In this blog post, we’ll examine what actually exploded during the accident at the Fukushima Daiichi Nuclear Power Plant, how hydrogen was generated and destroyed the reactor building, and what lessons this accident taught us about nuclear power plant safety.
How Do Explosions Occur at Nuclear Power Plants?
What kinds of explosion accidents can occur at a nuclear power plant? Generally, an “explosion” refers to a phenomenon in which a substance expands in a very short period of time or pressure rises sharply, transmitting a powerful shock to the surrounding area. It is easy to understand damage caused by a pressure increase if you imagine a situation where you are watering your yard with a rubber hose connected to a faucet, and someone steps on the hose, causing the internal pressure to rise and the hose to eventually pop off the faucet. A nuclear power plant consists of various structures, including the reactor—which contains nuclear fuel and coolant—and the reactor coolant system; the system that uses heat generated by the reactor to produce steam and drive the turbine and generator; and the containment vessel and reactor building, which prevent the release of radioactive materials into the environment. Therefore, the statement “a nuclear power plant exploded” alone makes it difficult to determine exactly where and what kind of explosion occurred. In particular, while both the Chernobyl and Fukushima nuclear accidents were serious reactor accidents, the course of the accidents and the nature of the explosions differed significantly. In the Chernobyl accident, a steam explosion and fire destroyed the reactor after its power output rapidly spiraled out of control, whereas in the Fukushima accident, hydrogen generated by core damage—which occurred after the reactor lost its cooling function due to the earthquake and tsunami—exploded inside the reactor building. So, what exactly exploded at Fukushima?
To put it simply, the massive explosion witnessed on video at the Fukushima Daiichi Nuclear Power Plant was not a nuclear explosion of the reactor itself, but a “hydrogen explosion.” Specifically, at Units 1 and 3, hydrogen generated during the core damage process migrated into the reactor buildings, accumulated, and then exploded; at Unit 4, which had been shut down, an explosion also occurred due to the accumulation of hydrogen believed to have migrated from Unit 3. Therefore, it is inaccurate to simply describe this as “the containment building exploding.” The explosion primarily destroyed the upper structure of the reactor building surrounding the containment vessel; the containment vessel is a critical barrier—separate from the reactor pressure vessel—that prevents the external release of radioactive materials. Hydrogen is not the fuel used to operate the reactor under normal conditions. To continue the rubber hose analogy, it was not simply that the water inside the hose increased in pressure and caused it to burst; rather, it is more accurate to say that flammable gas newly generated during the accident accumulated in another space and then exploded. So how was such a large amount of hydrogen—which was not originally part of the reactor’s fuel—produced? To understand the reason, we need to examine the sequence of events that led to the Fukushima accident.
How did the earthquake and tsunami lead to core damage?
At the time of the Fukushima accident, a massive magnitude 9.0 earthquake struck the Pacific Ocean off the coast of Japan’s Tohoku region, followed by a massive tsunami that swept over the coastal areas. When the earthquake struck, Units 1–3 of the Fukushima Daiichi Nuclear Power Plant, which were in operation at the time, automatically shut down in accordance with safety protocols, and the nuclear fission chain reaction also ceased. However, a reactor does not cool down immediately just because nuclear fission has stopped. This is because radionuclides accumulated within the nuclear fuel continue to decay, generating “decay heat.” While a car’s engine cools down naturally over time after being turned off, a reactor immediately after shutdown continues to emit significant amounts of decay heat, so its cooling functions must be maintained. However, the situation deteriorated rapidly when the tsunami that followed the earthquake—which had already cut off the external power supply—flooded the emergency power sources and electrical equipment. Traces of flooding at a depth of approximately 14 to 15 meters were confirmed in the main building area of the Fukushima Daiichi Nuclear Power Plant, and a wide area was submerged. As emergency diesel generators and electrical equipment ceased to function, it became difficult to secure the power needed to cool the reactors stably, and some steam-driven and battery-powered safety systems also failed over time. Ultimately, Units 1 through 3 were unable to maintain sufficient cooling, causing the reactor water levels to drop and the nuclear fuel temperature to rise significantly, leading to core damage and meltdown.
