Researchers said Thursday they have developed a system that uses cosmic rays to get images of melted nuclear fuel inside reactors at Tokyo Electric Power Co.'s Fukushima No. 1 plant.

In an experiment conducted between February 2012 and last month, the team succeeded in using the rays to produce images of nuclear fuel at an offline nuclear plant in Tokai, Ibaraki Prefecture.

Nuclear fuel melted down at three reactors at the Tepco plant as a result of the March 11 earthquake and tsunami. Inaccessible inside the reactors, the melted nuclear fuel's exact condition remains unknown.

"The (cosmic ray) measurement system can be installed easily," said team member Hidekazu Kakuno, associate professor at Tokyo Metropolitan University. "We are ready to use it at the Fukushima No. 1 plant if Tepco cooperates."

The team, which also includes researchers from the government-affiliated High Energy Accelerator Research Organization in Tsukuba, Ibaraki Prefecture, tracks muons, which make up much of the cosmic radiation in the atmosphere, and change course when coming into contact with nuclear fuel.