Fukushima reactor sitting on shaky base raises quake concerns
May 28, 2022
Alarm bells are sounding over signs the heavily damaged structure of the Fukushima No. 1 nuclear power plant may be too flimsy to withstand another major earthquake.
Photos taken by a remote-controlled robotic device sent into the No. 1 reactor found that a large portion of the concrete base supporting the pressure vessel appears to have melted, leaving only a metal framework holding up the pressure vessel.
Experts are now saying the remaining structure may not be strong enough to withstand a big earthquake, a troubling prediction given that the region has been hit by a number of strong temblors in recent months.
An official with the Agency for Natural Resources and Energy who is handling decommissioning work said at a May 26 news conference the remaining structure could not be described as safe, noting that a large portion of the concrete base only had the metal framework remaining.
At a news conference the previous day, Toyoshi Fuketa, chairman of the Nuclear Regulation Authority, said, “We remain concerned about whether it will withstand a strong quake.”
The Fukushima plant went into triple meltdown after the magnitude-9.0 Great East Japan Earthquake in 2011 that generated devastating tsunami.
The No. 1 reactor bore the brunt of the damage at the nuclear complex. Photos taken by the remote-controlled robot showed not only the exposed metal framework but what looked like a pile of melted fuel on top of the framework.
Officials believe the meltdown at the No. 1 reactor caused most of the fuel to melt through the bottom of the pressure vessel. The plant’s operator, Tokyo Electric Power Co., suspects the tremendous heat from the nuclear fuel may have melted the concrete of the base supporting the pressure vessel.
The cylindrical base is 1.2 meters thick with a diameter of about 6 meters. It supports the pressure vessel, which weighs about 440 tons.
A fiscal 2016 estimate by the International Research Institute for Nuclear Decommissioning said that the seismic resistance of the structure would not be an issue even if one-quarter of the base was structurally compromised, along with other damage to the interior of the pressure vessel.
But the latest photos captured only about 25 percent of the base, prompting the agency official to speculate the concrete around the entire base had melted.
TEPCO officials plan to send in another robot to take photos of the interior of the base to better grasp the seismic resistance of the structure.
Fumiya Tanabe, a former senior researcher at what is now the Japan Atomic Energy Agency, offered an even scarier possibility.
Noting that the interior of the base lies directly under the reactor core there, he said it was possible that piping hot nuclear fuel flowed into the interior.
“The metal framework in the interior of the base may even have melted,” Tanabe said.
That is a particularly worrisome thought in light of the fact that quakes with a seismic intensity of upper 6 on Japan’s maximum scale of 7 hit off the coast of Fukushima Prefecture in February 2021 and March 2022.
Tanabe said that in a worst-case scenario, another strong quake could cause the pressure vessel to topple over, making work to remove the melted fuel that much more difficult.
He recommended that work start quickly to assess the extent of damage.
Debris in sediment, bottom of Fukushima Unit 1 Neutron radiation detected at high levels
May 26, 2022
On May 26, TEPCO announced that it had detected high levels of neutron radiation, which is emitted when uranium and plutonium contained in nuclear fuel undergo nuclear fission, in sediment found at the bottom of the containment vessel at the Fukushima Daiichi Nuclear Power Plant’s Unit 1 reactor. TEPCO announced that it had detected high levels of neutron radiation, which is emitted when uranium and plutonium contained in nuclear fuel fuse. A TEPCO representative said, “It is presumed to be derived from molten nuclear fuel (debris). It is natural to assume that the debris is contained in the sediment.
TEPCO will focus on examining the thickness of the deposits near where the neutron rays were detected and the types of radioactive materials contained in the deposits.
