Tepco's latest step in decommissioning Fukushima Daiichi has been to remove unit 4's containment vessel head from storage on the service floor. The lift is a milestone in a plan to remove nuclear fuel from the building.
The 52 tonne dome was lowered down to ground level by two crews during a 15 minute operation on 10 August.
During normal operation the containment vessel head is bolted in place as part of an airtight boundary around the reactor system's major components, but the tsunami of 11 March 2011 happened to occur during a maintenance outage at unit 4. To enable work to take place inside the reactor vessel, the containment head had been removed and placed on the service floor and the core load of fuel moved to the cooling pond.
A few days into the accident sequence, the reactor building was wrecked by a hydrogen explosion and the subsequent clearing of debris has now left the service floor as essentially the roof of the building. Tepco's plan is to build an airtight secondary structure alongside and over the top of unit 4 to enable engineers to safely remove nuclear fuel from the pond, but to do this the service floor has to be completely cleared. Lifting away the vessel head represents a major milestone in the project.
|The yellow containment vessel head is visible in its normal position
The relatively large amount of nuclear fuel in the pond has made unit 4 one of the top priorities for Tepco in assuring overall safety of the site. The pond has been covered with a steel platform, while a range of analyses all indicate that the fuel inside is largely undamaged, although the pond contains a lot of dust and debris.
Researched and written
by World Nuclear News