In this blog post, we’ll examine the safety of South Korea’s nuclear power plants by comparing the principles and pros and cons of pressurized water reactors and boiling water reactors.

Background and Purpose of This Article

Nuclear power plants, commonly referred to as “nuclear plants,” have been the subject of public interest and concern for several years. In particular, following the Fukushima nuclear accident in Japan on March 11, 2011, radiation leaks and the prolonged cleanup efforts amplified public anxiety, and concerns spread that a similar accident could occur in South Korea.
The purpose of this article is to alleviate public anxiety by comparing the reactors primarily used in South Korea with those at the Fukushima plant where the accident occurred, and to clearly explain the operating principles, as well as the advantages and disadvantages, of both reactor systems.

Current Status of Nuclear Power Plants in South Korea

A total of 23 nuclear power plants were in operation in South Korea, including Kori Units 1–4, Shin-Kori Units 1–2, Wolseong Units 1–4, Shin-Wolseong Unit 1, Hanbit Units 1–6, and Hanul Units 1–6. However, in the wake of corruption scandals involving nuclear power plants, the operation of Shin-Kori Unit 2 and Shin-Wolsong Unit 1 was suspended; currently, 21 units are in operation, and the electricity they generate amounts to approximately 13 million kWh, accounting for about 24% of total electricity production.
With alternative energy sources not yet sufficiently commercialized, nuclear power continues to play a vital role in South Korea’s electricity production.

Classification of Reactors and Key Terms

Reactors are primarily classified by their coolant and moderator. Those that use water—the most common coolant and moderator—are called light-water reactors, while those that use heavy water (water containing deuterium) are called heavy-water reactors. The coolant is a substance that carries heat generated in the core to cool the reactor, and the moderator is a substance that slows down the neutrons emitted during the fission process to sustain the chain reaction.
Light-water reactors are further divided into two types based on the presence or absence of a steam generator. A pressurized water reactor (PWR) separates the primary and secondary systems using a steam generator, while a boiling water reactor (BWR) drives a turbine using steam generated directly from the boiling of coolant water in the core.

Operating Principle of a Pressurized Water Reactor (PWR)

Pressurized water reactors (PWRs) are the most common type of light-water reactor, accounting for approximately 60% of the world’s nuclear reactors. They use low-enriched uranium-235 (approximately 2–5%) as fuel and are designed to maintain the water in a liquid state even at high temperatures of about 300°C by applying high pressure (approximately 150 atmospheres) to the coolant (light water).
The high-pressure hot water from the primary system, which flows through the reactor core, enters the steam generator, where it heats the water in the secondary system to produce steam; this steam from the secondary system then drives the turbine to generate electricity. After passing through the turbine, the steam is cooled in a condenser, where heat is exchanged and dissipated into seawater or other media. This separation of the reactor system and the turbine system is a defining characteristic of pressurized water reactors.

Operating Principle of a Boiling Water Reactor (BWR)

Boiling water reactors account for approximately 22% of the world’s reactors, and the type used at Fukushima belongs to this category. Boiling water reactors also use low-enriched uranium (approximately 2%) as fuel, but the steam generated when the coolant boils directly within the reactor core drives the turbine.
While they are similar to pressurized water reactors in that the steam is condensed in a condenser—where heat is transferred to seawater or another medium—there is a structural difference: the primary system and the turbine system are not separated.

Comparison of the Advantages and Disadvantages of Pressurized Water Reactors and Boiling Water Reactors

One of the advantages of pressurized water reactors is the direction in which control rods are inserted. In pressurized water reactors, control rods are often designed to be inserted from the top of the fuel rods downward; this design provides a safety feature whereby, even if power is lost due to an accident, the control rods naturally descend under the force of gravity to slow down the fission reaction. Control rods are made of materials that absorb neutrons well—such as boron, cadmium, and hafnium—to regulate the rate of fission.
Another advantage is the separation of the reactor system from the turbine system. This separation significantly reduces the likelihood of radiation reaching the turbine system and minimizes corrosion issues inside the reactor, resulting in higher safety in terms of radiation leakage risk and maintenance. On the other hand, due to additional systems such as steam generators, complex design, and large plant scale, initial investment and maintenance costs are high, which can be a disadvantage in terms of cost and efficiency.
Boiling water reactors (BWRs) have the advantage of high thermal efficiency because steam is generated directly in the core to drive the turbine, and their relatively simple structure helps reduce design and construction costs. However, since the primary system and the turbine system are not separated, radioactive material can migrate to the turbine side, which is a disadvantage in terms of radiation shielding and maintenance.
Furthermore, the control rod insertion mechanism in boiling water reactors involves either pushing the rods upward from beneath the fuel rods or relying on electrical power; consequently, automatic insertion by gravity becomes difficult if power is lost during an accident. Therefore, if automatic control rod insertion fails during a loss-of-power situation, nuclear fission may continue, posing a risk of the situation worsening; the continued fission during the Fukushima accident has been cited as one of the factors that exacerbated the damage. Core corrosion caused by the phase change of the coolant is also a significant drawback of boiling water reactors.

Conclusion

In summary, while pressurized water reactors may be at a disadvantage compared to boiling water reactors in terms of cost and efficiency, they can be considered safer in terms of design. On the other hand, while boiling water reactors offer advantages such as cost savings and efficiency due to their direct cycle, they have vulnerabilities in terms of safety, including the risk of radioactive leakage and difficulties in controlling the reactor in the event of an accident.
Considering that most reactors used in South Korea belong to the pressurized water reactor family, it can be concluded that relatively safer designs and operating procedures are in place to prevent incidents like the Fukushima accident in Japan, and there is no need for excessive concern.