According to TEPCO, on March 20 and 21, underwater robots were used to survey four locations at the bottom of the containment vessel, and neutron rays were detected in all of them. The values at three locations near the openings in the base of the pressure vessel were particularly high.
https://www.tokyo-np.co.jp/article/179742?fbclid=IwAR0oljxEJF2Q5XzUE859cfr1DfSmtvct8xkg6FcU2uIEpIKvtlrh3qm9aag
If a major earthquake were to occur… “Will it be safe?” concerns the chairman of the regulatory committee after the damage to the foundation of the pressure vessel at the Fukushima Daiichi Unit 1 reactor is discovered
May 25, 2022
At a press conference on May 25, Chairman Toyoshi Sarada of the Nuclear Regulation Authority of Japan (NRAJ) said, in response to an investigation that found rebar exposed in part of the reinforced concrete foundation supporting the Unit 1 reactor pressure vessel at TEPCO’s Fukushima Daiichi Nuclear Power Plant (Okuma and Futaba, Fukushima Prefecture), “I have concerns about whether it will be safe when a major earthquake occurs. I have concerns about whether it will be safe in the event of a major earthquake. If we could reinforce it, we would.
Sarada pointed out that “it is possible to speculate” about the possibility that nuclear fuel (debris) melted down during the accidental meltdown (core meltdown) and broke through the pressure vessel, melting the concrete (1.2 meters thick) of the foundation. He expressed a sense of crisis, saying, “It is necessary to consider what would happen if the foundation collapsed.
He also recognized that it would be difficult to reinforce the foundation in a nuclear reactor with extremely high radiation levels, and said, “We cannot take too long when considering earthquake resistance, and we should remove the debris as soon as possible, even if it is just an earpick, and analyze its condition.
TEPCO placed an underwater robot into the containment vessel from March 17 to 21 to investigate its interior. Near the base of the bottom of the containment vessel, several lumpy deposits were found that appeared to be debris. Although the accident caused the meltdown of Units 1-3, this was the first time that the exposure of rebar inside the concrete was confirmed. (Kenta Onozawa)
https://www.tokyo-np.co.jp/article/179502?fbclid=IwAR1Oq1_rEddpUnUyTDUjJXd494WNQk3_XfAyPMVz3zFFWVqZGcsV3X10qSY
TEPCO to Remove Contaminated Pipes at Fukushima Daiichi Nuclear Power Plant by “First Half of FY2022” Due to Continuing Troubles
April 18, 2022
On April 18, at a meeting of the Nuclear Regulation Authority to review the status of the accident at the Fukushima Daiichi Nuclear Power Plant, TEPCO announced that it had changed its target date for the completion of work to “the first half of FY2022 (April to September)” regarding the removal of pipes between Units 1 and 2 that were contaminated with high concentrations of radioactive materials. Previously, the target was “within FY 2009. The scope of the removal work includes the areas that interfere with the installation of rainwater inflow countermeasures in the waste treatment buildings of Units 1 and 2 and the installation of a large cover in Unit 1.
The removal work began on February 24, but has not progressed at all due to a series of problems with the cutting equipment. The timing for the resumption of work is not clear, as investigations are still underway to determine whether the cutting equipment and method used to lift the pipes up by a large crane are appropriate.
The pipes to be removed were used in the venting process immediately after the accident to release contaminated air inside the reactor to prevent the containment vessel from rupturing. The pipes are 30 cm in diameter and measure 65 meters on the Unit 1 side and 70 meters on the Unit 2 side. The current plan is to cut the piping into 26 sections and remove them. The surface dose at the connection with the exhaust stack is 4 sievert per hour, which is high enough to kill a person if he or she stays there for several hours. For this reason, all work will be carried out remotely. (Shinichi Ogawa)
https://www.tokyo-np.co.jp/article/172472?rct=national&fbclid=IwAR1DYTcIpK–IpNqQfheOVBWKG8-G1Eonb274DLuS8FOMWxZ9ciYQLdmaiM
Latest look inside Fukushima’s ruins shows mounds of melted nuclear fuel
February 16, 2022
A remote-controlled robot has captured images of melted nuclear fuel inside Japan’s wrecked Fukushima nuclear plant.
