In southeastern Sweden, the three reactors that make up the Oskarshamn nuclear power plant provide about a 10th of the nation's total power. One of the three reactors is the largest of its type in the world. And, temporarily, the reactor was felled by jellyfish.

The Oskarshamn plant is a boiling-water reactor, just like the Fukushima plants in Japan that failed so disastrously two years ago. (For more on what a boiling-water reactor is and how it works, check out our explainer.) In brief, a boiling-water reactor uses thousands of thin tubes full of fuel, commonly known as fuel rods, which contain the little pellets that feed the nuclear reaction.