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“We put in cameras and robots and obtained valuable images, though they were partial . . . but it’s still unclear what is really going on there,” said Shunji Uchida, who became chief of the crippled plant last July. “We first need to know the situation of the debris.”

Last month, the utility inserted a 10.5-meter rod with a camera on its tip into a hole in the No. 2 reactor’s primary containment vessel and discovered black lumps sticking to the grating directly underneath the suspended pressure vessel, which holds the core.

Tepco claims it is still unsure whether the lumps are really melted fuel that burned through the bottom of the pressure vessel. Although it is still years away from actually trying to remove the fuel, Tepco, the government and related parties are planning to decide on a basic strategy this summer and go into more detail next year.

Uchida described last month’s surveillance operation as “just peeking.”

Engineers are playing with the idea of refilling the primary containment vessel with water during debris cleanup operations to reduce the intensity of the radiation, but since the PCV was probably damaged during the meltdown crisis in March 2011, the water that’s being pumped in 24/7 to keep the fuel cool is just leaking back out.

According to past analyses, some of the melted fuel rods penetrated the pressure vessel and fell into the containment vessel surrounding it after the March 11 quake and tsunami caused a station blackout at the plant, crippling all cooling functions.