http://www.digitaljournal.com/tech-and-science/technology/fukushima-robot-finds-possible-fuel-debris-on-friday-probe/article/498214

Fukushima — Robot finds possible fuel debris on Friday probe

Posted Jul 22, 2017 by Karen Graham
An underwater robot captured images inside reactor 3 at the Fukushima No. 1 power plant on Friday that appears to be melted fuel debris. On Saturday, a third exploration hopes to reach the basement of the reactor pedestal.
This image  taken near the concrete wall of the pedestal  appears to be corium  a mix of melted fuel...
This image, taken near the concrete wall of the pedestal, appears to be corium, a mix of melted fuel rods and other structural debris.
TEPCO press handout
The discovery on Friday is the first time that TEPCO has found something that looks like it could be melted fuel. In January, they sent a different robot into reactor 2 and found black lumps sticking to parts of the grating in the primary containment vessel, but the substance was difficult to identify, reports the Japan Times.
Locating the radioactive fuel is essential to completing the decommissioning process. The objects spotted on Friday look like icicles hanging around a control rod that is attached to the bottom of the pressure vessel, which holds the core, said TEPCO officials at an evening news conference Friday.
The pressure vessel, which contains the fuel rod assemblies, is enclosed by the huge primary containment vessel. But in March 2011, after the earthquake and monster tsunami swamped the nuclear power plant causing it to lose power, the fuel rods melted into a puddle at the bottom of the pressure vessel, burning through the bottom.
Simplified cross-section sketch of a typical BWR Mark I containment as used in units 1 to 5 at Fukus...
Simplified cross-section sketch of a typical BWR Mark I containment as used in units 1 to 5 at Fukushima power plant. RPV: reactor pressure vessel DW: dry well enclosing reactor pressure vessel. WW: wet well - torus-shaped all around the base enclosing steam suppression pool. Excess steam from the dry well enters the wet well water pool via downcomer pipes. SFP: spent fuel pool area SCSW: secondary concrete shield wall
Sandia National Laboratories, posted by User:84user
At the time of the accident, unit 3 contained 548 fuel assemblies, while unit 1 had 400, and unit 2 had 548 fuel rod assemblies. Used fuel rods at the Fukushima facility were normally stored on-site for a period of 18 months in an area adjacent to the reactors. Fukushima's storage area contains 6375 fuel assemblies. After further cooling, fuel can be transferred to dry cask storage, which has shown no signs of abnormalities.
According to TEPCO spokesman Takahiro Kimoto, some of the images captured by the robot appeared to show that lumps of material had melted and resolidified near the wall of the pedestal, the concrete structure that supports it.
“From the pictures taken today, it is obvious that some melted objects came out of the reactor. This means something of high temperature melted some structural objects and came out. So it is natural to think that melted fuel rods are mixed with them,” he said.
The ROV image shows melted materials that are consolidated and
have fallen. This image was taken  ne...
The ROV image shows melted materials that are consolidated and have fallen. This image was taken near the platform in the pedestal.
TEPCO Press handout
“In that sense, it is possible that the melted objects found this time are melted fuel debris or probably around it,” he said, adding the utility will think about how they can be analyzed to determine if they are the former fuel rods. Fuel from a nuclear meltdown is a lava-like combination of the melted fuel rods and other structural materials. It is called corium.
Lake Barrett, a former official at the U.S. Nuclear Regulatory Commission, who was involved in the clean up at Three Mile Island, says, “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."