It was a rather normal day back on March 11, 2011, at the Fukushima Daiichi nuclear plant before 2:45 p.m. That was the time when the Great Tohoku Earthquake struck, followed by a massive tsunami that caused three reactor meltdowns and forever changed the nuclear power industry in Japan and worldwide. Now, 10 years later, much has been learned and done to improve nuclear safety, and despite many challenges, significant progress is being made to decontaminate and defuel the extensively damaged Fukushima Daiichi reactor site. This is a summary of what happened, progress to date, current situation, and the outlook for the future there.
Tokyo Electric Power Company’s (TEPCO’s) Fukushima Daiichi facility had for many years been the largest nuclear power station in the world, with its six 1970s vintage General Electric boiling water reactors. Unit 1, a 460-MWe BWR 3, was commissioned in 1971; Units 2, 3, 4, and 5 were 750-MWe BWR 4s; and Unit 6 was a 1,100-MWe BWR 5 that was finished in 1979. On March 11, 2011, Units 1, 2, and 3 were at full power, and Units 4, 5, and 6 were shut down and undergoing springtime maintenance. The Unit 4 reactor vessel was defueled, with all spent fuel in its spent fuel pool. All the units were well maintained and had been upgraded to the extent required under Japanese regulations of that time.
The earthquake, one of the largest ever recorded in human history, and the following tsunami were well beyond projections. The initial huge seismic shocks were slightly beyond the site seismic design bases; however, all the reactors successfully scrammed and were experiencing an as-designed safe shutdown sequence without any significant safety system damage or problems. All off-site power connections were lost due to transmission system failures, but the site’s 13 emergency diesel generators started powering all safety systems as designed. So, despite the great earthquake shock, the reactors were being safely shut down in a controlled manner.
Immediately following the initial seismic shocks, the Japan Meteorological Agency issued a tsunami warning for a 3-meter-high wave. Being in a major outage situation, there were approximately 6,000 workers on-site, and evacuations were initiated from the lower plant areas. Initially, there was not much concern about a tsunami, as the site’s tsunami protection design had been upgraded from 3 meters to 6.2 meters, and most vital equipment was located at the 10-meter elevation level.
However, approximately 45 minutes after the initial seismic shock, a series of tsunami waves hit the site, flooding it up to the 15-meter level (Fig. 1) and disabling 12 of the 13 emergency diesel power supplies and most of the emergency DC power for Units 1, 2, and 3. The massive flooding created a beyond-station blackout situation, with virtually all emergency AC and DC power systems lost. Reactor buildings were flooded with seawater (Fig. 2), tanks were washed away, control rooms were dark, virtually all instrumentation was lost, and electronic communications were nonexistent (Fig. 3). As core cooling was uncertain, a major emergency condition was declared and off-site emergency plans were initiated, followed by a series of public evacuations as the situation deteriorated.
The operators struggled to restore safety instrumentation and to find ways to inject water into the reactor vessels to cool the cores. They creatively scavenged batteries, including those from vehicles in the parking lots, to restore vital instrumentation, such as reactor water levels and pressures (Fig. 4).
Figure 1: A tsunami wave hits Fukushima Daiichi.
Figure 2: Flooding of the reactor buildings.
Figure 3: Fukushima interior with no power.
Figure 4: Batteries used to restore power to vital systems.
The functionality of the Unit 1 emergency core cooling isolation condensers was very difficult to determine because of the uncertainty of containment isolation valve positions due to the sporadic AC and DC power loss sequencing. However, some of the isolation condenser valves were in the closed position, which resulted in the loss of core cooling, core metallic component oxidation, core melting, reactor vessel breach, primary containment overpressure and leakage, and high radiation levels that evening.
Operators were able to keep the Unit 2 and 3 high-pressure turbine-driven reactor core isolation cooling (RCIC) pumps and the Unit 2 high-pressure coolant injection (HPCI) pumps operating to inject water into the reactor vessels from the torus wet well for several days. These variable RCIC and HPCI injections helped delay the overheating of the Unit 2 and 3 cores; however, since there was no available ultimate heat sink for the torus wet wells, the containment pressures and temperatures continued to rise, making low-pressure injection difficult.
