The Decommissioning Facilitation Corporation is reconsidering the approach to be used for removing melted fuel at Fukushima Daiichi. The early established plan was to try to fill the containment structures with water and retrieve the melted fuel with robotic equipment through the top of the reactor via the reactor well.
Last year IRID (International Research Institute for Nuclear Decommissioning) questioned this approach and urged that an alternative option be established in case the flooding plan would not work. IRID’s ongoing work has included multiple plan options. The description from the Decommissioning Corporation indicates they are considering “Plan D”. This is the one IRID describes as the side entry, non flooding approach.
The NHK article does not explicitly say why the Decommissioning Corporation made this announcement now. It does mention that they assume the melted fuel inside unit 1 has fallen to the bottom of “the vessel” but do not clarify if they mean the reactor vessel or containment vessel. Muon scans have been underway for the last month and were expected to have results around March 11th. So far no public announcements about the muon scans have been made.
Based on the information from IRID, plan d would be more likely to be used if the fuel was located outside of the reactor vessel. Plan d would resolve many of the challenges involved in the flooding plan but would have a new set of big challenges. Robotic equipment that can work in close proximity to melted fuel will be required. Such high working levels of radiation and contamination could make doing maintenance on any such equipment difficult. Sufficient shielding and containment for this option will also present challenges.
IRID’s most recent report:
IRID research progress 140914
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