Fukushima today8 June 2017
Takafumi Ihara gives an update on the Fukushima decommissioning work and the key challenges including management of contaminated water, and removal of spent fuel.
Six years have passed since the Fukushima accident occurred. Tokyo Electric Power (Tepco) is undertaking the decommissioning of Fukushima Daiichi steadily and safely, with the help of domestic and international expertise, to fulfil its responsibility for the March 2011 accident. This article describes the main progress of on-site activities and future plans.
A new internal entity whose function is to deal with decommissioning and contaminated water was established within Tepco on 1 April 2014. The Fukushima Daiichi Decontamination & Decommissioning (D&D) Engineering Company (FDEC) will clarify the allocation of responsibilities and focus solely on the handling of decommissioning and contaminated water at Fukushima Daiichi.
Through all the Fukushima decommissioning work, the new company will clarify lines of responsibility and demand accountability for performance. Project managers will be assigned to each project, and will be responsible for meeting targets and schedules, as well as managing risks and budgets.
At the same time, the company intends to strengthen cooperation with international and domestic experts. They include the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) and the International Research Institute for Nuclear Decommissioning (IRID), along with the experts already serving on Tepco’s Nuclear Reform Monitoring Committee.
Tepco is continuing its efforts to secure sufficient numbers of workers for future decommissioning, considering their competence and their workload. The average number of workers per day was around 6000 in 2016. Cumulative dose values for most workers on site is within the regulatory effective dose limit and allows them to continue engaging in decommissioning work safely.
Revision of the mid-and-long-term roadmap was approved by the government, Tepco and other relevant parties on 12 June 2015, at the Inter-Ministerial Council for Contaminated Water and Decommissioning Issues. The overall goal, including “completion of decommissioning 30-40 years from now”, etc. was maintained. Based on the approved roadmap, the following five fields were revised:
i) Emphasising risk reduction, as well as increasing speed, and prioritising the risks to ensure steady long-term risk reduction.
ii) Specifying milestones in response to the views of local stakeholders, setting specific targets over the next few years.
iii) Enhancing relationships with local citizens and increasing trust with thorough information disclosure.
iv) Further reducing the dose of exposed workers and strengthening labour, safety and hygiene control.
v) Reinforcing the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF), which is the central organisation that manages research and development and incorporates domestic and international expertise.
Based on this revised roadmap, decommissioning will proceed steadily, including managing contaminated water.
Groundwater flows continuously into the reactor and turbine buildings, and the volume of contaminated water to be managed has accordingly increased. Although measures have been taken to reduce the volume recently, the inward flow of water continues to be around 400m3/day.
The strategy for managing the contaminated water accumulating in the buildings is as follows. The water is first pumped from the buildings and then treated to remove caesium isotopes using two parallel systems, “Caesium adsorption apparatus” and “2nd Caesium adsorption apparatus” (named “Kurion” and “Sarry”, respectively). After the gamma-emitting caesium isotopes have been removed the treated water can be managed and reused. These two caesium removal systems have consistently achieved caesium decontamination factors of 10,000 or more. The two systems have now been enhanced by adding strontium-removal capabilities. The systems can now remove caesium and strontium from the water at a rate of 1800 m3/day.
Following caesium removal, the water is treated to remove dissolved salts using reverse osmosis. Around half of the feedwater is desalinated and used for cooling the damaged cores of units 1, 2 and 3. The remaining half is a concentrated salt solution, which is highly radioactive. It contains strontium because this is still present even after the water has been treated by the two caesium removal systems with their strontium removal capabilities. This water is stored in above-ground tanks. Multi- nuclide removal systems (so-called ALPS) are being used to treat the highly radioactive water and remove 62 radionuclides to below detectable levels. A number of additional systems have also been deployed specifically to remove strontium from water stored in the bolted flange-type tanks.
By March 2015 these treatments had achieved the goal of reducing the radiation dose at the site boundary, attributed to the water tanks, to less than 1mSv/year.
Tepco is also making progress in preventing contaminated groundwater from leaking into the sea, by removing some contamination sources from the cooling water trench and tunnel system.
After removing all radionuclides (except tritium) the contaminated water will be stored in above-ground tanks. Several measures have also been taken to improve storage safety, including replacing bolted flange-type tanks with fully welded tanks and construction of dykes with enhanced water- holding capacity. Rainwater deflection covers have been provided to keep rainwater away from the dykes, so that any leak from the tanks can be contained.
Significant steps have been taken to stop groundwater entering the reactor and turbine buildings from the mountainside, by installing a series of bypass pumping wells. A subdrain system around the buildings has been redeveloped to allow greater control of groundwater entry in the near future. Additional measures, such as installation of a land-side impermeable wall installation, are in partial operation.
Tepco is also making progress in preventing contaminated groundwater leaking into the sea, by installing a sea-side impermeable wall. While this wall has blocked the natural flow of groundwater, it means more water has to be pumped out and treated with the groundwater drain to prevent overflow into the sea.
Spent fuel removal from the spent fuel pools
The removal of spent fuel assemblies from the spent fuel pool at unit 4 commenced on 18 November 2013 and was successfully completed on 22 December 2014.
At unit 1, a cover exclusively for spent fuel removal will be installed over the refuelling floor. The existing building cover has been removed and the next step will be removing the rubble on the floor. After the rubble is removed, the new cover and a fuel handling machine will be installed.
At unit 2, the options to remove spent fuel assemblies were narrowed down to two. In the first option an integrated container will be installed over the refuelling floor to remove spent fuel assemblies and fuel debris. In the second option a cover will be installed over the refuelling floor to remove spent fuel assemblies, and afterward another cover will be installed for fuel debris removal.