For all three units, operators made heroic efforts by entering extremely high-radiation areas inside of the dark, flooded reactor buildings to manually open valves to vent the containments to reduce pressures to allow low-pressure water injection. These venting efforts were only partially successful.
Many courageous attempts were made to reestablish core cooling by pulling temporary electrical cables, manually carrying batteries and portable air compressors to operate valves, installing new ultimate heat sink seawater pumps, and utilizing fire engines to inject fresh water (Fig. 5) and then seawater when freshwater supplies were exhausted. Efforts to cool the reactor vessel cores of shut-down Units 5 and 6, all six reactor spent fuel pools, and the large common spent fuel pool were successful, but the cores of Units 1, 2, and 3 could not be saved.
Despite these great operator efforts, the cores in Units 1, 2, and 3 overheated and melted. Fuel cladding and other metals oxidized, creating exothermic hydrogen gas, which breached the reactor vessels and overpressurized the primary containments, causing leakage such that explosive hydrogen gas and radioactive fission products entered the reactor buildings. The Unit 1 and 3 reactor buildings’ upper floors were destroyed by internal hydrogen gas explosions (Figs. 6 and 7). Hydrogen gas also backflowed from the Unit 3 ventilation system into the Unit 4 reactor building, causing an explosion on the upper floors of that building as well. No explosion occurred in the Unit 2 reactor building because the shock wave from the Unit 1 hydrogen explosion dislodged the Unit 2 reactor building blowout panel, dispersing the hydrogen gas generated by the Unit 2 core oxidation into the atmosphere before it could explode. However, airborne fission products vented to the environment along with the heated steam (Fig. 8).
Figure 5. Fire engines inject fresh water into the heat sinks.
Figure 6. An internal hydrogen gas explosion.
Figure 7. An internal hydrogen gas explosion.
Figure 8. Heated steam escapes the Unit 2 reactor building.
Fission products escaping from the three units, primarily cesium and iodine, created extremely high radiation levels on the site, hampering on-site mitigation efforts. On-site gamma radiation levels were in the sievert per hour range (100 rem/hr) in many areas, making emergency work difficult and dangerous.
During the immediately following days and weeks, TEPCO amassed a large skilled team to establish control over the site. Seawater had to be injected by fire trucks during the first week and then new freshwater supplies were brought in for improved injection cooling. Extensive airborne mitigation efforts were made to minimize off-site releases (Fig. 9). Special water injection pumping systems were created to ensure that all spent fuel pools were flooded (Fig. 10). Silt fences were installed to mitigate fission products, primarily cesium, from flowing into the ocean from building basements that were filled with contaminated water flowing from the severely damaged reactor buildings (Fig. 11). Further information is provided in the ANS special Fukushima report at fukushima.ans.org.
Containment of highly contaminated water leaking from the reactor building basements into the turbine building basements and then to the seawater intake structures via a maze of underground tunnels was a major early challenge. Some of the underground pipe tunnels allowed direct leakage into the sea (Fig 12).
Figure 9. Airborne mitigation efforts.
Figure 10. Water injection to ensure fuel pools remain filled.
Figure 11. Silt fences to mitigate fission products from flowing into the ocean.
Figure 12. Underground pipe tunnels leaking into the sea.
Early efforts were made to minimize such leakage with concrete sealing (Fig. 13) and the installation of special tank capacity (Fig. 14). New storage capacities for high volumes of very radioactive wastewater were created by quickly preparing the basements of radwaste and incinerator buildings to become de facto contaminated water storage tanks. Zeolite bags were placed in submerged areas to minimize cesium mobility and to minimize sea contamination (Fig. 15).
After the first several weeks, the site was stabilized, with core debris cooling established and with airborne and water containment/mitigation efforts proceeding. A comprehensive personnel radiation protection system was put in place to support an on-site workforce of approximately 5,000 workers, with many outside support people constantly coming and going.
A major early priority was the creation of cesium removal water systems to allow the recycling of highly radioactive water from the melted fuel debris cooling water injection. TEPCO engaged Kurion to develop a zeolite cesium adsorption water processing technology similar to that used for processing highly radioactive water in the Three Mile Island (TMI) accident cleanup 40 years ago. Through effective teamwork, this new processing system was designed, constructed, transported, installed, and safely operated within a three-month period. With subsequent improvements, this system is still in use today (Fig. 16).
