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Clean power from the Earth used to use a lot of water. But a new discovery means that water can be replaced with CO2, which gets left in the ground and doesn't alter the climate

Japan's Fukushima Daiichi nuclear reactor accident has focused new attention on how much ionizing radiation people are exposed to from different sources (see list below). By far the largest source is medical imaging technology (see " Graphic Science: Exposed " in the May 2011 issue). Americans, on average, are exposed to 3.1 millisieverts of radiation a year from natural background factors such as radon gas from the Earth and cosmic rays from the universe