TEPCO released three videos of its robot probe inspections inside unit 3. The videos provided some interesting information, showing some unexplained “blobs”, thick adhered substances, concrete spalling seen in fires, possible melted fuel formations, and thought to be solidified melted fuel around the lower end of a control rod.
TEPCO think the graphite gaskets sealing the control rod holes in bottom of the reactor vessel melted allowing molten fuel to flow through these holes to drip down into the reactor pedestal. Structures in the pedestal show some of the patterns created by the thick substances that appear to have splattered around the containment structures.
TEPCO cites 364 tons of fuel debris (melted fuel, internal reactor parts and control rods) to be expected at unit 3. The videos only show very small views of the damage found making it difficult to determine how much fuel debris was actually found inside unit 3’s pedestal.
Source : Tepco
TEPCO revised unit 2’s containment inspection radiation readings done earlier in 2017, claiming a set of instrument and calibration errors caused the inaccurate readings.
TEPCO claims that their camera based radiation estimates were too high due to an oversight where they forgot to reset the sensitivity threshold on the equipment that was reading camera interference.
The final claim made in the report for downgrading the radiation readings was that one of the 4 sensors was reading considerably higher than the other 3. When they took that sensor out of the readings the other 3 sensors read considerably lower.
Source : Tepco’s handout
Underwater robot finds likely melted fuel heap inside Fukushima reactor
.This image captured by an underwater robot provided by International Research Institute for Nuclear Decommissioning on Saturday, July 22, 2017 shows heaps of solidified lava-like rocks believed to be nuclear fuel.
TOKYO (AP) — Images captured by an underwater robot showed massive deposits believed to be melted nuclear fuel covering the floor of a damaged reactor at Japan’s crippled Fukushima nuclear plant.
The robot found large amounts of solidified lava-like rocks and lumps in layers as thick as 1 meter on the bottom inside of a main structure called the pedestal that sits underneath the core inside the primary containment vessel of Fukushima’s Unit 3 reactor, said the plant’s operator, Tokyo Electric Power Co.
On Friday, the robot spotted suspected debris of melted fuel for the first time since the 2011 earthquake and tsunami caused multiple meltdowns and destroyed the plant. The three-day probe of Unit 3 ended Saturday.
Locating and analyzing the fuel debris and damage in each of the plant’s three wrecked reactors is crucial for decommissioning the plant. The search for melted fuel in the two other reactors has so far been unsuccessful because of damage and extremely high radiation levels.
During this week’s probe, cameras mounted on the robot showed extensive damage caused by the core meltdown, with fuel debris mixed with broken reactor parts, suggesting the difficult challenges ahead in the decades-long decommissioning of the destroyed plant.
TEPCO spokesman Takahiro Kimoto said it would take time to analyze the debris in the images to figure out debris removal methods.
Melted nuke fuel images show struggle facing Fukushima plant
What is believed to be nuclear fuel debris has accumulated at the submerged bottom of the containment vessel in the No. 3 reactor at the Fukushima No. 1 nuclear power plant in an image taken on July 22. Part of the collapsed metal scaffolding is seen at back right.
Images captured on July 22 of solidified nuclear fuel debris at the bottom of a containment vessel of the crippled Fukushima No. 1 nuclear power plant show the enormity of decommissioning of the facility.
Plant operator Tokyo Electric Power Co. said it will closely study the images from the No. 3 reactor’s containment vessel to determine the spread and amount of nuclear fuel debris.
After analysis, TEPCO will decide on a policy to retrieve the fuel debris.
The government and TEPCO plan to start the retrieval process in one of the three crippled reactors at the plant from 2021.
It will be a formidable task, given that a method of recovering debris that is stuck to the floor has yet to be considered.
The recent images were taken by a submersible robot, which was sent into the containment vessel on July 19, 21 and 22.
The No. 3 reactor’s containment vessel is filled with water to a depth of 6.4 meters.
On the final day, the remote-controlled robot was dispatched to the deepest part of the containment vessel.
The images showed that pieces that fell from the structure and deposited material accumulated to a height of about 1 meters at the bottom of the containment vessel.
