Impacts and challenges for Japanese nuclear operators as a result of legislative change to extend the legal lifetime of nuclear power plants in Japan

Abstract

The purpose of nuclear regulation is not to impede the sound development of industry's competitiveness. Therefore, the regulations must be coherent, transparent and most of all, logical.

On May 31st, 2023, a bill including rules to change the legal lifetime of nuclear power plants was approved and endorsed at the 211th ordinary session of the National Diet in Japan. An important characteristic of this legislative change is that it does not simply grant additional operating years to the original lifetime; rather, it considers the length of time that the plant was under shutdown status due to reasons not attributable to the plant owners and excludes this period from the countdown of the predetermined lifetime, thus extending the original legal lifetime.

Three major problems could be listed in the procedures for changing the rules and in the new rules themselves. Firstly, the period during which this matter was discussed was too short to discuss issues raised by the change in detail. Secondly, the definitions of the period to be excluded from the predetermined lifetime are neither scientifically logical nor clear and would likely cause a great deal of confusion for both licensees and regulators when the new rule comes into effect. Thirdly, under this rule, the legal lifetime of a plant which performance is worse would be longer than that of the one with better operating performance. In particular, the second and the third problems deviate greatly from the purpose of nuclear regulation, which is not to impede the sound development of industry's competitiveness. The author strongly urges all parties involved to reconsider their decisions even after the bill has got an official approval at the National Diet.

1. Introduction

The basic concept of nuclear regulation is that it should not impede the sound development of the nuclear industry as long as radioactive and other hazardous materials do not adversely affect human health. The Atomic Energy Act of 1954, as Amended in NUREG-0980, the cornerstone of the United States Nuclear Regulatory Commission (NRC), states that "the development, use, and control of atomic energy shall be directed so as to promote world peace, improve the general welfare, increase the standard of living, and strengthen free competition in private enterprise [1].” It means, therefore, that the purpose of regulation is to promote the welfare of the people and to strengthen the competitiveness of business activities. In order not to impede the sound development of industry's competitiveness, regulations must naturally ensure coherency, transparency, and above all, a logical basis. The NRC's Principles of Good Regulation clearly state that "Regulations should be coherent, logical, and practical”.

The foundation of Japan's nuclear regulatory system is the Law “Regulation of Nuclear Source Materials, Nuclear Fuel Materials and Reactors” (hereafter "Nuclear Regulation Law") enacted in 1957. The purpose of this law, however, is not to "strengthen the competitiveness of the nuclear industry" as stated in NUREG-0980 of the United States, although there are provisions for the protection of the public's health and property. From its inception, Japan's nuclear regulations have been designed solely to regulate the industry, and not, as in the United States, to position the development of industry competitiveness as a primary goal.

Based on this premise, this research paper reviews the evolution of the rules for the legal lifetime of commercial nuclear power plants in Japan, looking back to the revision of the Nuclear Regulation Law in 2012 and the establishment of the Nuclear Regulation Authority, followed by a detailed review of the discussion on the legal lifetime extension starting in 2022. What should be cleared is the process of how the revised law was enacted without logical rationale, transparency, and coherency. In addition, as a case study of changes in the definition of legal lifetime in other countries, the 2002 revision in Germany and its results are discussed, and a comparison with Japan is made to show that, although the peculiar rules may have led the incentives of operators in the wrong direction, the German law was logically based and transparent as a matter of principle. Germany is the only country which adapted a definition of the legal lifetime different from “the number of years since the beginning of commercial operation”. That is the reason why the case study of Germany is crucial. Furthermore, the discussion surrounding the subsequent licensing renewal in the United States will also be reviewed, and points that the discussion suggests "strengthening the competitiveness of the industry," which have never been discussed in Japan, will be clarified.

On March 1, 2023, a bill including rules to change the legal lifetime of nuclear power plants was submitted to the 211th ordinary session of the Diet, and it was approved at the House of Councilors of Japan on May 31st. An important characteristic of this bill is that it does not simply grant additional operating years to the original lifetime; rather, it considers the length of time that the plant was under shutdown status due to reasons not attributable to the plant owners and excludes this period from the countdown of the predetermined lifetime, thus extending the original legal lifetime.

The objective of this paper is to clarify the problems which this new regulation would bring about and to openly ask whether it is in accordance with the “Principles of the Good Regulation” by NRC [2], and also whether it contributes to the sound development of the nuclear industry's competitiveness. In this paper, the author will explain in detail the policy change, which would greatly affect the operation of nuclear power plants and have a large impact on the management strategy of nuclear power operators. The author will also discuss situations in other countries and try to pose relevant questions that would contribute to furthering the discussion on a better business environment for nuclear power operations.

2. Methodology

The unique aspect of this study is that the definition of the legal lifetime of nuclear power plants in Japan under the revised law in 2023 is unprecedented in the world, and there is no existing literature that explores this issue, making it very difficult to apply the usual research framework of 1) reviewing existing research literature and 2) finding new perspectives which has not been suggested in the existing ones. Therefore, the author had to apply the less common research framework in this paper as follows.

2.1. Review of developments concerning operating lifetime regulations

The history of how the 60-year limit was clearly defined in 2012, along with the consolidation of the organization into the Nuclear Regulation Authority, will be reviewed mainly through official documents such as governmental documents and operator announcements.

2.2. Review of the discussion surrounding change of the legal lifetime in 2022

The author reviews the discussions concerning the proposed amendment, focusing on the discussions in the Nuclear Energy Subcommittee, a Japanese government council. The proposed amendments have been exclusively discussed by the Nuclear Energy Subcommittee, an expert committee of private-sector experts established by the Ministry of Economy, Trade and Industry of Japan (METI). Although the history of the discussions is disclosed in the materials and minutes of the Subcommittee, there are no precedents of studies that have analyzed in detail the pros and cons of excluding the shutdown period from the statutory service life, which is the issue in this paper, and the possible impact on operators, except for an analysis by the Institute of Energy Economics, Japan (IEEJ) in January 2023. The issues discussed in this paper are based on the opinions of the members of the Nuclear Energy Subcommittee and the IEEJ analysis.

2.3. Review of the cases in overseas

As mentioned above, the way the legal lifetime is defined in Japan is unprecedented in the world. In the meantime, Germany is the only country in the world with a precedent for defining the lifetime in terms other than calendar years, i.e., in terms of total generation. This paper analyzes the history of this provision and the results it has caused, based on government documents and press releases from operators, and reviews the differences from the recent legal revision in Japan. In addition, the discussion on the second licensing renewal rule in the United States will be reviewed to clarify the differences with Japan.

