Plant operator Tepco has begun the process of changing the status 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 involve reducing 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 2011 crisis, including the seismic isolated building, because of the state of its surroundings and because of airborne ingress by radioactive particulate matter.

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 have been 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 required sevral measures. Leaded boards were fixed 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.

• 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 the existing seawall, and will be filled with about 60 000 m3 of soil. The sheet piling is expected to last for two years.