How is radiation measured?

The unit for quantifying radiation dose is a Sievert (Sv). It's the ratio of radiation energy (joules) to total body mass exposed (kilograms). A Sievert is a lot of radiation -- an instantaneous dose of 1 to 3 Sv could cause you severe nausea and cripple your immune system, and 10 Sv all at once could kill you. Normally, people are exposed to much smaller doses, which are measured in millisieverts (mSv) that amount to 0.001 of a Sievert. The average person is exposed to about 6.2 mSv annually from natural and manmade sources [source: National Institutes of Health].

How Safe Is a Nuclear Reactor?

The answer to that question is pretty complicated, and it depends who you ask and how you define "safe." Are you concerned about radiation routinely leaking from plants or about the radioactive waste generated by reactors? Or, are you more worried about the possibility of a catastrophic accident? What degree of risk do you consider an acceptable trade-off for nuclear power's benefits? And to what extent do you trust the government and the nuclear power industry to keep things safe and provide accurate information to the public?

"Radiation" is a loaded word, mostly because we all know that big doses of radiation -- from a nuclear bomb blast, for example -- can kill scores of people or make them horribly ill.

Advocates of nuclear power, however, point out that we all routinely are exposed to radiation from a variety of sources, including cosmic rays and radiation naturally emitted by the Earth itself. The average annual exposure to radiation is about 6.2 millisieverts (mSv), half of it from natural sources and half from man-made sources, ranging from chest X-rays to smoke detectors and luminous watch dials [source: Nuclear Regulatory Commission]. How much do we get from nuclear reactors? A tiny fraction of a percent of our typical annual exposure -- 0.0001 mSv [source: Nuclear Regulatory Commission].

While all nuclear plants inevitably leak small amounts of radiation, in the United States, regulators hold operators to a stringent standard. They can't expose people living around the plant to more than 1 mSv of radiation per year, and workers in the plant have a threshold of 50 mSv per year. That may sound like a lot, but according to the Nuclear Regulatory Commission, there's no medical evidence to show that annual radiation doses below 100 mSv pose any health risks [source: Nuclear Regulatory Commission].

But it's important to mention that not everybody agrees with that assessment of radiation risks. For example, Physicians for Social Responsibility, a longtime critic of the nuclear industry, cites a 2009 study of children living around German nuclear power plants. The study found that those living within 3.1 miles (5 kilometers) of the plants had twice the risk of contracting leukemia as those living farther away [source: Physicians for Social Responsibility].

There's also the problem of radioactive waste generated by reactors. We'll cover that challenge on the next page.