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It has just a couple of protons less than uranium, and it's common as dirt.
Thorium-fueled reactors are supposed to be much safer than uranium-powered ones, use far less material (1 metric ton of thorium gets as much bang as 200 metric tons of uranium, or 3.5 million metric tons of coal), produce waste that is toxic for a shorter period of time (300 years vs. uranium's tens of thousands of years), and is hard to weaponize. In fact, thorium can even feed off of toxic plutonium waste to produce energy. And because the biggest cost in nuclear power is safety, and thorium reactors can't melt down, argues Michael Anissimov in Accelerating Future, they will eventually be much cheaper, too.
How cheap would it be?
If a town of 1,000 bought a 1-megawatt thorium reactor for $250,000, using 20 kilograms of thorium a year with almost no oversight, every family could pay as little as $0.40 a year for all their electricity, Anissimov predicts. And small reactors like that aren't just potentially cost-effective, he says; they're much safer, too.
So why haven't we been using it instead? History. We needed to make bombs, and Thorium just didn't get us there.