Nuclear energy is the most concentrated source of energy on Earth. Its fearsome power was first demonstrated in 1945, when the U.S. military dropped atomic bombs over the Japanese cities of Hiroshima and Nagasaki. The bombs brought an end to World War II, but they also wreaked havoc on an entire nation of civilians: radiation sickness, cancers that killed slowly and to which young children were especially vulnerable, infertility in some, and birth defects in others.

In 1953, President Eisenhower made his famous “Atoms for Peace” speech, laying out a plan by which this destructive force could be harnessed for good: Instead of building bombs, we would produce cheap, reliable energy. In the years that followed, nuclear physics indeed gave rise to a litany of technologies that have benefitted humankind, from radiocarbon dating to X-rays to radiation therapy for cancer.

But while there are more than 400 nuclear power plants around the world, nuclear energy itself remains mired in controversy.

Proponents argue that uranium ore (uranium-containing rock) is both more abundant and produces a more efficient fuel than any fossil fuel. For example, 1 kilogram of uranium produces the same amount of energy as about 100,000 kilograms of coal. And the operating costs, per kilowatt-hour, for nuclear power are comparable to those of coal.

Of course, as critics are quick to point out, that estimate does not factor in the great expense of building, maintaining, and then decommissioning nuclear plants. In the United States, it costs about $4 billion to build a nuclear reactor (about twice the cost of building a coal power plant), and anywhere from $200 million to $1 trillion to decommission one. (Most nuclear reactors have an expected life span of 40 to 60 years, after which they need to be disassembled and the radioactive components stored and guarded.)

One thing both sides agree on is that using nuclear energy is a cleaner way to produce electricity. The processes of generating it emit much less CO₂ than the analogous processes for any fossil fuel, and nuclear power creates virtually none of the other problematic combustion by-products, like sulfur dioxide, nitrogen oxides, and particulate matter. According to the Department of Energy, switching from fossil fuels to nuclear energy would be the single most effective way to reduce greenhouse gas emissions in the United States. In the United States alone, existing nuclear power plants already prevent roughly 650 million metric tons of CO₂ per year from being released. Worldwide, they prevent close to 2.5 billion metric tons of CO₂ from entering the atmosphere.

And despite some persistent fears, research suggests that living near a nuclear power plant is actually safer than living near a coal-fired one. A 2011 study found no increased risk of birth defects for those living within 10 kilometers (6 miles) of a nuclear facility compared to the risk for those living farther away. Meanwhile, epidemiologist Javier García-Pérez has found that in Spain, the number of cancer-related deaths does increase as one moves closer to coal-fired power plants. “You still have some environmental hazards, from mining uranium and from radioactive waste and water,” says Charles Powers, a professor and nuclear energy scientist at Vanderbilt University. “But on balance, nuclear is far cleaner than any fossil fuel.”

Still, the debates over safety remain unresolved. Proponents point out that, considering the number of existing plants and the length of time they have been operating, accidents have been exceedingly few and far between. But opponents say that such safety claims ignore two key points: the vulnerability of nuclear power plants to natural disasters (which at Fukushima led to nuclear meltdown and the release of radioactive material into the environment) and the potential for nuclear fuel to be stolen and weaponized. The radioactive waste produced by nuclear power plants is another huge safety issue: It’s extremely dangerous; there’s a lot of it, and we have yet to come up with a plan for disposing of it safely.

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France now generates more than 75% of its electricity from nuclear power. But while the United States has the most nuclear reactors of any nation, Americans themselves have been divided over nuclear energy since the 1980s, after two infamous nuclear accidents made global headlines. The first was a partial meltdown at the Three Mile Island plant near Middletown, Pennsylvania, in 1979 (due to an electrical failure followed by a flurry of operator errors). The second was a full nuclear meltdown at the Chernobyl reactor in what is now Ukraine, in the spring of 1986.

The Three Mile Island incident did not result in any deaths or major public health problems. The Chernobyl meltdown was considerably more severe. A steam explosion triggered by a test that went awry sent tremendous amounts of radiation wafting over much of western Russia and Europe. More than one-fifth of the surrounding farmland remains unusable to this day, and the World Health Organization estimates that the radiation will ultimately be responsible for some 4,000 deaths when all is said and done. That figure does not include cancer-related deaths, which range from 60,000 (according to one European report) to nearly 1 million (according to a Russian report).

The radioactive waste produced by nuclear power plants is another huge safety issue: It’s extremely dangerous; there’s a lot of it, and we have yet to come up with a plan for disposing of it safely.

For their part, the Japanese were terrified of nuclear power after the bombings of Hiroshima and Nagasaki. (The popular Godzilla movies were actually based on a fictional reptile that had been mutated by a nuclear reaction and was coming to exact his revenge on humankind!) But as their country entered its own era of industrialization and economic growth, they were forced to overcome those fears. “Japan had no other natural energy source,” says Frank N. von Hippel, nuclear physicist and arms control expert at Princeton University. “Nuclear was their only ticket to becoming a world superpower.”