Reviewing the Safety of Nuclear Power Plants — Part 1: Resuming nuclear power generation is a highly dangerous choice that may cause another Fukushima-scale disaster

By Goto Masashi, Doctor of Engineering, Former Nuclear Plant Engineer, Member of Citizens’ Commission on Nuclear Energy

Introduction

After the Fukushima nuclear power disaster in 2011, the Japanese government adopted the policy of “reducing dependence on nuclear power generation as much as possible.” In February 2023, however, it adopted the Basic Policy for the Realization of Green Transformation (GX), in which it clearly intends to build new nuclear power plants and/or add reactors to existing sites (including the replacement of old reactors with next-generation innovative reactors). No matter how environmental conditions may change, nuclear power plants continue to generate radioactive wastes, and how to dispose of these wastes is unknown. More importantly, the fundamental understanding about the safety of nuclear plants, which is to do everything possible to prevent another Fukushima-like disaster occurring, is being forgotten. In this series of articles, I would like to return to the basics and clarify the philosophy concerning nuclear accidents and dangers that have been discussed since the Fukushima disaster. I would like to speak out to win freedom from nuclear power generation by encouraging each individual to seriously examine how much the public tendency to accept the resumption of nuclear power is increasing the risk of a severe nuclear accident.

Status quo of restart of nuclear power plants and impaired safety

1. Status-quo of restart of nuclear power plants after the Fukushima disaster

1-1 The Fukushima disaster is not over — risks of radioactivity

The Fukushima Daiichi Nuclear Power Station disaster emitted immense volumes of radioactive substances all over the eastern part of Japan, and many people throughout this country have been concerned about the impact of these emissions. However, the Japanese government underestimated the impact of radioactivity: The annual limit of radioactive exposure to an ordinary person is supposed to be one millisievert (1 mSv) (the limit of the dose to which a person may be exposed in one year; there is a controversy about the scientific meaning of this limit, but this article does not examine this aspect). After the Fukushima disaster, however, the government adopted an emergency annual dose limit of 20 mSv, which is 20 times as high. In addition, deliberately ignoring opposing opinions from local fishers and people in and outside this country, the government boldly decided to discharge the contaminated water into the ocean. Although treated by the Advanced Liquid Processing System (ALPS) and diluted, the water still includes radioactive substances such as tritium. In fact, nuclear power plants and spent-fuel reprocessing plants discharge great amounts of tritium during normal operation, even when they are not experiencing an accident. The government calls nuclear power plants “environment-friendly and sustainable,” while, to the contrary, they emit radioactive substances into the environment. This is a totally brazen contradiction and reckless policy, which is both illogical and immoral. Furthermore, the government plans to use the soil resulting from decontamination work following the Fukushima disaster for public civil engineering projects, such as the construction of embankments and roads, unless the radioactivity of the soil is higher than a given level.

The nuclear fuel debris, which is the principal cause of the Fukushima disaster, is extremely radioactive. Only 0.7 gram has been experimentally examined so far, and how to decommission the reactors is totally unknown. As can be seen from this fact, it is apparent that the attitudes of plant operators, the Nuclear Regulation Authority (NRA) and the government toward the safe control of post-disaster radioactive substances are totally untrustworthy.

1-2 Fukushima disaster is not over — the causes of the disaster are only half-known