A massive earthquake and tsunami in 2011 damaged cooling systems at the power plant, causing the meltdown of three reactor cores.
Most of their highly radioactive fuel fell to the bottom of their containment vessels, making its removal extremely difficult.
A previous attempt to send a small robot with cameras into the Unit 1 reactor failed, but images captured this week by a ROV-A robot show broken structures, pipes and mounds of what appears to be melted fuel.
Other debris was also submerged in cooling water, according to Tokyo Electric Power Company Holdings (TEPCO), the plant operator.
About 900 tonnes of melted nuclear fuel remain inside the plant’s three damaged reactors, including about 280 tons in Unit 1.
Its removal is a daunting task that officials say will take 30-40 years. Critics say that’s overly optimistic.
The robot, carrying several tiny cameras, obtained the internal images of the reactor’s primary containment vessel while on a mission to establish a path for subsequent probes, TEPCO said.
TEPCO spokesperson Kenichi Takahara said the piles of debris rose from the bottom of the container, including some inside the pedestal — a structure directly beneath the core — suggesting the mounds were melted fuel that fell in the area.
Takahara said further probes will be needed to confirm the objects in the images.
At one location, the robot measured a radiation level of 2 sievert, which is fatal for humans, Takahara said. The annual exposure limit for plant workers is set at 50 millisievert.
The robot probe of the Unit 1 reactor began on Tuesday and was the first since 2017 when an earlier robot failed to obtain any images of melted fuel because of the extremely high radiation and interior structural damage.
The fuel at Unit 1 is submerged in highly radioactive water as deep as 2 meters (6.5 feet).
TEPCO said it will conduct additional probes after analyzing the data and images collected by the first robot.
The investigation at Unit 1 aims to measure the melted fuel mounds, map them in three dimensions, analyze isotopes and their radioactivity, and collect samples, TEPCO officials said.
Those are key to developing equipment and a strategy for the safe and efficient removal of the melted fuel, allowing the reactor’s eventual decommissioning.
Details of how the highly radioactive material can be safely removed, stored and disposed of at the end of the cleanup have not been decided.
TEPCO hopes to use a robotic arm later this year to remove an initial scoop of melted fuel from Unit 2, where internal robotic probes have made the most progress.
Robot photos appear to show melted fuel at Fukushima reactor
MARI YAMAGUCHI – February 10, 2022
TOKYO (AP) — A remote-controlled robot has captured images of what appears to be mounds of nuclear fuel that melted and fell to the bottom of the most damaged reactor at Japan’s wrecked Fukushima nuclear plant, officials said Thursday.
A massive earthquake and tsunami in 2011 damaged cooling systems at the power plant, causing the meltdown of three reactor cores. Most of their highly radioactive fuel fell to the bottom of their containment vessels, making its removal extremely difficult.
A previous attempt to send a small robot with cameras into the Unit 1 reactor failed, but images captured this week by a ROV-A robot show broken structures, pipes and mounds of what appears to be melted fuel and other debris submerged in cooling water, plant operator Tokyo Electric Power Company Holdings said Thursday.
About 900 tons of melted nuclear fuel remain inside the plant’s three damaged reactors, including about 280 tons in Unit 1. Its removal is a daunting task that officials say will take 30-40 years. Critics say that’s overly optimistic.
The robot, carrying several tiny cameras, obtained the internal images of the reactor’s primary containment vessel while on a mission to establish a path for subsequent probes, TEPCO said.
TEPCO spokesperson Kenichi Takahara said the piles of debris rose from the bottom of the container, including some inside the pedestal — a structure directly beneath the core — suggesting the mounds were melted fuel that fell in the area.
Takahara said further probes will be needed to confirm the objects in the images.
At one location, the robot measured a radiation level of 2 sievert, which is fatal for humans, Takahara said. The annual exposure limit for plant workers is set at 50 millisievert.
The robot probe of the Unit 1 reactor began Tuesday and was the first since 2017, when an earlier robot failed to obtain any images of melted fuel because of the extremely high radiation and interior structural damage.
