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(PhysOrg.com) -- In any kind of nuclear reactor, there is a small amount of the radioactive isotope strontium-90 that is formed as part of the regular fission process.

Nestled more than half a kilometer deep in a salt mine, the plutonium slowly decays, taking some 250,000 years to become uranium. As the U.S. debates what to do with the nuclear waste produced by its fleet of 104 reactors, the radioactive legacy of decades of nuclear bomb-making sits entombed in the U.S.

A full meltdown has been avoided so far at Japan's 40-year-old Fukushima Daiichi nuclear plant.

The 8.9 magnitude earthquake in Japan is causing problems for at least one of its fleet of nuclear reactors--and authorities have shut down 10 of the country's 55 units.