Tokyo Electric Power Company Holdings Research Articles

Statistical evaluation of individual external exposure dose of outdoor worker and ambient dose rate at evacuation ordered zones after the Fukushima Daiichi Nuclear Power Station accident

Following the accident at the Fukushima Daiichi Nuclear Power Station, evacuation orders were issued for the surrounding communities. In order to lift the evacuation order, it is necessary to determine individual external doses in the evacuated areas. The purpose of this study was to determine the quantitative relationship between individual external doses and ambient dose rates per hour as conversion coefficients. More specifically, individual external doses of Tokyo Electric Power Company Holdings employees in difficult-to-return zone were measured broadly over a long period (fiscal year 2020 to fiscal year 2022). To obtain highly accurate estimates, we used not only ambient dose rates based on airborne radiological monitoring data, but also Integrated dose rate map data that had been statistically corrected to correspond to local ambient dose rate gradients on the ground. As a result, the conversion coefficients based on the ambient dose rate map measured by airborne radiological monitoring were 0.42 for the Evacuation-Order Lifted Zones (ELZs), 0.37 for the Special Zones for Reconstruction and Rehabilitation (SZRRs), and 0.47 for the Difficult-to-Return Zones without SZRRs (DRZs). On the other hand, the conversion coefficients based on the Integrated dose rate map which is a highly accurate dose rate map based on statistical analysis of various types of monitoring that have been studied in government projects in recent years, were 0.78 for the ELZs, 0.72 for the SZRRs and 0.82 for the DRZs. Using these conversion coefficients, the individual external dose can be estimated from two representative ambient dose rate maps provided by the government.

Japan Atomic Energy Agency: Contribution to the Decommissioning of the Fukushima Daiichi Nuclear Power Station and the Reconstruction of Fukushima Prefecture at the Naraha Center for Remote Control Technology Development

Naraha Center for Remote Control Technology Development was established for the development and demonstration of remote control equipment and apparatus to promote the decommissioning of the Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Company Holdings, Inc. The center has been in operation since 2016. The mission of the center is to contribute to the decommissioning of the Fukushima Daiichi Nuclear Power Station and the reconstruction of Fukushima Prefecture. This report describes the center’s full-scale mock-up test facility, component test equipment for the development of remote control equipment and apparatus, and virtual reality systems and their utilization for the realization of the mission.

Comprehensive Analysis and Evaluation of Fukushima Daiichi Nuclear Power Station Unit 3

The estimation and understanding of the state of fuel debris and fission products inside the plant is an essential step in the decommissioning of the Tokyo Electric Power Company Holdings Fukushima Daiichi Nuclear Power Station (1F). However, the direct observation of the plant interior, which is under a high radiation environment, is difficult and limited. Therefore, in order to understand the plant interior conditions, a comprehensive analysis and evaluation are necessary, based on various measurement data from the plant, analysis of plant data during the accident progression phase, and information obtained from computer simulations for this phase. These evaluations can be used to estimate the conditions of the interior of the reactor pressure vessel (RPV) and the primary containment vessel (PCV). Herein, 1F Unit 3 is addressed as the subject to produce an estimated diagram of the fuel debris distribution from data obtained about the RPV and PCV based on the comprehensive evaluation of various measurement data and information obtained from the accident progression analysis, which were released to the public in November 2022.