The fuel at Unit 1 is submerged in highly radioactive water as deep as 2 meters (6.5 feet).
TEPCO said it will conduct additional probes after analyzing the data and images collected by the first robot.
Five other robots, co-developed by Hitachi-GE Nuclear Energy and the International Research Institute for Nuclear Decommissioning, a government-funded consortium, will be used in the investigation over the next several months.
The investigation at Unit 1 aims to measure the melted fuel mounds, map them in three dimensions, analyze isotopes and their radioactivity, and collect samples, TEPCO officials said.
Those are key to developing equipment and a strategy for the safe and efficient removal of the melted fuel, allowing the reactor’s eventual decommissioning.
Details of how the highly radioactive material can be safely removed, stored and disposed of at the end of the cleanup have not been decided.
TEPCO hopes to use a robotic arm later this year to remove an initial scoop of melted fuel from Unit 2, where internal robotic probes have made the most progress.
https://www.yahoo.com/now/robot-photos-appear-show-melted-134212334.html
Delayed robot probe of Fukushima reactor begins
Feb. 8, 2022
The operator of the damaged Fukushima Daiichi nuclear power plant has begun its delayed robot probe of the inside of the facility’s No.1 reactor.
Tokyo Electric Power Company, or TEPCO, had planned to start the survey of the reactor’s containment vessel on January 12, but postponed it due to mechanical trouble.
Engineers noticed during preparations that data from radiation-measuring equipment installed in a robot was not shown correctly.
The engineers later found out that electromagnetic waves emitted from the robot’s device for extending and winding up cables had affected its radiometers. They solved the problem, and confirmed that the data was then shown accurately.
TEPCO started the survey on Tuesday morning. Officials of the utility say they initially planned to end the probe by August, but that the schedule will be moved back due to the delay.
The probe is part of efforts to remove molten fuel debris from inside the reactor.
Fukushima Operators Send Robot Into Worst-Hit Melted Reactor
Feb. 8, 2022
By MARI YAMAGUCHI, Associated Press
TOKYO (AP) — A remote-controlled robot on Tuesday was used to probe the hardest-hit nuclear reactor at Japan’s wrecked Fukushima plant, as officials push forward with clean-up operations that have been mired in delays and controversy.
An earthquake and tsunami in 2011 unleashed a disastrous meltdown at Fukushima Daiichi’s three reactors that partly sunk their radioactive cores into the plant’s concrete foundations, making removal extremely difficult.
The plant’s operator, Tokyo Electric Power Company Holdings, said the submersible robot was sent into Unit 1’s primary containment vessel to install a guiding path for five subsequent robots, which will attempt to asses and take samples of the melted fuel that emits fatally high radiation.
Tuesday’s probe followed five years after the operators sent another robot into the same and badly-damaged reactor, but failed to get any images of the melted fuel.
The robot-led work, which was postponed from mid-January due to mechanical glitches, is expected to last for a few days before full-fledged probes begin.
Earlier probes showed that the fuel at Unit 1 is submerged by highly radioactive water as deep as 2 meters (6.5 feet).
Five other robots, co-developed by Hitachi-GE Nuclear Energy and the International Research Institute for Nuclear Decommissioning, a government-funded consortium, will be separately sent in for the investigation over the next several months.
The probe at Unit 1 aims to measure the melted fuel mounds, map them in three dimensions, analyze isotopes and their radioactivity, and collect samples, Tokyo Electric officials said.
Those are key to developing equipment and a strategy for a safe and efficient melted fuel removal.
About 900 tons of melted nuclear fuel remain inside the plant’s three reactors, including about 280 tons in Unit 1, and its removal is a daunting task that officials say will take 30-40 years. Critics say that’s overly optimistic.
Remote-controlled robots with cameras have provided only a limited view of the melted fuel in areas too dangerous for humans to reach. In 2017, super-high levels of radiation and structural damage hampered investigating Unit 1.
