* Astronomy

Named for the Norse god of thunder, thorium is a lustrous silvery-white metal. It's only slightly radioactive; you could carry a lump of it in your pocket without harm. On the periodic table of elements, it's found in the bottom row, along with other dense, radioactive substances - including uranium and plutonium - known as actinides.
Actinides are dense because their nuclei contain large numbers of neutrons and protons. But it's the strange behaviour of those nuclei that has long made actinides the stuff of wonder. At intervals that can vary from every millisecond to every hundred thousand years, actinides spin off particles and decay into more stable elements. And if you pack together enough of certain actinide atoms, their nuclei will erupt in a powerful release of energy.
To understand the magic and terror of those two processes working in concert, think of a game of pool played in 3-D. The nucleus of the atom is a group of balls, or particles, racked at the center. Shoot the cue ball - a stray neutron - and the cluster breaks apart, or fissions. Now imagine the same game played with trillions of racked nuclei. Balls propelled by the first collision crash into nearby clusters, which fly apart, their stray neutrons colliding with yet more clusters. Voila: a nuclear chain reaction.
Actinides are the only materials that split apart this way, and if the collisions are uncontrolled, you unleash hell: a nuclear explosion. But if you can control the conditions in which these reactions happen - by both controlling the number of stray neutrons and regulating the temperature, as is done in the core of a nuclear reactor - you get useful energy. Racks of these nuclei crash together, creating a hot glowing pile of radioactive material. If you pump water past the material, the water turns to steam, which can spin a turbine to make electricity.

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