After the Fukushima disaster, the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Committee (NAIIC) and other organizations compiled disaster investigation reports. The NAIIC proposed that an organization should be established to investigate the disaster further, but no such organization was established, putting an end to investigation efforts regarding the causes of the disaster. A limited number of prefectures established nuclear power safety technical committees (named differently from prefecture to prefecture) and continued discussions, but because of the prefectures’ policy (under the control of national government policy), no “proper investigation” was conducted. By the word proper investigation, I refer to an investigation by an organization in which technically correct discussion is possible, independent of the “Nuclear Power Village.” An example of such a body was the early days of the Niigata Prefecture Technical Committee, which was composed of experts, various civil organization members, and others. Under the NRA, the Committee on the Analysis of the Accident at TEPCO Fukushima Daiichi Nuclear Power Station has been acting continuously but obscurely, focusing on the problems left unsolved by the NAIIC. More than ten years has passed since the Fukushima disaster, but many new facts are still being discovered. As an example, the shield plug made from reinforced concrete on the operation floor (placed immediately above the containment chamber to block radiation), has been found to be as highly radioactively contaminated as nuclear fuel debris, and measures to counter the radiation and an investigation of the cause are being conducted. In 2022, a large part of the concrete of the reactor pressure vessel base (pedestal) of Fukushima Daiichi Unit 1 was found to have disappeared, which shocked me. Although the reactor itself has already been damaged, it was staggering to learn that a thickness of nearly one meter in the concrete of the base that supports the reactor was missing. In addition, the explosion of Unit 3 was initially said to have occurred due to hydrogen gas, but today a more dominant opinion is that it occurred in two stages, involving another flammable gas. The causes of the disaster are far from being clarified, while once-stopped reactors in other power stations are being allowed to restart one after another, having been found acceptable under the new regulatory standards, which are unreliable.

1-3 On the verge of trouble due to reactor aging and restart

The Japanese government abolished the 40-year limit of reactor service, having decided to allow reactors to run for up to 60 years after a special inspection is conducted at 40 years. The government stated that the 40-year design life of reactors is not a matter of technology but is a matter of governmental policy, and the extension of the reactor life was approved by engineering experts at the technical committee. This is an outrageous misunderstanding about technology. In the production and design of a device, the number of years of use is one of the most critical specifications. Depending on the materials and service environments, components start to fail frequently after a given number of years, which will make punctual repair impossible.

In the case of a nuclear power plant, such failures may result in a severe disaster. Nuclear plants are subject to neutron irradiation embrittlement: The reactor and internal structures in the core embrittle as they are exposed to neutrons. If a failure such as piping rupture occurs and cooling water enters the embrittled reactor vessel, a rapid temperature drop occurs, producing tensile stress, resulting in a destructive reactor breakdown, which would lead to an unimaginably severe accident. In addition, aging materials are damaged in various ways, such as corrosion, stress corrosion cracking, wear, and fatigue. A failure that may threaten the safety of nuclear power plants is a failure of the electrical system, especially of the instrumentation control system. This may occur anywhere and anytime, and because the location of the failure is invisible, the cause often remains unknown. For more details about the destruction of metal materials, please refer to How Nuclear Power Plants Break Down — Review from the Basics of Metals¹⁾. For neutron irradiation embrittlement, The Aging of Nuclear Power Plants — Focusing on the Neutron Irradiation Embrittlement of Pressure Vessels²⁾ carries detailed explanations.

1-4 Boiling-water reactors (BWRs) that restarted after a suspension of 13 years following the Fukushima disaster

After the occurrence of the Fukushima disaster, new regulatory standards took effect in July 2013. The first reactors that restarted according to the new standards were Sendai Nuclear Power Station Units 1 and 2, Kagoshima Prefecture, which came online in August and September 2015, respectively.

As of September 2024, 12 nuclear power reactors have been restarted, including the Sendai units. However, all of these have been pressurized-water reactors (PWRs), which have a different mechanism from Fukushima’s boiling-water reactors (BWRs). No BWRs were restarted until October 2024, when Onagawa Nuclear Power Station Unit 2 was restarted, and this was followed by Shimane Nuclear Power Station Unit 2, restarted at the end of 2024. It is extremely problematic that the government permitted the restart of the BWRs while the investigation into the causes of the Fukushima disaster has been insufficient and accident countermeasures have not been properly taken. Both Onagawa Unit 2 and Shimane Unit 2 experienced critical problems as they were being prepared for restart, and I would like to explain about these and their significance.

(1) A neutron detector in the reactor was found to be abnormal when Onagawa Unit 2 was being prepared for restart, and the startup aborted

On November 3, 2024, when the calibration devices, which had been inserted into the reactor to detect neutrons, were being pulled out, one of the four detectors of the traversing incore probes (TIP) became abnormally stuck (Tohoku Electric Power calls this phenomenon an “event”). (Refer to the figure below. In the figure, the calibration devices are being pulled out from the reactor vessel and primary containment vessel.)

