Radiation still too high in reactor#2 building

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July 2, 2018
A robotic probe has found that radiation levels remain too high for humans to work inside one of the reactor buildings at the damaged Fukushima Daiichi nuclear power plant.
 
Tokyo Electric Power Company, the operator of the plant, plans to relocate 615 units of nuclear fuel from the spent fuel pool, which is located on the top floor of the No. 2 reactor building and is separate from the reactor itself.
 
TEPCO says the relocation will help reduce risks, including possible damage caused by earthquakes.
 
The No. 2 reactor underwent a meltdown, but did not experience a hydrogen explosion in the 2011 nuclear accident. The building is likely to still have a high concentration of radioactive materials.
 
Last month, TEPCO drilled a hole in the wall of the building in order to use a camera-equipped robot to create a detailed map of the radiation on the top floor.
 
On Monday, workers started the survey and measured radiation levels at 19 points, mainly near the opening. Up to 59 millisieverts were detected per hour.
 
That’s above workers’ allowable annual exposure of 50 millisieverts and more than half of their 5-year exposure limit. TEPCO has concluded it cannot let humans work inside the building.
 
TEPCO will use the results to determine specific ways to remove the fuel from the pool. It plans to start the work in fiscal 2023.

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TEPCO to gauge radiation in reactor N°2 building

 

June 21, 2018
The operator of the crippled Fukushima Daiichi nuclear power plant intends to send a robot into the No.2 reactor building as early as next week to measure interior radiation levels in detail.
It is a key step toward removing all 615 nuclear fuel rod units that remain in a storage pool in the building, and eventually decommissioning the reactor.
The pool is located on the top floor of the building. The No.2 reactor experienced a meltdown after the major earthquake and tsunami that hit eastern Japan in 2011.
Tokyo Electric Power Company, or TEPCO, plans to transfer the fuel units to reduce the risks posed by possible earthquakes and other factors.
TEPCO needs to map radiation levels and other detailed conditions inside the building before retrieving the fuel units.
The utility on Thursday finished breaching a wall of the building to allow entry to a robot and heavy machinery. Work on the 5-meter wide and 7-meter high hole started last month.
TEPCO plans to send a robot fitted with a camera and a radiation measurement device through the opening as early as next week.
And TEPCO could start removing the fuel around fiscal 2023 based upon the survey results.
TEPCO also seeks to begin retrieving nuclear fuel from the No.1 reactor around fiscal 2023 and from the No.3 reactor as soon as this autumn. Both reactors had a meltdown following the natural disaster.

 

TEPCO prepares to remove fuel from damaged reactor

 

May 28, 2018
The operator of the damaged Fukushima Daiichi nuclear power plant has started laying the groundwork to retrieve fuel from one of the plant’s reactor buildings. It’s a crucial step toward scrapping the complex.
 
Tokyo Electric Power Company began the work on Monday to move 615 fuel rod units from a storage pool on the top floor of the No. 2 reactor to a more secure location.
 
The No. 2 reactor is one of 3 at the plant that melted down in the 2011 accident. Radiation levels inside the reactor building remain high.
 
TEPCO will open a hole measuring 5 by 7 meters in the building’s wall and send a robot through it to measure radiation levels inside.
 
A section of the wall will be divided into 29 blocks, each with a handle to facilitate its removal.
 
From a control room some distance from the reactor building, TEPCO officials will remotely operate a machine to remove the blocks.
 
The work is expected to continue until mid-June.
 
The plant operator will measure radiation levels before deciding how to retrieve the fuel rods. The company plans to start retrieving the fuel in fiscal 2023.
 
TEPCO official Hiroshi Noda says that although the decommissioning work for the No. 2 reactor has just started, it’s a major step forward.
 
He says the company will make sure that the work will have no impact on the environment.

New Data for Unit 2’s Missing Fuel

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TEPCO published a Roadmap document right before leaving for Golden Week vacation. In this document is a 30+ page section of new data for unit 2’s missing fuel.
 
TEPCO has given varying explanations for unit 2’s meltdown and fuel location. Two muon scans have been completed for unit 2. The first found no fuel remaining in the RPV. A second scan by TEPCO claimed to have found some fuel in the bottom of the RPV, our analysis of the scan found otherwise. It is likely that all of the fuel inside the reactor vessel melted and all of it except for some residues is no longer in the RPV.
 
Fuel debris volume:
The volume of fuel debris inside unit 2 is difficult to calculate due to a number of factors. The debris is spread between multiple areas including the floor grate level, the pedestal floor and whatever debris may have burned down into the pedestal floor. The total volume of the fuel core is known for unit 2 but the exact size of the pedestal diameter is not known.
 
