problem is that as nearby seawater temperatures rise, they are approaching the design seawater temperature, the equipment limit for the plants.
If the cooling water temperature rises to the design seawater temperature, operations must stop. The design seawater temperatures for Hanul Units 1–6 are set at 31.5–32.5℃, and for Shin-Hanul Units 1 and 2 they are 31℃.
All domestic nuclear plants use seawater as cooling water. After turning the turbine with heat generated by the reactor, seawater is fed into the key system that cools the used steam back into water. Nuclear plants cluster on the coast because cooling water is easy to supply.
When the temperature of seawater used as cooling water rises, heat exchange efficiency falls and key safety systems cannot shed heat, undermining their function. KHNP sets a design seawater temperature for each plant, and if incoming seawater exceeds it, the plant must be shut down.
Following Uljin in North Gyeongsang, Shin-Wolsong Units 1 and 2 in Gyeongju also have a relatively low design seawater temperature of 31.5℃. As of 2:30 p.m. on the 7th, the surface seawater temperature at the nearby Guryongpo Hajeong station was 28.9℃.
A rise in surface seawater temperature means the overall seawater temperature is increasing. The design seawater temperature used for cooling is based on the thermometer on KHNP's intake pipe, so it may differ from surface temperatures. Farther offshore and deeper waters are cooler.
As seawater temperatures near the limit, KHNP is first responding by improving cooling system performance to raise the design seawater temperature. In other words, it is raising the shutdown threshold.
Shin-Wolsong Units 1 and 2 increased the number of heat transfer plates in the heat exchangers to enhance cooling performance. The company is currently pursuing a plan to raise the design seawater temperature limit through the Nuclear Safety and Security Commission's "reassessment of seawater temperature limits." Hanbit Units 1–6 and others also plan to improve heat exchanger facilities by 2029 to raise the design seawater temperature.
Under KHNP's step-by-step procedures, enhanced monitoring applies within minus 3℃ of the design seawater temperature, and output reduction and shutdown apply when exceeding the limit.
A KHNP official said, "In anticipation of rising seawater temperatures, we have conducted various studies and assessments since 2022 and are reviewing facility improvements and licensing options."
However, beyond adding heat exchangers and reducing output, there is no fundamental solution. Jung Bum-jin, a Kyunghee University professor, said, "Seawater temperatures are rising rapidly, and there are only stopgaps like adding more heat exchangers to nuclear plants. Other countries are in a similar situation, and it was an unforeseen issue when plants were built."
He added, "If seawater temperatures rise further, there is a possibility that the plants clustered along the east coast belt will see a series of output reductions."
Kim Beom-nyeon, a visiting professor at KEPCO International Nuclear Graduate School, said, "If rising seawater temperatures raise cooling water temperatures, power generation efficiency declines and equipment anomalies become a problem," and added, "This does not occur year-round, so it can be addressed flexibly with technical measures such as output control."
Meanwhile, on the 6th, the Nuclear Safety and Security Commission checked instruments and heat exchanger performance in preparation for possible rises in seawater used as plant cooling water, and reviewed KHNP's on-site response status and procedures.