In this blog post, we will examine the causes of the accident at the Fukushima Daiichi Nuclear Power Plant and the enhanced safety measures implemented since then, and explore the perspectives from which we should evaluate the risks and safety of nuclear power.

Questions About Nuclear Safety Raised by the Fukushima Accident

In the past, a severe nuclear accident occurred in Fukushima, Japan, caused by a massive earthquake and tsunami. The Great East Japan Earthquake, measuring 9.0 on the Richter scale, and the massive tsunami that followed caused the Fukushima Daiichi Nuclear Power Plant to lose power and cooling capabilities, ultimately leading to core meltdowns in Units 1 through 3. Leading to hydrogen explosions and the release of radioactive materials, the Fukushima accident—along with the Chernobyl accident—was classified as a Level 7 incident, the highest level on the International Nuclear Event Scale (INES).
Immediately following the accident, debates surrounding nuclear power plant safety spread beyond Japan to countries around the world. In South Korea as well, the seismic resistance of nuclear power plants, along with emergency power supplies, cooling systems, and the ability to respond to severe accidents, emerged as major social issues. At the time, some media outlets explained the risks of nuclear power plants by simply comparing the magnitude of earthquakes with the seismic design standards of the plants; however, since the magnitude of an earthquake and the ground acceleration actually observed at a specific facility are different concepts, it is inappropriate to judge the seismic safety of a nuclear power plant based solely on a simple numerical comparison.
In the wake of the Fukushima accident, the debate over the pros and cons of nuclear power intensified once again. However, to assess the safety of nuclear power, rather than starting from the simplistic conclusion that “nuclear power is dangerous” or “nuclear power is safe,” it is necessary to examine the actual sequence of events leading to the accident and identify where the existing safety systems failed.

Are Nuclear Power Plants a Perfectly Safe Source of Energy?

Nuclear power plants are not perfect. There are still several issues to be resolved in nuclear power generation, the most prominent of which is the long-term management of high-level radioactive waste, including spent nuclear fuel. Since high-level radioactive waste remains radioactive for an extremely long time, the problem is not solved simply by storing it in on-site storage facilities at the power plant. In the long term, deep geological repositories that can isolate the waste from human habitation must be constructed, and countries around the world are currently engaged in technology development, site selection, and social consensus-building processes for this purpose.
Therefore, we must not overlook these issues when discussing the safety of nuclear power plants. However, it is also inaccurate to explain the Fukushima accident simply as an incident that occurred “because the technology of the reactor itself is dangerous.” A comprehensive review of the accident investigation findings reveals that a combination of factors contributed to the disaster, including a large-scale natural disaster, flooding that exceeded anticipated levels, a prolonged loss of power, vulnerabilities in safety systems, insufficient preparedness for severe accidents, and problems with risk assessment and accident response systems. The International Atomic Energy Agency’s (IAEA) analysis of the accident also identified key issues, such as the failure to adequately prepare for a prolonged loss of power and the loss of instrumentation capabilities. Therefore, rather than categorizing the Fukushima accident solely as an act of God or solely as human error, it is more accurate to understand it as a complex major accident in which an extreme natural disaster exposed the vulnerabilities of existing design and safety management systems.

Why Did the Reactor Core Melt Even After the Reactors Were Shut Down?

Through news reports at the time, people around the world witnessed the immense power of the magnitude 9.0 earthquake. Seeing scenes of buildings and roads destroyed and cities swept away by tsunamis, it is easy to assume that a nuclear power plant could not have withstood such a natural disaster. However, the direct sequence of events that led to the Fukushima accident escalating into a severe accident was different from the reactor being immediately destroyed by the earthquake.
When the earthquake struck, Units 1 through 3 of the Fukushima Daiichi Nuclear Power Plant, which were in operation at the time, automatically shut down in accordance with the reactor protection system. When a reactor shuts down, power generation through a sustained nuclear fission chain reaction ceases. However, a significant amount of “decay heat” continues to be generated in the nuclear fuel due to the decay of fission products. This is why a reactor is not immediately in a state where cooling is no longer required simply because it has shut down. Therefore, even a shut-down reactor must maintain its cooling function for a certain period of time.
The problem was the massive tsunami that struck shortly afterward. After losing external power, the Fukushima Daiichi Nuclear Power Plant had been receiving the necessary electricity from emergency diesel generators; however, when a tsunami measuring approximately 14 meters struck the plant, numerous emergency generators and electrical systems were submerged. Some battery systems were also damaged. Ultimately, a so-called “station blackout”—a situation where power could not be supplied to multiple reactors for an extended period—occurred.
The reason a loss of power at a nuclear power plant is particularly dangerous is that various safety systems designed to remove decay heat from the reactor core are closely linked to the power supply. If power is lost and cooling functions cannot be maintained for an extended period, the temperature inside the reactor continues to rise, damaging the nuclear fuel and, in severe cases, leading to a core meltdown. This process actually unfolded at the Fukushima Daiichi Nuclear Power Plant, resulting in severe damage to the reactor cores in Units 1 through 3. Hydrogen generated at high temperatures accumulated, causing hydrogen explosions in the reactor buildings of Units 1 and 3, as well as in the building of Unit 4.
A station blackout itself is not a risk that was first identified through the Fukushima accident. The nuclear safety community had long recognized that a loss of both external power and emergency AC power could threaten the ability to remove residual heat from the reactor core, and safety measures had been put in place to address this. The key lesson from the Fukushima accident was not whether these risks were known, but rather how adequately the plant was prepared for a situation where multiple safety systems failed simultaneously due to a natural disaster exceeding previously anticipated parameters.

