A video taken by micro-drones sent into the damaged Unit 3 reactor at Japan’s Fukushima Daiichi nuclear power plant has given TEPCO its first confirmed look at the bottom of the Unit 3 pressure vessel since the 2011 meltdowns, the company said Thursday. The operator said the footage showed a hole in the thick-walled steel container of the core and lumps of likely melted fuel debris hanging from it, along with damaged internal structures around the vessel.
TEPCO said the drones flew into the primary containment chamber during multiple flight missions, with one micro-drone operating at a time to navigate around debris and broken equipment. The company said the drone probe began March 5 and ran for about two weeks, with the results released late Thursday. TEPCO described the bottom of the pressure vessel as an important target for the mission.
The micro-drones were small enough to fit inside the reactor’s harsh, high-radiation environment, with each unit measuring about 12 by 13 centimeters (4.7 by 5.1 inches) and weighing about 95 grams (3.3 ounces), according to TEPCO. Through the mission, the drones recorded video and other measurements from inside the Unit 3 reactor where human entry is not feasible.
TEPCO spokesperson Masaki Kuwajima said officials confirmed there was a hole at the bottom of the vessel and that hanging objects and deposits are believed to be melted fuel debris. Kuwajima also said the drones gathered radiation measurements and used the data to produce a detailed three-dimensional map of the Unit 3 interior, which TEPCO said it can use for follow-on investigations and to plan melted fuel debris removal.
The mission arrived as TEPCO continues a long-running effort to understand what remains inside Unit 3 after the March 11, 2011 earthquake and tsunami destroyed cooling systems at the Fukushima Daiichi plant and caused meltdowns at reactors No. 1, 2 and 3. The three reactors contain at least 880 tons of melted fuel debris, TEPCO said, with radiation levels still high enough to require remote approaches.
TEPCO said it has previously taken tiny samples of melted fuel from the Unit 2 reactor, but internal details in Unit 3 have remained limited. The new drone footage provides a rare internal view, with TEPCO saying it also showed ruptures and other damaged structures that used to be inside the vessel, plus objects described as hanging like “giant icicles” in the video.
Looking ahead, TEPCO said it plans more remote-controlled probes and additional sampling to analyze the melted fuel and to develop robots for future fuel debris removal, a process that experts have said could take decades more. The company said the latest drone probe also came nearly a decade after an earlier underwater robot probe provided a less clear picture of the Unit 3 interior.