Details of how the highly radioactive material can be safely removed, stored and disposed at the end of the cleanup have not been decided.
Tokyo Electric hopes to use a robotic arm to remove a first scoop of melted fuel later this year from Unit 2, where internal robotic probes have made the most progress.
Fisherman and residents of Fukushima’s outlying areas have protested the operator’s plans to discharge into the nearby sea radioactive waters from the reactors, after treating and diluting them to safely releasable levels.
“We don’t know if it’s debris.” Bumpy deposits at the bottom of the reactor.
February 9, 2022
On February 9, Tokyo Electric Power Company (TEPCO) released a video of the bottom of the containment vessel of the Unit 1 reactor at the Fukushima Daiichi Nuclear Power Plant (Okuma and Futaba towns, Fukushima Prefecture) taken by an underwater robot during an internal investigation. From the images, it was confirmed that orangeish bumpy deposits were spreading and adhering to the structure inside the vessel. It is possible that it is nuclear fuel (debris) that melted down during the accident. We don’t know if it is debris at this stage,” said Kenichi Takahara, a spokesman, at a press conference.
A video of the bottom of the containment vessel of the Unit 1 reactor shows a bumpy deposit spreading from the bottom center to the right side of the vessel at TEPCO’s Fukushima Daiichi Nuclear Power Plant (Courtesy of TEPCO)
According to TEPCO, it is unclear how high the confirmed deposit is; a survey in March 2017 confirmed a deposit of 90 centimeters high near this location. According to TEPCO, the height of the debris is unknown.
A part of the piping that was cut off for the introduction of the robots has sunk to the bottom of the containment vessel of the Unit 1 reactor (Courtesy of TEPCO)
By around 1:50 p.m. on the 9th, the underwater robot that was deployed on the afternoon of the 8th had moved through the water inside the containment vessel and attached four guide rings with a diameter of 30 centimeters at intervals to the structure inside the vessel. The purpose of these rings was to prevent cables from getting tangled in the structure when the robot, which will be used in the future, moves around, and we were finally ready for a full-scale investigation.
In the containment vessel of the Unit 1 reactor, an underwater robot moves through the vessel while attaching rings to the structure (TEPCO)
Oil-like suspended matter was observed on the water surface inside the containment vessel of the Unit 1 reactor. The yellow glow in the center is a pipe illuminated by a light (TEPCO)
The radiation level in the water was 1 to 2 sievert per hour. The radiation level in the water was 1 to 2 sievert per hour. The exposure limit for workers at the nuclear power plant is 50 millisieverts per year, and even if one were to enter the water, it would take only 1 to 3 minutes to reach the exposure limit, so people are not allowed to go near the water.
A worker opens a valve leading to the containment vessel of the Unit 1 reactor to insert a robot (Courtesy of TEPCO)
Workers insert an underwater robot outside the containment vessel of the Unit 1 reactor (Courtesy of TEPCO)
Of the three reactors that suffered core meltdowns in the March 2011 accident, video footage shows deposits of what appears to be debris in Units 2 and 3, but no debris was found in Unit 1 during the 2005 survey.
Removing the debris is the most difficult task, and it is estimated that a total of 880 tons has melted down in Units 1-3. TEPCO is aiming to collect a few grams of debris from the Unit 2 reactor by the end of this year. (Shinichi Ogawa, Kenta Onozawa)
https://www.tokyo-np.co.jp/article/159265?fbclid=IwAR08nc1U0duYKVLHyBgjGnoRnhVI944BMtWrN-6F8HNVGUlKXTpkkFAItm4
Fukushima Unit 1: First internal investigation in 5 years
2022/02/08
The Tokyo Electric Power Company (TEPCO) began an internal investigation of the containment vessel at the Unit 1 reactor of the Fukushima Daiichi Nuclear Power Plant on February 8. An underwater robot was used to collect information on the sediment accumulating at the bottom of the vessel and the molten nuclear fuel (debris) underneath. This is the first time in about five years since March 2017.
