The effort to decontaminate the area around Fukushima Daiichi is gathering pace with the goal of enabling the return of residents evacuated some six months ago.
While steady progress at the Fukushima Daiichi site has secured its basic safety after the triple meltdown that followed the natural disasters of 11 March, a huge job of decontamination awaits Japanese authorities. A total of 80,000 people were evacuated from their homes during the first week of the accident sequence.
Most of these were from the 20 kilometre radius of compulsory evacuation, while more people left the next 10 kilometre band where official instructions were to prepare for evacuation should the accident worsen. About another 20,000 in a separate sector extending about 50 kilometres to the northwest were recommended to evacuate because radioactivity deposited there was leading to dose rates of over 20 millisieverts per year.
Government decontamination guidelines noted that the screening of 220,000 people has shown "no case of adverse health effects." Nevertheless, some ¥78.2 billion ($1.01 billion) has been allocated for a health fund related to the nuclear accident. This will provide for "detailed health checks for evacuees and ultrasound thyroid examinations for children in the "mid- to long-term."
This compares to a global average background dose rate of 2.4 millisieverts per year from natural sources to which all people are exposed with no apparent ill effects. The figure includes places with far higher doses from natural background, such as igneous rock areas where dose rates may be over 10 millisieverts per year, as well as the highest known area, Ramsar in Iran, where doses are 250 millisieverts per year - again with no measurable increase in cancer rates.
Having conducted aerial and ground surveys of radiation levels, the Japanese authorities want to focus first on the locales with the highest dose rates and reduce these to a maximum of 20 millisieverts per year. The goal for areas already below that is to reduce the dose rate to 1 millisievert per year "as soon as possible, eventually undercutting that figure."
With a huge and varied task covering a very wide area, Japan wants to prioritise decontamination on homes, schools and places children play. The pace and scale of this work is increasing and the plan is to lift the 'evacuation-prepared' order at once when all municipalities are ready. A team of experts has been organised by the International Atomic Energy Agency to liaise with Japanese planners. Between 7 and 15 October it will provide assistance and review the Japanese strategy before reporting back to the international community.
Drains, gutters and ditches will be cleared in people's homes, with some topsoil to be removed in some cases. High-pressure hoses will then be used to wash down the area with "a massive amount of water" in order to dilute and flush away contaminants.
Weeds and trees will be pruned back in gardens, streets, parks and forests, but consideration will be given to the ecosystem before forest leaf mould is removed. A survey in May "did not detect any radioactive materials from river water" therefore no decontamination of rivers is planned.
Roadways will be pressure-washed, although resurfacing is an option as is the removal of the current tarmac for disposal. Play areas are the priority for schools and surface soil is being removed from many places. However, authorities are concerned about the potential build-up of huge volumes of lightly contaminated material.
Contaminated soil and detritus will be kept dry and isolated from uncontaminated ground using plastic sheeting. Much of this could be incinerated to reduce the volume of waste for disposal. Dose rates in the storage areas can be controlled with layers of soil or concrete: five centimetres is enough to reduce dose rates by over half, while 15 centimetres reduces them by nearly 90%, according to government guidelines.
Dose rates for decontamination workers should be limited to 20 millisieverts per year - the same dose rate as allowed for nuclear workers.
Among the more difficult areas will be farmland, where topsoil could be removed from fields before they are ploughed. However, some farmers have already ploughed their land, and the role of farmland in the ecosystem is complex. Decisions on how to tackle farmland are to follow after more detailed study. Another problem could come from brick-built environments where contamination may lie in the porous surfaces that are difficult to clean.
The government guidelines did not put forward a timeline for the clean-up project, and noted that some places might need repeated decontamination as radioactive particles are displaced from other locations.
Researched and written
by World Nuclear News