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A Step Toward Fuel Debris Removal: Robotic Arm Arrives at TEPCO’s Fukushima Daiichi Nuclear Power Plant

January 31, 2022

Akira Onoda: “The robot arm, which weighs 4.6 tons, is about 8 meters long when folded, but when a device is attached to the end, it can extend up to 22 meters. It can extend up to 22 meters.

On the morning of March 31, the robot arm was brought to Naraha Machi. The development of the arm in the UK was delayed for a year due to the new corona, but the final test will be conducted at the facility in Naraha Machi.

Akira Onoda: “The robotic arm will be placed in the upper part of the facility that mimics the interior of Unit 2 here, and will pass through a hole 55 centimeters in diameter to enter the pedestal where the debris is located.”

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First, the telescopic arm is extended to enter the containment vessel. It is assumed that the tip of the arm, which is equipped with a device for extraction, will be used to access the bottom where the debris is located. The final test will be conducted over a period of six months from mid-February using a full-scale model.

Tomoki Kamigaki, chief engineer of MHI’s Decommissioning Project Office: “One important thing is that the device works exactly as we intend it to, so we think it is important to make sure that both the device and the operating system are working properly.

The only thing we know about the fuel debris is that even in Unit 2, the most advanced reactor in the study, it is possible to grab and lift some of it. The only thing we know about the fuel debris is that it can be grabbed and lifted, even in the Unit 2 reactor, which is the most advanced.

Akira Ono, President of TEPCO’s Fukushima Daiichi Nuclear Decommissioning Promotion Company: “I think we will start with a small amount of fuel debris, maybe one or two grains. I think we will be able to understand what the fuel debris is through analysis. I think this is the first step.

TEPCO plans to start removing fuel debris from the Unit 2 reactor by the end of 2022.

Fukushima TV

https://news.yahoo.co.jp/articles/954f51c243fecd5b3ead67b60974e8c6b01a567c?fbclid=IwAR3SYoOWHj2oWTudnOcDXkcigzmdQkTjoU48XTTUMsYlxYJ–BD__wvYXpw

February 3, 2022 Posted by | Fuk 2022 | , , , | Leave a comment

Can reactor fuel debris be safely removed from Fukushima Daiichi?

January 25, 2022

Source: University of Helsinki

Decommissioning and clean-up are ongoing at the Fukushima Daiichi Nuclear Power Plant (FDNPP); however, many difficult problems remain unaddressed. Chief amongst these problems is the retrieval and management of fuel debris.

Decommissioning and clean-up are ongoing at the Fukushima Daiichi Nuclear Power Plant (FDNPP); however, many difficult problems remain unaddressed. Chief amongst these problems is the retrieval and management of fuel debris. Fuel debris is the name given to the solidified mixture of melted nuclear fuel and other materials that now lie at the base of each of the damaged reactors (reactor Units 1 — 3). This material is highly radioactive and it has potential to generate enough neutrons to trigger successive nuclear fission reactions (uranium-235 breaks into two elements after capturing neutrons, emitting enormous amounts of energy, radiation, and more neutrons). Successive fission reactions would present a serious safety and material management risk.

One of the materials in nuclear reactors that can lower the number of neutrons interacting with uranium-235 is boron carbide (B4C). This was used as the control rod material in the FDNPP reactors, and it may now remain within the fuel debris. If so, it may limit fission events within the fuel debris.

Can the fuel debris be safely removed?

On March 11th 2011, the control rods were inserted into the FDNPP reactors to stop the fission reactions immediately after the earthquake, but the later tsunami destroyed the reactor cooling systems. Fuel temperatures soon became high enough (>2000 °C) to cause reactor meltdowns. Currently, the fuel debris material from each reactor is cooled and stable; however, careful assessment of these materials, including not only their inventories of radioactive elements but as well their boron content, a neutron absorber, is needed to ascertain if successive fission reactions and associated neutron flux could occur in the fuel debris during its removal. Many important questions remain: was boron from the control rods lost at high temperature during the meltdown? If so, does enough boron remain in the fuel debris to limit successive fission reactions within this material? These questions must be answered to support safe decommissioning.

Study shows direct evidence of volatilization of control rods during the accident.