There were failures not only in natural disasters but also in the preparedness system

Another critical issue to examine regarding the Fukushima accident is the pre-accident risk assessment and the post-accident response system. There was insufficient preparation for a scenario in which a massive tsunami flooded the plant site, causing the simultaneous loss of multiple power sources and cooling functions; furthermore, with power and instrumentation functions lost, operators faced significant difficulties in accurately assessing the conditions inside the reactor and responding appropriately.
In particular, during the progression of a severe accident, it is crucial to accurately monitor key information such as reactor pressure, water level, and temperature. However, in the Fukushima accident, the simultaneous loss of power and instrumentation made it difficult to assess the reactor’s condition. The IAEA’s accident investigation also concluded that the accident management guidelines at the time were not adequately designed to handle prolonged power outages at the plant or situations where critical instrumentation was unavailable, and that training based on such extreme conditions was insufficient.
Therefore, the Fukushima accident cannot be viewed simply as an “accident that occurred unavoidably due to the tsunami.” Conversely, it would be overly simplistic to attribute the accident solely to a single flaw in the reactor design. It is more reasonable to view the accident as having escalated due to a chain of factors: a natural disaster exceeding anticipated parameters, power supply systems vulnerable to flooding, insufficient preparedness for prolonged power loss, and limitations in the severe accident management system and emergency response.
One of the most important lessons the global nuclear power industry has taken to heart since the Fukushima accident is precisely the need to prepare for such “multiple failures.” This means that safety measures must not only account for the failure of a single safety system but also consider the possibility that multiple power sources and safety systems could simultaneously fail due to external events such as earthquakes, floods, or tsunamis. Accordingly, many countries have strengthened their defense systems by utilizing separate equipment—such as mobile generators and mobile pumps—to supply power and cooling water, rather than relying solely on fixed safety systems.

What Is the Difference Between Boiling Water Reactors and Pressurized Water Reactors?

To understand the accident at the Fukushima Daiichi Nuclear Power Plant, it is necessary to examine the types of reactors involved. Units 1 through 6 at the Fukushima Daiichi plant, where the accident occurred, were boiling water reactors (BWRs). In a boiling water reactor (BWR), water is boiled inside the reactor, and the resulting steam is sent to a turbine to generate electricity. In contrast, the primary type of reactor operated in South Korea is the pressurized water reactor (PWR). In this design, the water in the primary circuit circulating through the reactor is maintained at high pressure to prevent it from boiling, and heat is transferred from the steam generator to the secondary circuit to produce steam.
Although the two reactor types differ in the configuration of their systems for handling coolant and steam, it is not appropriate to simply conclude, based on this alone, that boiling water reactors are dangerous and pressurized water reactors are safe. Each reactor has its own safety systems, containment building design, and accident response procedures; therefore, actual safety must be evaluated comprehensively, taking into account not only the reactor type but also the specific design, construction quality, operational management, regulatory framework, level of preparedness for natural disasters, and capability to manage severe accidents.
While pressurized water reactors (PWRs) form the core of South Korea’s nuclear power fleet, not all reactors in the country are PWRs. The Wolsong Nuclear Power Plant also operates CANDU-series reactors, which are pressurized heavy-water reactors (PHWRs). Furthermore, the OPR1000 and APR1400—both developed independently by South Korea—also belong to the PWR family.

How Has Nuclear Power Plant Safety Changed Since Fukushima?

Even before the Fukushima accident, nuclear power plants had established multiple layers of safety systems based on assumptions regarding design-basis accidents and various failure scenarios. However, the Fukushima accident clearly demonstrated the need to account for extreme natural disasters—exceeding the scope anticipated in existing designs—as well as the simultaneous loss of multiple systems.
Following the accident, nuclear regulatory agencies and operators around the world reassessed the risks of earthquakes and flooding and installed additional equipment to ensure that critical safety functions could be maintained even in the event of a prolonged loss of off-site power. Key examples include mobile generators and pumps, additional power supply systems, monitoring equipment for spent fuel storage pools, and severe accident response systems. The U.S. Nuclear Regulatory Commission (NRC) also required additional response measures following the Fukushima accident to ensure that core safety functions could be maintained even in the event of a total loss of all existing power sources.
In South Korea as well, the Fukushima accident prompted a series of measures to inspect and improve nuclear power plants’ earthquake and tsunami response capabilities, power supply systems, cooling water supply capacity, and severe accident response systems. Therefore, when evaluating the safety of nuclear power plants today, rather than simply applying the facilities and response systems in place at the time of the Fukushima accident to current plants, we must also examine how the lessons learned from that accident have been incorporated into actual design, regulations, and operating procedures.
However, this does not mean that the possibility of a nuclear accident has disappeared since Fukushima. In engineering, it is difficult to find technology that guarantees absolute safety. Particularly in facilities like nuclear power plants, where the consequences of an accident can be severe, the concept of multiple layers of defense is crucial—not only to reduce the probability of an accident but also to prevent damage from escalating should one occur. Rather than relying on a single piece of equipment or a single decision for safety, this approach involves layering multiple independent safety devices and procedures so that even if one fails, the next layer can halt the progression of the accident.