According to the images taken by the camera mounted on the robot, the bottom of the containment vessel was bumpy, as if something had accumulated there. A TEPCO official said, “We don’t know yet whether it is a deposit or not, and we will proceed with the investigation.
The investigation was scheduled to start on January 12, but was postponed due to problems during the preparation work. TEPCO is now reconsidering the process, which was set to last until August.
https://www.minpo.jp/globalnews/moredetail/2022020801001260?fbclid=IwAR0YiLHqKhJNuprOuxehkLnPzsAYBMFHgQ_0hcLHnWwpllUn9CFD2zSjR10
Robotic failure: “We don’t know the cause, and the outlook is unclear…” High barrier to internal investigation of high radiation dose at Fukushima Daiichi Nuclear Power Plant Unit 1
January 15, 2022
Due to a robot malfunction, an internal inspection of the Unit 1 reactor at TEPCO’s Fukushima Daiichi Nuclear Power Plant (located in the towns of Okuma and Futaba in Fukushima Prefecture) has not been able to begin. The radiation level inside the containment vessel, where melted nuclear fuel (debris) remains, is too high for people to approach. The work, which requires remote control, has had a series of problems. As the eleventh anniversary of the accident approaches, a high wall continues to block the way. (Kenta Onozawa)
Advance preparations were too lax.
We didn’t know the cause of the accident. We don’t know the cause, we don’t know the prospects for countermeasures, and we haven’t decided when to resume the investigation.
At a press conference on March 13, a TEPCO spokesman gave a vague answer. The internal investigation of the Unit 1 reactor, which was delayed for more than two years from the original plan, was supposed to start on the 12th, but it stalled right from the start.
Of the three reactors that suffered meltdowns, Unit 1 is the only one where no debris has been found. The survey this time has been planned with a lot of effort to make up for the delay, including the use of six different types of robots with multiple functions, and the survey will take about seven months.
The first underwater robot (25 centimeters in diameter and 111 centimeters in length) will be used to create a survey route. A 30-centimeter-diameter guide ring will be attached to the robot so that subsequent robots can pass through it to prevent cables from getting tangled, which the spokesperson stresses is essential for the survey.
However, the preparations in advance were lax. The preliminary tests were limited to confirming the operation of each piece of equipment, and the team was unable to immediately respond to problems with the dosimeters that occurred when they were operated simultaneously.
In the past, there have been cases of “leaving things behind.
It is expected to take some time to identify the cause of the problem. If similar problems occur with other robots, plans to take images of the inside of the containment vessel, grasp its three-dimensional structure, and collect sand-like deposits in the water will not be able to proceed and may be abandoned.
We know from previous studies that complex devices are less effective, such as the underwater robot that photographed debris deposits inside the containment vessel of Unit 3 in 2017. The underwater robot that photographed the debris in the containment vessel of the Unit 3 reactor in 2017 was about a quarter of the length and had a simpler structure. It also focused on photographing as its main purpose.
In the 2006 survey that succeeded in photographing the debris in the Unit 2 reactor, a worker inserted a pipe (13 meters long) with a camera attached to the end, rather than a robot. In the previous year, a camera-equipped pipe was inserted. In the previous year, a camera-equipped robot called a “scorpion” was deployed, but it climbed up on the sediment and could not be retrieved, remaining in the reactor.
Although “human power” can be used to deal with the problem outdoors…
Remote-controlled operations are always fraught with difficulties, even outside the building where the reactor is located.
The exhaust stack near the Unit 1 and 2 buildings, which was in danger of collapsing due to the earthquake and was highly contaminated, was cut down to about 60 meters, half the height of the original stack (1 In one case, the saw blade of a cutting device lifted by a large crane got stuck in the cylinders and could not be moved. At that time, a worker climbed up to the cutting device installed at the top of the 110-meter-high cylinder with a crane and cut it with a power tool.
