10 years after Fukushima: Are Japanese nuclear power plants safe?

By Jun Tateno 

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Immediately after the Fukushima Daiichi Nuclear Power Station accident in 2011, the Japanese government shut down all of its nuclear power plants. Following, they reviewed their nuclear regulations that had been widely criticized as influenced by promotion groups and the former nuclear regulatory body. Since then, the Nuclear Regulation Authority, which was established in 2012, set new standards, examined all plants, and allowed those that passed to restart. While this process remains in progress, it is worth asking: Are the restarted Japanese nuclear power plants safe?

Sole reliance on expert judgment. “It is natural for professionals to determine safety,” the Nuclear Regulation Authority argued following the Fukushima disaster. Toward this end, the authority’s staff conducted a review to determine whether individual facilities satisfied earthquake and severe accident countermeasure requirements. Then the authority restarted plants based on the results, without clarifying for the public what had caused the Fukushima accident. The authority also did not provide information or form a consensus on nuclear power use. This sole reliance on a closed group of experts may reproduce conditions that led to the Fukushima accident. Perhaps it is no surprise that, in the aftermath of the Fukushima disaster, more than 60 percent of Japanese citizens opposed restarting nuclear power plants. Most Japanese remain concerned that a severe accident will occur again.

To be sure, the Nuclear Regulation Authority set stricter and more rational standards following the accident. However, in practice, Japanese regulators have often compromised their reviews. For example, they granted operating permits for countermeasures such as adding complementary facilities without requesting fixes to structural flaws. Map of Japan showing the March 11, 2011 earthquake. Credit: W. Rebel via Wikimedia Commons. CC BY-SA 3.0.

What happened in Fukushima in 2011? At 2:46 pm on March 11, 2011, a magnitude 9 earthquake hit the Fukushima Daiichi Nuclear Power Station, which shut down all reactors at the power plant. The quake also knocked down the towers of the transmission lines, cutting off electricity from the outside, according to the government accident investigation commission report. Emergency diesel generators fired up to supply electricity. However, when tsunamis arrived 27 and 35 minutes later, they too experienced a total loss of power known as a “station blackout.”

After the reactors shut down, their cores continued to generate large amounts of heat from fission product decay. Since the electrical cooling equipment no longer worked, a non-electrical emergency cooling system started to cool the core. However, this system also soon failed. Without cooling, the temperature and pressure inside the boiling water reactors rose. The reactors’ water then transformed into steam that was ejected into the containment vessel, causing water levels in the reactors to drop, exposing the core.

The operators tried to maintain the cooling function in darkness and in danger from aftershocks, but to no avail. The three reactors failed catastrophically one after another. What follows is a closer look at what this failure looked like, as well as eight unanswered questions concerning whether current Japanese nuclear power plants are safe.

Light water reactors come in two types: boiling water and pressurized. A boiling water reactor heats water in its core, which it then turns to steam that powers a turbine to generate electricity. A pressurized water reactor sends heated water to a steam generator that turns the turbine to generate electricity. The nuclear power plant that caused the Fukushima accident was the earliest type of boiling water reactor equipped with a containment vessel to prevent radioactive gas from entering the environment in the event of damage to the reactor. Boiling water reactors have extremely small containment vessel volumes—about one-fifth that of pressurized water reactors. As a result, the possibility of damaging a boiling water reactor’s containment vessel is greater than that of a pressurized one. To overcome this shortcoming, boiling water reactors are equipped with pressure suppression chambers. Regrettably, during the Fukushima accident, the suppression chamber also failed.

When the temperature inside the reactor rose, internal steam was ejected into the suppression chamber through a safety valve. When a suppression chamber’s temperature is low, ejected steam condenses into water, preventing pressure in the containment vessel from rising. However, when too much steam is ejected into a suppression chamber, the temperature rises, which prevents the steam from condensing. When this happened repeatedly during the Fukushima accident, the containment vessel was damaged.

