Five years after an earthquake and tsunami caused a series of meltdowns at the Fukushima Dai-ichi nuclear plant in Japan, there are signs of progress. Many workers cleaning up the ruined plant no longer need to suit up in full respirators. Some nearby villages that were evacuated are open to residents.

But there are still plenty of problems.

"Fukushima Dai-ichi is a complicated clean-up site," says Dale Klein, a former chairman of the U.S Nuclear Regulatory Commission who now consults for the Tokyo Electric Power Company, or TEPCO, which owns the plant.

Perhaps the biggest problem is water, Klein says. Groundwater from nearby mountains constantly seeps through the ruined building before spilling into the ocean. TEPCO has tried a number of different approaches ( including a massive underground ice wall) to try and slow the incursion, but so far efforts to stop it have been only partially successful.

Instead, the company must send the groundwater through a complex filtration system that removes radioactivity. The system is effective at removing some of the most dangerous elements, but one isotope of hydrogen, called tritium, can't be removed because it's literally embedded in the H2O of the water molecules.

"Tritium is part of the water itself, so how do you filter water out of water?" Klein says.

The best solution might be to dilute the radioactive groundwater and then release it into the ocean. But fishermen and the public would have to be convinced. For now, nearly a million tons of water is being stored all over the site.

"There are about a thousand containers, steel containers that hold this water," he says.

The land around the plant was also contaminated by the accident. Azby Brown is with the nonprofit organization Safecast, which independently monitors radiation levels in Fukushima prefecture. He says readings by Safecast and the government show that radiation levels in the region around the plant have fallen by roughly half.

"There's no question that the radiation levels have decreased compared to 2011," he says.

Some of that drop is due to the natural radioactive decay, but there has also been a huge cleanup effort. Workers across Fukushima have been scraping up contaminated topsoil and storing it in bags. And that's created its own problem, Brown says.

"There are now about 9 million bags of decontamination waste from all over the prefecture that are being consolidated into these vast fields with these pyramids of radioactive waste," he says.

Just like with the water, regulators aren't quite sure what to do with all that soil. Japan doesn't have a centralized radioactive waste dump to take it to.

In the longer term, the biggest issue will be what to do with the highly radioactive cores of the reactors themselves, each filled with melted uranium fuel. After the Chernobyl nuclear accident in Ukraine, Soviet officials opted to pour concrete over the melted reactor core. But TEPCO's chief nuclear officer, Takafumi Anegawa, says Fukushima's fuel has got to be removed because the plant's presence on the coast poses a threat to the environment. "This condition cannot continue for many years," he says.

Unfortunately, TEPCO doesn't really have any firm ideas about how to get the melted fuel out. "So far, there's no specific clear solution," Anegawa says.

In fact, the company hasn't even been able to look inside to see what needs to be done. Anegawa says the radiation is so intense that it's fried the circuits of several robots sent in to investigate.

Dale Klein, the safety consultant, thinks that eventually robots will be used to cut up the molten cores of the reactors and seal them inside concrete containers. But he says it's not going to happen overnight.

"This will be a several-decades process of cleanup," he says.

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