Measurement of individual external doses of Tokyo Electric Power Company Holdings employees working in Fukushima Prefecture and the relationship between individual external doses and air dose rates in areas including difficult-to-return zones

Since the Fukushima Daiichi Nuclear Power Station accident, evacuation orders have been lifted except for the difficult-to-return zones (DRZs). Within the DRZs, there has been designated a special zone for reconstruction and revitalisation (SZRR). Decontamination of the SZRR has been promoted so that evacuation orders may be lifted. Previous studies measured individual external doses in the evacuation order-lifted zones (ELZs) and other living areas where the annual additional individual external dose was overall less than approximately 5 mSv y−1. However, there have been few reports about the measurement of individual external doses in a SZRR or outside of an SZRR (O-SZRR). In SZRRs and O-SZRRs, Tokyo Electric Power Company Holdings employees work mainly outdoors. Therefore, the employees’ individual external doses and air dose rates were measured in these zones from March 2020 through January 2021. Our key results were:The median (minimum to maximum) individual external doses at outdoor locations were 0.16 μSv h −1 (0.05–0.63 μSv h−1 ), 0.57 μSv h −1 (0.15–3.92 μSv h−1 ), and 1.36 μSv h −1 (0.14–11.91 μSv h−1 ) for the ELZ, SZRR, and O-SZRR, respectively.The conversion coefficients for the air dose rate measured by airborne monitoring to individual external dose were 0.23, 0.38, and 0.50 for the ELZ, SZRR, and O-SZRR, respectively. The conversion coefficients were below 0.6, which was used in the national government model for estimating external exposure dose from air dose rate. In addition, the conversion coefficients for the SZRR and O-SZRR in air dose rates of less than 1.5 μSv h−1 differed from those obtained for the entire measurement range of this study.The conversion coefficient from air dose rate at a height of 1 m above ground level to individual external dose was researched across a broader and higher range of air dose rates than in the previous study (0.24–20.89 μSv h−1 ). The conversion coefficient is confirmed to be 0.7, similar to previous studies.

Relationship between haematological data and radiation doses of TEPCO workers before and after the FDNNP accident.

We evaluated the correlation between radiation dose and the medical examination data of Tokyo Electric Power Company Holdings, Inc (TEPCO) employees working during the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. This study included 2164 male TEPCO workers who received periodic medical examinations from March 2006 to January 2013. First, we conducted log-linear regression analyses using the haematological data of 585 emergency workers and confounding factors to examine the effect of internal radiation exposure in March 2011. Since external radiation exposure was a major influence, we then evaluated the correlation between both internal and external radiation dose and the haematological data of 1801 emergency workers and confounding factors before and after the accident. Among 585 workers, internal radiation exposure in March 2011 alone was mainly due to thyroid doses (0.1-10Gy) but not to bone marrow (BM) doses (0.01-1mGy). Compared to before and after the accident, we found that the levels of monocytes, eosinophils (Eos) and basophils increased slightly, whereas the frequency of smoking and alcohol consumption decreased substantially. External dose exposure was positively correlated with haemoglobin (Hb), red blood cell and Eos but negatively correlated with age, haematocrit and frequency of alcohol consumption. Among these variables, Hb exhibited the strongest correlation with external dose. Regarding the correlation with Hb, although there is a possibility that confounding factors other than exposure were not evaluated, our findings on emergency workers can serve as a reference for the evaluation of health conditions during the emergency period of future nuclear-related accidents.

A numerical investigation into Metallic-Melt continuous drainage in the core support plate region of a BWR for the initial phase of core melt progression

