Hitting a potential milestone in the ongoing cleanup of one of the world’s worst atomic disasters, Japan has captured images of melted nuclear fuel inside the reactor 3 at the Fukushima No. 1 power plant.

The images of mounds of black rock & sand-like substances inside the reactor containment vessel at the nuclear power plant were captured by a remotely controlled robot operated by Tokyo Electric Power Co. Holdings Inc., which is Japan’s largest utility.

Lake Barrett, a former official at the U.S. Nuclear Regulatory Commission who was involved in the cleanup at the Three Mile Island nuclear electricity plant in America, stressed o the importance of knowing the exact locations of the melted fuel to develop a defueling plan.

Speaking on the topic, Barrett said, “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. The recent investigation results are significant early signs of progress on the long road ahead.”

However, the utility noted that it would take some more time for experts to confirm whether this debris contains melted nuclear fuel.

Removing the nuclear fuel is one of the most crucial steps in a cleanup that may consume up to four decades. According to the Ministry of Economy, Trade & Industry, decommissioning the reactors will cost nearly 8 trillion yen or US$72 billion.