TEPCO has begun work to change most of the main operational headquarters of the Fukushima Daiichi decontamination & decommissioning project, the seismic isolated building, from a controlled to an uncontrolled zone.
Doing so will reduce average levels of radiation from 1.6 microSv/hr to 0.7 microSv/hr.
The entire Fukushima site was named a controlled zone in the days after the March crisis, including the seismic isolated building, because of its surroundings and because of radioactive particles that floated into the building through windows.
Although the expert staff who returned to work in the building were protected by short-term measures such as the installation of lead panelling on stairwells, they could be subjected to more radiation than the standard legal limit of 100 mSv/year; by a special ministerial ordinance their limit was temporarily raised to 250 mSv/year for decontamination work. In fact, the annual exposure for workers performing 2000 hours of work indoors is expected to drop from 3.2 mSv to 1.4 mSv.
To achieve the required dose reductions inside the building, many actions were carried out. Leaded boards were stuck to internal walls, ceilings, floors and windows; air-conditioning ducts and filters were removed; contaminated roof concrete was removed, and whole-body contamination monitors were installed to prevent radioactive material being carried into the building.
In other news, construction work has begun on a 780m-long steel sheet pile wall that will block any further contaminated water leaks from reaching the ocean. The 600-pile wall will sit several metres outside of the existing seawall, and be filled with about 60,000 m3 of soil. Sheet piling is expected to last for two years.
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