Fukushima: Overview of the work on the plant site nine years after the nuclear accident

10.03.2020

Nine years have passed since a tsunami on 11 March 2011 caused a serious nuclear accident at the Japanese nuclear power plant site of Fukushima Daiichi. Since the remediation of the immediate consequences of the accident, power utility Tepco, the plant’s operator, has been focusing for several years now on the work of dismantling and decommissioning the damaged reactors. To illustrate this ongoing effort, every day in January 2020, around 3,900 workers were busy working on the site towards this end. Public attention has recently been drawn to the debate about possible disposal options for the contaminated water stored on the plant premises.

Recovering the fuel assemblies from the spent fuel pool of Unit 3

The still image from a video recording shows how the loading crane lifts a fuel assembly from the spent fuel pool of Unit 3 (source: Tepco)In April 2019, Tepco began unloading the fuel assemblies from the storage pool of Unit 3. 70 of the total of 566 fuel assemblies have so far been removed and transferred to the joint storage pool at the site. As at the beginning of February 2020, these comprise 52 unused fuel elements and 18 used ones. In summer 2019, the work had to be interrupted because faults were discovered in the containers for the transport of the fuel assemblies and also in the loading crane. Tepco is currently working with dummy fuel assemblies in order to check and, if necessary, adjust the procedure. The work is scheduled for completion in spring 2021.

Unit 2: Recovery of molten core to start here

According to the present state of knowledge, core melt from the reactor pressure vessels in Units 1-3 reached into the respective containments during the course of the accidents. However, since the core melt fragments are distributed differently in the three buildings as regards their location, condition and quantity, their recovery poses great challenges for the operator in the course of the dismantling process. In addition to aspects of radiation protection, it is in particular questions concerning technical implementation that must be clarified in advance.

Tepco considers Unit 2 to be suitable as the first place to start with the recovery of the core melt. The decision was based among other things on the results of the exploration rounds of various robots inside the containment (e.g. camera recordings and removal of debris samples), which have been more extensive than in the other units so far, and the working environment as a whole (access to the containment is possible from outside, the reactor building was not damaged by a hydrogen explosion). In a next step, Tepco plans to use a robot arm to remove core melt fragments via a lateral containment penetration as a test. Should the method prove successful, the recovery of the core melt fragments could begin in 2021, according to Tepco.

Planning for the recovery of core melt: With the help of a robot arm, core melt fragments are to be recovered from the interior of reactor 2 via an opening in the containment (photo: Tepco)

Handling of contaminated water

At present, an additional 100 cubic metres of contaminated water are produced on the plant site each day (as at 3 February 2020) due to groundwater inflow into the reactor buildings and precipitation. In recent years, Tepco has been able to significantly reduce the amount of groundwater entering the plant through various measures such as the so-called "ice wall" and the operation of drainage pumps. In mid-2014, for example, the volume was still around 550 cubic meters per day.

According to Tepco, it wishes to maintain the level inside the reactor buildings at a level below the water table outside the buildings. With this measure, the company accepts that groundwater will continue to penetrate the buildings. In addition, however, it will also prevent water that is already contaminated from escaping from the buildings.

Still, the steadily increasing overall volume of contaminated water to be stored continues to pose a major challenge for Tecpo in terms of the space required for storage tanks. According to Tepco, as at 23 January 2020, a total of around 1,185,000 cubic metres of water was stored in 1,000 tanks on the plant premises (see Tepco portal on the handling of contaminated water). The majority of these tanks (863) is used for water that has passed through the multi-nuclide filter systems (Advanced Liquid Processing Systems - ALPS). These systems filter 62 radionuclides from the water. The radioactive tritium cannot be filtered out.

Last year, however, it became apparent that this water does not only contain radioactive tritium - as previously communicated by Tepco - but also, in some cases, still relevant quantities of other radioactive substances such as iodine, caesium and strontium. Thus, more than 70 percent of the stored water exceeds the limits for a discharge into the environment. Tepco then noted that it intended to repeatedly treat the water again in the multi-nuclide filter systems. According to current knowledge, this renewed treatment has not yet taken place. Due to the constantly increasing water volumes, Tepco is planning to expand the tank capacities to 1.37 million cubic meters by the end of this year.

In early February 2020, the Japanese government informed foreign diplomats about the options being considered for the disposal of the "tritium" water stored in the tanks. According to this information, a group of experts appointed by the Ministry of Economics METI considers two options to be "feasible" (cf. presentation and report of the commissioned sub-committee). One option provides for the evaporation of the water. In this process, the water vapour, which also contains tritium, is released into the air or the atmosphere and is dispersed there. Since the dispersion is weather-dependent, it is less predictable than, for example, a discharge into the sea.