2.4. Discussions and conclusions

Based on a review of domestic and international cases, this paper will discuss issues and problems in the Japanese legal change process, and recommendations for a better regulation.

3. Review of developments concerning operating lifetime regulations

3.1. History of regulatory standards for lifetime extension of nuclear power plants

Prior to the accident at the Fukushima Daiichi Nuclear Power Plant (hereafter referred to as the “1F accident’) in March 2011, there were no legal or regulatory provisions regarding the maximum operating period of nuclear power plants in Japan, and it was believed that safety could be guaranteed through appropriate maintenance and inspections alongside strict reviews. In fact, the Nuclear and Industrial Safety Agency (NISA) approved the safety regulations for the long-term maintenance management policy for Tsuruga Unit 1 (commercially operating from March 1970), Mihama Unit 1 (commercially operating from November 1970), and Mihama Unit 2 (commercially operating from July 1972) of the Japan Atomic Power Company (JAPC) in September 2009, June 2010, and July 2010, respectively [3], and the operators had planned to continue after 40 years of commercial operation.

In the wake of the 1F accident, the regulatory framework for nuclear power plants was fundamentally reviewed, and historic changes were made to the Nuclear Reactor Regulation Law regarding operating periods. The Act for Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors (hereafter “the Nuclear Regulation Act”) was revised in June 2012 [4]. The revised Nuclear Regulation Act stipulates that the operating period of a nuclear reactor shall be 40 years from the date of passing the pre-use inspection, and that this lifetime may be extended for a period not exceeding 20 years on one occasion only, with the approval of the Nuclear Regulation Authority (NRA).

3.2. The post-Fukushima change of the regulatory body and the regulatory standards

Following the 1F accident, significant changes were also made to the nuclear safety regulatory system. NISA, which had been in charge of nuclear regulation and licensing since 2001, was a body established under the Agency for Natural Resources and Energy (ANRE) [5]. After the accident, this regulatory body was abolished from the viewpoint of organizationally separating bodies promoting and regulating nuclear development, and in September 2012, the NRA and its technical agency, the Nuclear Regulation Agency were established independently from ANRE [6]. The purpose of the NRA is to “restore domestic and international confidence in Japan's nuclear regulatory organization, rebuild nuclear safety management and establish a true culture of safety, with the safety of the public as the top priority.”

In July 2013, the “New Regulatory Standards for Commercial Power Reactors and Nuclear Fuel Facilities,” which were revised based on the lessons learned from the 1F accident, came into effect [7]. Since then, all commercial nuclear power plant operators are subject to review by the NRA if they wish to continue to operate their reactors beyond 40 years.

3.3. Management decisions of the operators based on the new regulations

In June 2016, Units 1 and 2 of the Takahama Power Plant owned by Kansai Electric Power Co. received approval from the NRA for an extension of its operational period beyond 40 years of commercial operation to 60 years [8], and in November of the same year, Unit 3 of the Mihama Power Plant, also owned by Kansai Electric Power Co., received a similar approval [9]. In November 2018, Tokai Daini Power Station owned by JAPC received the same approval as the fourth plant being licensed for the lifetime extension to 60 years [10]. As of the end of July 2023, four plants have obtained approval to extend their lifetime beyond 40 years–60 years, and four plants have submitted application (units 1 and 2 of Sendai Nuclear Power Station of Kyushu Electric Power Co. and units 3 and 4 of Takahama Nuclear Power Station of Kansai Electric Power Co.).

On the other hand, since the revision of the Nuclear Regulation Act in 2012, there have been 11 plants that have decided to end commercial operation, excluding the 10 units at Fukushima Daiichi and Daini power stations. As mentioned above, Tsuruga Unit 1 and Mihama Units 1 and 2 had already been in commercial operation for 40 years and were planning to continue operation, before the regulatory revision. However, in March 2015, the decision to decommission was announced by the operators, JAPC [11] and Kansai Electric Power Co. [12], following the tightening of the criteria for renewal of operating licenses. Subsequently, a series of similar decisions were made for plants with smaller output that began commercial operation in the 1970s and early 1980s, including Kyushu Electric Power's Genkai Unit 1 and 2, and Chugoku Electric Power's Shimane Unit 1. These operators that made the decision to decommission their plants before its 40th birthday stated that the decision was based on a comprehensive consideration of various factors, including “the large-scale and long-term construction costs to obtain approval, the operating period after the approval, and scale of output if the plant is restarted” [13]. Even after obtaining approval to extend the lifetime beyond 40 years, the plant will not be able to operate until the restoration work is completed and the plant passes a comprehensive load test. The remaining operating period will therefore be much shorter than 20 years. It is undeniable that such factors had a significant impact on these decisions.

3.4. Discussions leading up to the new rule to exclude the shutdown period from the countdown of operating lifetime

The invasion of Ukraine by Russia that began in February 2022 and the consequent global energy price hikes had a major impact on Japanese society. On August 24, 2022, Yasutoshi Nishimura, the Minister of Economy, Trade and Industry, made a presentation titled “Rebuilding a Stable Energy Supply for Japan” at the second meeting of the Green Transformation (GX) Implementation Council, chaired by Prime Minister Kishida. Nishimura, concurrently serving as the GX Implementation Minister, stated that the “maximum utilization of existing nuclear power plants, including extending their operation periods” would constitute a key part of the “political decisions to resolve 'stagnation in energy policy'” within the nuclear energy field [14]. The GX Implementation Council is a high-level meeting attended by the prime minister and the minister in charge of energy policy, where policy issues that “should be implemented immediately” are referred to lower-level councils in each field for further discussion.

The direction set forth by Minister Nishimura was taken up by the Nuclear Energy Subcommittee, a council in charge of discussing nuclear energy policy, at its 31st meeting on September 22, 2022. At this meeting, one of the “points for consideration upon setting the lifetime” of reactors, as stated in a document produced by the ANRE, mentions the “consideration of outage periods etc. due to heteronomous factors.” It has been explained that the word “heteronomous” here means “beyond the operators' responsibility” [15]. The idea of excluding outage periods from the countdown of the given lifetime, thus adding to the operational period, was discussed once before in April 2021 [16], but this was the first time the word “heteronomous” appeared in a publicly available governmental document as a condition for such exclusion. At the 31st Subcommittee meeting, Commissioner Shinsuke Toyonaga, an attorney at law, discussed possible instances that could be considered “heteronomous,” such as delays in the review process, lawsuits, and temporary injunctions to halt operations. He also commented that “this would be a tricky issue because licensing processes take a certain amount of time, and there is difficulty in determining whether an injunction to halt operations is a legitimate exercise of the plaintiff's rights. In any case, relevant institutions should start discussions to clear such issues” [17]. His statement is noteworthy as it points out the difficulty that would arise in trying to strictly define the period of suspension that should be excluded.