Figure 13. Concrete sealing in progress.
Figure 14. Installed special tank capacity.
Figure 15. Zeolite bags being placed in submerged areas.
Figure 16. The zeolite cesium adsorption water processing system.
Given the importance of cesium removal, TEPCO also developed other redundant and diverse systems. Areva (now Orano) developed and operated a less successful cesium precipitation removal system, and Toshiba also developed a slightly different follow-on zeolite adsorption system called SARRY. The SARRY system, like the Kurion system, has been improved over the years and still operates today (Fig. 17). The Areva cesium precipitation system was discontinued due the complexities of having to manage extremely high levels of radioactive cesium sludges in its receiving tank. TEPCO currently has a major engineering effort to develop robotic equipment to remove and solidify this high-gamma (in the range of tens of Sv/hr (1,000+ rem/hr) sludge. A lesson learned has been that the waste management aspects of these special highly radioactive systems need to be constantly considered in all stages of design, construction, operation, and decommissioning.
The core melting and containment leakages caused considerable radioactive releases off-site. During the early phases of the accident, the winds were blowing toward the Pacific Ocean, so there was little impact (Fig. 18). However, later cesium releases were blown westward toward the mainland, causing extensive land contamination (Fig. 19). Early evacuations prior to these releases protected the public. Extensive Japanese and World Health Organization studies have concluded that there were no radiation fatalities, and no observable increases in cancer above the natural variation in baseline rates are anticipated (who.int/publications/i/item/9789241505130). Unfortunately, the psychosocial effects of the initial evacuation of approximately 160,000 people have been significant (www.niph.go.jp/journal/data/67-1/201867010007.pdf).
The off-site contamination of Cs-137 requires extensive land and building decontamination and new solid radwaste management capabilities. The Ministry of the Environment, working with Fukushima Prefecture and townships, is financing the reconstruction of earthquake and tsunami damages and decontamination efforts to allow people to return to their homes. Much progress has been made, with most of the evacuation areas now released for people to return to their homes. However, repopulation is a challenge, as many are not returning due to their having moved forward with their lives in other places and the psychosocial feelings about returning. This situation is certainly made more difficult by unrelated Japanese cultural changes that are simultaneously taking place. There is a decreasing overall national population and a desire of young people to live in metropolitan areas, which the Fukushima Daiichi area is not.
A by-product of the off-site decontamination work has been the accumulation of large volumes of low-level cesium-contaminated soils in fabric bags. Gamma radiation levels were reduced to “able-to-return levels” by removing the top 5 cm (2 inches) of soil. Altogether, this has resulted in up to 20 million 1-cubic-meter fabric bags that require storage somewhere (Fig. 20). Progress has been made to negotiate for temporary storage in an annular ring around the Fukushima Daiichi site for the time being, and transfers are currently taking place to special lined, capped storage trenches (Fig. 21).
After the first several months, it became clear that the on-site and off-site recovery from the accident was going to require a coordinated major national and international effort. Similar to the United States’ response to the TMI-2 accident, TEPCO, the Japanese government, the Fukushima prefectural government, and the nuclear industry organized to meet the challenge at Fukushima and across the entire Japanese nuclear complex. These changes focused on not just the on-site and off-site Fukushima Daiichi accident recovery but on ensuring safe nuclear energy across Japan and globally as well.
Japanese laws were changed, and a stronger independent regulator, the Japan Nuclear Regulatory Authority (NRA; www.nsr.go.jp/english/index.html), was created to ensure reactor safety. Utilities committed billions of dollars to improve safety to restart nuclear reactors. The Japanese nuclear industry followed the post-TMI example of establishing its own safety organization, the Japan Nuclear Safety Institute (www.genanshin.jp/english/), which is modeled after the Institute of Nuclear Power Operations. Thanks to these and other improvements, nuclear power remains an important, although lesser, component of Japan’s clean energy needs for the future.
For decontamination of the off-site area, the Ministry of the Environment is working with Fukushima Prefecture to accomplish that task with extensive government and TEPCO support. Further information is located here: josen.env.go.jp/en/decontamination/.
The extensive Fukushima Daiichi on-site decontamination and decommissioning (D&D) activities remain the responsibility of TEPCO, with substantial government support. A new comprehensive structure of organizations under the leadership of the Ministry of Economy, Trade, and Industry (METI) has been set up to ensure proper financing and support for on-site D&D.
