This month, Japan shut down the last of its 54 nuclear reactors. When and if any of those reactors are to be restarted is uncertain. One thing is for sure, though: as long as it is without nuclear power, Japan will be almost completely dependent on imported fossil fuels.

Japan has the third most nuclear generating capacity in the world, behind the U.S. and France. Just before the devastating earthquake and tsunami in March 2011, nuclear power was the source of just under 30 percent of the country's electricity. Hydropower and other renewable power sources accounted for less than 10 percent. The rest came from fossil fuels—the vast majority of which came from foreign nations, since Japan has few fossil-fuel resources of its own.

Japan's heavy dependence on foreign oil was exposed as a major vulnerability in 1973, after oil-producing countries in the Middle East imposed an oil embargo. In order to help protect itself from future shocks, the country accelerated its nuclear program, which had begun in the 1950s. Still, half of the nation's primary energy supply in 2010 came from oil, around 85 percent of which was imported from the Middle East.