Specific examples of outage periods that should be excluded from the given lifetime appeared in the documents presented at the 33rd Nuclear Energy Subcommittee meeting on November 8, 2022, based on the short discussions at the previous meetings so far. At this meeting, the ANRE presented the following three options for considering the lifetime of a reactor “from the perspective of utilization policy, such as securing a stable power supply and pursuing the green transformation,” as distinguished from regulatory standards and processes such as periodic licensing reviews [18].

Proposal 1: Maintain the current regulation of 40 years of operation, and allow only one additional extension of up to 20 years.

Proposal 2: Eliminate the cap and set no time limit for additional extensions after 40 years of operation.

Proposal 3: After 40 years of operation, additional extensions would be allowed, subject to certain limits.

As specific examples of the “additional extensions” mentioned in Proposal 3, the document by ANRE lists “shutdown periods based on heteronomous factors that should be excluded from the given lifetime,” such as shutdowns based on changes in regulatory standards, administrative orders or court orders for temporary injunctions. Henceforth, this paper discusses the idea proposed here to “exclude periods of shutdown from the countdown of the given lifetime, and extend the operation period by the same length,” which will be referred to as “stop-counts.”

About three weeks later, at the 34th Nuclear Energy Subcommittee meeting, the ANRE compared the three proposals from several perspectives including “public sentiment in light of the 1F accident,” “stable energy supply,” and “ensuring predictability for operators” [19]. According to this assessment, in terms of public sentiment in light of the 1F accident, Proposal 1, which maintains the intent of the current law as formulated in 2012, is preferable. It also would be conservative, because aged reactors generally have some problem on safety issues. Proposal 2, which sets no limit on the operation period, is better from the perspective of stable supply. However, the ANRE recommended Proposal 3, which they claimed was neither largely problematic nor particularly outstanding from any perspective, and it was this proposal that was eventually finalized.

On December 22, 2022, the 35th Nuclear Energy Subcommittee summarized the discussion so far and released a draft policy paper titled “Direction of future nuclear energy policy and action guidelines” on the following day. The cabinet office called for public comments on the draft paper for a month and it was officially finalized on April 28, 2023 [20]. The following three conditions are shown as examples of stop-counts.

A: Shutdown periods that occurred in accordance with changes in legal systems (safety regulations, etc.) after the Great East Japan Earthquake (including review and preparation periods after changes in circumstances).

B: Shutdown periods that occurred in accordance with administrative orders, recommendations, or administrative guidance, etc. after the Great East Japan Earthquake (excluding those issued against inappropriate actions of operators).

• C: Shutdown periods that occurred in accordance with court orders of temporary injunction after the Great East Japan Earthquake, or other reasons unpredictable for the operator (but limited to those that have been corrected by a superior court).

More detailed descriptions cannot be found anywhere in public documents, which means that defining “stop-count” periods for individual plants would be a fairly arbitrary procedure in reality. Before this problem is investigated in more detail in Chapter 2, the following must be noted. What is most alarming is the fact that, from the 31st meeting to the 35th meeting, when the “Direction of future nuclear energy policy and action guidelines (Draft)” was compiled, there were very few comments from the committee members discussing the advantages and disadvantages of the stop-count system (Proposal 3, recommended by the ANRE) as compared to Proposals 1 (as before) and 2 (no operation period restriction), or pointing out the difficulty of strictly defining such a “stop-count” period. One of the few instances include statements made at the 34th meeting, one by Commissioner Heigo Sato (a Professor at Takushoku University) that “If the conditions for extension periods are not clarified, confusion about lifetime extensions will arise, due to the possibility of political considerations,” and another by Commissioner Sae Ochi (Professor at Jikei University School of Medicine) that “It is important to provide a clear and transparent definition of the conditions for granting an extension so as not to leave any ambiguity and to prevent the loss of predictability” [21]. However, there have been no remarks that go as far as to point out in which cases ambiguity may arise in reality. As noted above, Commissioner Toyonaga, who pointed out the “difficulty in defining the shutdown period to be excluded” at the 31st meeting, has not raised the same point since, and the ANRE did not follow up on his point after that meeting. The author would like to emphasize the seriousness of the fact that this obvious issue was never properly discussed during these multiple meetings.

3.5. The revision of the Electricity Business Act

From late February 2023, moves to realize the necessary legal amendments to put the “stop-count” policy into practice began to gear up. On February 28, a series of bills (officially titled the “Bill for Partial Amendment of the Electricity Business Act, etc. to Establish an Electricity Supply System for Realizing a Decarbonized Society”), including a bill to amend the Electricity Business Act to incorporate provisions related to the “stop-count,” were approved by the Cabinet and submitted to the 211th ordinary Diet session [22]. The bill has been debated in the Committee on Economy, Trade and Industry in the House of Representatives of Japan since February 28, in the House of Representatives plenary session since March 30, and in the Committee on Economy, Trade and Industry at the House of Councilors since May 10 and finally approved at the House of Councilors of Japan on May 31 [23]. The revised section of the act is Article 27-29-2, where “the lifetime of a nuclear reactor for power generation” is newly established, and the stop-count period is defined as follows. Since the article itself is lengthy and difficult to understand, a brief version is presented here:

… In the case where the given lifetime to be extended exceeds 20 years, the period exceeding 20 years must be less than the total of the following periods (of which occurred after March 11, 2011).

• (a)Shutdown period to comply with changes in the Nuclear Regulation Act, etc.
• (b)Shutdown period due to exceeding the grace period for the construction of “specialized safety facilities (hereafter “SSFs”)” for severe accidents [24].
• (c)Shutdown period due to compliance with administrative guidance
• (d)Shutdown period due to a temporary injunction order
• (e)Other shutdown periods in accordance with ministerial ordinances that could not have been predictable by operators

While several items have been added, the definition stated above hardly differs to the policy paper produced by the Nuclear Energy Subcommittee in its ambiguity regarding how shutdown periods will be counted. The complexities and implications of such obscure regulations will be investigated in detail in the following sections.