Does a High Operating Rate at a Nuclear Power Plant Equate to Safety?

In the past, South Korea’s nuclear power plants have cited high utilization rates and relatively stable operating records as key achievements. However, there are limitations to judging a nuclear power plant’s safety based solely on high utilization rates or a low number of unplanned outages. While the fact that a plant has operated continuously for a long period can serve as one indicator of operational reliability, it does not fully account for the plant’s ability to withstand severe accidents, its capacity to respond to natural disasters, the integrity of its facilities, its safety culture, or the independence of regulatory oversight.
The safety of a nuclear power plant must be assessed by considering a comprehensive range of factors. Not only the design of the reactor itself, but also the emergency core cooling system, emergency power supply, containment building, severe accident management systems, radiation protection, natural disaster response, operator education and training, maintenance quality, and regulatory oversight must all function in concert. The Fukushima accident demonstrated that what matters most is not the mere presence of a single safety measure, but rather how thoroughly the possibility of multiple lines of defense failing simultaneously is taken into account.
The principle of prioritizing safety over efficiency is also crucial during the actual operation of a reactor. This is why reactors are designed and operated to reduce power or shut down the reactor and investigate the cause whenever abnormal signs are detected. Nuclear power plants continuously monitor key operating parameters using various instrumentation and control systems as well as reactor protection systems, and are equipped with protective functions that automatically shut down the reactor if conditions deviate from established safety standards.
However, no matter how sophisticated the equipment may be, safety cannot be achieved through technology alone. The concept that has been emphasized even more since the Fukushima accident is “safety culture.” Even the possibility of a low-probability accident must not be taken lightly; unfavorable information and warning signs must be actively reviewed, and situations exceeding the scope anticipated in the design must be constantly monitored. This is why, in addition to safety equipment, the organization’s decision-making processes, regulatory frameworks, and the training and response capabilities of on-site operators are so important.

What Should We Learn from the Fukushima Accident?

The Fukushima accident has significantly intensified global debate surrounding the safety of nuclear power plants. In the wake of the accident, some countries have reviewed their nuclear policies or opted for a phased phase-out of nuclear power, while others are continuing to utilize nuclear power or constructing new plants while strengthening safety measures at existing facilities. Recently, with the addition of issues such as climate change, energy security, and rising electricity demand, discussions regarding the role of nuclear power have once again become lively.
Nuclear power is not a perfect energy source. While it has the advantages of relatively low greenhouse gas emissions during the power generation process and the ability to produce large-scale electricity reliably, issues such as the risk of major accidents, spent nuclear fuel management, enormous construction and decommissioning costs, and long-term safety management must also be taken into account. Therefore, when discussing the necessity and safety of nuclear power plants, it is necessary to consider all these factors together rather than emphasizing only the advantages or risks of one side.
The most important lesson from the Fukushima accident is not a simple conclusion such as “nuclear power plants are absolutely safe” or “nuclear power plants cannot be inherently safe.” The fact is that even if the probability of an accident is extremely low, if the consequences are severe, that possibility must be constantly reviewed, and preparations must be made for situations that go beyond existing predictions and designs. Nor does a history of safety automatically guarantee future safety.
During the Fukushima accident, although the reactor detected the earthquake and shut down normally, a major accident occurred when the massive tsunami caused a cascading loss of power and cooling functions. This fact starkly demonstrated that safety is not ensured merely by shutting down the reactor; it is also necessary to be able to remove decay heat and maintain power and cooling functions even after shutdown. Nuclear power plant safety is not determined by the performance of a single piece of equipment; it can only be ensured when design, facilities, operation, regulation, organizational culture, and emergency response interlock in multiple layers.
Ultimately, the questions we must ask in the wake of Fukushima go beyond simply “Are nuclear power plants safe?” We must continuously verify whether safety functions can be maintained even when unexpected disasters occur, whether we are prepared for situations where multiple safety systems fail simultaneously, and whether new risks, once discovered, are reflected in design and operations without being concealed or downplayed. The safety of nuclear power generation is not something that is proven once and for all; rather, it is a task that must be continuously verified and improved for as long as the power plant exists. This is precisely why the Fukushima accident remains so important today.