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updated 3/31/2011 12:48:41 PM ET 2011-03-31T16:48:41

Japan's struggle to regain control over the nuclear power reactors damaged in the tsunami that followed a 9.0-magnitude earthquake on March 11 has people worldwide wondering and worrying about the impacts of radioactive fallout. Here's our top 5 questions and answers culled from experts at the Union of Concerned Scientists, a nuclear safety advocacy group.

1. Should people in the United States be concerned about their drinking water?

Edwin Lyman, senior scientist: "In my judgment, at this point, no. We have to use a disclaimer that no level of radiation is safe, but the risk is proportional to dose, and the dilution that's experienced as a plume travels many thousands of miles is highly significant."

2. How did radioactive water end up in the basement and in some tunnels around the reactor buildings?

Lyman: "It's not clear where it came from. During the last week or the last two weeks, workers have periodically vented the reactor vessel into the secondary or into the primary containment structure. It appears that some of those ventings occurred after fuel damage occurred. So, that would have been a pathway for highly radioactive water to get into the containment buildings. So, it's possible that it came from that structured and vented pathway, as opposed to some holes on the bottom of the reactor vessel or some other containment breach.

3. Are the leaks of radioactive water likely to continue and/or worsen?

David Lochbaum, Nuclear Safety Program director: "Over the past few weeks, the focus has been on cooling the reactor cores and the spent fuel pools. They've been doing that by a non-preferred method, which is just adding more and more water. The preferred method would be to remove some water from the reactor vessel or the spent fuel pool, cool it, and return it, so your inventory of water is staying the same. But they didn't have that option, so they just continued to add water to the reactor vessels and to the spent fuel pools.

That water had to go somewhere, so it's found its way into the containment buildings, the basements, the reactor buildings, the turbine buildings. It's piling up or flooding up all kinds of places that it shouldn't be, and because it's radioactively contaminated now, anything that leaks out carries that radioactivity with it. For the reactor buildings and the turbine buildings, in addition to the leakage pathways for water, you also have evaporation of material with radioactive isotopes. So, it's going to be a problem, and it's going to take a while for them to, first of all, get their hands around it, and second, to clean it up.

4. What about the detection of plutonium. Is that a concern?

Lochbaum: "Plutonium is a longer-lived isotope than even cesium 137, and so the presence of plutonium does present an additional hazard and complicates it and would require more expensive cleanup. There have been sites in the United States that have had plutonium contamination that have been successfully remediated, like the Rocky Flats plant in California, and that, you know, is an expensive and difficult enterprise. Plutonium, when it becomes aerosolized and inhaled, is a particularly potent carcinogen, and so you need additional respiratory protection if there's significant plutonium that could potentially be inhaled. So, I think it does add another wrinkle to the problem.

5. Is is safe to eat Pacific seafood?

Lyman: I would think it's unlikely for seafood that is not caught relatively close to the Japanese shore to be contaminated, but we haven't done any analysis on this. Even dilute levels of contamination can be enhanced in certain marine life, like how mercury concentrates in large fish, like tuna. Also, plants like seaweed are known to concentrate certain isotopes, and so are certain types of shellfish. But I would think certainly in the fishing industry in the region, they're most likely going to have to take measures to inspect their catches.

© 2012 Discovery Channel

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