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Fukushima nuclear mess 2021 – the tasks ahead

The Fukushima Nuclear Disaster: Then and Now, The Chemical Engineer 25th February 2021 by Geoff Gill 

“………..Decommissioning and contaminated water management

The work to decommission the plants, deal with contaminated water and solid waste, and remediate the affected areas is immense. A “Mid-and-Long Term Roadmap”2 was developed soon after the disaster to set out how this will be achieved. Also, to facilitate decommissioning units 1-6, and dealing with contaminated water, TEPCO announced, at the end of 2013 the establishment of an internal entity: the Fukushima Daiichi Decontamination & Decommissioning Engineering Company, which commenced operations in April 2014. The entire decommissioning process will take 30–40 years, and, as noted above, the volume of tasks is gigantic. Therefore, the Government of Japan and TEPCO have prioritised each task and set the goal to achieve them. Essentially, it is a continuous risk reduction activity to protect the people and the environment from the risks associated with radioactive substances by:

  • removing spent fuel and retrieval of fuel debris from the reactor buildings;
  • establishing measures to deal with contaminated water; and
  • establishing measures to deal with radioactive waste material.

Fuel removal from the reactor buildings

In the Fukushima Daiichi design of reactor, used and new fuel rod assemblies are stored in the upper part of the reactor. The used fuel rods are highly radioactive and continue to generate heat, and thus require continued cooling.

Depending on the degree of damage, the process of removing the fuel assemblies presents different challenges in each of the reactors. For example, one of the significant challenges is to firstly remove the large quantities of rubble caused by the hydrogen explosions. As noted above, reactors 5 and 6 were shut down at the time of the accident. The reactor cores were successfully cooled, and thus suffered no damage. Given that the conditions of the buildings and the equipment for storing the fuel are stable, and risks of causing any problem in the decommissioning process are estimated to be low compared to the other units, the fuel assemblies of units 5 and 6 continue to be safely stored in the spent fuel pool in each building for the time being. The next step will be to carefully remove the fuel from the fuel pools in units 5 and 6 without impact on fuel removal from units 1, 2 and 3.

All the remaining units are going through a number of stages to achieve fuel removal. They differ slightly for each unit, but essentially the stages are: survey of internal state, removal of rubble, installation of fuel handling facility, and removal of fuel. By way of example, the position regarding unit 3 is shown in Figure 3 [on original].

At unit 3, rubble removal and other work at the upper part of the reactor building, together with installation of a cover for fuel removal was completed in February 2018.

After all preparations were in place, work to remove the 566 fuel rod assemblies, including 52 non-irradiated fuel assemblies, began in April 2019. The process of fuel removal is shown diagrammatically in Figure 4.

The four stages are:

  • Fuel rod assemblies stored on fuel racks in the spent fuel pool are transferred in the water one at a time to transport casks, using fuel handling equipment;
  • after closing the cask cover and washing, a crane is used to lower the cask to ground level and load into a trailer;
  • the cask is transported to a common pool on the site; and
  • the fuel in the cask is stored in the common pool.

As of 8 January 2021, 468 assemblies including the 52 non-irradiated fuel assemblies had been removed from unit 3. Measurements of airborne contamination levels are being monitored in the surrounding environment throughout the fuel removal operations. The plan is that all fuel will have been removed from all of the reactor units by sometime during 2031.

Retrieval of fuel debris

At the time of the accident, units 1–3 were operating and had fuel rods loaded in the reactors. After the accident occurred, emergency power was lost, preventing further cooling of the cores. This resulted in overheating and melting of the fuel, together with other substances. Fuel debris refers to this melted fuel and other substances, which have subsequently cooled and solidified, and, of course, still remains dangerously radioactive. This clearly poses a very complex and difficult decommissioning challenge.

Currently the state inside the containment vessel is being confirmed, and various kinds of surveys are being conducted prior to retrieval of the debris. The current aim is to begin retrieval from the first unit (unit 2), and to gradually enlarge the scale of the retrieval. The retrieved fuel debris will be stored in the new storage facility that will be constructed within the site.

The distribution of debris between the pressure and containment vessels differs in each of the 3 units. By way of example, Figure 5 [on original] shows the current position in unit 2. Large amounts of debris are located in the bottom of the pressure vessel, with little in the containment vessel.

The investigation to capture the location of fuel debris inside unit 2 was conducted from 22 March–22 July 2016. This operation applied the muon transmission method, of which effectiveness was demonstrated in its appliance for locating the debris inside unit 1. (Muon transmission method is a technique that uses cosmic ray muons to generate three-dimensional images of volumes using information contained in the Coulomb scattering of the muons.) These operations used a small device developed through a project called “Development of Technology to Detect Fuel Debris inside the Reactor’’.

 Use of remote operations for decommissionings;
  • establishing measures to deal with contaminated water; and
  • establishing measures to deal with radioactive waste material.

…….. Understanding of the situation inside the stricken reactors was urgently needed following the accident in order to prevent the spread of damage and to mitigate

the disaster. Tasks had to be carried out in a very complicated, difficult and unpredictable environment. In particular, the environment inside the reactor buildings reached high radiation levels due to the spread of radioactive contamination. To reduce the risk of radiation exposure to operators, remote control technologies  have proved indispensable for examining the reactor buildings and subsequently for decommissioning work. Thus, remote control technology, including robot  technology has been heavily utilised in response to the accident. Figure 6 [on original]shows a typical configuration of remotely-controlled robotic systems for
decommissioning work.
Reducing the risks associated with contaminated water
Water has posed a very demanding challenge for the operators. The problem stems from groundwater flowing from the mountain side of the site toward the ocean.
This flows into the reactor buildings and becomes mixed with radioactive water accumulated in the buildings, increasing the amount of contaminated water already
 there. The solution to the contaminated water problem is being tackled through a three-pronged approach. These are redirecting groundwater from contamination
sources, removing contamination sources, and prevent leakage of contaminated water.
In order to achieve this, barriers have been installed on the land -side and sea-side of the plant. An impermeable barrier on the land-side has been achieved by
freezing the ground. The frozen soil “wall” (which has a circumference of about 1,500 m) has been achieved by piping chilled brine through pipes to a depth of 30 m,
which freezes the surrounding soil. On the sea-side, a wall has been constructed, consisting of 594 steel pipes (see Figure 7   -on original)………

Purification treatment of contaminated water and management of treated water…….

Treatment and disposal of solid radioactive waste
Waste materials resulting from the decommissioning work are sorted based on their radiation levels and are stored on the premises of the Fukushima Daiichi
 Nuclear Power Station. Along with strict safety measures and studies on treatment and disposal methods, a solid waste storage management plan is drawn up
 based on waste generation forecasts for around the next ten years, so that measures to deal with waste materials will be carried out effectively

The storage management plan is updated once a year, while reviewing the waste generation forecasts, taking account of progress of the decommissioning work.

 The illustration in Figure 9 [on original] shows the various facilities planned for treatment and storage of solid waste. TEPCO’s Mid and Long Term Roadmap shows all these
facilities being completed by 2028. The amounts of waste generated are huge.

For example, the latest edition of the roadmap estimates the amount of solid waste which will be generated over the next 10 years to be 780,000 m3

 

February 27, 2021 - Posted by | Fukushima continuing, Reference

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