Under a typical uncovered core scenario in a Boiling Water Reactor (BWR), it is not clear that a large core molten pool will result as occurred in the Three Mile Island, Unit 2 (TMI-2) accident. Sandia National Laboratories (SNL) has propounded the alternative “Continuous Drainage” melt progression for the scenario, where molten material drains from the core region without the formation of any stable crusts or blockages. Hence the present simulation study was conducted to investigate the behavior of the metallic-melt continuous drainage and the drainage pathways during the initial phase of core melt progression in a BWR under an uncovered core condition like that encountered in the Fukushima Daiichi Nuclear Power Plants (NPPs) accident experienced by Tokyo Electric Power Company Holdings (TEPCO). Researchers around the world have conducted numerous sophisticated and comprehensive studies using simulation codes and experiments to investigate the Fukushima accident. Melt drainage with deforming components is subject to many uncertainties in a core support plate region possessing rather complicated geometries. Additionally, there is very limited experimental and numerical simulation knowledge on melt drainage in such a region. Thus, in recent years, Japan Atomic Energy Agency (JAEA) has experimented on melt drainage using a simplified apparatus to reproduce the behavior. The present paper provides simulation results on melt drainage in the core support region using SAGITTARIUS, an in-house three-dimensional simulation code capable of consistently simulating melt drainage with crust formation, local blockage, and structural damage. The in-vessel simulation target with a prototypic BWR-composition and -geometry is composed of four fuel assemblies (4 × 4 fuel rods per assembly), an absorber blade, a support piece, a core support plate, an Instrument Guide Tube (IGT), and a Control Rod Guide Tube (CRGT) with a Control Rod Drive Housing (CRDH). We estimated the drainage speed for the JAEA experiment to validate the SAGITTARIUS function. The drainage speed was evaluated to be 3.1 m/s, a speed corresponding to that observed in the core support plate region. We also used SAGITTARIUS to simulate the behavior of the metallic-melt drainage and temperature rise of the structure. The results from SAGITTARIUS on the trend and values of the temperature rise agreed with the measured values. On the basis of this finding, we conducted numerical investigations on the metallic-melt drainage of B4C-Stainless Steel (SS) eutectic from the absorber blade and Zry-melt from the fuel rods. The simulation demonstrated the capacity to describe the following features: (i) visual demonstration of metallic-melt drainage from the absorber blade and fuel rods, (ii) aggressive Zry-melt attack to the inlet orifice, (iii) crust formation with local blockage in the narrow drainage paths, and (iv) sedimentation of solidified melt on the bottom faces. These findings were consistent with the concept of the “Continuous Drainage” proposed at SNL and we supported the SNL scenario by the numerical investigation. These investigations probably provide useful knowledge during the discharge of damaged fuels or crust in the Fukushima-Reactor Pressure Vessels (RPVs).

News From Japan

The history of the use of electricity in Japan began as telecommunication, which started in 1869, one year after the Meiji Restoration. The first electric arc lamp shone in Ginza, Tokyo, in 1882, the same year as the start of operation of Edison's Pearl Street Power Plant in New York. Only four years later (1886), Tokyo Electric Light Company, one of the predecessors of Tokyo Electric Power Company Holdings, started its business as the first electric power company in Japan. Many similar companies started their businesses in the following several years. In these early days, electric power companies in eastern Japan imported 50-Hz generators from Allgemeine Elektricitäts-Gesellschaft (AEG) in Germany, and those in western Japan imported 60-Hz generators from General Electric Company in the United States. From this historical background, AC 50 Hz is still used in eastern Japan and 60 Hz in western Japan.

Final activity report of the “Workshop of Advanced Studies for Steam Flow Measurement”

The "Workshop of Advancement of Steam Flow Measurement" started its activities in April 2017 within the Power and Energy Systems Division of the JSME and finished as planned in March 2022. The industrial sector has a steam demand of 1.12 million TJ/year, which is the second largest energy source after electricity, and steam accounts for about 80% of the heat energy use in the industrial sector and is widely used in large quantities in the main production processes: sterilization, drying, cleaning and distillation. Steam is thus a major heat source in the industrial sector and there is a strong need for flow measurement from the perspective of energy conservation. However, flow measurement of steam is very difficult due to its high temperature and susceptibility to wetness. In addition, there are very few measurement standards. Therefore, the study group has made efforts to clarify the condition of application and accuracy of each measurement method with regard to the flow measurement method of steam. The number of committee members is 30 (chief: Shuichi Umezawa (Tokyo Electric Power Company Holdings, Inc.), vice-chair: Ryo Morita (Central Research Institute of Electric Power Industry), secretary: Tatsuya Funaki (National Institute of Advanced Industrial Science and Technology)), who come from universities, companies, research institutes and others. The frequency of its activities is as follows: it holds study groups once or twice a year, summarizes its results and publishes a report. The predecessor of this study group, the "Workshop of Wet Steam Flow Measurement" (April 2012 - March 2017), also published a report summarizing its overall activities. On the basis of these results, an academic standards development committee has been set up this year and is preparing guidelines for the correction of wet steam flow rate indications from the perspective of thermal energy utilization.