While TEPCO remains the owner of the site, it has set up within TEPCO Holdings a new D&D implementing organization called the Fukushima Daiichi Decontamination and Decommissioning Engineering Company (FDEC) to focus on Fukushima. This concept is similar to what the United States set up to achieve D&D success at TMI.
METI established a new technology research association composed of 17 organizations (currently, 18), the International Research Institute for Nuclear Decommissioning (IRID), to coordinate national and international resources to develop new remote D&D technologies that can be used at Fukushima and elsewhere. The Japan Atomic Energy Agency is also a major supporting resource for D&D and safety technologies and the advanced scientific D&D work at Fukushima.
To ensure overall integration, financing, and policy guidance, METI established the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) to focus on the Fukushima D&D program. The NDF, on behalf of the Japanese government, provides financial support, policy guidance, and coordination for the Fukushima recovery. Further information is on the NDF website (dd.ndf.go.jp/eindex.html). Figure 22 shows the interrelationship of these organizations within Japan to safely accomplish the Fukushima D&D recovery effort.
Reactor safety lessons
As with the TMI accident 40 years ago, the Fukushima accident has yielded a wealth of reactor safety lessons that are being internationally captured and acted upon to make nuclear power safer. Here in the United States, the nuclear industry and the Nuclear Regulatory Commission did major generic and site-specific reviews to ensure and improve safety for all U.S. reactors, with many safety enhancements made, e.g., implementation of a flexible and diverse strategy (FLEX) to address virtually any possible reactor safety challenges. Further information on FLEX is provided here: nrc.gov/docs/ML1222/ML12221A205.pdf.
The Department of Energy’s Office of Nuclear Energy has a program that allows nuclear safety and operation experts from industry, academia, and the national laboratories to work closely with their Japanese and other international colleagues to extract data from the ongoing characterization and cleanup efforts to learn and gain design and operational insights to further enhance safety for existing and future reactors. These insights are used to update guidance for severe accident prevention, mitigation, and emergency planning. A status report on this work is provided at the anl.gov site here: publications.anl.gov/anlpubs/2019/09/154944.pdf.
The D&D approach
Once the site was stabilized after 2011, the long process to safely contain radioactive materials by removing them from damaged, undesigned conditions and placing them in controlled, engineered configurations began. The general approach and schedule for achieving this is presented in the METI-issued 30–40-year plan called the Fukushima D&D Roadmap (meti.go.jp/english/earthquake/nuclear/decommissioning/). Additional further information is provided in the supporting NDF strategic plan with annual updates (dd.ndf.go.jp/en/strategic-plan/index2020.html).
In general, the D&D approach is to proceed along the major areas below. Much progress has been made over the past 10 years in each of these areas. Here are some of the major accomplishments by area:
Maintain worker safety and improve working conditions
The site has been significantly decontaminated, allowing over 90 percent of the area to be accessed with normal work clothing (Fig. 23). Only the highly contaminated reactor and turbine buildings and some waste management facilities require respirators and special protective clothing.
A very comprehensive radiation protection and occupational worker safety program is fully in place.
New on-site buildings have been constructed to support tradesmen and engineering functions.
Reduce site radiological risks in a risk-informed manner
A detailed site-wide risk analysis has been performed for all risk areas, and work prioritization is risk-informed (Fig. 24).
The Unit 1 explosion damaged the seismic braces near the top of Unit 1/2 100-meter-high exhaust stack. The stack internals were highly radioactive due to primary containment venting, as determined by surveys and drone investigation into the stack. The top 50 meters of the stack were remotely cut and removed in sections last year (Fig. 25).
Remote decontamination activities continue daily in contaminated buildings to reduce radiation and contamination. An example of progress is shown in Fig. 26A, with a robotic crawler to remove and collect radioactive sludges. Figures 26B and 26C are examples of a debris-clearing robot and a vacuuming robot, and 26D is a floor-washing robot working in the Unit 2 reactor building.
Figure 26A. Sludge cleanup before (left) and after (right) robotic crawlers.
Figure 26B. A debris-cleaning robot.
Figure 26C. A vacuuming robot.