4. Estimation of the “stop-count” period for the 33 existing reactors in Japan

4.1. Calculation process and issues regarding the estimation of “stop-count” periods

As mentioned in subsection 1.5., a bill defining the stop-count provision has been approved by the Cabinet and was already approved in the ordinary Diet session. Meanwhile, there have been no reports estimating the actual stop-count periods, i.e., the duration for which each plant would be able to extend their lifetime without any additional licensing procedure if the aforementioned bill were enacted, except for an estimation produced by the Institute of Energy Economics, Japan (IEEJ) [25]. According to their report, the IEEJ set the conditions of the calculations for the 33 existing reactors as follows.

• ✓In addition to the period from shutdown to restart after the 1F accident, the calculation includes any suspensions that were required to complete the construction of SSFs and to abide by court rulings.
• ✓For plants that had been out of service prior to the 1F accident, the shutdown date is set as March 11, 2011.
• ✓
  For plants that have not reached restart as of the time of calculation (December 2022), an assumption was made that their restart date shall be December 31, 2023. An exception was Units 1 and 2 of Takahama Station, where the expected dates of the conclusion of regular inspections have been announced; therefore, this date was used in the calculation.

  • ✓
    For plants that have already restarted, the restart date is defined as that of resumption of commercial operation. For plants that did not have such a specified date as of December 2022, the date of resumption of commercial operation was defined as one month after the restart of power generation. The same rule was applied to plants coming back from suspensions related to SSFs or court rulings.

The IEEJ paper points out that there are many considerations to be made regarding the conditions assumed in this estimation. The following two main problems are both concrete examples of the “ambiguity of the system” that was pointed out by several commissioners at the Nuclear Energy Subcommittee. The lifetime of a reactor, down to every single day, is a critically important factor for plant owners to make business decisions, and any ambiguity should not be allowed in the definition of such systems.

The IEEJ sets the beginning of the stop-count period as March 11, 2011 for the nine units that had been shut down prior to the 1F accident due to periodic inspections or equipment trouble. This is likely to be the case when the bill comes into effect, as the aforementioned article clearly states, “Only for the period after March 11, 2011.” However, it is doubtful, for example, whether the entire 12-year-plus shutdown from 2011 up to now of Kashiwazaki Kariwa Units 3 and 4, which were shut down after the Chuetsu-oki Earthquake of July 16, 2007, can be regarded as a “shutdown period to comply with changes in the Nuclear Regulation Act.”

Another example would be the Tokai Daini Power Station, which suffered severe damage to its turbine shafts and blades during the Great East Japan Earthquake. Even if the Nuclear Regulation Act had not been changed, it is highly likely that they would not have been able to restart operations for more than one year. This leads to the critical question - Who can decide how long the Tokai Daini Power Station was shut down solely out of the need to comply with the change in the Act, and how?

(2) The definition of the “stop-count” period of the plants which had been in periodic inspection before the end of the grace period for the completion of the SSFs.

The “grace period for the installation of the SSFs” in the aforementioned bill refers to the period that the plant can continue operations even if the SSFs are not yet completed, a temporary transitional measure limited to five years from the date of obtaining construction approval for the plant in question [26]. In other words, the reasoning of the bill is that if the SSFs are not completed within five years from the date of obtaining construction approval and the plant is shut down in accordance with the Nuclear Regulation Act, that period is also outside of the operators’ responsibility and should be subject to the stop-count.

In practice, however, there are cases in which suspension due to the lapse of the grace period for SSFs and suspension due to other troubles, etc., occur simultaneously. For example, Takahama Unit 3 and Genkai Unit 3 was shut down for periodic inspections about seven months earlier than the end of their grace periods for the completion of SSFs. Since it can be understood that work necessary for the construction of the SSFs was underway during the grace period as well as after its deadline, the IEEJ includes the seven months of shutdown prior to the deadline as stop-count periods.

However, there are more complicated factors to be considered when we take a closer look. In the case of Takahama Unit 3, the original schedule had been to finish the periodic inspection within the grace period, and resume operation for several months before its expiration. In reality, the periodic inspection was delayed due to damage to the heat transfer tubes of the steam generator, and the unit was still under repair work on the final day of the grace period in August 2020. The original plan for Genkai Unit 3 was to have its SSFs completed by the August 2022 deadline, but in reality the construction period was extended, reportedly due to a fire at the construction site and other issues related to COVID-19 [27], and the unit subsequently resumed operation in December 2022. If the stop-count periods for these cases are to be calculated strictly, it is necessary to clarify how much of the shutdown period is attributable to work related to the SSFs. In the case of Genkai Unit 3, it is also necessary to determine whether the cause of the delay is attributable to the operator or not. Since the actual business decisions concerning the scheduling of regular or irregular maintenance activities is a highly strategic and complex matter, it will be exceedingly difficult for authorities to assess these questions and give an impartial judgement on how long the stop-count period should be. Regulations should be coherent, logical and practical, as the “Principles of Good Regulation” by Nuclear Regulatory Commission says [28]. People must think further whether the “stop-count” system is in line with the principles of good regulation, and also with the purpose of the regulation to enhance the industry’s competitiveness, just as the NUREG-0980 suggests.

4.2. Results of the estimations and the emerging paradox: “the shorter the licensing period, the shorter the additional lifetime”

According to IEEJ calculations, the average stop-count period for the 12 reactors which passed the licensing criteria and restarted early, including two which are expected to restart in 2023, is 7.8 years. On the other hand, the average stop-count period for the 21 units that are not expected to have restarted by 2023 is 12.5 years, a difference of nearly five years from the average for the aforementioned 12 units. This is the difference based on the assumption that the 21 reactors will be restarted on December 31, 2023, so in reality, this gap will widen further year by year.

The IEEJ report illustrates this difference more visually as shown in Fig. 1. The “actual age” of each plant is defined as the period from the date of commencement of commercial operation to December 31, 2023. By subtracting the stop-count period from the actual age of each plant, the “legal age” as of December 31, 2023, can be calculated. The figure plots the “actual age” versus the “legal age after application of the stop-count” for the 33 existing reactors to visualize the paradoxical effect of the stop-count system. Plants that are slower at clearing the licensing process will have a longer stop-count periods than plants that have already been restarted or are expected to be restarted during 2023, so their “legal age” will be much younger.

