Fukushima: An overview of the work at the site on the occasion of the 8th anniversary

08.03.2019

© TEPCO

11 March 2019 marks the 8th anniversary of the nuclear accident at Fukushima. More than 4,000 people still work on the power plant site every day. Parallel to the ongoing work, plans for clearing the fuel storage ponds of Units 1 to 3 and for salvaging the molten nuclear fuel are being concretised. TEPCO is currently planning to establish a salvage method for one of the reactors as early as in 2019*, to be applied in 2021. Some techniques and equipment have yet to be developed. Plant operator TEPCO therefore estimates that it will take a total of 30 to 40 years for the destroyed reactors at the Fukushima Daiichi site to be completely dismantled.

What work was carried out at the site last year?

Investigations and tests in Unit 2. For the further planning of the dismantling activities, additional devices for recording the radiation dose were installed on the service floor of Unit 2 in November 2018. These devices will be used, among other things, to determine the dispersion of radioactive substances and possible hotspots. A robot with a gripper arm was deployed in mid-February 2019 (see Figure 1 and TEPCO report). This was used to examine the properties and mobility of deposits in the containment, which are presumably molten nuclear fuel. According to TEPCO, the robot managed to deform and grip the deposits with the gripper arms in five locations. This finding is important for later recovery of the melt. Sampling for further laboratory testing is planned for the second half of 2019.

Figure 1: Sketch of the exploration robot and its functions (left). Photo taken during the reconnaissance mission inside the containment of Unit 2 (right): The reconnaissance robot grips a single piece of the deposits with its "finger". Further photos of the reconnaissance mission can be found here (source: TEPCO).

In November 2018, TEPCO announced in its roadmap tests in connection with the injection of water into the reactor pressure vessel (RPV) to take place at end of March 2019. The amount of water injected into the reactor pressure vessel will initially be reduced by half over a period of seven days. In a further test, cooling is to be completely stopped over a period of seven hours. Currently, the temperatures in the bottom area of the RPV of Units 1 to 3 are between 14 and 20 degrees Celsius (as at 15 February 2019). TEPCO intends to use the test results to validate its decay rate calculation codes.

In order to prevent the formation of larger quantities of hydrogen, TEPCO continuously injects nitrogen into the reactor pressure vessels of Units 1 to 4. This also reaches into the containment through leaks. In Unit 2, a pressure of 4.25 kilopascals (corresponds to 0.0425 bar) above ambient pressure had previously prevailed inside the containment. Although the measure prevents a possible hydrogen detonation, the increased internal pressure also increases the risk of a release of radioactive substances. That is why in autumn 2018, TEPCO only injected nitrogen up to a pressure of 2 kilopascals (equivalent to 0.02 bar) as a test. According to TEPCO, the test did not lead to any significant changes in the plant values, which is why the lower injection rate has been maintained since December 2018.
 

How does hydrogen form in reactors 1 to 3 at Fukushima?Hydrogen is generated as a product of so-called radiolysis. Water molecules are split by ionizing radiation in several steps into hydrogen and oxygen. If a certain concentration of these so-called radiolysis gases is reached, an explosive reaction can occur together if there is an ignition source (oxyhydrogen explosion). In order to avoid the formation of such gas mixtures in the damaged reactors at Fukushima, nitrogen is continuously injected into the reactor pressure vessels. The property of nitrogen is made use of to displace the oxygen.

Recovery of fuel assemblies from Unit 3. TEPCO is planning to start salvaging the 566 fuel assemblies from the storage pool of Unit 3 from the end of March 2019. The commissioning of the refuelling machine and the crane, both of which are needed for the work, should be completed by this time. Figure 2: The refuelling machine that is to be used in Unit 3 to recover the fuel assemblies from the fuel pool (Source: TEPCO).The machines are housed in the cylindrical structure on the roof of the reactor building. In the past few months, repairs have repeatedly been necessary. recently, a faulty cable on the crane had to be replaced. According to TEPCO, the recovery of the fuel elements in Units 1 and 2 is to begin in 2023.

Protection against tsunamis. To protect the buildings from tsunami waves, TEPCO plans to extend the outer quay wall of the harbour by an 11-metre-high concrete wall. The wall is to be completed by 2020. So far, only a 2.4 to 4.2-m-high temporary barrier has been erected. In addition, the reactor buildings were protected against the ingress of water by individual measures. In November 2018, work also began on anchoring the so-called Mega Float to the quay wall of the port. Water is still stored on this raft, which had accrued in Units 5 and 6 immediately after the reactor accident. The anchoring is intended to prevent the raft from being entrained and damaging buildings in the event of a tsunami.

