Dry Removal Plan Confirmed For Fukushima Daiichi

A state-backed entity is expected to soon compile a plan for decommissioning the crisis-hit Fukushima nuclear power plant, unveiling how to extract fuel debris from three reactors for the first time, sources close to the matter said Tuesday.

The Nuclear Damage Compensation and Decommissioning Facilitation entity was established after the Fukushima crisis, the worst nuclear disaster since Chernobyl, to help the utility pay damages for the calamity. The state-backed entity holds a majority stake in Tepco.

The Nuclear Damage Compensation and Decommissioning Facilitation Corp., tasked with providing technical support for decommissioning the complex, has confirmed they will likely use a method to attempt to remove melted nuclear fuel that does not include flooding the containment structures.

Until now, the NDF had considered employing the submersion fuel retrieval method — filling the containment vessels with water — alongside the dry method. In the submersion method, water shields plant workers from radiation. The three units at Fukushima Daiichi all suffered full meltdowns and failure of the reactor vessels. This meant they would have to determine a way to flood the containment structures. This concept was found to have multiple challenges including preventing criticalities in the fuel debris, preventing leaks of highly contaminated water and preventing a structural failure of the then flooded containment.

A method to fulfill reactor containers with water first is effective in blocking radiation from spreading but the entity decided not to adopt the approach as the three reactor containers are believed to have been damaged and water would likely leak.

The NDF, however, determined that repairing all damaged areas of the containment vessels in order to be able to fill the reactor wells to the top with water would be too difficult. Instead, for the time being, the NDF decided to prioritize dry removal of the nuclear fuel debris using robotic arms.

“It isn’t that we’ve decided to completely do away with the submersion method, but we have to think about how best to distribute the technological resources we have,” said one source closely involved with the NDF.

When using the dry nuclear fuel retrieval method, it is crucial to implement measures to prevent microscopic radioactive substances from spreading in the air. To counter this, the NDF is considering spraying water on the fuel as robotic arms are used to sever and retrieve the fuel debris.

Shown in red is melted nuclear fuel, or nuclear debris. Shown in black is a drill or laser for scraping off the debris.

The damage differs from reactor to reactor at the Fukushima No. 1 plant. Probes of the reactor interiors conducted by TEPCO have yet to directly observe the nuclear fuel, meaning that the shape and distribution of the debris remain unknown. Fuel removal methods specific to the state of each reactor must be decided before moving ahead.

A series of photos taken on Jan. 30 shows the inside of the Fukushima No. 1 nuclear plant’s reactor 2 pressure vessel. A specific method for removing debris is set to be revealed soon.

In the No. 1 reactor, much of the nuclear fuel is believed to have melted through the pressure vessel onto the floor of the reactor containment vessel. Inserting a robotic arm through the side of the containment vessel to remove the melted fuel is under primary consideration to deal with this situation.

It means the debris inside the Nos. 1 to 3 reactors at the crippled Fukushima Daiichi complex is likely to be shaved off gradually with a drill or laser equipment, while pouring water shower under a remotely controlled operation, the sources said.

Under the method the entity currently envisions, some part of debris would remain in the air during the operation so a major challenge facing the debris extraction work is how to shield radiation and prevent debris from flying off.

While debris in the reactors has yet to be directly confirmed and information on the exact locations and conditions is limited, the extraction work, the most difficult part of the decommission project, is expected to proceed in stages from the side of the bottom part of each reactor container while ensuring safety measures, the sources said.

Based on the decommission plan to be compiled by the entity, the government and the plant operator Tokyo Electric Power Company Holdings Inc. are expected to determine a debris extraction scheme for each reactor building this summer and possibly review a road map for decommissioning the complex as well, the sources added.

The various parties involved in decommissioning research are required to publish a clear plan for removing the melted fuel by the summer of 2017. To publish such a plan necessitates admissions of the conditions within the three reactor units. Admitting that only a dry method of fuel removal will be proposed is a major departure from Tepco 2011 claims that only partial meltdown took place, and tells much about the extent of the damage within the units even as they have been unable to identify the location of any of the fuel.

july 5 2017 fuel debris removal 2

* Estimated that most fuel melted and almost no fuel rod remains based on the muon measurement, analysis result, and the fact that a water level is not formed

* Estimated almost no heat source remained in core region from the fact that sub-cooling conditions were achieved before starting CS injection (12/10/2011)

The current road map calls for completion of a plan on how to extract debris from each reactor this summer and finalizing a detailed method for at least one of the three units in the first half of fiscal 2018 to begin the extraction operation in 2021.

Sources:
https://english.kyodonews.net/news/2017/07/ddee4ff9fec7-debris-extraction-method-at-fukushima-nuclear-plant-to-be-revealed.html

https://mainichi.jp/english/articles/20170705/p2a/00m/0na/008000c

http://www.japantimes.co.jp/news/2017/07/05/national/group-mulls-fukushima-no-1-melted-fuel-debris-extraction-without-filling-containment-vessels-water