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MELISSA BLOCK, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

ROBERT SIEGEL, host:

And I'm Robert Siegel.

Today at the crippled Fukushima Dai-ichi nuclear plant in Japan, workers tested out a system that will start cleaning up radioactive water. As NPR's Richard Harris reports, the water has flooded many buildings at the complex, and it has seriously complicated efforts to bring the crisis there to an end.

RICHARD HARRIS: The utility in Japan has been pumping water continuously into the crippled nuclear plant since the tsunami knocked out the system that was supposed to keep the reactor cores from overheating. The result is that there's now enough radioactive water there to fill something like 40 Olympic swimming pools.

Mr. PER PETERSON (Chair of Nuclear Engineering, University of California Berkeley): You have this large amount of contaminated water, which is a good thing.

HARRIS: Yes, Per Peterson, chair of nuclear engineering at UC Berkeley says it's a good thing to have all that water flooding the reactor buildings if you consider what could have happened instead.

Mr. PETERSON: Most of the radioactive material that was released from the damaged fuel was actually washed out into this water instead of going out into the environment, and that's good.

HARRIS: But it's also a huge problem when it comes to cleaning up the plant because the radioactive water keeps workers at bay. So the Japanese utility hired several companies to build a water decontamination plant. They're hoping the plant will be fully operational later this week.

The challenge is to remove radioactive cesium and other elements that are dissolved in the water. The water is being pumped from the flooded basements into holding tanks. From those tanks it will go through a filtration system, something like a charcoal filter, that captures some of the radioactive material.

Next, the water will run into a system built by the French nuclear company Areva. Spokesman Jarret Adams explains that they use a chemical reaction to turn the dissolved cesium into a solid material.

Mr. JARRET ADAMS (Spokesman, Areva): In our step of the process, the radioactive material precipitates out like rain and settles in the bottom of the tanks, where it forms a radioactive sludge. And that sludge can be removed from the tanks and then sent for long-term storage.

HARRIS: They use this process at other nuclear facilities, Adams says, and it works quite well.

Mr. ADAMS: We can reduce the radioactivity of the contaminated water by a factor of at least 1,000. So it will be significantly less radioactive than it was coming in.

HARRIS: Cleaning up all this water is likely to take a couple of months. If the water is clean enough, Japanese officials could decide to dump some of it into the ocean. But in the short term, they plan to run it back into the plant. That will keep the cores relatively cool. And as long as they stay cool, they won't ooze more radioactive cesium into the water.

Mr. ADAMS: I think this is an important step forward because once they begin treating this water, then they'll be able to get into the plant and start doing significant repairs.

HARRIS: And one of the most important repair jobs will be to stop the reactor vessels from leaking water. Those leaks mean the utility has to keep pumping more and more water into the reactors to keep them cool. Per Peterson from Berkeley says several ideas are in play to plug the leaks.

Mr. PETERSON: One of the ones that's been suggested is to literally fill the space between the containment vessel and the building walls with cement or grout. It's possible that one might also be able, using robots, to get in and patch places where the leaks are occurring as well.

HARRIS: Once the leaks are patched, the utility can fill the three damaged reactor vessels completely with water, without flooding the rest of the plant. That would not only keep the reactor cores cool, but water - clean water, that is - works as an effective radiation shield. In fact, Peterson says a water shield proved to be key to cleaning up the Three Mile Island nuclear meltdown in Pennsylvania, back in 1979.

Mr. PETERSON: It was possible to remove the fuel because you could flood water up above the level of the fuel, provide shielding, and then workers could get in to manipulate damaged fuel and get it out.

HARRIS: Eventually, they'll need to do the same thing at Fukushima Dai-ichi, though like Three Mile Island, the process is likely to take many years.

Richard Harris, NPR News.

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