A fuel debris volume estimate was made for unit 1 based on known data and meltdown events at that reactor. Unit 1 is smaller than unit 2 in both fuel core size and size of the reactor structures. The general reactor building sizes and the fuel core sizes should be something that could roughly scale up for unit 2. Unit 1 estimate showed a fuel volume of all of the fuel and related melted structural materials as 60-100 cm deep.
 
Inside unit 2 about 50% of the pedestal floor was found to be covered with 70 cm of fuel debris. Additional fuel debris in an unknown volume is on the floor grate level. An unknown amount is burned down into the pedestal concrete basemat. Further fuel debris may be in lower reactor piping systems or the outer drywell floor. Unit 2’s fuel debris volume would also be reduced as the control rod drive array and bottom head of the reactor vessel are still intact. That large amount of metal structural material is known to not be part of the melted fuel debris in unit 2.
 
What has been found on inspection may be all of the fuel debris for unit 2 if a portion of the material is burned down into the pedestal basemat concrete. In most meltdown scenarios that is a given assumption unless the containment structure was heavily and repeatedly flooded with water at the time the fuel first dropped into the pedestal. With unit 2 that is an unlikely scenario.
 
There is an alternative possibility that a large amount of the radioactive materials in the fuel vaporized during the meltdown and escaped containment. This concept requires more investigation to confirm vaporization but this possibility for unit 2 is not completely ruled out. Fused microparticles containing nuclear fuel and other meltdown related materials have been found over a wide swath of Fukushima and beyond. Unit 2’s refueling floor blow out panel and reactor well containment gasket are one escape path for micro materials, steam and other gasses. Unit 2’s venting attempts are another concern. TEPCO has claimed the direct drywell venting of unit 2 didn’t work and the rupture disc for this system did not break as intended. TEPCO has provided no conclusive proof of this claim such as photos, video or other tangible evidence for this claim. Due to this, there is still the possibility that unit 2’s venting released some of these fused microparticles of fuel.
 
Radiation levels:
The radiation levels found in unit 2’s pedestal including a reading close to the fuel debris pile were between 7-8 Sieverts/hour. The high reading found along the CRD rail in 2017 was between 200-300 Sieverts/hour. These pedestal readings are drastically lower than what would be expected near an unshielded large pile of fuel debris.
By comparison, radiation levels along the outer containment wall in 2012 were within a similar range of the lower readings found on the CRD rail in 2017.
 
The elephant’s foot at Chernobyl, measured within the first year of the disaster, converted to Sieverts was 100 Sievert/hour.
 
Underwater readings in unit 1’s torus room near what is suspected fuel debris, taken in 2012 were 100,000 to 1 million Sieverts/hour.
 
Radiation levels near the fuel debris indicate that the top layers of debris may be mostly metallic materials with little fuel.
 
Radiation levels indicate that fuel bearing debris is not in the visible layer in the pedestal. (other possible locations – vaporized/vented, beneath the metallic layer, sml amounts in piping).

 

Melted fuel Deposits in Fukushima N°2 Reactor

TEPCO footage shows deposits inside damaged Fukushima reactor
TOKYO — Tokyo Electric Power Co. (TEPCO) on April 26 released footage taken inside the containment vessel of the No. 2 reactor at the disaster-hit Fukushima No. 1 Nuclear Power Plant, showing spots where molten fuel appears to have fallen through the reactor pressure vessel.
Photos and video footage show deposits — believed to be fuel debris — at the bottom of the containment vessel. In two spots, the debris is piled higher than in other places. TEPCO officials said they suspect that the bottom of the pressure vessel, situated at the upper part of the containment vessel, was damaged in more than one place, which allowed melted fuel to fall into the containment vessel.
TEPCO captured the images in January using a camera attached to a pipe inserted into the containment vessel, and found deposits spread across the bottom of the vessel. The nuclear plant operator will use a robot arm to analyze the containment vessel in more detail.
(Japanese original by Ei Okada and Riki Iwama, Science & Environment News Department)
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Full extent of melted fuel in Fukushima No. 2 reactor revealed
Inside the bottom of the containment vessel in the No. 2 reactor of the Fukushima No. 1 nuclear power plant in January, revealed fully by new image processing.
The bottom of the inside of Fukushima No. 1 nuclear plant’s crippled No. 2 reactor has been revealed in a much clearer and wider range in footage released by plant operator Tokyo Electric Power Co. on April 26.
The film shows the clearest pictures yet inside the containment vessel just below the pressure vessel of the nuclear reactor, which went into meltdown due to the 2011 earthquake and tsunami disaster.
Melted nuclear fuel debris is seen attached to pillars, walls and the ceiling, and accumulations between approximately 40 and 70 centimeters thick are piled up and cover the whole floor.
TEPCO captured the footage on Jan. 19 by attaching a remote-controlled camera to an extendable rod with a span of 16 meters into the containment vessel from an opening in its side.
Excerpts were released at the time, but new processing of the footage has revealed a much clearer picture.
In the bottom of the containment vessel, fuel debris has fused to some areas particularly thickly. It is possible that the bottom of the reactor has several holes that caused the debris to fall and solidify as it cooled.
The improved knowledge of the nuclear reactor’s state will help to calculate an estimate of the amount of the debris inside, and suggest at how it could be removed in the future. TEPCO hopes to start its next investigation inside the reactor within this fiscal year.

