In this blog post, we’ll examine the principles and safety of pressurized water reactors, as well as the key issues surrounding the controversy over the extension of Unit 1 of the Kori Nuclear Power Plant’s operating life.

Nuclear Power and the Image of Fear in South Korea

Nuclear power accounts for about 33% of South Korea’s electricity production and has played a major role in industrial and economic development by providing a cheap supply of electricity. However, a vague fear of nuclear power remains widespread. Historical images of atomic bombs and mushroom clouds, along with frightening stories about birth defects or cancer deaths linked to radiation, remain deeply ingrained in people’s minds. These anxieties and controversies have intensified even further since the Fukushima accident.

The Historical Significance of Kori Unit 1 and the Crossroads

Kori Unit 1 was the first nuclear power plant completed in South Korea in 1977 and served as a symbolic facility that signaled the start of the country’s economic development at the time. Now standing at a crossroads between permanent shutdown and extended operation, it has become the center of social and policy debates.

Structure and Operating Principles of a Pressurized Water Reactor (PWR)

A pressurized water reactor (PWR) is a type of reactor in which the reactor core and steam generators are physically separated. Unlike boiling water reactors (e.g., the type used in Fukushima), pressurized water reactors have physically separate primary and secondary systems. In the primary system, cooling water circulates to absorb heat generated by the reactor core, and that heat is transferred to the secondary coolant through numerous U-tubes inside the steam generator.
The secondary coolant, having absorbed heat from the steam generator, turns into steam and drives the turbine; the spent steam is then cooled and condensed by seawater before returning to the steam generator. The primary coolant is maintained at high temperature and high pressure (approximately 150 atmospheres) to prevent evaporation and is pumped back into the reactor to repeat the cycle. This design safely isolates core heat through the primary system.

Why pressurized water reactors are considered safer than boiling water reactors

One of the advantages of pressurized water reactors is the separation of the primary and secondary systems. Although the primary system contains radioactive material, it is maintained under high pressure; therefore, if a leak occurs, a drop in pressure is detected, automatically shutting down the reactor and allowing corrective measures to be taken. Furthermore, since the secondary system is located within the turbine building and typically does not contain radioactivity, the risk of radioactive material escaping to the outside is relatively low.

South Korea’s Nuclear Power Strategy and Concerns Regarding Probabilistic Safety

Pressurized water reactors (PWRs) have become the standard over the long history of nuclear power. The combination of government policies that opted for this standardization early on and sustained industry investment has made South Korea a global leader in nuclear power. Among nuclear engineers, there is a widespread view that South Korea’s PWRs offer higher design safety than Japan’s boiling water reactors (BWRs) and are more likely to prevent the release of radioactive material even in the event of an accident.
During the nuclear power plant design phase, various limit values are set with sufficient margins based on accident scenarios, and manuals and guidelines are prepared. Efforts have also continued to estimate the probability of a severe accident (core meltdown) as extremely low through probabilistic safety assessments. The design goal is to reduce the risk to a level where a severe accident occurs only once every several hundred thousand years.
Nevertheless, the limitations of probabilistic safety analysis are sometimes revealed in reality. The fact that three severe accidents—Chernobyl, TMI, and Fukushima—have occurred over the past 30 years, beginning with the TMI accident in the 1980s, demonstrates the gap between theoretical probabilities and actual accidents.
In particular, during an accident, even manuals and procedures often prove useless. In the case of Fukushima, while many experts believed the earthquake itself could be withstood by the seismic design, a 11-meter tsunami—exceeding design standards—swept over the seawall and flooded the plant, causing the situation to spiral out of control. The problem is that when external impacts exceed design standards, a chain of failures can occur that were not anticipated by preliminary calculations.

The Controversy Over the Extension of Gori Unit 1’s Operation and Communication Issues

While the aging of Gori Unit 1 is undeniable, nuclear engineers argue that an extension of its operation by about 10 years is technically feasible, provided that proper operation and thorough safety management are guaranteed. Since the concept of a nuclear power plant’s “lifespan” is not defined by a fixed standard from the outset, the decision to extend its operation is made only after undergoing extremely rigorous inspections and procedures.
However, the public does not fully trust quantitative safety analyses based on probability. The disconnect between engineers and the general public is not merely a technical or engineering issue, but one of trust and communication. The reason the debate over extended operation continues is that scientific arguments alone are insufficient, and universal reassurance has not been achieved through empathy and communication. The controversy surrounding the Kori Nuclear Power Plant therefore involves philosophical and social issues that go beyond technical judgments.