In particular, what is believed to be nuclear fuel debris is scattered in the form of rocks in the area directly beneath the pressure vessel.
The latest investigation has confirmed TEPCO’s assumption made through analyses that most of the reactor’s nuclear fuel melted through the pressure vessel and accumulated at the bottom of the containment vessel.
It also discovered that the nuclear fuel debris has spread throughout the containment vessel.
The images marked the first confirmation through a robot probe of a large amount of nuclear debris in any of the embattled No. 1 through No. 3 reactors.
New images show what is likely to be melted nuclear fuel hanging from inside one of Japan’s wrecked Fukushima reactors, a potential milestone in the cleanup of one of the worst atomic disasters in history.
Tokyo Electric Power Co. Holdings Inc., Japan’s biggest utility, released images on Friday showing a hardened black, grey and orange substance that dripped from the bottom of the No. 3 reactor pressure vessel at Fukushima, which is likely to contain melted fuel, according to Takahiro Kimoto, an official at the company. The company sent a Toshiba-designed robot, which can swim and resembles a submarine, to explore the inside of the reactor for the first time on July 19.
“Never before have we taken such clear pictures of what could be melted fuel,” Kimoto said at a press briefing that began at 9 p.m. Friday in Tokyo, noting that it would take time to analyze and confirm whether it is actually fuel. “We believe that the fuel melted and mixed with the metal directly underneath it. And it is highly likely that we have filmed that on Friday.”
Pictures taken on July 21 inside of Fukushima reactor. Source: Tokyo Electric Power Co. Holdings Inc.
If confirmed, the substance — which has the appearance of icicles — would be the first discovery of the fuel that melted during the triple reactor accident at Fukushima six years ago. For Tokyo Electric, which bears most of the clean-up costs, the discovery would help the utility design a way to remove the highly-radioactive material.
The robot, which is about 30 centimeters (12 inches) long, will search for melted fuel at the bottom of the reactor on Saturday. It is possible that the company will take more pictures of what could be melted fuel spread across the floor and lower levels, according to Tokyo Electric’s Kimoto. Fuel from a nuclear meltdown is known as corium, which is a mixture of the atomic fuel rods and other structural materials.
“It is important to know the exact locations and the physical, chemical, radiological forms of the corium to develop the necessary engineering defueling plans for the safe removal of the radioactive materials,” said Lake Barrett, a former official at the U.S. Nuclear Regulatory Commission who was involved with the cleanup at the Three Mile Island nuclear power plant in the U.S. “The recent investigation results are significant early signs of progress on the long road ahead.”
Because of the high radioactivity levels inside the reactor, only specially designed robots can probe the unit. And the unprecedented nature of the Fukushima disaster means that Tepco, as the utility is known, is pinning its efforts on technology not yet invented to get the melted fuel out of the reactors.
The company aims to decide on the procedure to remove the melted fuel from each unit as soon as this summer. And it will confirm the procedure for the first reactor during the fiscal year ending March 2019, with fuel removal slated to begin in 2021.
Decommissioning the reactors will cost 8 trillion yen ($72 billion), according to an estimate in December from the Ministry of Economy, Trade and Industry. Removing the fuel is one of the most important steps in a cleanup that may take as long as 40 years.
Similar to the latest findings on Friday, Tepco took photographs in January of what appeared to be black residue covering a grate under the Fukushima Dai-Ichi No. 2 reactor, which was speculated to have been melted fuel. However, a follow-up survey by another Toshiba-designed robot in February failed to confirm the location of any melted fuel in the reactor after it got stuck in debris.
A robot designed by Hitachi-GE Nuclear Energy Ltd. also failed to find any melted fuel during its probe of the No. 1 reactor in March.
The significance of Friday’s finding “might be evidence that the robots used by Tepco can now deal with the higher radiation levels, at least for periods of time that allow them to search parts of the reactor that are more likely to contain fuel debris,” M.V. Ramana, professor at the Liu Institute for Global Issues at the University of British Columbia, said by email.
“If some of these fragments can be brought out of the reactor and studied, it would allow nuclear engineers and scientists to better model what happened during the accident.”