This overheating of the nuclear fuel was the direct cause of the production of large amounts of hydrogen. It is easy to understand if you recall the science experiments from middle and high school where bubbles form when a metal reacts with a specific solution. However, the reaction that occurred in the Fukushima accident was different from that between a metal and hydrochloric acid; the key factor was the chemical reaction between high-temperature zirconium alloy and water vapor. Nuclear reactor fuel consists of thin fuel rods, and zirconium alloy is used for the cladding tubes that surround the fuel. Under normal operating conditions, these cladding tubes act as a barrier between the fuel and the coolant; however, if the cooling function is lost and the temperature of the fuel rods rises significantly, the zirconium reacts rapidly with water vapor, oxidizing and generating large amounts of hydrogen. At Fukushima, this reaction produced large amounts of hydrogen as the reactor core overheated and was damaged; the hydrogen then moved from the reactor pressure vessel toward the containment vessel and subsequently entered the reactor building through leaks or venting processes. The hydrogen accumulated in the reactor building mixed with oxygen in the air to form an explosive concentration and ignited, causing powerful hydrogen explosions at Units 1 and 3, as well as a hydrogen explosion at Unit 4. Therefore, rather than explaining that “hydrogen accumulated at the top of the concrete containment vessel, causing it to explode,” it is more accurate to state that hydrogen generated during the core damage process migrated into the reactor building, accumulated there, and exploded, causing significant damage to the upper part of the building. During the accident, radioactive materials were released into the environment through various pathways, and the damage to the reactor building caused by the hydrogen explosions further complicated the accident response.
What lessons did the Fukushima accident teach us about nuclear power plant safety?
The Fukushima accident sent a major shockwave through the global nuclear industry and academic community. This is because it demonstrated that even though the reactors successfully shut down automatically upon detecting the earthquake, the massive tsunami that followed simultaneously threatened the plant’s power supply and cooling systems, potentially escalating the incident into a severe accident. At the time of the accident, the main building area of the Fukushima Daiichi Nuclear Power Plant was flooded to a depth of approximately 14–15 meters, and most of the key zones housing the reactor and turbine buildings were affected. In addition to the loss of external power caused by the earthquake, the flooding of emergency power sources and electrical equipment led to a prolonged power outage and a decline in cooling capacity, ultimately resulting in core damage and meltdown in Units 1 through 3. Since the Fukushima accident, it has become increasingly important in nuclear power plant safety to consider not only the failure of a single piece of equipment but also external disasters—such as earthquakes, tsunamis, and floods—that can simultaneously render multiple safety systems inoperable, along with prolonged power outages. Alternative equipment such as mobile power sources and pumps, flood prevention measures, diversification and physical separation of power systems, as well as hydrogen management and the maintenance of containment functions during severe accidents, are also being addressed as critical safety challenges. While the risks associated with hydrogen were known even before the Fukushima accident, the incident dramatically demonstrated how the generation, movement, accumulation, and explosion of hydrogen in an actual severe accident scenario can significantly complicate accident response efforts.
The Fukushima accident also demonstrated that the safety of nuclear power should not be explained solely on the premise that “accidents do not occur.” In nuclear power plant safety, it is important not only to reduce the likelihood of an accident but also to establish a multi-layered defense system capable of continuing to cool the nuclear fuel, contain radioactive materials, and prevent the accident from escalating when an event occurs that exceeds anticipated parameters. That said, it is also insufficient to view nuclear issues through the lens of either vague optimism or pessimism. Discussions on nuclear power can become more concrete and productive only when we examine both the advantages and risks of nuclear technology, analyze the causes and consequences of potential accidents, and transparently clarify the extent to which safety measures can function and what limitations they have. One of the key lessons from the Fukushima accident is that risks do not disappear simply because they were “unforeseen,” and that we must continuously review how to prepare for events that, even if unlikely to occur, could have catastrophic consequences.
How should we handle “the second fire” that is nuclear power?
In Greek mythology, Prometheus stole fire from the gods and gave it to humanity; thanks to this, humanity was able to develop a more prosperous civilization than before, but Prometheus himself suffered a harsh punishment. This is why nuclear power is often likened to “the second fire” that humanity has acquired. Fire provides humans with heat and light, cooks food, and serves as a means to advance civilization; however, if mishandled, it causes fires and destruction. Similarly, nuclear power is a technology capable of producing immense energy, but it is also one that requires strict management of the consequences in the event of an accident. What to create using this new fire, what safety measures to put in place, what level of risk to accept, and—if necessary—what alternative energy sources to use are ultimately issues that human society must decide based on scientific facts and technical limitations. Gaining an accurate understanding of what actually exploded in the Fukushima accident goes beyond simply identifying the causes of a past incident. It also serves as a starting point for considering how we will understand and manage the immense power of nuclear energy in the future.