During the Fukushima accident, the plant operator opened the exhaust pipe valve many times for an intentional release of radioactive gas into the environment. A Tokyo Electric Power Company investigation committee considered this venting, which was intended to prevent damage to the containment vessel, successful.

When the reactor core temperatures rose higher than 1,200 ℃, zirconium in the fuel cladding reacted with water to generate large amounts of hydrogen gas. This hydrogen accumulated in the reactor buildings and ignited there, causing an explosion. Once again, large amounts of radioactive materials were released into the environment. Once the core temperatures hit 1,800 ℃, the core meltdown began.

At first, the molten core accumulated at the bottom of the pressure vessel. But soon after, it melted through the bottom and fell into the containment vessel. Though the United Stated had warned of “unexpected” dangers associated with this kind of station blackout, Japanese authorities were unprepared.

The aftermath of the Fukushima nuclear accident. When the accident occurred, the government issued an evacuation order for residents in the surrounding area. Communication about the accident was extremely poor, as most local government disaster prevention officers first learned about the accident on TV. In particular, evacuating the elderly and the sick was extremely difficult. More than 140,000 people fled and nearly 30,000 have yet to return. While authorities are engaged in ongoing efforts to decontaminate the land, some areas remain inaccessible. More than 2000 people died as a result of the nuclear accident, including some whose deaths resulted from the prolonged evacuation. Among survivors, many lost the foundation of their lives.

Meanwhile, the accident is not over. The Fukushima Daiichi Nuclear Power Station remains contaminated with difficult-to-remove fuel debris. Also, the accident site continues to generate contaminated water.

The molten core that reached the bottom of the containment vessel during the disaster later solidified into fuel debris. This debris contains fission products that emit radiation strong enough to cause death after a few hours of exposure. As a result, cleanup work is performed by remote control. Though the Tokyo Electric Power Company plans to develop technology and remove the fuel debris in upcoming decades, some nuclear engineers doubt that the core can be removed.

Since the fuel debris continues to generate heat, water is still circulated to cool the reactor buildings. This circulating water is exposed to debris, leaving it contaminated. This contaminated, circulating water later mixes with groundwater flowing into the reactor buildings. Without intervention, this process would have led to large-scale groundwater pollution. Instead, the power company installed a wall in the ground around the building to limit the inflow of groundwater. Also, though part of the incoming groundwater is purified, the water remains radioactive with tritium. For this reason, the water is stored on site in tanks. Though tanks for this radioactive water cover the ground tightly on the site, the power company is considering diluting the water and releasing it into the ocean. Needless to say, local fishermen and others strongly oppose the release.

The Nuclear Regulation Authority restarted nuclear power plants. Even before the accident, the regulators and the regulated appeared to be on the same side, which had a negative impact on regulation. After the accident, the investigation committee pointed out that the old regulation authority had not applied the latest knowledge concerning natural disasters such as earthquakes and tsunamis and that severe accident countermeasures had not been taken.

The Nuclear Regulation Authority has begun a nuclear conformity assessment based on the new standards. Recognizing the criticisms, they mentioned “independence, neutrality, and expertise” as a feature of the new regime and emphasized that the new requirements are in line with the International Atomic Energy Agency’s strategy to use multiple security measures to prevent accidents.

Under the new requirements, the Nuclear Regulation Authority incorporates severe accident countermeasures that were not previously required. Also, they will pay particular attention to natural disasters such as earthquakes and tsunamis that could cause internal fires or flooding. They will also work to diversify safety facilities and plan for anti-terrorism measures.

To date, the regulation authority and the electric power company have met in nearly 1,000 review meetings. Their meeting records reveal some of the problems at the restarted nuclear power plants, particularly with boiling water reactors that caused the Fukushima accident.

Still, the conformity assessment leaves many important unanswered questions.

What dangers do vents pose? Prior to the Fukushima accident, regulators and plant operators did not understand the importance of vents—the intentional release of radioactive gas from a containment vessel—for accident response measures. However, those who conducted the conformity review learned lessons about venting from the Fukushima accident.

