TOKYO -- The outline of a plan for the decommissioning of the Fukushima Daiichi Nuclear Power Station has recommended that the extraction of nuclear fuel debris from the facility begin with the No. 2 reactor.
The plan covers the removal of debris created by nuclear fuel that has melted and become mixed with the surrounding structures at the No. 1 to 3 reactors, which suffered meltdowns in the aftermath of the March 2011 Great East Japan Earthquake and tsunamis.
The technical strategic plan was released on Aug. 8 by the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF), an organization founded to support Tokyo Electric Power Company Holdings' (TEPCO) decommissioning works.
"It would be appropriate for us to start with the No. 2 reactor," the report reads. Based on its contents, the government and TEPCO will amend their plans for decommissioning works by the end of this financial year, stating, "From 2021 we will begin extraction work from the No. 2 reactor."
Under the existing working schedule, debris extraction is planned to commence on either of the reactors in 2021 after methods of storage and other issues are decided by the end of fiscal 2019.
An NDF expert committee carried out technological studies to determine from which reactors debris will be extracted first. In February, a mass of pebble-shaped objects that are thought to be nuclear fuel debris was confirmed at the bottom of the No. 2 reactor's containment vessel. Experts then used a machine to successfully pick up and move the objects.
Additionally, the area around the containment vessel, which would become a working area, has relatively low radiation dosages. When considering the decommissioning of the entire power station, the No. 2 reactor was judged a better candidate to start from over the No. 1 and No. 3 reactors. The report concludes, "It (working on the No. 2 reactor first) will enable us to safely and reliably extract the debris, and allow us to obtain necessary information and experience for the subsequent expansion of work."
Due to the large amount of damage to the containment vessels, extraction by "submersion method" techniques that use water to shield radiation were considered difficult. For this reason, TEPCO and other bodies looked into "atmospheric method" debris extractions using implements such as a robotic arm to perform the removals.
As it is a method that has no precedent anywhere in the world, the decommissioning corporation was particularly careful to investigate that it would not have an effect on the nuclear reactor that was damaged in the original disaster.
It highlighted that it is desirable for the work to be done on the smallest scale possible, such as by using existing piping as opposed to opening up new holes to introduce extraction equipment for the first removals.
To obtain a greater understanding of the issues, TEPCO says it intends to conduct a test removal of a small amount of fuel debris using a robot arm and other implements in the second half of this fiscal year.
To prevent the reactor from going critical again, the report says the extracted debris should be placed in storage units to be newly established on the site of the power station, and placed in small containers. They can then be temporarily stored and cooled by atmospheric conditions under dry cask storage methods.
(Japanese original by Riki Iwama and Suzuko Araki, Science & Environment News Department)