Dealing with debris at Fukushima Daiichi 3

Friday, 28 September 2012

A steel beam slipped into the fuel pond of Fukushima Daiichi 3 during an operation to remove tonnes of debris. Engineers confirmed by inspection that no damage was caused.

The steel-frame section that sheltered the service floor of unit 3 was wrecked by a hydrogen explosion three days after the tsunami of March 2011 - leaving the fuel pond covered by debris including many twisted steel beams. Concrete dust in the pool has made the water very murky and the debris inside also inhibits the flow of water that cools the used nuclear fuel.

Over several days from 17 September, Tepco engineers used a large crane and set of hydraulic forks to grab, cut and lift away debris from the service floor. They eventually cleared the bulk of debris from the pond area.

Radiation levels varied as beams were removed and lifted out of the pond water, but without a pattern to indicate the work was disturbing contaminated dust.

The top of Fukushima Daiichi 3 seen from directly overhead in November 2011 (left) and this month (right) after debris removal. The purple rectangle indicates the fuel pond

Slipped beam

 
An incident occurred in the course of the work on 22 September when a truss about seven metres long and weighing about 470 kilograms slipped into the fuel pond during remote cutting and handling.

Tepco said that efforts to remove a truss near the pond had been halted when the contractor carrying out the work realised it was connected to other adjacent pieces of steel which could not be cut. Tepco noticed that the truss had moved into an unstable position, with half of it already the fuel pond. It was decided to lift the piece away, but the tips of hydraulic forks touched the piece and it slipped all the way into the pond.

An underwater video inspection on 25 September showed the beam lying on top of dust-covered fuel racks having caused no apparent damage.

Researched and written
by World Nuclear News