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The earthquake and tsunami that hit Japan one year ago lasted for many terrifying minutes. The multiple nuclear meltdowns that followed caused an emergency that lasted for weeks and a legacy that will last for decades.

NPR's Richard Harris reports on the nuclear plant one year later and the work that lies ahead.

RICHARD HARRIS, BYLINE: The tsunami knocked out power to the Fukushima Daiichi nuclear power plant. As a result, the cooling systems failed and three reactors melted down. Steam laced with radioactive material poured into the air. Water contaminated with radiation also flowed into the sea. The plant looks like a ruin.

But Per Peterson, chair of Nuclear Engineering at UC, Berkeley says the situation is actually stable.

PROFESSOR PER PETERSON: The risks of further releases from the plant are pretty low. And the main issue going forward is going to be decontamination, and ultimately cleaning out and removing the damaged fuel from these reactors.

HARRIS: More easily said than done. Engineers haven't even been able to look into the reactors, but they suspect at least one of the nuclear cores melted through its reactor vessel and now sits on a concrete and steel floor below. It's no longer boiling hot, but it is extremely radioactive.

Peterson says one of the biggest challenges will be to seal up the whole system, so they can fill it with water. Water will serve as a radiation shield and let workers get close enough to break up and collect the ruined fuel.

PETERSON: One option would be to essentially inject cement around the outside of the vessel. And by doing that, it's called grouting, you can essentially fill up and plug all of that space, and then you could get the vessels to be watertight.

HARRIS: Right now, the heavily damaged reactor vessels don't hold water. In fact, the broader containment buildings are also leaking.

Finis Southworth, chief technology officer at Areva North America, says water comes into the reactor basements when it rains.

DR. FINIS SOUTHWORTH: You want to just completely stop that groundwater intrusion to make sure that you have no leakage in either direction.

HARRIS: That contaminated water is being collected and cleaned in a massive filtering system. It was cobbled together quickly in the months following the accident. Southworth says they are now upgrading that system, taking out hoses and putting in pipes. And he says the power plant is gradually looking a lot less like a disaster zone.

SOUTHWORTH: The progress is stunning. You know, the degree to which Japan and TEPCO have been able to clean this site up in under a year is phenomenal.

HARRIS: The good news is the utility, TEPCO, is keeping on its schedule to clean up the plant. The bad news is that schedule stretches out for decades. And the cost is anybody's guess, but certainly in the tens of billions of dollars. That's a sobering thought for the global nuclear industry which is hoping to expand.

But Michael Corradini at the University of Wisconsin says it's still something like five to 10 percent of the overall cost of the tsunami disaster.

MICHAEL CORRADINI: It's a large impact economically. But given how large the whole event is, it's a small part of it - only a few percent.

HARRIS: The utility is getting government bailouts. That money helps pay for cleanup at the plant. But that's just a start, says Jacopo Buongiorno at MIT.

JACOPO BUONGIORNO: The most difficult challenge that they face is really to decontaminate the area that is currently being evacuated, and so that people can go back to their homes. So I would say their biggest challenge is going to be offsite.

HARRIS: That involves hosing down roads and buildings to wash contamination into the dirt. It also involves scraping up huge volumes of soil. It's not clear what it will take to bring radiation levels back down to the local residents' comfort zone. And this isn't simply a technical question. It's a judgment call ultimately left to people who have been through a lot over the past 12 months.

Richard Harris, NPR News.

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