Fig. 1.

Actual age of plants and age after the application of “stop-count”.

Source: The Institute of Energy Economics, Japan (2023)

The unreasonableness of the situation becomes more obvious once you put yourself in the operators’ shoes. For example, Kashiwazaki Kariwa Unit 1 started commercial operation in September 1985 and Sendai Unit 2 in November 1985, and both are 38 years old as of December 31, 2023. However, as a result of the application of the stop-count periods, 4.9 years is added to Sendai Unit 2 and 12.4 years to Kashiwazaki Kariwa Unit 1, making the “legal ages” of the two units 33.2 years and 25.9 years, respectively. This difference of over seven years is the difference between the end of the licensed lifetimes of the two units. If the given lifetime is 60 years, the end for Sendai Unit 2 would be 2050, whereas that for Kashiwazaki Kariwa Unit 1 would be 2058.

Fig. 2 illustrates the situation explained above. In reality, Kashiwazaki Kariwa Unit 1 has not applied for licensing as of July 2023, and has not even started to go through the safety review process in order to restart operations. Therefore, the additional years obtained under the stop-count system would be more than 12.4 years, possibly as much as 15 or 20 years.

Fig. 2.

The legal lifetime of Sendai Unit 2 and Kashiwazaki-Kariwa (KK) Unit 1.

Sendai Unit 2 will run out of operating time in 2050, no matter how good the plant performances are at that point, and will not be allowed to operate beyond that point. Conversely, Kashiwazaki Kariwa Unit 1, which took longer to pass the licensing procedure, or whose restart was less of a priority for its owners for whatever reason, will have more time left, and can operate until 2058 if it passes the NRA's review. Is this a reasonable differentiation?

4.3. Discussions and decisions by the NRA regarding lifetime extensions and concurring concerns of the stop-count system

A similar apprehension was voiced from an NRA commissioner during the process of discussion and decision by the regulators regarding the timeframe of licensing procedures that would accommodate the proposed stop-count system.

At a regular meeting on November 2, 2022, the NRA proposed the following changes to safety regulations for older power reactors.

(current regulation) After 40 years of commercial operation, all reactors must pass a one-time review before an extension of up to 20 years is allowed.

(proposed change) After 30 years of commercial operation, and every 10 years thereafter, operators must evaluate the deterioration of their facilities, and operation can only be continued if the NRA deems the results of the evaluation to be appropriate [29].

This proposal is positioned as a response to discussions of lifetime extensions coming from the ANRE, to lay out the conditions of regulation changes for such extensions should such utilization policies become legalized. This demarcation is important to the NRA as a regulatory body, since they have been careful not to get involved with issues on the promotion of nuclear use, such as issues of given lifetimes and their extensions [30]. The NRA believes that its role is solely to set standards from a scientific and technical perspective, to review facilities for compliance with those standards, to conduct audits through inspections, and to issue appropriate licenses. While this demarcation made the NRA hesitant to speak up on lifetime extension issues, it became undeniable that there will be an increasing need for strict regulations for ageing management of commercial reactors if operations exceeding 60 years become legalized, hence, the proposal for a new timeframe of regulatory reviews was discussed.

On February 13, 2023, the NRA, presented a draft amendment to the Nuclear Regulation Act based on the proposed changes stated above at an extraordinary meeting [31], and it was passed by a majority vote [32]. This was highly irregular, and was the first time since the NRA was established in 2012 that it had to resort to a majority vote for its decision-making. In fact, when the same proposal was presented by the NRA at its regular meeting on February 8, the week before, the conclusion was carried over because Commissioner Akira Ishiwatari, one of the five committee members, expressed opposition to the proposal. The same agenda was re-presented at the extraordinary committee meeting, where Commissioner Ishiwatari again spoke against the motion. However, Chairman Shinsuke Yamanaka and three other Commissioners, four in total, voted in favor, so the proposal was adopted by Chairman Yamanaka.

As the reason for his opposition to this proposal, Commissioner Ishiwatari stated, “From my position as a regulatory reviewer, it seems very problematic that the legal lifetime would get longer and longer with the length of the review period, as a result of a rigorous review process” [33]. This criticism is in the same vein as the IEEJ, which problematized the fact that “the earlier a plant is restarted, the shorter its remaining lifetime will be.” It is noteworthy that the concerns of members of the IEEJ and the NRA, with no organizational similarities, personal interactions or common interests in this matter coincided.

Furthermore, after the majority vote at the extraordinary meeting, Commissioner Tomoyuki Sugiyama, who voted in favor of the proposed amendment to the article, expressed his views that “We have been discussing this proposed amendment to the act under pressure to meet deadlines. Since the NRA is an independent body, we should have discussed this at length among ourselves,” to media reporters. These words suggest the possibility that someone may have pressured the NRA to pass the proposed amendment to the act as soon as possible. As Commissioner Sugiyama says, the NRA is an independent regulatory body, and the first Guiding Principle for activities puts forth the rule of “Independent Decision Making: We shall make decisions independently, based on the latest scientific and technological information, free from any outside pressure or bias” [34]. Obviously, it is imperative that there is no form of outside “pressure” on the NRA's activities. The author would like to point out that, together with the fact that the discussion period at the Nuclear Energy Subcommittee was very short, there may have been some unacceptable political influences at work in this case.

4.4. Possible impacts of the stop-count system to the decisions of operators

The stop-count system adds a certain period to the given lifetime of a reactor without licensing review, and this unarguably has a significant impact on the decision-making of operators.

It must first be noted that while “back-fitting” regulations is common in nuclear, nowhere else in the world is there a rule that says a plant must be shut down until it has fully complied with changes in regulation, and Japan was no exception until the accident. However, Shun-ichi Tanaka, then-Chairman of the NRA, expressed his informal opinion that “plants that have passed the licensing reviews based on the new regulatory standards should be allowed to be restarted” [35]. All the operators in Japan obeyed this informal opinion and shut down all plants, and as a result, many plants are still offline after more than 10 years. The stop-count system will save these plants, which will undoubtedly contribute to the stable supply of power generation and environmental sustainability in Japan. Such effects are demonstrated quantitatively in the aforementioned IEEJ research paper.

