Tepco on Monday found what may be melted nuclear fuel debris that penetrated the reactor 2 pressure vessel at the Fukushima No. 1 plant.
Tokyo Electric Power Company Holdings Inc. said more analysis and investigation is needed to confirm that the black lumps detected in the reactor’s containment vessel are indeed fuel debris.
The steel pressure vessel houses the nuclear fuel rods and is set up inside the surrounding containment vessel.
“At this point, it’s difficult to clearly identify what they are,” said Yuichi Okamura, general manager of Tepco’s nuclear power and plant siting division, during an evening news conference at the utility’s Tokyo headquarters.
Video footage from Monday’s probe showed black lumps that looked like something that had melted and then congealed, sticking to parts of a steel grating area at the base of the containment vessel.
The material could be melted paint, cable covers or pipe wrappings, Okamura said.
Still, this is the first time Tepco has detected anything in any of the facility’s three wrecked reactors that might be melted fuel rods since the outbreak of the crisis in March 2011. Okamura described the finding as “valuable information.”
The location of the debris and what form it is in are critical to eventually recovering the fuel.
Tepco plans next month to send in a remote-controlled robot equipped with a thermometer and dosimeter. Analyzing the temperature and radiation level will help identify whether the lumps are fuel debris, Okamura said.
The fuel melted after the March 11, 2011, earthquake and tsunami knocked out Fukushima No. 1’s power supply, including the vital cooling functions.
It is believed that reactor No. 2’s fuel rods melted and penetrated the bottom of the 20-cm-thick pressure vessel and fell in to the containment vessel.
Tepco has been conducting an investigation to check the interior of the containment vessel since last week.
In a previous try, workers inserted a rod equipped with a small camera as a precursor to sending in the remote-controlled robot.
The first attempt turned up nothing of note, but the utility then tried a longer rod — 10.5 meters long — on Monday that could capture images of the area beneath the pressure vessel.
The video footage also showed that water droplets were falling, which Tepco said must be cooling water being injected into the damaged pressure vessel.
Reactor 2 is one of three reactors, including 1 and 3, that experienced fuel meltdowns.