Figure 26D. A floor-washing unit working in the Unit 2 reactor building.
Control and minimize airborne releases
As the decontamination and deconstruction of damaged and contaminated building structures proceeds, there is always a risk of activities creating cesium radioactive dusts that may enter the air and spread. An extensive active airborne monitoring array is being operated, and specific activities are closely monitored. When necessary, large, remotely constructed temporary enclosures are built, such as the Unit 1 reactor building enclosure (Fig. 27). When work access is needed, panels can be removed with airborne mitigation actions (e.g., water sprays) taken as necessary (Fig. 28).
Decontaminate and temporarily store radioactive waters
The continuous injection of recycled water into the three damaged cores over the past 10 years has required the constant processing of highly contaminated cesium and strontium wastewater from the basement floors. The initial gross cesium removal systems, Kurion and SARRY, have processed over 2.4 million tons of water, removing over 99.99 percent of cesium. To date, these systems have discharged over 1,000 highly radioactive zeolite adsorption vessels (Fig. 29), which are stored on-site.
After the gross cesium removal, gamma levels are reduced to allow salt removal by using primarily reverse osmosis (RO) systems, allowing the purified water to be reinjected onto the tops of the reactor core debris. The RO concentrate stream is high in salts, Sr-90, and other isotopes. Three special advanced liquid waste processing systems (ALPS) have been created to process these concentrates to remove Sr-90 and 62 other isotopes (Fig. 30) to levels well below international standards for a controlled ocean release. Tritium is not removed, but tritium levels are low enough to allow normal dilution to well below international safety and environmental protection standards. To date, these systems have processed over 1.2 million tons of water. Further information is provided here: www.TEPCO.co.jp/en/decommission/progress/watertreatment/index-e.html.
Over 1,000 large welded steel tanks (Figs. 31 and 32) have been built that now contain over 1 million tons of processed water awaiting a government decision for final disposition. A Japanese study group and many other organizations, including the International Atomic Energy Agency, have recommended a controlled, monitored discharge into the ocean. The Japanese government is currently in a dialog with interested groups (e.g., fishery cooperatives) regarding socioeconomic concerns that might arise from unscientific, emotionally based rumors. A final disposition decision is expected soon. The current planned tank capacity will be full in approximately mid-2022.
Figure 29. Toshiba SARRY cesium removal system.
Figure 30. Advanced liquid waste processing systems installed at Fukushima.
Figure 31. Over 1,000 steel tanks hold processed wastewater awaiting government decision for final disposition.
Figure 32. Closeup of the wastewater tanks.
Seal and mitigate underground contaminated water sources to control ocean releases
During the initial accident phases, there was some fission product contamination that entered the on-site groundwater aquifer from underground structure leaks and rainwater infiltration from surface depositions. To mitigate further ocean contamination, a comprehensive special concrete sealing operation of underground equipment tunnels has taken place. To date, several hundred meters of underground tunnels have been sealed with special sealing concrete.
To further prevent underground water flows into the ocean, a 780-meter-long, 30-meter-deep steel seawall has been built (Fig. 33).
A 1.5-kilometer-long, 30-meter-deep ice wall has been constructed around the Unit 1–4 reactor and turbine buildings to isolate the contaminated basements and better control groundwater levels (Figs. 34 and 35).
A sophisticated subdrain groundwater level pumping system has been built to control groundwater levels within the ice wall boundary to ensure that the groundwater level is always slightly above the reactor building basement water levels, which are being constantly reduced to dry building basements to ensure that there is no radioactive water leakage into the groundwater while minimizing the amount of groundwater flowing into the contaminated building basements. Groundwater and rainwater inflows have been reduced from over 400 tons per day to about 100 tons per day (Fig. 36).
A line of groundwater bypass intercept pumps has been installed to divert natural groundwater from flowing down from the hillside above the Unit 1–4 reactor buildings to minimize groundwater flows and building intrusion. To date, over 600,000 cubic meters of water have been monitored and released.
A 20-meter-deep underground wall of apatite/zeolite columns was placed downgradient of an older tank farm of flanged tanks that had leaked water containing significant levels of Sr-90. The purpose is to retard possible Sr-90 groundwater movement toward the ocean (Fig. 37).
