TEPCO revealed some new plans in the long term effort to find the melted nuclear fuel at Fukushima Daiichi.
IRID, the international research body tasked with developing technology to deal with the Fukushima disaster, shared new plans for this work.
Research work to estimate the composition of fuel debris and to what extent it dissolves or can be dissolved is underway. Efforts to understand the thermal behavior of fuel debris is being conducted with unit 2 as the current research candidate. Both projects will help refine plans for future fuel debris removal.
IRID has established three options to investigate inside the reactor pressure vessel (RPV). One would drill through the top of the containment vessel and RPV from the refueling floor level to look down into the location where the fuel core used to be. The second option would drill at an angle into the RPV to look at the same area. Unit 2 already has a platform in place that the angle drilling could be done from. The third option would be to use the access points already in place from other investigation work and attempt to reach up into the RPV from below, through the pedestal region. Each option has challenges. A final candidate has not been selected. It is most likely this will be attempted at unit 2 first.
Plans for actual fuel removal have focused on a design that uses a mechanical arm to retrieve the fuel and a small transport vehicle with a shielded container. IRID has been working on designs, prototypes and testing.
The research plan appears to be focused on unit 2 due to the mention of the X-6 port. This port has already been used for investigations in unit 2 but has not been a viable option for units 1 or 3.
The translated diagram from IRID shows some of the research work underway. A pair of neutron monitors are planned. Other research reports mention this will be used to detect criticalities if they occur while they move fuel debris. A variety of fuel cutting and collecting ends have been planned that would be mounted on the remote access arm. The remote transport vehicle will remove shielded canisters of fuel debris from the hot cell installed at the X-6 port. These basic concepts have been around for a few years. This newer reporting confirms that this has become the main plan for removing fuel debris. The plans have also seen more refinement as documented in this report from IRID.
A machine translated version of the IRID report can be found here:
The original is the last 3 pages in this larger report from TEPCO
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