Work is continuing to decommission the crippled Fukushima Daiichi nuclear power plant in Japan's northeast.
The triple meltdown it suffered eight years ago after it was hit by an earthquake and tsunami is considered one of the world's worst nuclear accidents. The process is expected to take decades.
Engineers are determining the best way to do remove the piles of fuel debris still inside the reactors.
Last month, a robotic probe sent inside managed to touch the debris for the first time, but was unable to move it.
Another problem is the growing volume of contaminated water. Coolant being poured over the debris is mixing with groundwater that seeps into the compound.
However, Japan's nuclear authority says the water can be treated and safely released into the ocean.
That idea has been met with protests from the fishing industry. The final plan has yet to be decided.