There are two types of vents: filter vents and hardened vents. A filter vent reduces radioactivity to about 1/1000 by way of a filter, whereas a hardened vent releases radioactive gas without a filter. In the conformity assessment, the power company suggests not using hardened vents after the core is damaged as they are likely to make holes in the containment vessel and release a large amount of radioactivity into the environment Under these circumstances, the Nuclear Regulation Authority’s recommendation to use venting as a countermeasure against severe accidents is extremely irresponsible.

Are alternative circulation cooling systems sufficient? As mentioned above, the relatively small volume of the boiling water reactor containment vessel poses a risk when internal pressure rises during an accident. Though a suppression chamber is intended to provide backup, this also failed during the Fukushima accident.

In the conformity review, the Nuclear Regulation Authority mandated installing a new cooling system to compensate for this flaw. In this system, water is removed from the suppression chamber, passed through an additional heat exchanger to lower the temperature, and then returned to the containment vessel. However, since virtually all severe boiling water reactor accidents result from a failure to remove decay heat, this type of reactor may have a design flaw. In light of this, the Nuclear Regulation Authority should have addressed the design flaw rather than allowing plants to restart with alternative circulation cooling systems.

Can hydrogen explosions be prevented? In the Fukushima accident, hydrogen and oxygen mixed in a certain ratio to produce a terrible phenomenon known as “hydrogen detonation.” In the conformity review, the electric company insisted that the hydrogen generated in the event of an accident could be removed by adding more recombiners, and the Regulation Authority accepted this. However, recombiners remove only a relatively small amount of hydrogen which is generated during normal operation. Researchers have not demonstrated that recombiners can remove a large amount of hydrogen generated during a nuclear accident—and many doubt this is possible.

Are mobile facilities effective? In the event of a severe accident, the Nuclear Regulation Authority relies on mobile safety facilities, including a large number of water injection vehicles, power supply vehicles, and more. But is this realistic in the event of an earthquake? According to a Tokyo Electric Power Company survey after the Fukushima accident, the water injected into the reactor cooling system from the fire engine spilled out of the reactor pipes and did not reach the core. Temporary mobile facilities may appear flexible, but their reliability has been poor in past accidents.

Is retrofitting flammable cables appropriate? In 1975, the Browns Ferry Nuclear Power plant in the United States experienced a serious accident when a candle used to detect leaks started a fire that burned the covering material of electric wires, making it difficult to cool the reactor. In light of this, the Nuclear Regulation Authority requested that all existing flammable electrical wiring be replaced with flame-retardant cables. However, in reality, this kind of retrofitting is difficult on wiring that is routed like a spider web. The electric power company did not replace all wiring out of concern for miswiring, opting instead to apply flame-retardant paint. This is one of many examples in which an established requirement was ignored in practice.

Is it acceptable not to install a core catcher? Given that authorities found it difficult to treat the meltdown core at the Fukushima accident site, the Nuclear Regulation Authority might have required all nuclear power plants to be equipped with a core catcher—a device for receiving and treating the meltdown core. However, they made no such requirement. As this compromising attitude has been consistent throughout their assessment, they may fail to prevent accidents and disasters.

What is the civilian evacuation plan? The regulatory commission has emphasized that their new standards are in line with the International Atomic Energy Agency’s concept of multiple protection, which attaches great importance to evacuating residents as a final measure. Nevertheless, the requirements in Japan do not include an evacuation plan for the residents. Both before and after the Fukushima accident, the local government was in charge of evacuation. Though the Nuclear Regulation Authority has issued a permit to operate nuclear power plants in densely populated locations, the authority’s review does not mention the difficulty of evacuating residents.

Are Japanese nuclear power plants safe 10 years after the Fukushima disaster? In Japan, seismologists are concerned that earthquakes as significant as the one that struck Nankai, which registered magnitude 8, will occur in the future. Will the restarted nuclear power plants that have passed the conformity assessment following the Fukushima disaster be able to withstand such an earthquake? In light of current assessment practices, there is reason for extreme pessimism.

The author would like to thank Professor Subrata Ghoshroy for encouraging him to write this article.