Figure 1

The cable was then pulled manually, and the calibration device extracted. The reactor was shut down on November 4, and an investigation was begun to discern the cause of the trouble.

According to the power company, the cause was that the cable connection was lost when the calibration device in the guide pipe was being pulled out. The explanation also mentioned that the easily removable nut at the cable connection was not sufficiently tight. The explanation implies that there might have been a human error. The company also mentioned that the guide pipe was curved and friction was high. However, the fundamental problems are that the reactor is designed such as not to be operational without such incore detectors for precise neutron detection, and that the detector mechanism itself is vulnerable: The detector drive is provided outside the primary containment vessel, and detectors are inserted into and pulled out of the vessel isolation valve via the guide pipe. More specifically, when the calibration devices are in the reactor, the primary containment vessel isolation valve is open, namely, it cannot be closed, creating an imperfectly shielded condition, even if temporary. The isolation valve is closed after the calibration device is extracted, but the primary containment vessel is supposed to have all isolation valves closed immediately when trouble occurs to prevent radioactivity leakage.

Neutron detection in the reactor is called “Nuclear instrumentation, which is a facility that monitors the nuclear fission in the core through the entire output range of the reactor, from standstill status to activation status (confirmation of criticality), and then up to full-output status. Because the reactor output range is far wider than that of industrial use, the detector is supposed to respond quickly and to accommodate a high-precision measurement range. It is imperative for nuclear plant safety and protection” (quoted from the “Nuclear instrumentation” web page of Wikipedia, with additions by the author). The TIP is not at all a secondary device, but is responsible for safety of the heart of the nuclear power plant, namely, the control of the reactor, and “detects an anomaly and shuts down the reactor when required.” In the investigation of the recent accident, aging-related problems were ignored, such as the cause of the friction increase in the TIP guide pipe, which was reported to have been the cause of the trouble. Onagara Unit 2, not in use for 13 years and six months after being shut down, may develop more problems. Not only the instrumentation system but also the equipment and mechanisms that have been out of service for so many years are highly likely to have abnormalities due to age deterioration or for other reasons. Concerning the systems that are not in service during normal operation, including the emergency core cooling system (ECCS), if an anomaly occurs and an attempt is made to operate the safety device, it may fail to activate, causing a failure which may then develop into an accident. In addition, Onagawa Unit 2 was damaged by the 2011 earthquake. After the trouble with the TIP, it was restarted in haste only after the implementation of stopgap measures, such as the preparation of manuals and the issuing of cautions about human errors, instead of the implementation of full-scale measures, leaving the possibility of a recurrence of similar troubles.

In August 2010, a problem was reported at Hamaoka Power Station Unit 4: After TIP detectors were retrieved, one primary containment vessel isolation valve, which was supposed to close automatically, could not be closed. The cause of this trouble was a control circuit failure: After the TIP detectors were pulled out in a normal manner, the valve-closing signal to close the isolation valve was not generated by the control circuit. The failure of a control circuit cannot be found in advance, and it may have posed a danger, causing an operational inactivation or error.

(2) Reactor cooling system trouble (accident) at the time of restart of Shimane Nuclear Power Station Unit 2, resulting in a temporary suspension

On December 7, 2024, Shimane Nuclear Power Station Unit 2 was restarted after a suspension of 13 years. Shortly after 11:00 o’clock on the morning of the 12th, a problem was reported: the water gauge in the reactor was unable to measure the water level. The company announced that the water gauge was showing abnormal values for about one hour. According to the company, a subsequent investigation found that there was no abnormality with the equipment, and that the problem was the result of insufficient understanding of the instrumentation.

The Chugoku Shimbun newspaper reported that “According to the Chugoku EPCO Nuclear Power Headquarters, the rate of water flow in the piping of the pump that adjusts the output, was increased to raise reactor output, and at that time the water gauge indicated a value beyond the upper limit. The company found the water level unmonitorable at 11:21 a.m. When the flow rate was reduced, the indication returned to normal. The indication of the water gauge normally used showed no abnormality. The company said that the phenomenon could have occurred due to the  other water gauge, which had been newly installed in accordance with the new regulation standards, and that operating staff had not understood the gauge readings correctly.