A further variant that is mentioned is the discharge of water into the sea. However, this is under the proviso that the water to be discharged contains no other radioactive substances apart from tritium, or only such quantities that are below the limit values. In its presentation, the expert group refers in this respect i.a. to the extensive empirical data available from the operation of nuclear installations regarding the regular discharge of comparable wastewater. For example, 1,540 terabecquerel of tritium from the reprocessing plant at the British Sellafield plant were discharged into the sea on the basis of the relevant licences in 2015 (see presentation of the subcommittee). Tepco, on the other hand, estimates the activity of the water stored in the tanks to be 860 terabecquerel.

The Japanese government had made the report available to the International Atomic Energy Agency IAEA with the request for review. The latter attested the government a comprehensive assessment based on scientific methods. In addition, the IAEA considers both of the above options to be technically feasible and has pledged its support to Japan, i.a. with regard to monitoring radiation levels.  

Plant condition and general radiological situation

The situation inside the reactor buildings is similar to that of the previous years. For example, Tepco states that in recent months it has constantly measured temperatures between 20 and 30 degrees Celsius inside the containment vessels and at the bottom of the reactor pressure vessels. In May 2019, in a trial in Unit 2, Tepco tested the temperature development after an interruption of coolant injection. The injection quantity had previously been around 3 cubic meters of water per hour. After 8 hours, the supply was gradually resumed. Within these 8 hours, the temperature at the bottom of the reactor pressure vessel rose by 1.2 °C. The evaluation of the test was intended to help improve the advance calculations and emergency measures. Tepco had planned the same test run for Unit 3 in February 2020. Results have not yet been published.

The local dose rates recently determined (between 18 December 2019 and 28 January 2020) at the measuring points at the plant fence are between 0.370 and 1.324 microsievert per hour. This corresponds approximately to three to ten times the amount of natural background radiation that a person in Germany receives per hour for example in the town of Forbach in the Black Forest.

The Japanese regulatory authority NRA continues to record i.a. the measured values for the activity concentration (in becquerel per litre, or Bq/l) in seawater within a radius of 5, 10 and 20 kilometres around the plant site. The values for caesium-134 determined in 2019 were thus up to a maximum of 0.011 Bq/l within a radius of 5 kilometres around the plant. The highest measured value for strontium-90 was 0.0052. By way of comparison, the World Health Organization (WHO) indicates a guideline value for drinking water of 10 Bq/l for caesium-134 and strontium-90.

A team of French, Japanese and Canadian researchers evaluated the effectiveness of the decontamination work carried out in the regions affected by the fallout in the wake of the accident. Between 2011 and 2016, the Japanese government decontaminated agricultural and inhabited areas covering a total area of more than 9,000 square kilometres. Particularly with regard to caesium-137, the researchers assess in their study the removal of soil as an effective measure to prevent further mobilisation of the radionuclide by surface water on agricultural land. The scientists estimate that a total of around 20 million cubic metres of contaminated soil was removed during the decontamination measures. The authors of the study conclude that the transport and storage of the contaminated soil, along with the recultivation of the decontaminated areas, will be the main challenge for the coming years. A summary of the study was published in December 2019 in the scientific journal Soil, which is published by the European Geosciences Union (EGU).

Current work of GRS on Fukushima

Currently, GRS is cooperating with numerous other international expert organisations in the research project ARC-F "Analysis of Information from Reactor Buildings and Containment Vessels of Fukushima Daiichi NPS". This work is funded by the Federal Ministry of Economics and Technology (BMWi). The project is coordinated by the Nuclear Energy Agency of the Organisation for Economic Co-operation and Development (OECD/NEA) and focuses on the behaviour of radioactive fission products during accident sequences and on sensitivity calculations for the accident sequence. The experts calculate, among other things, the accident sequences in the identical Units 2 and 3 at Fukushima Daiichi, using the AC² (ATHLET-CD/COCOCOSYS) simulation code that was developed by GRS. Furthermore, they make so-called back-calculations concerning the release of fission products from the plants. The findings obtained in this way are to allow more detailed statements on the accident sequence - for example on the amount of radioactive materials still remaining inside the plants - and thus support the planning of the recovery of the nuclear fuel or the dismantling of the affected reactors.

An overview of further work of GRS on Fukushima can be found here.

Find out more

>> Tepco: Summary of Decommissioning and Contaminated Water Management (as at: 30 January 2020)
>> Technical Strategic Plan 2019 der Nuclear Damage Compensation and Decommissioning Facilitation Corporation (as at: September 2019)
>> Report of the Subcommittee on the management of water purified in the ALPS facilities (as at: February 2020)
>> Effectiveness of landscape decontamination following the Fukushima nuclear accident: a review (Study in the technical journal Soil of EGU, December 2019)
>> Emergency Centre of GRS
>> Fukushima Daiichi – Accident sequence, radiological consequences (GRS Report, 5th revised edition)
 

(Update 31.03.2020: Paragraph about the Summer Olympics, scheduled to take place in Japan 2020, was removed. The games have been postponed to 2021 due to the corona pandemic)