To reduce rainwater infiltration that may transport residual ground surface cesium contamination (from the early accident period) into the aquifer, which flows to the ocean, the site has been extensively covered with asphalt or shotcrete. To date, approximately 1.5 million square meters have been covered (Fig. 38).
Figure 33. The 780-meter long sea wall in place.
Figure 34. Diagram of the frozen-soil wall system.
Figure 35. Workers inspecting the deep ice wall.
Figure 36. Diagram of the groundwater pumping system.
Figure 37. The apatite/zeolite wall being built.
Figure 38. The ground covered by shotcrete to reduce rainwater infiltration.
Remove spent fuel from the damaged reactor buildings’ spent fuel pools
Early on, plans were made to defuel the spent fuel pools in damaged Unit 1–4 reactor buildings. The Unit 4 spent fuel pool had the most spent fuel and had the highest heat load; thus, it had a higher-risk source term. It was also structurally weakened because explosive hydrogen that flowed from Unit 3 via interconnected piping accumulated and exploded on the fourth and fifth floors of Unit 4. In addition, since Unit 4’s nuclear fuel was not damaged, the radiation levels there were much lower, so conventional manual pool defueling could take place. The top of the damaged Unit 4 reactor building was removed and a new self-supporting, seismically engineered spent fuel pool defueling building (Fig. 39) containing a new fuel handling machine and cask handling crane was built (Fig. 40). The pool was subsequently emptied of 1,535 nuclear fuel assemblies in 2014.
Unit 3 was the next spent fuel pool to be defueled. The highly radioactive Unit 3 reactor building top (Fig. 41) had rubble removed and was remotely decontaminated to allow a new self-supporting defueling structure to be placed over the spent fuel pool. A significant milestone in the process was the lifting of the fallen original fuel handling machine from the top of the spent fuel racks (Fig. 42). A new shield floor was remotely installed, and a new pool defueling building enclosure was built above the existing spent fuel pool (Fig. 43). To perform rubble removal from the tops of the spent fuel racks and to remove the spent fuel assemblies, a new remotely controlled robotic fuel handling machine was installed (Figs. 44 and 45). As of January 22, 510 fuel assemblies have been removed, and the pool is scheduled to be emptied this spring.
The refueling floor of the Unit 2 reactor building has been remotely accessed, and robots have cleaned the defueling floor. Plans are proceeding to install a new side-entry defueling building (Fig. 46) for special remote/robotic spent fuel defueling machine access. Pool defueling is scheduled to begin in the 2024–2026 timeframe.
The severely damaged top of the Unit 1 reactor building is being remotely accessed to prepare for spent fuel pool defueling (Fig. 47). The general pool defueling approach is shown in Fig. 48. A special floating concrete shield blanket has been remotely placed on top of the spent fuel pool surface to provide a safety barrier for the future remote lift of rubble and heavy objects, such as the original 70-ton crane that is currently over the spent fuel racks. Once the area above the pool is cleared of heavy objects, a remote/robotic defueling machine will be installed. Pool defueling is scheduled for 2027–2028.
Figure 39. Reconstruction of the top of the Unit 4 reactor building.
Figure 40. The new fuel handling machine and cask handling crane.
Figure 41. The remains of the top of the Unit 3 reactor building.
Figure 42. Removing the original fuel handling machine.
Figure 43. The new pool refueling building enclosure.
Figure 44. The new remotely-controlled robotic fuel handling machine.
Figure 45. Closeup of the robotic fuel handling machine.
Figure 46. The planned side-entry refueling building.
Figure 47. The severely damaged top of the Unit 1 reactor building.
Figure 48. The general pool defueling approach.
Investigate and characterize the internal primary containment vessel (PCV) and core debris conditions
Extensive human and robotic surveys and investigations have taken place inside all reactor buildings (but outside of the PCV), and much has been learned.
In the Unit 1 reactor building torus room, robotic boats and underwater explorers (Figs. 49A and 49B) have performed visual and sonic measurements to identify PCV leak points, e.g., sand drainpipe leakage, implying that relocated molten core material damaged the PCV liner.
Inside the Unit 1 PCV, shape--changing crawler robots have explored internal conditions, taking radiation and physical measurements (Fig. 49C).