At the same time, this system may also affect the economic rationale of commercial operation and result in changes of behavior of the operators. For instance, the Japan Atomic Power Company submitted licensing application for a safety review of Tsuruga Unit 2 to the NRA in November 2015, which had been under review, but the review has been suspended as of April 2023 [36]. The stated reason for the suspension of the review is the repeated inappropriate handling of documents. However, it must be noted that since the cause of the long shutdown of Tsuruga Unit 2 falls into the category of “shutdowns necessary to comply to changes in the Nuclear Regulation Act” as stipulated in the bill introduced in 1.4., the entire shutdown period would be added to the licensed lifetime of Tsuruga Unit 2 according to the stop-count system.

Of course, the ultimate purpose of the power business —to generate electricity and sell it on the market to earn income—will not be fulfilled as long as the plant is under shutdown status. However, there are many possible ways that the operators could take advantage of the stop-count system in their complex strategies for plant operation. For example, Tsuruga Unit 2 has already been shut down for 12 years since May 2011. If there had been no stop-count system, Tsuruga Unit 2 would have reached the end of its given lifetime in 2027. Meanwhile, the cost of Tsuruga Unit 2 is covered by contracts with the power purchaser, so as long as the minimum maintenance costs can be covered, the owners could even consider pending the current situation until 2050, in a wait-and-see strategy that would not have been possible without the stop-count system. The author would like to point out here that there is a nonnegligible possibility that this system could undermine operators’ incentives to efficiently proceed with the review process, promptly restart operations, and supply electricity to the market.

5. Similar regulations and their impact to operational decisions in other countries

5.1. Approaches to reactor lifetime regulations in other countries utilizing nuclear power

In 2019, the Nuclear Energy Agency of the Organisation for Economic Co-operation and Development (OECD/NEA), an intergovernmental agency that facilitates co-operation among countries with advanced nuclear technology infrastructures, held an expert committee on “Long-term operations of nuclear power plants” and the output was published as a report titled “Legal Frameworks for Long-Term Operation of Nuclear Power Reactors” [37]. The report summarizes how the lifetime of commercial nuclear power plants is conceived in 24 of the 33 NEA member countries currently using nuclear power. Out of the 24 countries, 20 provided comments on operating period provisions and other related issues. Table 1 summarizes for each country whether or not a finite lifetime is specified (“Specific” or “Indefinite”) and whether or not there is a limit on the number of years or number of times the initial lifetime can be extended.

Table 1.

Legal lifetime of nuclear power plants in NEA member countries.

Country	Term type	Initial term length	Term length for lifetime extension
Argentina	Specific	10 years	10 years + another 25–30 years
Belgium	Indefinite	Indefinite	10 years
Canada	Specific	10 years	10+ years
Czech Republic	Indefinite	Indefinite	Indefinite
Finland	Specific	30 or 40 years	Case-by-case basis
France	Indefinite	Indefinite	Indefinite
Hungary	Specific	30 years	20 years for once only
Japan	Specific	40 years	20 years for once only
Korea	Specific	30 years, 40 years, 60 years	10 years x n
Netherlands	Indefinite	Indefinite	Case-by-case basis
Romania	Specific	30 years	Case-by-case basis
Russia	Specific	30 years	Case-by-case basis
Slovak Republic	Indefinite	Indefinite	Indefinite
Slovenia	Specific	40 years	10 years for twice only
Spain	Specific	10 years	10 years for case-by-case basis
Sweden	Indefinite	Indefinite	Indefinite
Switzerland	Indefinite	Indefinite	Indefinite
Ukraine	Specific	30 years	10–20 years x n
United Kingdom	Indefinite	Indefinite	Indefinite
United States	Specific	40 years	20 years x n

Source: OECD/NEA, “Legal Frameworks for Long-Term Operation of Nuclear Power Reactors”, 2019 https://www.oecd-nea.org/jcms/pl_15154/legal-frameworks-for-long-term-operation-of-nuclear-power-reactors

It should be noted from this table that while there are 12 countries that set a finite lifetime at the start of operation, 8 countries, including the Czech Republic, France, Sweden, and the United Kingdom, do not set a finite lifetime. France has one of the longest histories of nuclear energy use in the world, with 59 operating reactors as of 2023. The French nuclear regulator, the Nuclear Safety Authority (Autorité de sûreté nucléaire; ASN), evaluates the integrity of the plant throughout its lifetime in accordance with Article L.593-6 of the Environmental Code [38], and can order the plant to be shut down if there is a problem, thus guaranteeing safety. Similarly, Sweden does not set a specific time limit at the start of operations, but ensures safety by having the nuclear regulator, the Swedish Radiation Safety Authority (Strål Säkerhets Myndigheten; SSM), conduct a periodic safety review (PSR) every 10 years during the common use period.

Even in countries where a finite lifetime is specified in the initial operating license (e.g., the United States, Canada, and South Korea), the initial lifetime can be extended prior to its deadline, upon application by the operator and a safety review by the regulatory body. In the United States, Part 54 of Title 10 Chapter I of the Code of Federal Regulations provides for an initial licensed lifetime of 40 years, which may be extended for a period not exceeding 20 years with approval from the Nuclear Regulatory Commission (NRC), with no limit on the number of times the license may be renewed [39]. The recent discussion concerned with the lifetime extension in the United States will be further discussed in 4.2.

According to the NEA report mentioned above, Hungary, Japan, and Slovenia are the only countries where the legal lifetime of a nuclear power plant, including the initial operating license and its renewal after expiration, has a set time limit. In Hungary, the legal lifetime of a nuclear power plant is 30 years for the initial license and not more than 20 years for renewal, i.e., a total of 50 years, while in Slovenia the legal lifetime is 40 years with a 20-year-limit for extensions, totaling 60 years. In Japan, as mentioned above, the act established after the 1F accident states 40 years plus 20 years, totaling 60 years.

Among these countries, Japan stands out in terms of the number of reactors that would be affected, as well as the urgency of the situation. Krško, Slovenia's only nuclear power plant and jointly operated with neighboring Croatia, is a Westinghouse-type PWR that began commercial operation in 1983. On January 13, 2023, Krško was licensed by the Ministry of Environment and Spatial Planning of Slovenia to operate until 2043 [40]. In Hungary, on December 7, 2022, a bill submitted by Deputy Prime Minister Zsolt Semjen to extend the operation of four nuclear power plants was passed by the parliament by 170 votes in favor, 8 against, and 1 withheld [41]. The four plants began commercial operation between 1983 and 1987, and now their operating lifetimes have been extended to 2033–2037. Based on the above, both Hungary and Slovenia will not be facing an imminent operating deadline for the next 10–15 years. In contrast, Japanese operators will soon have to face tough decisions of whether or not to apply for extensions, as more and more plants reach their initial lifetime deadline of 40 years.

