The damage to the Fukushima Dai-ichi nuclear power plant (NPP) by the Great East Japan earthquake and tsunamiresulted in considerable radioactive contamination, both on- and off-site. After decay of shorter-lived nuclides, the contamination is now dominated by radio-caesium (134, 137Cs), which is the focus for clean-up actions. Caesium tends to bind strongly to soil, especially clays. Dose rates are generally low, except at a few locations, and result predominantly from external gamma irradiation. These dose rates are decreasing with time as a result of washoff and soil mixing.
After the accident, a staged clean-up of contaminated areas was initiated first in populated areas (especially sensitive areas such as schools and playgrounds) and then, following stabilisation of the reactors and decay of shorter-lived nuclides, extending into evacuated zones.
The overarching “Special Measures” laws to manage radioactive contamination from this incident and establish an overall policy for decontamination were promulgated on 30th August and 11th November 2011. These specified responsibility for conducting a range of decontamination pilot projects to examine applicability of clean-up technologies to the higher levels of contamination within the evacuated zone. Based on such projects, the Government will develop the technical basis for efficient and effective clean-up technologies, assuring worker safety, establishing a regional remediation plan and advancing to stepwise implementation of decontamination, with a special focus on reducing dose rates andallowing evacuees to return to re-establish their normal lifestyles as quickly as possible.
The Japan Atomic Energy Agency (JAEA) was chosen by the Government to conduct decontamination pilot projects at model sites. The first project included two residential sites with lower contamination levels, and ran from August 2011 until March 2012. The second project was implemented from September 2011 until June 2012 at 16 sites in 11 municipalities, including highly contaminated sites in evacuated zones. Despite tight constraints in terms of timescale and resources, the decontamination pilot projectsprovided a good basis for developing recommendations on how to assure clean-up efficiency and worker safety and reduce time, cost, subsequent waste management and environmental impacts.
The regional decontamination presently being initiated must be well planned, rigorously implemented and clearly presented to all stakeholders. However, decontamination on such a large regional scale in a highly populated region has never been attempted before. The main challenges to implementing full-scale decontamination are lack of both real-world examples and also experience in planning and implementing decontamination procedures appropriate to Japanese conditions. Therefore, the decontamination pilot projects are playing a key rolein support of the drafting of guidelines and manuals that can be used as a source of reference by the national government, local municipalities and the contractors performing regional decontamination. This paper discusses this application of the decontamination pilot projects, focusing on those carried out in theevacuated zones.