In Unit 3, an underwater robot, called Sunfish, explored the drywell and swam under the reactor vessel and identified molten core debris (Fig. 49D). Second-generation submarines that can take samples are being developed for further use in Unit 3 (Fig. 49E).
In Unit 2, a shape–changing crawler, named Scorpion (Fig. 49F), tried to enter under the pedestal area by traveling down the control rod changing rail but got stuck on hard debris on the rail.
Later, in Unit 2, an extendable, remotely operated pole with a camera, sensors, and movable fingers did explore the pedestal area under the reactor vessel and was able to move small core debris objects. Fuel debris was clearly seen on the basement floor as a fuel assembly lifting handle is clearly visible (Fig. 50). An overall picture of the highly damaged area underneath the failed reactor vessel has been developed (Fig. 51). Note the hole in the floor grating below the apparent reactor vessel breach where the molten core mixture melted through the steel grating.
Based on data obtained, coupled with extensive computer modeling, conceptual internal debris projections are being made to guide defueling plans. Figure 52 is a simplified generic projection of internal reactor conditions.
Figure 49A. A robotic boat.
Figure 49B. An underwater explorer.
Figure 49C. A shape changing crawler robot.
Figure 49D. The Sunfish underwater robot.
Figure 49E. A second-generation submarine robot.
Figure 49F. A shape-changing robot called Scorpion.
Figure 50. An image captured by a camera mounted on an extendable pole shows the pedestal area under the reactor vessel.
Figure 51. An overall picture of the highly damaged area underneath the failed reactor. Note the hole in the floor grating below the apparent reactor vessel breach where the molten core mixture melted through the steel grating.
Figure 52. A simplified generic projection of internal reactor conditions.
Prepare to defuel and store the damaged core debris
FDEC and IRID are currently working to remove the first core debris samples from Unit 2. The plan is to install a hot cell box outside the X-6 penetration that will hold a 22-meter extendable remote arm with end effectors to obtain a sample from the floor (Fig. 53). The 6-ton sampler arm and internal trolley system are shown in Fig. 54. The special arm is under development in the United Kingdom and Japan. Debris sampling is scheduled for later this year, although COVID-19–related delays in the United Kingdom may extend the schedule.
FDEC is developing conceptual fuel debris removal plans and designs, and IRID is developing higher-capacity robots for that purpose. Current defueling plans are for side entry as well as top entry options. Given that there are substantial differences and uncertainties concerning the conditions inside the Unit 1–3 PCVs (e.g., water levels and damaged core debris locations), the consideration of multiple defueling options is very appropriate for this stage.
Safely process and store solid waste materials
An exceptionally large array of radioactive solid wastes has arisen and will further accumulate over the coming years. A comprehensive on-site storage plan has been developed for the north end of the site. More than 10 major buildings have been built or are planned to be built (Fig. 55), including two large nuclear-grade incinerators to reduce the volume of combustible wastes. The first unit will be used to burn protective clothing and similar materials, and the second will burn the 130,000 cubic meters of trimmed, contaminated trees (Fig. 56) that had to be cut down to make room for the many water storage tanks.
There had already existed a large amount of spent fuel stored at the site from operations prior to the accident. Most of that fuel is stored in the large common spent fuel pool, but there did exist nine loaded dry storage casks before the accident. During the tsunami, these casks were flooded over with seawater (Fig. 57), but there was no damage to the casks themselves. These and newer spent fuel storage dry casks are being placed in a newly designed spent fuel storage area at a higher elevation on-site.
The Kurion and SARRY cesium removal systems have generated more than 1,000 highly radioactive shielded spent adsorption containers (Fig. 58). These are stored in a vented condition to control any possible hydrogen gas buildup.
The operation of the ALPS strontium removal system has generated over 3,500 high-integrity containers that contain highly radioactive Sr-90 sludges, which are kept in shielded concrete vaults (Fig. 59). These are also vented to control hydrogen gas, and a major waste processing project is proceeding to dry these sludges and incinerate their polyethene inner containers to reduce storage volumes and hydrogen gas explosion risks (Fig. 60).
Figure 55. Major buildings built or planned to be built on-site.
Figure 56. Some of the 130,000 cubic meters of contaminated trees to be disposed of.
Figure 57. The existing spent fuel casks that were flooded during the tsunami.
Figure 58. Spent cesium adsorption containers stored on-site.