Development of the Multi-Cubic [formula omitted]-ray spectrometer and its performance under intense 137Cs and 60Co radiation fields

The number of nuclear facilities being decommissioned has been increasing worldwide, in particular following the accident of the Tokyo Electric Power Company Holdings’ Fukushima Daiichi Nuclear Power Station in 2011. Large amounts of radioactive wastes and spent nuclear fuels are stored in these facilities. Furthermore, advanced radiation therapies such as proton therapy, ion therapy, and boron neutron capture therapy using charged-particle accelerators have become popular, but charged-particle accelerators generate radiators during operation. In these nuclear and radiation facilities, proper management of radioactive materials is required until the end of the deactivation, cleanup, or disposal. Then, γ-ray spectroscopy techniques have been useful tools because they can obtain significant information on radioisotopes. Besides, space information of radioisotopes is also important, and γ-ray imaging is also required. However, intense radiation fields are located at these facilities, causing γ-ray spectroscopy and imaging difficultly. A γ-ray spectrometer with four segmentations using small-volume CeBr3 scintillators with a dimension of 5×5×5 mm3 was developed. The four scintillators were coupled to a multi-anode photomultiplier tube specific to intense radiation fields by individual supplied voltages to the last three stages. We performed the γ-ray exposure to the spectrometer under 137Cs and60Co radiation fields at the National Institute of Advanced Industrial Science and Technology in Japan. Under the 137Cs radiation field, the relative energy resolution at 1375 mSv/h was 9.2 ± 0.05%, 8.0 ± 0.08%, 8.0 ± 0.03%, and 9.0 ± 0.04% for the four channels, respectively. As the dose rate increased, significant spectral downward shifts were observed for the four channels under the 137Cs and60Co radiation fields because of the space charge effects in the photomultiplier tube. However, the Multi-Cubic sensor could provide the total throughput rate for the four channels of greater than 6 Mcps (Mcps: 106 counts per second), which increases the detection efficiency under intense radiation fields.

“Economizing” TEPCO’s Responsibility for the Fukushima Daiichi Nuclear Power Plant Accident

The purpose of this paper is to show that the responsibility of Tokyo Electric Power Company Holdings, Inc. (TEPCO) was “economized” in the wake of the accident. It is argued that responsibility for the nuclear accident was mainly linked to economic responsibility, and that social and moral responsibility were consequently obscured by the financial accounting that followed the accident. This paper also proposes that the responsibility for the accident should not be limited to TEPCO, but that the responsibility of the nuclear industry and regulatory bodies should be more strongly emphasized in order to prevent such accidents in the future. These conclusions are consistent with some of the literature that emerged following the Fukushima accident. The theoretical framework is derived from sociological studies on quantification and accounting literature on economizing. TEPCO’s accounts, the special business plans of TEPCO and the Nuclear Damage Compensation and Decommissioning Facilitation Corporation, and documentation from the Ministry of Economy, Trade and Industry are the main sources that are consulted in this paper.

Development of a Robot Simulator for Decommissioning Tasks Utilizing Remotely Operated Robots

This paper describes the development of a robot simulator for remote decommissioning tasks using remotely operated robots at the Fukushima Daiichi Nuclear Power Station of the Tokyo Electric Power Company Holdings. The robot simulator was developed to provide a remote operation training environment to ensure operator proficiency. The developed simulator allows for the calculation of physical aspects, such as the hydrodynamics of a remotely operated vehicle and the aerodynamics of an unmanned aerial vehicle. A disturbed camera view presented to an operator can be generated by setting parameters such as transparency, color, distortion, and noise. We implemented a communication failure emulator on the simulator in addition to functionalities for calculating the integral dose and generating the gamma camera image. We discuss the functional requirements and introduce the implemented functionalities. The simulator was built using the developed functions and can be executed integrally.