In late January, they plan to cut the contaminated pipes leading to this exhaust stack. The project was originally supposed to start four months ago, but there was a problem with the remote-controlled cutting device and the crane broke down, delaying the plan.
Debris collection is planned for Unit 2 by the end of the year. If a problem occurs in the reactor, we cannot rely on human power.
https://www.tokyo-np.co.jp/article/154269?fbclid=IwAR2E55S3DYLr7KiroYjxza6u_MX67pvMdrWioFykfOwgoxBdOlqhOraX9WI
Translated with http://www.DeepL.com/Translator (free version)
TEPCO to begin robot probe of Fukushima reactor
Jan. 6, 2022
The operator of the crippled Fukushima Daiichi Nuclear Power Station says it will launch a probe of the inside of the No.1 reactor on Wednesday using robots. The firm is seeking to clear debris from the reactor interior as part of the decommissioning process.
Tokyo Electric Power Company, or TEPCO, says the probe will involve six types of robots, each with a different function.
It says the survey will continue for more than six months. It will use ultrasonic devices to locate and measure the thickness of debris believed to be submerged under water inside the reactor containment vessel.
The utility says it also hopes to collect small samples of the debris.
TEPCO says it will use a robot to install a cover on a path for the survey machines to move smoothly under water.
The No.1, 2 and 3 reactors of the plant suffered meltdowns in the massive earthquake and tsunami of 2011.
TEPCO confirmed the existence of what is believed to be solid fuel debris inside the No.2 and 3 reactors, but not inside the No.1 reactor. The debris consists of molten nuclear fuel and metal parts.
Fukushima Daiichi Decontamination & Decommissioning Engineering Company, which was established by TEPCO, said on Thursday it will use the robots to gather information before considering how to remove the debris.
Upcoming Inspection of Fukushima Unit 1
November 25, 2021
TEPCO announced that the next phase of containment inspections inside unit 1 will finally take place in early 2022. This next phase of inspections had been delayed due to technical challenges, concerns of radioactive dust releases, then due to the pandemic.
Some of the explanatory materials provide new insight into the true situation inside unit 1. It has been speculated since 2011 that melted fuel (aka corium) flowed out of the pedestal of unit 1, across the containment floor, and burned through the thin connection edge of the containment structure. Diagrams included in this most recent report show that the entities tasked with decommissioning the damaged reactors think so too. Diagrams originally from NDF, the decommissioning authority, and IRID, the main agency tasked with research, show the same.
The graphic below shows a significant pile of melted fuel in the pedestal and along the outer edge of the containment structure. The right side graphic shows melted fuel a significant way up to the lower edge of the downcomer tubes that route into the torus tube. Evidence of fuel debris inside the torus tube was found in earlier inspections along with our early findings that appeared to show fuel debris under the water in the torus room outside of containment.
This side view of the same area shows what IRID and TEPCO assume to be the situation inside unit 1’s containment as they prepare for the upcoming inspections.The red color is solidified previously melted fuel, The brown layer is the sediment layer and the blue layer is standing water inside containment. The sediment layer on the right side where inspections have already taken place is significantly lower than the sediment bed directly adjacent to the assumed location of the fuel debris. The depth difference appears to be 3 times as much adjacent to the melted fuel locations.
The debris bed on the shallow right side was roughly 4-10 inches deep based on TEPCO estimates in 2017. If the left side adjacent to the solidified fuel is 3 times the depth, it would be 12-30 inches in depth. What exactly this debris bed is and how it developed had caused head-scratching for years. A TEPCO report in 2017 showed it contained stainless steel, materials related to shielding, cabling, and some low levels of reactor-based radioactive isotopes. If this material contained additional substances or not was a bit ambiguous. There had been some initial assumptions this was pulverized concrete. There may have been some involvement of the concrete structures into this debris pile through mechanical destruction or molten corium concrete interaction, but TEPCO provides insufficient data to confirm or rule this out.
