In a monumental undertaking the likes of which has never been attempted, it could take up to four decades to complete the decommissioning and dismantling of the four damaged reactors at Tokyo Electric Power Co.’s Fukushima No. 1 nuclear power plant.
In a monumental undertaking the likes of which has never been attempted, it could take up to four decades to complete the decommissioning and dismantling of the four damaged reactors at Tokyo Electric Power Co.’s Fukushima No. 1 nuclear power plant.
The entire process will be composed of three phases, according to the road map toward the decommissioning of the No. 1 to No. 4 reactors at the plant announced in December 2011 by the government and TEPCO. Phase 1 will last until the end of 2013, followed by Phase 2, which is likely to continue until around 2021. The third and final phase should conclude sometime between 2041 and 2051.
In Phase 1, the cooling water circulation loops will be curtailed step by step while cooling of nuclear fuel is continued. A method of establishing a closed and independent cooling system for each reactor building will be devised. Research and development efforts will be started for removing fuel and disposing of radioactive waste.
In addition to the fuel assemblies inside the reactors, totals of 392-1,533 assembly units are left in the spent fuel storage pools of the four reactor buildings. Full-scale operations to remove spent fuel from the No. 4 reactor will start in November 2013, one month earlier than initially scheduled.
In Phase 2, operations to remove fuel assemblies from the spent fuel storage pools of the No. 1 to No. 3 reactors will be launched and completed. The removed fuel assemblies will then be placed in the shared storage pool within the plant, which is relatively less damaged. After nuclear fission and resultant generation of heat have declined to certain levels, the fuel assemblies will be put in dry casks for storage within the compound.
Flooding of the reactors will also be done in Phase 2. Flooding the reactors is important not only for scaling down the cooling water circulation loops but also for the subsequent removal of melted fuel from the reactors. Removal of nuclear fuel must be done in water in order to ensure that radiation is contained and the fuel keeps being cooled. TEPCO tried but failed to flood the reactors immediately after the nuclear disaster broke out on March 11, 2011, following the Great East Japan Earthquake and resulting tsunami. That’s probably because the reactor containment vessels had been damaged, resulting in leakage of water.
Repair of the containment vessels will begin around 2016 so that a closed and independent cooling system can be installed in each of the reactor buildings, which are currently connected with each other with pipes and other equipment.
In Phase 3, which will begin around 2021 and last until around 2051, at the latest, the main task will be removal of the melted fuel from the reactors. If the flooding of the reactors to be done in Phase 2 goes as planned, work to remove melted fuel will start around 2021 and be finished for all the reactors around 2031 – 2036. The removed fuel will be stored for a certain period within the plant and then moved to a disposal site.
In the meantime, decontamination and monitoring of radioactive materials will continue in the surrounding sea and within the plant itself. After all the fuel is removed, operations to dismantle the facilities will start. The work will begin with areas, such as the turbine buildings, where levels of radiation are lower. The reactor pressure vessels will be the last to be dismantled. The whole process of dismantling the vessels is expected to require some 15 years.
No decision has yet been made on how the site of the damaged reactor buildings will be used after the facilities are dismantled. Since the land will remain contaminated, the possible options will be quite limited.
Since the plan for decommissioning the crippled reactors was crafted while the situation at the plant was unclear, the prospects for the mission remain murky.
The biggest challenge will be the removal of melted nuclear fuel. Since the condition of the fuel is unknown, there are many uncertainties surrounding this process.
The work to remove melted fuel will be carried out by drawing on the U.S. experience in the cleanup at the Three Mile Island nuclear power plant, where failure of the cooling system resulted in a partial nuclear meltdown in 1979.
But the Fukushima nuclear disaster is different in some important respects from the Three Mile Island accident, in which melted nuclear fuel remained within the pressure vessel. At the Fukushima plant, melted fuel went through pressure vessels and fell into the outer containment vessels.
Removing melted fuel from a containment vessel is a task that has never been carried out before anywhere in the world. The amounts of fuel that accumulated in the containment vessels have only been estimated without being confirmed. There are undoubtedly many difficulties lying ahead.
It is necessary to confirm the situations and decontaminate the facilities for protecting workers from unnecessary exposure to radiation. Remote-controlled robots are invaluable for the operation, but the efforts to develop necessary technologies have barely begun.
Since it will be a long process stretching over three to four decades, it is also important to make sure that experience and expertise obtained through operations will be handed down to the next generation and an effective worker training system will be established. The decommissioning plan will have to be revised many times due to unpredictable factors. At such junctures, development of new technology may be necessary. The total cost of the project will likely be astronomical, but the road map doesn’t contain any cost estimate.
Two years have passed since the severe nuclear accident broke out at the Fukushima No. 1 nuclear plant. In December 2011, then Prime Minister Yoshihiko Noda declared the devastated reactors at the plant to be in a state of “cold shutdown,” meaning the fuel cores are in a safe and stable condition, initiating the work to decommission the reactors.
But the work is still in the preparatory stage, with an uphill battle going on to deal with increasing amounts of radioactively contaminated water.
Relatively greater progress has been made in the operation to decommission the No. 4 reactor, which was off-line for regular safety inspections when the nuclear crisis erupted in March 2011.
