Home Environment Reflections on the Fukushima Daiichi Nuclear Accident
Management of Contaminated Water
To cool fuel debris stably, cooling water is continuously recirculated inside the primary containment vessel. Nevertheless, approximately 400 m3 of groundwater flows into power station buildings per day, and hence the amount of contaminated water increases daily.
In order to manage the contaminated water, three measures have been implemented [3, 4]: (i) “Remove” sources of contamination, (ii) “Isolate” water from contamination, and (iii) “Prevent leakage” of contaminated water. In order to reduce the risk from contaminated water, some measures have been implemented. For example, the contaminated water has been treated with a multi-nuclide removal equipment (ALPS), the groundwater has been pumped up from sub-drains near nuclear power station buildings, land-side frozen soil impermeable walls have been installed, and the soil has been improved with sodium silicate. Furthermore, some additional measures have been decided. For example, more multi-nuclide removal equipment will be installed, measures to prevent water leakage from tanks will be taken, broader area pavement (surface waterproofing) at the site will be implemented, and the length of contaminated water transfer piping will be reduced. Further detailed measures are illustrated in the Refs. [3, 4].
Also being addressed are the necessity and importance of accelerating the installation of further tanks to the extent possible with combined efforts of the publicand private-sectors, developing measures with high technical diffi such as methods to clean up the sea water in the harbor and to remove radioactive materials in the soil, and of making a comprehensive evaluation of all options for tritiated water containing residual risks as soon as possible and consider appropriate measures.
Management of Radioactive Wastes Generated Within Nuclear Power Station
Figure 15.2 illustrates the kinds of waste contaminated with radioactive materials and nuclear fuels emitted from the damaged nuclear reactors to the atmosphere, groundwater, and soil by the Fukushima accident, and an example of the flow of processing and disposal processes of the radioactive waste. Figure 15.2 roughly consists of three streams of wastes. The two upper streams show the management of wastes generated within Fukushima Daiichi Nuclear Power Station, and the bottom stream represents that generated outside of the Station.
The radioactive waste consists of liquid waste and solid waste. The liquid waste includes the liquid which was initially stored in Mega-Float and the barges and whose radioactivity level is not high, the waste fluid which will be generated from the decontamination processes in future, the zeolite containing waste liquid with high radioactivity level, the sludge with high radioactivity level, and sea water near a sluice gate which the silt fence prevents from diffusing to the open sea, and so on. The radioactive liquid waste will be separated into freshwater components, sea water origin components such as sodium chloride, and solid components through evaporation and condensation, and then the freshwater and sea water origin components will be released into the environment such as the ocean after confirming its safety to discharge into the environment. The radioactive solid waste generated will be classified into waste of which the disposal is judged to be appropriate and waste deemed unsuitable from the viewpoint of current available technologies or existing legal system.
The solid waste includes soil, rubble, and forest in the on-site areas of Fukushima Daiichi Nuclear Power Station. In addition to these, the solid waste also includes the dredged sludge, the soil and rubble on the sea floor, the filters of cesium adsorption facility, and so on. According to its radioactivity level, form, and characteristic, radioactive solid waste is considered to be decontaminated by, for example, washing, blasting, or exfoliation, if necessary. Consequently, waste which does not need to be managed as radioactive waste will be dealt with as industrial waste and suitably processed and disposed of, and waste, of which the safety is confirmed and which is able to be reused, will be recycled. The waste which needs to be managed as radioactive waste will be classified into the waste of which the disposal is judged to be appropriate and the waste to be unsuitable, as described above in the solid waste generated from the liquid waste. The former waste will be reduced in its volume, stored, solidified by appropriate methods and then disposed of. The latter waste, which is judged to be unsuitable for disposal by the current available technologies or in the existing legal system, will be reduced in its volume and stored until the technology development and the new legal system are in place. The adsorption material (ferrosyanide) used in ALPS and the slurry generated in the pre-processing stage of ALPS are examples of such. Hence, it is necessary to develop the technology and prepare the legal system, which include the concept of disposal, the concept of waste form, and the criteria
of release from the legal control on land use of the repository site and so on, to enable future disposal of this waste through social consensus and agreement.
Furthermore, the kinds and radioactivity levels of waste are widely distributed. For example, incombustible, flame-resistant, and combustible wastes intermingle and most are difficult to separate out from each other. There are wastes containing not only plutonium and/or an anti-scattering agent but also oil and sea water components, and lead, PCB, and asbestos which require special consideration in processing and disposal. It is also necessary to measure their contents in the waste. It is well known that it is difficult to elucidate the physicochemical characteristics of radionuclides in sludge.
In addition to these, there is the problem of the huge quantity of the waste. The processing for effi reduction of the volume of intermingled waste is indispensable even for temporary storage. The quantity of the sea water components such as sodium chloride is also huge, and these components have to be separated. For example, the sea water components have to be separated from the sludge which is produced in the processing facility of high-level liquid waste, and simultaneously the radionuclides have to be separated from the sea water components. It is also essential and critical to solve the problem of how to reserve the many skilled workers for long-term restoration, considering their radiation exposure management, and so on.
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