Once an option has been chosen, the fuel handling facility will be installed, and spent fuel assemblies are scheduled to be removed from the spent fuel pool by the end of March 2021. The unit 2 building itself suffered almost no damage, because the blow-out panels were open during the accident and there was no hydrogen explosion. But inside the building there are high radiation levels, which means operation inside is very difficult. Tepco is considering demolishing the highest floor of the building (with measures to prevent radioactive substances from scattering into the environment) to access the spent fuel pool at the earliest opportunity. This is seen as the best way to reduce that radiological risk.
In unit 3, rubble removal from the refuelling floor was completed in October 2013. By the end of 2016 rubble had also been removed from inside the spent fuel pool. Decontamination and radiation shielding on the refuelling floor had been completed, to reduce the occupational exposure dose during spent fuel removal. Now the cover for the spent fuel removal and the fuel handling machine will be installed over the refuelling floor.
Robotics to prepare for fuel debris removal
Decontamination and the installation of shielding has improved access to the primary containment. Now technology has been developed and data has been gathered to prepare for the removal of fuel debris.
Because of the high level of radiation, it is impossible to access the inside of the reactor buildings directly, so it is crucial
to use remote controlled devices or robots to ascertain the conditions inside and to implement decontamination work. Below are photos of some of the many robots that have been developed for use at Fukushima Daiichi. Some machines have been used for decontamination in the lower levels. These robots use high-pressure water or dry ice blasts for decontamination. Other robots have been tasked with measuring radiation and taking photos, navigating debris or steps. Although it has not yet been decided at which unit fuel debris removal will commence, fuel debris removal at the first unit is planned to start by the end of 2021.
At unit 1, the permeation method of muon technology was used to detect fuel debris inside the reactor. Measurement equipment was installed and measurement began on 12 February 2015. The data collected during the 26 days to 10 March showed no large fuel block at the core location. Measurements have continued, and more data has been collected since 25 May to obtain accurate three-dimensional information. To help formulate a fuel debris removal plan, the environment inside PCV was investigated by a shape-changing robot on 10-20 April 2015. These investigations revealed information including the extent of damage inside the vessel, temperature and radiation. In the next investigation, a robot will go further inside the vessel to get crucial information for fuel debris removal.
In 2014 an investigation was conducted at unit 2, to identify the location of the leaks on the lower external surface of the suppression chamber to seal the water leaks from inside the vesel in preparation for fuel debris removal. No aperture was found within the investigation range. Based on the noise shown on the monitor screen of the investigative equipment, there is increasing radiation on the bottom of the suppression chamber. On 31 January 2017, a camera placed inside the vessel of Fukushima Daiichi 2 captured intriguing images that may be fuel debris from the March 2011 accident, but further examination is necessary before that can be verified. Aside from the possible images of fuel debris, the imaging inside unit 2 found other important things:
- No changes in the positions of the control rod drive housing, position indication probe cable, or CRD exchanger;
- Deformities in a part of the grating;
- Deposits found on the CRD rail and grating
- Evidence of water dripping in some areas.
On 15 May 2014 a camera was inserted into the main steam isolation valve room at unit 3, for investigation of the status of the reactor building. It detected water flow from the expansion joint of one of the main steam lines. This was the first leak from the vessel detected in unit 3.
Prior to future investigation inside the vessel, a radiation-source survey was conducted on the first floor of the unit 3 reactor building. Since 5 January 2015, decontamination of the middle-level floor inside the reactor building has been under way, using suction, floor-wiping and sprinkling devices.
Improvement of work environment
The work environment has continuously been improved, with the needs of workers on site considered. On 31 May 2015 a large rest house with a capacity of approximately 1200 workers went into operation. On 1 June 2015 meal services began to improve working conditions for the workers at Fukushima Daiichi. Approximately 3000 meals are cooked at lunch each day and transported within 30 minutes to the rest house, via a specially insulated truck, from the kitchen at the Fukushima Revitalization Meal Service Center, located approximately 9km from the Fukushima Daiichi site. This replaces the previously provided lunch box.
The food service helps in revitalising the local community. It employs local residents but uses appliances made by companies that operate manufacturing plants in Fukushima. Nearly 100 people, almost all of them from Fukushima Prefecture, are employed at the Meal Service Center. It also helps demonstrate the safety and wholesomeness of locally sourced ingredients from Fukushima Prefecture. It is the first time since the accident that a rest house designed to offer hot meals has been provided for the workers.
Reflecting and looking ahead
Reflecting upon the lessons learned from the Fukushima accident, in March 2013 Tepco compiled a report, the “Nuclear Safety Reform Plan.” Based on a root cause analysis of the accident, Tepco concluded that the underlying factors were deficiencies in safety awareness, engineering ability and communication ability. The report said that preparations for accidents were deficient, because there was an assumption that safety was already guaranteed and that capacity factors were the management’s most important challenge. Measures have been adopted to resolve these intrinsic problems within the organisation. Based on the lessons learned, Tepco has taken measures to improve both the tangible and intangible aspects of nuclear safety.
There are still challenges onsite. These include radioactive waste management, optimised risk reduction corresponding
to the changes in the site environment, the R&D activities required for work to be undertaken in unprecedented and complex circumstances.
Tepco is making safe and steady progress with decommissioning at Fukushima Daiichi. Safety is always the overriding priority, while making every possible effort to facilitate early repatriation of disaster evacuees.