The major issues involved in this incident are as follows:

(1) The water gauge, which is the most critical gauge in the reactor, did not show the correct water level. This was an extremely critical problem (which could be called an accident). The water level is an important indicator that controls the flow rate to cool down the reactor.

(2) Whatever the cause may have been, the reactor water level was unknown for one hour. This in itself has a great potential for danger because, in the case of a PWR, the core starts to melt if cooling is stopped for slightly more than twenty minutes. In the case of a BWR, if the cooling is off for an hour or more, a core meltdown is possible.

(3) The water gauge that posed the problem is for use at the time of a severe accident. It measures the water level from the pressure of the recirculation system piping of about 60 centimeters in diameter. The recirculation system pump circulates a great volume of water all the time to control reactor output; therefore, the flow rate fluctuates, and its measurement precision is lower than for water gauges used in normal conditions. The staff erroneously judged from the measured water level that an abnormality was occurring because their understanding of the specifications of the water gauge was insufficient. The water gauge that caused the human error in judging an “abnormal condition” when the water level was normal is equivalent to a design error. The investigation into the causes of the Fukushima disaster reclarified the disadvantages of the BWR water gauge, but the new regulation standards allow the reactor to pass the qualification investigation if a water gauge for detection of a significant (severe) accident is provided.

(4) It should be judged that the new water gauge that was added to pass the qualification investigation according to the new regulation standards has a fundamental defect. The NRA’s judgement is a review error that should never happen.

(5) A nuclear power plant accident is not caused by a single failure. Multiple failures of devices of the same kind or erroneous operations of different types of gauges are involved in many cases. Especially when there is a failure in the power supply or electronic circuit, the cause of the failure cannot be identified immediately. As the accident proceeds, the seriousness of the situation becomes apparent, but as was seen in the recent case, unexpectable accidents may occur if a human error is involved, as learned from the Three Mile Island accident (TMI accident) in the U.S. and from the Fukushima disaster.

(6) The light-water reactor water gauges originally have many problems. At TMI the PWR water level could not be ascertained, causing an accident. The failure of the water gauge was attributed to the misunderstanding of the workers, and fundamental countermeasures were not taken. This can be considered a cause of the Fukushima disaster.

(7) Concerning BWRs, the reliability of water gauges was in question even before the Fukushima disaster. The Fukushima disaster occurred with the known problems left unsolved. The water gauge malfunction was overlooked at the time of the disaster, and Unit 1 melted down late at night on March 11. TEPCO, however, did not acknowledge this fact for two months, until the long holiday season in May. Currently, the water gauge fitted on all the BWR nuclear power plants are the same type as those on Fukushima Daiichi NPS, and if an accident occurs, there is the same possibility that the staff will be unable to grasp the water level. Thus the reliability of the new regulation standards is in question.

(8) Shimane Unit 2 is in a very dangerous state. It is unknown where and how a problem may occur. In addition, there is a news report that 60 percent of Chugoku Electric Power Company nuclear power plant operating staff are inexperienced in nuclear power (Asahi Shimbun newspaper). While a nuclear power plant is running normally, it may not be very problematic even if half the workers are inexperienced, but once an accident breaks out, the situation is completely different. Especially when the reactor core may melt down, there can never be enough staff no matter how many are present. When an inexperienced worker is directed to carry out a task, s/he would have no time to ask where the required equipment is located, how to handle it, and in what way the troublesome mechanism is working. Looking back on the Fukushima disaster, even highly experienced workers were unable to act appropriately. Seen from the position of responding to a nuclear power plant accident, it is an unimaginably horrible situation.

In the next article in this series, I would like to discuss the basic philosophy of the safety of nuclear power plants.

1) Koiwa Masahiro and Ino Hiromitsu. How Nuclear Power Plants Break Down — Review from the Basics of Metals (in Japanese). Citizens’ Nuclear Information Center, Tokyo, 2018.

2) Researchers on the Aging Problem of Nuclear Power Plant. The Aging of Nuclear Power Plants — Focusing on the Neutron Irradiation Embrittlement of Pressure Vessels (in Japanese). Citizens’ Nuclear Information Center, Tokyo, 2023.