Application of three-dimensional detailed geometry to simulation of melt progression in an intricate BWR lower head

This study analyzes melt progression with core degradation in the core and lower head of a boiling water reactor using a detailed simulation geometry of an in-house three-dimensional simulation code. Since the Fukushima Daiichi nuclear power plant accident of Tokyo Electric Power Company Holdings (TEPCO) in 2011, the condition of the lower head following melt progression has been a topic of interest worldwide. By taking advantage of the unique characteristics of the three-dimensional simulation code, we focus on achieving a more detailed simulation of the reactor pressure vessel in view of future Fukushima decommissioning processes. Molten corium forms in the core and progresses towards the lower head. When the corium reaches water remaining there, it is fragmented and forms fuel debris on the bottom of lower head. Hence evaluating molten corium breakup with water in the lower head is essential for the coolability of fuel debris and the integrity of the lower head to prevent the release of a large amount of radioactive isotopes. To simulate melt progression together with core degradation effectively, we focused on the development of the detailed simulation geometry comprising the core, the core support plate, and the lower head zones as well as the simplified pedestal of the primary containment vessel. The analysis capabilities due to the detailed simulation geometry were preliminarily evaluated under hypothetical conditions (i) melt progression from the core to the pedestal of primary containment vessel, (ii) failure of the core support plate zone due to molten corium, (iii) water pool behavior in the lower head, (iv) the relocation paths of molten materials via the control rod guide tube/control rod drive housing and instrument guide tube, and (v) mixing behavior of molten corium resulting from different fuel assemblies.

Gamma-ray spectroscopy with a CeBr3 scintillator under intense [formula omitted]-ray fields for nuclear decommissioning

Recently, an increasing number of nuclear facilities have been decommissioned worldwide following the 2011 accident of the Tokyo Electric Power Company Holdings’ Fukushima Daiichi Nuclear Power Station. However, large amounts of radioactive wastes and spent nuclear fuels remain on these facilities, leading to intense radiation fields. During the decommissioning processes, radioactive wastes and spent nuclear fuels have to be retrieved under proper management, and γ-ray spectroscopy supports this process. In this study, a small cubic CeBr3 spectrometer with dimensions of 5 mm × 5 mm × 5 mm was manufactured to perform γ-ray spectroscopy under intense γ-ray fields. Furthermore, thanks to both: (1) a digital process unit with sampling rate of 1 Giga samples per second; and, (2), a photomultiplier assembly featuring individual voltages supplied to the last three stages; the device successfully performed γ-ray spectroscopy up to at dose rates of over 1 Sv/h. The energy resolution (full width at half maximum) at 662 keV ranged from 4.4% at 22 mSv/h to 5.2% at 1407 mSv/h for a 137Cs radiation field. Correspondingly, at 1333 keV, it ranged from 3.1% at 26 mSv/h to 4.2% at 2221 mSv/h for a 60Co radiation field. The energy resolution met the requirements to solve the prominent γ-ray spectral lines of 134Cs, 137Cs, 60Co, and 154Eu. These results suggest that the detector has the potential to realize the γ-ray assessment of 134Cs, 137Cs, 60Co, and 154Eu at dose rates of over 1 Sv/h.

Landside tritium leakage over through years from Fukushima Dai-ichi nuclear plant and relationship between countermeasures and contaminated water

There has been tritium groundwater leakage to the land side of Fukushima Dai-ichi nuclear power plants since 2013. Groundwater was continuously collected from the end of 2013 to 2019, with an average tritium concentration of approximately 20 Bq/L. Based on tritium data published by Tokyo Electric Power Company Holdings (TEPCO) (17,000 points), the postulated source of the leakage was (1) leaks from a contaminated water tank that occurred from 2013 to 2014, or (2) a leak of tritium that had spread widely over an impermeable layer under the site. Based on our results, sea side and land side tritium leakage monitoring systems should be strengthened.