Contaminated water. In autumn 2018, it became public that the water declared by TEPCO as so-called "tritium water" and stored in tanks still contained noticeable amounts of radioactive substances such as iodine, caesium and strontium (see among others TEPCO Progress Report FY2018 Q2 and interview with a TEPCO manager in the online edition of the daily newspaper Mainichi). According to this, more than 80 percent of the stored water exceeded the limits for a discharge to the environment. Until then, TEPCO had stated that - with the exception of tritium - a large proportion of the radioactive substances would be removed by multi-nuclide filter systems. For example, up to 99 percent of the caesium contained in the water could allegedly be filtered out.
In its final report on the fourth Peer Review Mission in November 2018, the International Atomic Energy Agency (IAEA) has commented negatively on this "communication deficiency". At the same time, the IAEA urges that "urgent" solutions be found to the problem of storing contaminated water. TEPCO now intends to treat the "tritium water" several times over again in the Advanced Liquid Processing System (ALPS) in order to be able to comply with the necessary limit values for a possible discharge into the sea.

TEPCO also plans to completely transfer the contaminated water stored on the plant site to welded tanks by the end of March 2019. The remaining leak-prone flanged tanks are to be replaced, thus reducing the risk of radioactive substances leaking into the environment. As at December 2018, more than 1.1 million cubic meters of water in more than 940 tanks were stored on the plant site.

TEPCO claims, however, that it was possible to reduce the amount of new contaminated water accrued every day. According to TEPCO, this is partly due to the so-called "ice wall", which has been in operation since 2018 around the reactor buildings of Units 1-4. In this way, it has been possible to significantly reduce the inflow of groundwater into the buildings, which in turn results in less contaminated water that has to be treated and temporarily stored in tanks. According to TEPCO, instead of the original 400 tons, it is currently between 100 and 150 tons of groundwater per day that enters the buildings.

Further dismantling work. TEPCO plans to dismantle the now dilapidated exhaust stack used jointly by Unit 1 and 2 from March 2019. Corresponding preparations took place in August 2018 to test the tools to be used. In July 2011, a local dose rate of about 10 sieverts per hour was measured one meter above ground at a supply line to the exhaust stack. This pipeline was used to relieve the pressure in the containment of Unit 1 during the accident. As a result, larger quantities of radioactive substances were presumably deposited inside the pipeline.

Figure 3: Measurement of the local dose rate with a telescope detector at a supply line to the exhaust stack of Units 1 and 2 in July 2011 (source: TEPCO).

 

What happens to the contaminated waste from the dismantling?
During the dismantling of the destroyed reactor units, different types of radioactive waste accumulate, which TEPCO stores on the plant site and partly treats there as well. The handling and storage of the waste must be organised in such a way that the additional local dose rate at the plant perimeter is below the officially specified value of one millisievert per year.  

Depending on their dose rate, different storage concepts are used for the waste. For example, solid waste with a dose rate of 1 to 30 millisieverts per hour - including, for example, concrete and metal parts from clean-up work in and on buildings - is stored in tents or containers. Radioactive residues that the multi-nuclide filter systems filter out of the contaminated water, on the other hand, are stored in so-called high-integrity containers, which are temporarily stored on the plant site in a separate area in protective and shielding concrete containers. Undamaged fuel assemblies are stored for a certain period of time (approx. 3 to 5 years) in the so-called "common site storage pool". This is where also a large part of the fuel assemblies of Units 1 to 3 will be stored as soon as they have been recovered from the storage pools. For the period thereafter, there exists a dry storage facility in which fuel assemblies that were unloaded before the accident are already stored.

Figure 4: Section of an overview map of the plant site (north of the reactor buildings) on which TEPCO has colour-coded the storages of the various types of waste. Click on the map to enlarge it. (Source: TEPCO)

Research with GRS participation

Within the framework of various research cooperations, university institutions and TEPCO are dealing with issues arising from the dismantling of the plants or the treatment of contaminated water. For example, scientists at the University of Kindai are researching ways of filtering tritium from stored water on an industrial scale. In addition, investigations into the condition of the molten reactor cores continue to be of relevance for the planning of dismantling. A new project of the Organisation for Economic Cooperation and Development (OECD) could also contribute to this. Germany will be represented in the "Analysis of Information from Reactor Buildings and Containment Vessels of Fukushima Daiichi NPS" (ARC-F) project by GRS, whose experts will deal, among other things, with the calculation of accident sequences. The experts will use the AC² simulation code developed by GRS to analyse accidents involving core destruction and fission product release. In addition, GRS will also carry out so-called back calculations of the fission product release from the plants. Further details on the project can be found here.

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