On November 8th two machines arrived at Fukushima Daiichi : a bridge crane and a fuel handling machine. These two machines will be installed on the reactor 3’s platform in order to unload the pool of its fuel.
Source: Pierre Fetet http://www.fukushima-blog.com/
Some of the fuel in the damaged unit 3 of the Fukushima Daiichi plant has melted and dropped into the primary containment vessel, initial results from using a muon detection system indicate. Part of the fuel, however, is believed to remain in the reactor pressure vessel.
Structures within the reactor building of unit 3 can be seen in images obtained using muon data (Image: Tepco)
Muons are high-energy subatomic particles that are created when cosmic rays enter Earth’s upper atmosphere. These particles naturally and harmlessly strike the Earth’s surface at a rate of some 10,000 muons per square meter per minute. Muon tracking devices detect and track these particles as they pass through objects. Subtle changes in the trajectory of the muons as they penetrate materials and change in direction correlate with material density. Nuclear materials such as uranium and plutonium are very dense and are therefore relatively easy to identify. The muon detection system uses the so-called permeation method to measure the muon data.
Tokyo Electric Power Company (Tepco) installed a muon detection system on the first floor of unit 3’s turbine building. Measurements were taken between May and September this year.
Tepco said analysis of muon examinations of the fuel debris shows that most of the fuel has melted and dropped from its original position within the core.
Prior to the 2011 accident, some 160 tonnes of fuel rods and about 15 tonnes of control rods were located within the reactor core of unit 3. The upper and lower parts of the reactor vessel contains about 35 tonnes and 80 tonnes of structures, respectively.
The muon examination indicates that most of the debris – some 160 tonnes – had fallen to the bottom of the reactor pressure vessel and resolidified, with only about 30 tonnes remaining in the reactor core. Tepco said another 90 tonnes of debris remains in the upper part of the vessel.
The bulk of the fuel and structures in the core area dropped to the bottom of the reactor pressure vessel (RPV), Tepco believes. While part of the molten fuel is understood to have then fallen into the primary containment vessel (PCV), “there is a possibility that some fuel debris remains in the bottom of the RPV, though this is uncertain”, the company noted.
Similar muon measurements have already been conducted at units 1 and 2 at Fukushima Daiichi. Measurements taken at unit 1 between February and September 2015 indicated most of the fuel was no longer in the reactor’s core area. Measurements taken between March and July 2016 at unit 2 showed high-density materials, considered to be fuel debris, in the lower area of the RPV. Tepco said that more fuel debris may have fallen into the PCV in unit 3 than in unit 2.
The current understanding of fuel location in units 1-3 (Image: Tepco)
Tepco said the results obtained from the muon measurements together with knowledge obtained from internal investigations of the primary containment vessels using remote-controlled robots will help it plan the future removal of fuel debris from the damaged units.
Tokyo Electric Power Co. shows its installation work to cover the upper part of the No. 3 nuclear reactor building that was blown off by the March 2011 hydrogen explosion at the Fukushima No. 1 nuclear power plant.
OKUMA, Fukushima Prefecture–Tokyo Electric Power Co. showed reporters its progress in installing a new roof above Fukushima No. 1 nuclear plant’s No. 3 reactor building on Aug. 2, ahead of work to remove spent nuclear fuel from a storage pool.
The company demonstrated how it is carrying out the work, which is necessary because the upper section of the reactor building was blown off in a hydrogen explosion in the nuclear disaster at the plant in March 2011.
The roof project marks a step toward removing nuclear fuel assemblies in the spent nuclear fuel storage pool in the building.
A photograph taken from an Asahi Shimbun helicopter shows a section of the half-tubular shaped roof being installed over the No. 3 reactor building at the Fukushima No. 1 nuclear power plant on Aug. 2.
To prevent the spread of radioactive material, TEPCO started to set up the half-tubular shaped cover to shield the damaged reactor building at the end of last month.
A section of the roof is lifted by a crane to place it on top of the No. 3 nuclear reactor building at the Fukushima No. 1 nuclear power plant on Aug. 2.
An artist’s rendition of the completed roof that will shield the upper part of the No. 3 nuclear reactor building that was blown off in the March 2011 hydrogen explosion at the Fukushima No. 1 nuclear power plant.
The 566 nuclear fuel assemblies currently lying in the pool will become a significant risk if another major disaster strikes the area.
TEPCO is expected to start removing the fuel from around mid-fiscal 2018.
Early on Aug. 2, part of the roof measuring around 17 meters high and weighing 37 tons was lifted to the top floor of the reactor building with a large crane.
Workers connected the new part of the cover to another section that had been installed at the end of July, completing one eighth of the roof. When finished, it will be about 60 meters long.
Dedicated removal machines are needed to retrieve the fuel from the storage pool. The machines that had been used at the plant prior to the accident were removed because they were severely damaged by the hydrogen explosion.
Source : Tepco
2017.7.31 Installation of Unit 3 fuel removal cover dome roof at Fukushima Daiichi Nuclear Power Station
Installation of Unit 3 spent fuel removal cover dome roof at Fukushima Daiichi Nuclear Power Station (On-site demonstration) Photos taken on: Aug 2, 2017
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