Measuring individual external doses of Tokyo Electric Power Company Holdings employees living in Fukushima prefecture

Since the accident at Fukushima Daiichi Nuclear Power Plant, individual external doses of residents have been investigated. To accurately analyse survey data, a variety of information, including the activity patterns of many residents, needs to be integrated. However, such large-scale surveys have not yet been conducted and actual individual external doses in Fukushima are unclear. In this study, the individual external doses of approximately 300 Tokyo Electric Power Company Holdings employees, who live and work in Fukushima Prefecture outside the Fukushima Daiichi Nuclear Power Plant, were measured. The employees carried GPS loggers and personal dosimeters capable of measuring dose in counts per minute. The employees’ individual external doses were compared along with their activity patterns. It was found that the annual additional individual external dose estimated based upon actual measurements was 1 mSv or less, and the influence on the individual external dose was also revealed.

Construction of virtual reality system for radiation working environment reproduced by gamma-ray imagers combined with SLAM technologies

The Fukushima Daiichi Nuclear Power Station (FDNPS), operated by Tokyo Electric Power Company Holdings, Inc., experienced a meltdown as a result of a large tsunami caused by the Great East Japan Earthquake on March 11, 2011. At that time, it was necessary to understand the aspects of the decommissioning working environment inside the FDNPS, such as establishing how the radioactive substances were distributed across the site, for work to be done efficiently without exposure to large amounts of radiation. Therefore, virtual reality (VR) emerged as a solution. There have been previous reports done on a technique for visualizing the distribution of radioactive substances in three dimensions utilizing a freely moving gamma-ray imager combined with simultaneous localization and mapping (SLAM) technology. In this paper, we introduce imaging technologies for the acquisition of image data from radioactive substances and three-dimensional (3D) structural models of the working environment, using a freely moving gamma-ray imager combined with SLAM technology. For this research, we also constructed a VR system and displayed the 3D data in a VR space, which enables users to experience the actual working environment without radiation exposure. In creating the VR system, any user can implement this method by donning an inexpensive head-mounted display apparatus and using a free, or low-cost, application software.

Seven-year temporal variation of Caesium-137 discharge inventory from the port of Fukushima Daiichi Nuclear Power Plant: continuous monthly estimation of Caesium-137 discharge in the period from April 2011 to June 2018

ABSTRACT After direct discharges of highly contaminated water of the Fukushima Daiichi Nuclear Power Plant (1 F) from April to May 2011, Kanda suggested that relatively small amounts of run-off of radionuclides from the 1 F port into the Fukushima coastal region subsequently continued by his estimation method. However, the estimation period was limited to up to September 2012. Therefore, this paper estimatesthe discharge inventory up to June 2018. In the missing period, the Japanese government and Tokyo Electric Power Company Holdings have continued efforts to stop the discharge, and consequently, the radionuclide concentration in seawater inside the 1 F port has gradually diminished. We show the monthly discharge inventory of 137Cs up to June 2018 by two methods, i.e., Kanda method partially improved by the authors and a more sophisticated method using Voronoi tessellation reflecting the increase in the number of monitoring points inside the 1 F port. The results show that the former always yields overestimated results compared with the latter, but the ratio of the former to the latter is less than one order of magnitude. Using these results, we evaluate the impact of the discharge inventory from the 1 F port into the coastal area and radiation dose upon fish ingestion.