Since levels of radioactive contamination are relatively low in areas around the No. 4 reactor, efforts to remove debris are focused on these areas at the moment. In July last year, two fuel assemblies were removed from the spent fuel storage pool of the No. 4 reactor building on a trial basis.
Currently, a structure to cover the No. 4 reactor building equipped with a crane to lift fuel assemblies is being installed in preparation for the full-scale operation to remove spent fuel expected to begin in November this year.
Meanwhile, the No. 1 to No. 3 reactors, which suffered core meltdowns, are still inaccessible due to high levels of radiation. At the moment, robots and other equipment are being used to grasp the conditions inside these heavily damaged reactors.
In January last year, an inspection using an endoscope was launched to look inside the containment vessel of the No. 2 reactor. A similar endoscopic inspection of the containment vessel of the No. 1 reactor started in October.
Radiation measurements within the No. 2 reactor building were 73 sieverts per hour at maximum, while those for the No. 1 reactor building were 11 sieverts per hour, levels that could kill a human within an hour.
No fundamental solution to the problem of increasing amounts of contaminated water has been figured out yet. After two years of increasing constantly, the total amount of polluted water, stored in fields of tanks on the plant’s grounds, reached some 260,000 tons as of Feb. 19.
Water injections into the reactors to cool the melted fuel started immediately after the accident broke out. Groundwater has been flowing into reactor buildings, where it mixes with cooling water that leaks from the reactors.
As a result, immense amounts of contaminated water built up within the reactor buildings. To curb growth in the amount of polluted water, a system to cool the reactors with circulating recycled water was installed in June 2011.
The four kilometer-long jury-rigged cooling system is composed of water purifiers, tanks and other vulnerable equipment connected to the reactors by long rubber hoses. The makeshift cooling system has been plagued by water leaks and equipment glitches. TEPCO intended to shorten the hoses, but has so far failed to do so.
TEPCO also planned to start operating an advanced liquid processing system, dubbed ALPS, last autumn. But the utility postponed using the equipment, which it claims can remove almost all types of radioactive materials, after experts questioned the durability of storage containers.
Even if radioactive materials in contaminated water can be reduced to levels lower than the safety standards, it will be impossible, at least for the time being, to release the treated water into the sea, given strong opposition from the local governments and fishermen and fishing businesses concerned.
TEPCO plans to increase the total capacity of storage tanks to 700,000 tons.
Two years after the Fukushima accident broke out, many questions still remain unanswered with regard to the causes of the nuclear disaster and the massive radiation leaks it triggered. Available data is not enough to give clear answers to these questions. But it is still impossible to gather necessary data in areas close to the reactors because of dangerously high levels of radiation. Many of the explanations about what occurred that have been given are based on conjectures and inferences, and experts are divided over a number of important points.
One big question is whether the earthquake itself caused serious disruptions in the functioning of important equipment of the nuclear reactors and related systems. The Fukushima plant was hit by tsunami waves that were far higher than the assumed maximum height, but the quake itself, although very powerful, was not bigger than the maximum magnitude assumed in designing the power plant.
If a quake whose power and intensity was within the scope of the assumptions for plant design actually disabled any key equipment in a way that triggered such a severe accident, the credibility of the government’s safety standards for nuclear power plants would be called into question.
The accident investigation committees set up by TEPCO and the government ruled out the possibility that the reactor pressure vessels and the pipes around them were damaged so seriously as to cause them to lose their ability to contain radioactive materials. They also claimed that there were no ruptures in pipes used for reactor cooling systems.
As reasons for these claims, TEPCO cited the results of inspections by workers at the plant immediately after the accident occurred, as well as operation data. The utility also said the emergency cooling system for the No. 1 reactor, which failed, was not damaged in any serious way by the quake itself, either, pointing out that no signs of serious damage were found through rough inspections of the system’s exterior.
The Diet’s investigative committee, in contrast, argued the possibility of the quake causing serious damage to reactor-related equipment and facilities could not be ruled out. Small ruptures in pipes cannot be detected immediately by simply monitoring pressure and water levels within reactors, the panel said. If the cooling water keeps leaking through such small ruptures, fuel damage events could occur, according to the Diet panel.
A second key question is when and how the plant released such huge amounts of radioactive materials into the atmosphere.
With regard to the No. 1 to No. 3 reactors, which suffered core meltdowns, TEPCO has estimated that three-quarters of the radioactive materials that were spewed into the environment came from the No. 2 and No. 3 reactors.
It is believed that a vent system designed to lower pressure within reactors failed to work in the No. 2 reactor. As a result, radioactive materials within the reactor leaked through openings in the reactor building into the environment on March 15, according to TEPCO’s analysis. But the analysis also indicated massive radiation leaks from the No. 3 reactor on March 16. Since the venting of the reactor was done successfully before the hydrogen explosion, however, there is no good explanation for the leaks. TEPCO said it blamed the No. 3 reactor, as well as the No. 2 unit, for the radiation leaks by “the process of elimination,” meaning there is no other plausible explanation.
(This article was written by Yu Kotsubo and Naoya Kon.)