The major clean-up program of contaminated areas in Japan has progressed considerably. The levels of contamination have decreased csignificantly, which makes it possible to envisage a gradual return of evacuated areas inhabitants. However, in some areas, such as forests, decontamination is much more difficult and slower to implement.
One distinguishes for the clean-up : Special decontamination areas where people have been evacuated and where the government, responsible of decontamination, chooses the companies responsible for the work; Extensive surveillance zones whose populations have not been evacuated but where a decontamination is necessary and the communities are responsible for decontamination under the supervision of the Ministry of the Environment.
The Special Decontamination Zone covers 11 communes: the cleaning design is actively engaged for 10 of them. Places with an annual exposure greater than 50 millisieverts per year would be declared uninhabitable. Where this yearly exposure is below 50 millisieverts per year, decontamination operations are designed to bring down this level below 20 millisieverts and allow residents to return. Where the exposure dose is between 1 and 20 millisieverts per year, the next return of populations is expected.
The Extensive Surveillance Zone includes 100 communes, whose population has not been evacuated. The clean-up iomplementation of sanitation is planned to last up to 5 years in the Fukushima Prefecture. The initial principle was to take into account ground contamination levels, irrespective of the time of on-site presence. Now decontamination is based on the effective dose accumulated by the public. For instance, in forests where people do not live permanently, decontamination can be less rigourous than in the agricultural lands.
The Japanese National Safety Authority (NRA) has demonstrated that the risk of exposure due to radiation in non-evacuated areas is low. The doses actually measured by personal dosimeters are 2.6 to 7 times lower than the estimates based on ground and air measurements.
Radioactive waste, dumped on sites near decontamination areas must be stored for the long term in a safer manner. In December 2013, the Japanese State took over the creation of a storage site for these residues, with an area of 3 to 5 square kilometers. It is planned to store the soil, leaves, radioactive grasses collected there for a period of about 30 years. A sufficiently thick protective earth cover will absorb gamma rays and protects the vegetation. The important thing is to avoid a circulation of water.
In the fields, public and private gardens, and around schools with children, the priority was to scrape the top layer of soil about 5 cm deep. 75 to 97% of soil radioactivity have been eliminated. As a result: 22 million m3 of contaminated soil have since been stored on dedicated sites, collected in so-called big bags. The Japanese authorities are looking for the best technologies to get rid of their radioactivity, mainly due to cesium-137. Nine projects were selected in 2017 for testing.
An example of the decontamination actions undertaken by the municipalities of the Fukushima Prefecture was the use of bulldozers to scrape and remove contaminated soil around schools and schoolyards. The aim was to reduce the annual exposure of schoolchildren below the legal limit of 1 millisievert / year. By October 2013, 400 school yards had been properly decontaminated.
First returns. How many people one could have avoided to evacuate?
Among the 11 communes in the Special Decontamination Zone, the first clean-up to be completed was in Tamura, the first municipality where the evacuation order was lifted. The return of the inhabitants was authorized from the beginning of April 2014.
In September 2015, the Japanese authorities invited the inhabitants of Narana to return to their city. This small town, 20 km from the plant, was completely evacuated in 2011 and was the first to allow the permanent return of its population after several years of decontamination. According to a BBC report, only 46% of residents expected to return.
In an interesting interview given in March 2016 in Les Echos, M.Jacques Répussard, director of the IRSN (french Istitute of Radioprotection and Nuclar Safety), questions the social and economic cost of evacuations, sometimes based on an overestimation of risks: “Researchers at the Tokyo University equipped with dosimeters workers who used to go to an evacuated area during the day. To their surprise, their dose was found to be one-third to one-half lower than expected the dose limit which had motivated the evacuation of the area. This is due to the fact that experts are trained to take safety margins to be certain to properly prevent risks "..
Recognizing the high economic and social cost of evacuations, and the many lives shortened by uprooting of their environment, M.Repussard recommends not to evacuate as a precaution, but on the basis of a realistic estimate of the risk: "There were 200,000 evacuees , it may have been necessary to permanently evacuate only half ... ".