More importantly, all of the provisions regarding lifetime deadlines in these countries listed on Table 1 are based on “the number of years since the beginning of commercial operation,” and do not distinguish between whether or not power was generated during that time. The only exception that can be found is the “cap based on total generated electricity,” a scheme that used to be applied in Germany.

5.2. Regulations and discussions in the United States. With regard to legal lifetime

Since the 1990s, the United States has established rules for licensing renewals when the initial authorized lifetime exceeds 40 years, and has made a series of revisions based on the latest technology. Starting in 2016, the United States has issued rules establishing requirements for licenses up to 80 years beyond 60 years. The requirements for review include detailed rules on the tolerances and technical specifications that equipment must have for long-term operation [42], however, there is no statement in whether the equipment was generating electricity or was shut down during that period. Neither there is no indication that the period of shutdown may be added to the legal lifetime depending on the reason for the outage. At the public meetings that have been held repeatedly on the revision of the rules for licensing renewals, for example, the Nuclear Energy Institute (NEI) of the United States requested the NRC to promptly open the application documents for licensing renewal to the public and to reflect public comments in order to make the licenses more appropriate [43]. The discussion is directed toward the "better regulation" that is efficient and effective for operators.

Another unique feature of the United States lifetime extension rules is that they require operators to provide an explanation of "reasonableness relative to other power alternatives" when applying for lifetime extension. The NRC has indicated that alternative power options to be considered will take into account trends in energy portfolio trends, the outlook for utility integration plans, and regional transmission grids, among other factors. Industry (NEI), on the other hand, has indicated that in addition, economics and new situations that were not considered in the first license renewal should be taken into account, and the two parties will continue to discuss these issues in a positive manner [44]. This suggests that the United States regulations are based on the idea that as long as they are reviewed at a cost, they must be economically rational, which in turn contributes to strengthening the competitiveness of the industry.

As of September 2023, six units at three power plants in the United States have already received 80-year operating licenses, and 10 units at five plants are under review. While nuclear power plants are being decommissioned one after another due to loss of market competitiveness, especially in states where electricity deregulation is prevalent in the United States, their life is not determined by their legal lifetime or total electricity generation. In that operators can choose to continue operation as long as their nuclear power plants remain competitive in the market, it can be said that United States regulations are not hindering the industry from strengthening its competitiveness.

5.3. The German “cap based on total generated electricity”

While most of the countries that set an operating deadline for their nuclear reactors have based it on the number of years since the start of commercial operation, Germany used to set its deadlines not in terms of years (hours) but in terms of the amount of electricity generated.

The German government, which decided to phase out nuclear under the revised Atomic Energy Act of 2002 [45], allocated the remaining power generation quantities, a total of 2623.31 billion kWh, to each of the 19 nuclear power plants in operation at the time [46,47]. Table 2 lists the amount of electricity generated that was allocated to each plant as a result of the 2002 amendments to the Atomic Energy Act. Since utilities were allowed to transfer the allocations between plants, Stade and Obrigheim were shut down in 2003 and 2005, respectively, and the unused allocations were transferred to other plants in operation.

Table 2.

List of the residual electricity allocated to plants by Atomic Energy Act in Germany

*) The electricity listed for the Mülheim-Kärlich nuclear power plant 107.25 TWh can be allocated to the nuclear power plants Emsland, Neckarwestheim 2, Isar 2, Brokdorf, Gundremmingen B and C and up to an electricity volume of 21.45 TWh on the core Biblis B power plant.

Source: Elektrizitätsmengen nach § 7 Abs. 1a. https://www.bgbl.de/xaver/bgbl/start.xav?startbk=Bundesanzeiger_BGBl&jumpTo=bgbl102s1351.pdf#__bgbl__%2F%2F*%5B%40attr_id%3D%27bgbl102s1351.pdf%27%5D__1681116019363

On top of such strategic decisions, electric utilities made other efforts to postpone the deadlines of their finest plants, by prolonging the shutdown periods for maintenance and repairs, and using this additional time to challenge the legal barriers for extended operations. For example, when RWE, the operator of Biblis A, shut down the plant on September 15, 2006 to modernize the cooling system [48], it also submitted a request to the Ministry of the Environment that Biblis A be allowed to receive 30 TWh out of the 107.25 TWh of unused allocations of Mülheim-Kärlich, a plant which had already been shut down [49]. The additional 30 TWh would allow the unit to operate until 2011, otherwise, Biblis A would run out of remaining allocations in 2008 and have to be shut down permanently.

According to the amended Atomic Energy Act of 2002, the remaining generation of Mülheim-Kärlich could be transferred to Emsland, Neckarwestheim 2, Isar 2, Brokdorf, Gundremmingen B and C, and also to Biblis B up to a limit of 21.45 TWh; Biblis A was not included in this list. When the Minister of Environment refused RWE's request for this reason, RWE is said to have persisted, confronting the government with a hard-line stance that even hinted at the possibility of litigation [50]. Eventually, RWE postponed the restart of Biblis An until February 10, 2008 to prevent further decline of its allocations, with hopes that the next election might bring change to the government's nuclear phase-out policy [51].

In the end, the operation period of Biblis A and B was extended for eight years due to another amendment of the Atomic Energy Act in 2010 [52]. The following year, however, the 1F accident occurred and both reactors were permanently shut down as of August 7, 2011, by order of Chancellor Merkel.

5.4. The impact of the German “cap based on total generated electricity” regulations

There are no descriptions in open literature that explains why the German rule delimits the operating period by the amount of electricity generated, rather than by the years from the start of commercial operation, but its implications to operating decisions is clear. The German case explained above shows that operators who want to postpone operational deadlines in such a non-temporal regulation scheme are willing to postpone restart and sacrifice short-term economics in hopes of a change in circumstances, such as future policy revisions. From 2007 to 2010, the average annual outage time of German nuclear power plants exceeded 1500 h, and the average capacity factor dropped to 69 % in 2009, even though there were no cases which would normally require a shutdown of more than one year. This behavior can be attributed to the German regulation of a “cap based on total generated electricity” of the operating lifetime.