Comprehensive Analysis and Evaluation of Fukushima Daiichi Nuclear Power Station Unit 2

Estimation and understanding of the state of the fuel debris and fission products inside the plant comprise an essential step in the decommissioning of Tokyo Electric Power Company Holdings’ Fukushima Daiichi nuclear power station (1F). However, because of the plant’s high-radiation environment, direct observation of the plant interior is difficult. Therefore, in order to understand the plant’s interior conditions, comprehensive analysis and evaluation based on various measurement data from the plant, analysis of plant data during the accident progression phase, and information obtained from computer simulations for this phase are necessary. These evaluations can be used to estimate the conditions of the interior of the reactor pressure vessel (RPV) and the primary containment vessel (PCV). This paper addresses 1F Unit 2 as the subject to produce an estimated map of the fuel debris distribution from data obtained about the RPV and PCV based on comprehensive evaluation of various measurement data and information obtained from the accident progression analysis, which were released to the public in June 2018.

Remote detection of radioactive hotspot using a Compton camera mounted on a moving multi-copter drone above a contaminated area in Fukushima

ABSTRACTThe Fukushima Daiichi Nuclear Power Station (FDNPS), operated by Tokyo Electric Power Company Holdings, Inc., suffered a meltdown as a result of a large tsunami triggered by the Great East Japan Earthquake on 11 March 2011. To proceed with environmental recovery, a radiation distribution map that indicates the distribution of radioactive substances is extremely important to establish detailed decontamination plans. We developed a remote radiation imaging system comprising a lightweight Compton camera mounted on a multi-copter drone to measure the distribution of radioactive substances remotely. The proposed system can perform radiation imaging using the Compton camera while flying and moving. In addition, the distribution of radioactive substances can be visualized three-dimensionally by drawing the radiation images captured by the Compton camera on a three-dimensional (3D) topography model acquired by a 3D-LiDAR (Light Detection and Ranging) system. We conducted a survey of radioactive hotspots in difficult-to-return zones in the coastal area of Fukushima, Japan. The drone system realized 3D visualization of several hotspots. Such remote technology would be useful for monitoring the difficult-to-return zone as well as for monitoring the distribution of radioactive substances inside the FDNPS site where decommissioning work is ongoing.

Tax Losses and Excessive Risk Taking under Limited Liability: A Case Study of the TEPCO Bailout after the Fukushima Nuclear Disaster

It is commonly accepted in the tax law literature that, in theory, business tax losses resulting from risk taking should be fully refundable to taxpayers. Further, when such refunds are unrealistic, tax losses should instead be freely transferable or carried forward without time limitations to achieve tax neutrality on risk taking. The general position taken in the literature assumes that such risk taking is socially desirable. However, that assumption does not always hold. For example, the corporate law literature clearly establishes that the rule of limited liability of shareholders encourages corporations to take excessive risks that might harm social welfare. Accordingly, this study theoretically examines the relationship between tax losses and limited liability. Furthermore, this study conducts a case study to empirically demonstrate what kind of tax problem can happen with respect to excessive risk taking. Specifically, the bailout of Tokyo Electric Power Company Holdings (TEPCO) after the Fukushima Dai-ichi nuclear disaster caused by the Great East Japan Earthquake on March 11, 2011, provides an informative case study for a further analysis of this relationship. TEPCO ended up owing over 7.9 trillion yen of compensation liability for the damages caused by its shareholders’ excessive risk taking under limited liability. Although TEPCO’s lack of resources to fully compensate for the damages was easily foreseen, the company did not go into bankruptcy. Instead, the government rescued it without wiping out its existing shareholders. The problem does not end there because the TEPCO bailout created a tax problem. This study questions the appropriateness of allowing TEPCO to deduct its tax losses. This is because TEPCO can circumvent the strict time limitation of the tax loss carryforward period owing to the peculiar legal structure of its bailout scheme. In other words, TEPCO’s existing shareholders, who are assumed to have taken excessive risk can enjoy tax windfall that other corporations’ shareholders cannot. This study considers possible, although imperfect, legislative solutions to the problem.