Fresh analysis of Fukushima Daiichi ‘fuel debris’

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April 26, 2018
A fresh video analysis by the operator of the Fukushima Daiichi nuclear power plant shows that molten fuel at the No.2 reactor may have fallen along several paths. The unit is one of the 3 reactors that experienced a meltdown in 2011.
 
Tokyo Electric Power Company processed images that were taken inside the reactor’s containment vessel in January. The firm pieced them together so that the resulting footage can better show conditions within the entire vessel.
 
The footage shows all of the vessel’s bottom covered with what looks like pebbles and clay. A roughly 70-centimeter-high pile of such materials is located near where part of the fuel casing was spotted in the January probe.
 
Another heap is close to a pillar-like structure at the bottom.
 
The utility says the materials may be fuel debris, which is a mixture of molten nuclear fuel and structural parts.
 
The firm also released a 3-dimensional video that reconstructs the interior of the containment vessel of the No.3 reactor. The unit also suffered from a meltdown.
 
The footage shows a mound of materials near the center of the vessel’s bottom. The company says it may have emerged after fuel debris fell onto structural parts that had already dropped there during the 2011 accident.
 
The utility says the new images provide clues to determine the paths along which molten fuel fell. It plans to decide in the next fiscal year how to remove the fuel debris.
 
The company is seeking to remove the molten fuel as part of its effort to decommission the plant.

 

First samples of Fukushima plant nuclear fuel debris to be collected in FY 2019

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March 16, 2018
The government and Tokyo Electric Power Co. (TEPCO) are set to extract a small sample of melted nuclear fuel from the bottom of the No. 2 reactor’s containment vessel at the disaster-stricken Fukushima No. 1 nuclear plant as early as fiscal 2019.
 
The operation will be a test before starting full-scale collection of the fuel, targeted for 2021 or earlier. If development of technologies for debris retrieval shows promise, the operation may be moved up to the end of fiscal 2018. The government and TEPCO hope to ascertain the properties of the melted fuel and use the information for developing collection devices and debris containers.
 
This will be the first attempt to sample nuclear fuel debris from a reactor. Other materials, including those floating in contaminated water and substances stuck to robot probes, have been extracted from the plant’s reactors before. The No. 1, 2 and 3 reactors at the Fukushima No. 1 plant melted down in the March 2011 nuclear disaster.
 
The road map for collecting the melted fuel, last revised in September 2017, states that TEPCO would choose a first reactor to tackle by the end of fiscal 2019 and decide on a collection method. The utility would then start the retrieval process in 2021. As deciding on this process requires finalizing ways to contain, transfer and store the debris, the government and utility concluded that they would need to grasp the fuel’s current condition by extracting samples beforehand.
 
In January this year, a camera and dosimeter were sent into the containment vessel of the No. 2 reactor through an existing passage to find pebble- and clay-like masses at its bottom believed to be melted fuel. A source close to the government says the plan is to remotely guide a robot arm equipped with a camera and dosimeter into the containment vessel through the same passage, and extract a small amount of the suspected fuel debris.
 
The January probe of the containment vessel revealed radiation around the pebble-like masses measured 8 sieverts per hour — a level potentially lethal to humans after just one hour of exposure. Due to the ultrahigh radiation, the sampled material will be placed in a special radiation-shielded container before being removed from the reactor. After that, the sample will be brought to a Japan Atomic Energy Agency facility in Ibaraki Prefecture for analysis.
 
A government source told the Mainichi Shimbun that sampling the suspected fuel debris is different from the debris collection specified in the road map, and stressed that extracting samples should be beneficial to determine a method for retrieving the fuel.