This is also demonstrated by the fact that capacity factors improved when such regulations disappeared. Following the 1F accident in March 2011, Chancellor Merkel rescinded the extension of lifetimes decided the year before, and immediately shut down eight of the 17 reactors, while also deciding on a shutdown date for each of the remaining nine reactors. This effectively abolished the regulation that sets a cap on the amount of electricity generated, thus eliminating any reason for operators to keep operable power plants shut. As a result, Germany's nuclear power plants have dramatically improved their average capacity factors to 87%–89 %. Undeniably, the measure of delimiting the lifetime by the amount of electricity generated, rather than by a clear shutdown date, had an influence on the behavior of operators, and may have distorted the soundness of their business decisions. The German case suggests that operators subject to non-standard regulations may take unexpected measures, which will also affect people's lives and the economy through the generation mix and other factors.

Even so, it must be noted that Germany's regulations of reactor lifetimes are, albeit unconventional, not ambiguous like the Japanese “stop-count” system. As of September 2023, the amended Atomic Energy Act of 2002 is the only example in the history of the use of nuclear power where the lifetime was regulated by the amount of electricity generated rather than by the time from the start of commercial operation. However, there is no room for ambiguity in the sense that it is strictly measured by the production of generated electricity. There was also a basis for the allocation of remaining power generation for each plant, i.e., that it should be “equivalent to 32 years of operation,” leaving little room for doubt except as to whether a 32-year operation period is technically appropriate. Although this regulation had the problem of inducing needless shutdown for sound power plants, and also of causing contention among operators and the government over the transfer of the remaining generation allocations, the definition of the regulation itself was quite clear. It would also be pointed out that there were some open communications between the operators and the regulators while the regulation had been valid in Germany. Openness and transparency mean “information disclosure” and “stakeholder involvement”, ensuring the public are informed about the regulatory process [53].

It should be kept in mind that the Japanese stop-count system may trigger much more complex problems, not only those caused by the introduction of a “cap” system like Germany, but also due to the inherent ambiguity of how the shutdown periods will be counted.

6. Discussions and conclusions

The definition of the legal lifetime of nuclear power plants in Japan under the revised law in 2023 is, as stated so far, unprecedented in the world. That is why the author has applied the less common research framework in this paper, however, the research could clearly suggest what is the problem to be solved and what parties involved should think further to improve the regulation.

There are three main problems with the history of the discussions surrounding the change of rules for extending the legal lifetime of Japanese nuclear power plants, and the way the changed rules are set forth.

Firstly, the period during which this matter was discussed was extremely short: only three meetings were held during a four-month period from September to December 2022. Looking back on the discussions at each meeting, there is no indication that the apprehensions voiced by several Subcommittee members were deliberated by others, or that the ANRE provided any kind of response. Even though “Proposal 2” raised by the ANRE, which suggests no cap on the lifetime, is globally the ordinary regulation, and only involves removing the word “one-time” from the current regulation stating that “a one-time extension application may be filed for a period not to exceed 20 years,” the Subcommittee rejected this for a more complicated and unfamiliar solution. “Proposal 1” raised by the ANRE, which strictly limits the lifetime within 40 plus 20 years, could be regarded as conservative, because aged reactors are generally have some problem on safety issues. Anyway, the reason for such a decision has not been made clear. More time and deliberation should have been spent on discussions regarding changes to the “legal lifetime of power plants,” which greatly affects the long-term business strategies of operators. Additionally, the situation becomes even more problematic if it were true that there was outside pressure to the Subcommittee to change the act as soon as possible.

Secondly, as pointed out in Section 2, it is difficult to decide on a clear rule to determine the stop-count period for each plant, that would satisfy all the relevant stakeholders. The definition of the lifetime by only measuring the total hours passed from the first day of the commercial operation is a good example from the viewpoint that the regulation should be simple, crystal clear and easy to apply. In the first place, the shutdown period of the reactor for any given reason – whether it is within the operators' control or not – has the same impact of its aging. Therefore, selecting the shutdown period which is beyond the operators' control only is not logical. The definitions written out in the bill would likely cause a great deal of confusion for both licensees and regulators, and create industry-wide frustration regarding the ANRE's decisions of the stop-count period.

Thirdly, as pointed out in Section 2, the earlier a plant passes the review and is restarted, the shorter the additional lifetime it will gain under the stop-count system. As NRA Commissioner Ishiwatari pointed out, it is natural to assume that plants that take longer for safety reviews may have more technical or organizational problems. Moreover, not only large components such as reactor vessels or primary circuit pumps, but also hundreds of small electronic devices such as flowmeters or thermometers, are deteriorating as a function of the total hours passed from the first day of the installation. Is it really reasonable and would lead to strengthen the competitiveness to formulate a system where such plants will remain after more competent plants that restarted earlier have been shut down?

As noted in Section 3, the majority of countries in the world that use nuclear energy do not have an upper limit on the lifetime of reactors and the regulator judges whether the plant would be permitted lifetime extension based only on safety considerations. This is because there is still no established worldwide standard to determine the technical service life of such structures, hence each country guarantees safety by imposing periodic safety reviews at certain intervals. No other considerations, such as “energy security” or “carbon neutral”, should be taken into consideration in the regulators’ decision. The operators/owners of the plant could decide whether they apply for the lifetime extension to the regulators based on their corporate economical strategy.

Why does Japan not follow suit? Is it worth enforcing the stop-count system even at the risk of causing industry-wide confusion that would likely be larger and more complex compared to that caused by the former German regulation, and of even weakening industry's competitiveness with institutional friction almost guaranteed over the arbitrarily determined periods of extended lifetimes? Why not simply adopt the proposal to remove the upper limit on the operating period or the number of times a license can be renewed? These questions are valid from practical, historical and global contexts, yet they remain unanswered to the public eye. The author strongly urges all parties involved to reconsider their decisions getting back to the principles of the good regulation and to the purpose of the regulation to enhance nuclear industry's competitiveness, even after the bill has got an official approval at the National Diet.

The list of authors

Tomoko Murakami, Senior Fellow, Electric Power Industry Unit, The Institute of Energy Economics, Japan.

Ethical statement and informed consent

The author has acquired a certificate of the e-Learning Course on Research Ethics [eL CoRE] from Japan Society for the Promotion of Science on June 1, 2023. Informed consent was obtained from all participants for the author's survey shown in the paper.

Data availability statement

Data included in the article, tables and figures are all open available in electronic format.

CRediT authorship contribution statement

Tomoko Murakami: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.