Figure 59. Containers of SR-90 sludges.
Figure 60. The planned process to dry sludges to reduce storage volumes.
Future challenges and outlook
Much has been accomplished so far, but many difficult tasks and challenges remain. From a technology perspective, developing, installing, operating, and maintaining reliable remotely operated robotic tools to remove the melted core debris from inside the primary containments will be very challenging. Gamma radiation levels are extremely high inside the PCVs such that human entry is not feasible. The FDEC/IRID team are world leaders in state-of-the-art robotics, but the removal of the heterogenous mixtures of melted core material, melted structural materials, corrosion products, and degraded concrete—all located in a physically restricted and hostile radiation, temperature, and chemical environment—is very complex and extremely challenging.
Managing the complex array of radioactive wastes safely will also present continuing significant challenges, as there are so many large volumes of new and different types of wastes with complex radiological, chemical, and physical characteristics.
Time will be a continuing challenge as well, as existing equipment, structures, and buildings slowly degrade over the years. Although a lot of progress has been made, internal robotic core debris exploration/characterization has been relatively slow due to all the necessary development and safety precautions. At the current rate of progress, in my view, it will take many decades to remove most of the melted core debris. Except for the radioactive decay of Cs-134, time is not on the side of reducing the risks, so delays in getting to production defueling is a risk challenge in itself.
Nontechnical sociopolitical challenges are also major factors in achieving success. So far, the Japanese society has been united in supporting D&D progress, but there are growing negative social impacts that can adversely impact technical risk reduction progress. For example, TEPCO has had to spend the equivalent of many billions of dollars storing and managing processed water that contains comparatively low levels of relatively benign tritium. Any other international nuclear facility would have been allowed to have a monitored and controlled ocean release system functioning under existing protective environmental rules years ago. However, the public stigma (often referred to as “harmful rumors”) and concern in Japan that there may be an impact on fishery sales has been an exceedingly difficult issue to resolve. It has also been extremely unfortunate that the water release issue has become part of nonrelated historical international tensions in the Pacific region that have no relationship with nuclear (e.g., ongoing historical trade and financial disputes between South Korea and Japan from over 75 years ago).
These complex sociopolitical issues can have a significant negative effect on actual recovery progress because they divert scarce engineering and management time resources from the technical risk reduction needs that already exist, like fuel debris removal. Holding the FDEC technical team back by having to address these socially driven psychological-emotional perception requirements is a major challenge that is very counterproductive and further exacerbates the already challenging technical D&D tasks.
Due to regional social concerns, all waste must be stored on-site, as there are no capable off-site facilities available. Fortunately, the Fukushima Daiichi site is relatively large with good storage elevations. For the near future, once the processed water disposition issue is resolved, there should be room to store all waste and fuel debris materials at the site for many decades. But at some point in the future, off-site long-term storage/disposal facilities will have to be established. As it was for TMI radioactive materials, this will likely become another challenging sociotechnical issue that will have to be addressed.
Another future challenge will be the setting of “how clean is clean enough” standards for decontaminated areas of the site. This will be a delicate social/technical/economic balance that will eventually have to be resolved by the local and regional authorities, TEPCO, and the Japanese government working together.
Despite all these future challenges, the good news is that TEPCO and other Japanese teams are extremely dedicated and focused to safely accomplish the D&D of the Fukushima site. As a Westerner, I am constantly amazed at the organization, personal feelings of responsibility and dedication, and the willingness to perform hard work that is undertaken by all involved to rectify the unfortunate impacts of the reactor accident.
My personal benchmark is that in the aftermath of the TMI-2 accident, we here in the United States learned our lessons, made nuclear energy safer and more productive, and decontaminated and safely removed the melted fuel from inside the damaged Unit 2 reactor. Although the technical damage is more significant at Fukushima, the capabilities today of the Japanese team surpasses what we had available 40 years ago. So, despite the great challenges ahead for Fukushima Daiichi, I am optimistic that Japan, with its international supporters, can achieve the same successful outcome that we did.
Lake Barrett is a semiretired nuclear engineer who is a senior advisor to TEPCO and IRID. He is a 50-year emeritus ANS member and served as the Nuclear Regulatory Commission’s site director for the cleanup of the Three Mile Island accident.