Another 2017 report gave some rough estimates of the depth of the known parts of the debris bed. The known parts from an earlier set of inspections would be the general area marked by the A in the above graphic. This area is roughly 4 – 10 inches in depth. The left side adjacent to the solidified fuel would then be about 12 – 30 inches in depth. The deposits closer to the pedestal opening were close to 1 meter deep on a 2017 inspection. TEPCO could not determine if there was any solidified fuel beneath. They assumed the debris bed was providing extensive shielding that would prevent the detection of any layer of solidified fuel. This debris bed appears to reside 1/3 of the way up the downcomer cover. It is likely some amount of it has entered the torus tube and potentially the torus room.
Upcoming Inspections:
The upcoming inspections include a total of 6 ROV units. The remote operational vehicles are not true robots as each one has a control tether. The biggest concern with these units is having one become stranded, preventing the introduction of future ROV units to continue inspections. Each ROV unit has an assigned task. Due to internal equipment inside containment, a series of rings will be placed by the first ROV to help guide the ROV units and prevent entanglement.
ROV-A
ROV-A will attempt to traverse the south direction to the pedestal doorway.
ROV-A2 will attempt to enter the pedestal to capture imagery of the conditions and potential fuel location.
ROV units B to E each have similar tasks tied to characterizing the fuel debris and sediments.
Each ROV has about an 80-hour high radiation tolerance. They will be introduced by the level of risk with ROV-A2 going into the pedestal last due to the high risk. Preparation work begins in January. The entire series of inspections are currently scheduled to take 10 months to complete.
Robots to probe Fukushima No.1 reactor from Jan.
Thursday, Nov. 25, 2021
NHK has learned that the operator of the Fukushima Daiichi nuclear plant plans to start a delayed robot survey of a damaged reactor from mid-January.
Officials of Tokyo Electric Power Company say preparations are well under way to send submersible robots inside the containment vessel of the No.1 reactor.
The probe is part of efforts to remove molten fuel debris from the reactor that suffered a meltdown accident due to the 2011 earthquake and tsunami.
The utility originally planned to start the robot survey of the reactor in 2019.
It has been postponed because preparations, such as making a hole in the door of the vessel for the robots to go through, have taken time.
The officials say they are now installing equipment to remotely control the robots, and expect to carry out a survey for more than six months from mid-January.
They plan to use a total of six robots with different functions to find and examine nuclear debris, or deposits of a mixture of molten fuel and reactor parts, inside the containment vessel.
The robots will use ultrasonic devices to locate and measure how much debris there is, and how thick the deposit is. They are also expected to collect a small amount of samples.
Previous surveys at the plant confirmed the presence of deposits believed to be fuel debris in the No.2 and No.3 reactors, which also suffered meltdowns, but not in the No.1 reactor.
Protective sheet positioned at damaged reactor
June 12, 2020
The operator of the crippled Fukushima Daiichi nuclear plant has released footage showing part of the time-consuming steps needed to remove nuclear fuel from a pool at its No.1 reactor.
In the video, workers used remote-controlled tools to place a wide sheet over the surface of the pool. The radiation level there remains high.
Tokyo Electric Power Company has been engaged in removing debris from the upper part of the reactor building where the pool is located. The debris was caused by an explosion during the 2011 nuclear accident.
The sheet is meant to protect the pool and 392 nuclear fuel assemblies still inside from further damage that could result from the possible falling of debris or large machinery.
The six-by-eleven meter sheet is inflatable to a thickness of 50 centimeters. It will be filled with cement to increase its strength.
The operator plans to begin clearing the debris from around the pool by the end of the month, as soon as the sheet’s cement has solidified.
The operator plans to start removing the fuel from the pool of the No.1 reactor in fiscal 2027. Similar work at the No.2 reactor will start in fiscal 2024. Fuel removal from the pool at the No.3 reactor will be completed by fiscal 2020. The removal work is finished at the No.4 reactor.
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