Fukushima units enter decommissioning phase

Wednesday, 21 December 2011

A roadmap setting out the mid- to long-term activities needed for the decommissioning of the damaged Fukushima Daiichi reactors has been released. The plan envisages decommissioning activities being completed within 30 to 40 years.

 

The roadmap - drafted by Tokyo Electric Power Company (Tepco); the Ministry of Economy, Trade and Industry's Agency for Natural Resources and Energy; and the Nuclear and Industrial Safety Agency - received government approval today.

 

Japanese prime minister Yoshihiko Noda announced on 16 December that the Fukushima Daiichi reactors had attained 'cold shutdown' conditions bringing to a close the 'accident' phase of events at the plant triggered by the 11 March tsunami. In announcing the decommissioning roadmap, Tepco said that it will now "shift our approach from the stabilization of the plant to the maintenance of stable condition."

 

The roadmap is split into three phases, the primary targets of which are: the removal of fuel from all four used fuel pools; the removal of melted fuel from the three damaged reactor cores; and finally the demolition of the reactor facilities. Work to carry out these tasks will be hampered by the amount of debris and contamination present on the site, not least that on top of units 1, 3 and 4.

 

 

Debris will need to be removed from atop the reactor buildings to allow decommissioning activities to proceed (Image: Tepco)

 

 

Tepco said that it plans to start removing the fuel from the used fuel pool at the top of the reactor building of unit 4 within the next two years, while the removal of fuel from the pool at unit 3 will begin by the end of 2014. At unit 1, the company plans to develop a used fuel removal plan based on its experience at units 3 and 4, while for unit 2 it will develop a plan once the inside of the reactor building has been decontaminated and the condition of existing used fuel handling equipment has been assessed.

 

Tepco plans to remove all of the used fuel from the four pools within ten years, during which time it will determine what reprocessing and storage methods it will use to deal with it.

 

With regards to fuel within the reactors themselves, its exact location and condition is still not known, but an earlier analysis suggested that most of unit 1's fuel melted and went through the bottom of the reactor vessel, as well as about 70 centimetres of the drywell concrete below. The cores of units 2 and 3 are thought to have overheated badly, with a large portion having melted or softened enough to slump to the bottom of the reactor vessel. A relatively small amount of this is thought to have passed through holes in the pressure vessel and fallen to the drywell floor. No fuel was in the core of unit 4 at the time of the accident.

 

Tepco plans to begin an investigation by the end of March 2015 into where water is leaking from the primary containment vessels (PCVs) of units 1, 2 and 3, as well as beginning work to decontaminate the interior of the reactor buildings. Within about a year, the company aims to have sealed those leaks found in the lower parts of the PCVs, allowing it to partly fill the vessels. This would then allow an investigation of the interior of the PCVs to be conducted. After this, repairs will be made to the leaks in the upper parts of the PCVs, allowing them to subsequently be filled with water. Once covers have been placed over the reactor buildings, the caps of the reactor pressure vessels (RPVs) will be removed so that examination of their contents can start around mid-2019. The removal of the damaged fuel is expected to start in about ten years' time, but Tepco estimates that this will take 10-15 years to complete. Demolition of the reactor facilities of units 1 to 4 is expected to be completed within 30 to 40 years.

 

In the meantime, Tepco said that in order to "stably maintain a condition equivalent to cold shutdown," it will continue injecting water into the reactors until all the molten fuel has been removed. New facilities for processing contaminated water will be installed by 2012. The processing of all the accumulated water in the reactor and turbine buildings is expected to be completed within ten years. Installation of water shielding walls to prevent possibly contaminated groundwater entering the ocean should be completed by late 2014.

 

"Step-by-step decontamination measures" would be implemented on-site in order to reduce exposure to the public and workers, Tepco said. These would be taken in conjunction with efforts to reduce radiation dosage outside the site. The company aims to maintain the effective radiation dose at the site boundary to below 1 millisievert per year by April 2012.

 

Tepco said that it would establish the necessary frameworks for on-site project operations and research and development activities to "ensure steady implementation" of the roadmap. The company noted, "As we are facing many difficult research and development issues that are unprecedented and challenging even from a global perspective, we will work hand-in-hand with our domestic and overseas supporters, and compile wisdom and knowledge from all over the world as we move forward." Tepco said that it would regularly assess the progress being made at the Fukushima Daiichi site and would update the roadmap or change its activities accordingly.

 

Researched and written

by World Nuclear News