Despite the importance of this topic, the state and stability of the FDNPP control rod material has remained unknown until now. However, work just published in the Journal of Hazardous Materials now provides vital evidence that indicates that most of the control rod boron remains in at least two of the damaged FDNPP reactors (Units 2 and/or 3).

The study was an international effort involving scientists from Japan, Finland, France, and the USA. Dr. Satoshi Utsunomiya and graduate student Kazuki Fueda of Kyushu University led the study. Using electron microscopy and secondary ion mass spectrometry (SIMS), the team has been able to report the first-ever measurements of boron and lithium chemistry from radioactive Cs-rich microparticles (CsMPs). CsMPs formed inside FDNPP reactor units 2 and/or 3 during the meltdowns. These microscopic particles were then emitted into the environment, and the particles hold vital clues about the extent and types of meltdown processes. The team’s new results on boron-11/boron-10 isotopic ratios (~4.2) clearly indicate that most of the boron inside the CsMPs is derived from the FDNPP control rods and not from other sources (e.g., boron from the seawater that was used to cool the reactors). Dr Utsunomiya states that the presence of boron in the CsMPs “provides direct evidence of volatilization of the control rods, indicating that they were severely damaged during the meltdowns.”

Ample boron likely remains in the reactors, but more research is needed

In the study the team also combined their new data with past knowledge on CsMP emissions. From this, they have been able to estimate the total amount of boron released from the FDNPP reactors was likely very small: 0.024-62 g.

Prof. Gareth Law, a co-author from the University of Helsinki emphasized that this “is a tiny fraction of the reactor’s overall boron inventory, and this may mean that essentially all of the control rod boron remains inside the reactors.” The team hopes that this should prevent excessive fission reactions in the fuel debris. Utsunomiya stresses that “FDNPP decommissioning, and specifically fuel debris removal must be planned so that the extensive fission reactions do not occur. Our international team has successfully provided the first direct evidence of volatilization of B4C during the FDNPP meltdowns, but critically, our new data indicated that large quantities of boron, which adsorbs neutrons, likely remains within the fuel debris.”

Prof. Rod Ewing, a co-author from Stanford University acknowledged the importance of these new findings but highlighted that the team’s measurements now need to be “extended in follow-up studies, where the occurrence and distribution of boron species should be characterized across a wide range of debris fragments.”

Prof. emeritus Bernd Grambow, a study co-author from SUBATECH, Nantes, France,highlights that the work “paves the way for improving the safety assessment of debris retrieval during decommissioning at FDNPP,” with the team’s methods “providing a template for further studies.” Utsunomiya concludes that “it is nearly 11 years since the FDNPP disaster. In addition to tireless efforts from engineers at the FDNPP, scientific contributions are becoming more and more important as tools to address the major difficulties that will be faced during decommissioning.”

Journal Reference:

  1. Kazuki Fueda, Ryu Takami, Kenta Minomo, Kazuya Morooka, Kenji Horie, Mami Takehara, Shinya Yamasaki, Takumi Saito, Hiroyuki Shiotsu, Toshihiko Ohnuki, Gareth T.W. Law, Bernd Grambow, Rodney C. Ewing, Satoshi Utsunomiya. Volatilization of B4C control rods in Fukushima Daiichi nuclear reactors during meltdown: B–Li isotopic signatures in cesium-rich microparticles. Journal of Hazardous Materials, 2022; 428: 128214 DOI: 10.1016/j.jhazmat.2022.128214

University of Helsinki. “Can reactor fuel debris be safely removed from Fukushima Daiichi?.” ScienceDaily. ScienceDaily, 25 January 2022. www.sciencedaily.com/releases/2022/01/220125093041.htm.

January 27, 2022 Posted by | Fuk 2022 | , , , | Leave a comment

Robot for removing nuclear fuel debris at Fukushima Daiichi

19 janv. 2022

Footage of a robot developed to remove nuclear fuel debris from the No. 2 reactor at the Fukushima Daiichi nuclear power plant is released.

January 20, 2022 Posted by | Fuk 2022 | , , , | Leave a comment

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.

jhhklm.jpgShown 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.

n-fukushima-a-20170706-870x328.jpgA 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

 

 

July 10, 2017 Posted by | Fukushima 2017 | , , , | Leave a comment