Six years after the largest nuclear disaster in a quarter-century, Japanese officials have still not solved a basic problem: what to do with an ever-growing pile of radioactive waste. Each form of waste at the Fukushima Daiichi Nuclear Power Station, where three reactors melted down after an earthquake and a tsunami on March 11, 2011, presents its own challenges. The Tokyo Electric Power Company is pumping water nonstop through the three reactors to cool melted fuel that remains too hot and radioactive to remove. About 400 tons of water passes through the reactors every day. The water picks up radiation in the reactors and then is diverted into a decontamination facility. All this water is being stored in 1,000 gray, blue and white tanks on the grounds. The tanks already hold 962,000 tons of contaminated water. Within a few years, though, and no one is sure exactly when, the plant may run out of room to store the contaminated water. The process of decontaminating the water leaves radioactive sludge trapped in filters, which are being held in thousands of containers of different sizes. Tokyo Electric says it cannot quantify the amount of radioactive sludge being generated. The ultimate goal of the cleanup is to cool and, if possible, remove the uranium and plutonium fuel that was inside the three reactors at the time of the disaster. Tokyo Electric hopes to begin removing fuel debris from the reactor cores in 2021. The entire effort could take decades.