The Dukovany Nuclear Power Station in the Czech Republic. (Credit: zhangyang13576997233/Shutterstock) There are 99 nuclear reactors currently operating in the United States. The power they generate is free of carbon dioxide emissions, but as a byproduct, they also generate small amounts of nuclear waste in the form of spent uranium. Even after the uranium in the fuel reactors is spent it remains radioactive, and that means storing it is difficult. Controversy over a permanent storage facility deep within Yucca Mountain in Nevada has left most of reactor waste stored on-site at nuclear power plants in ponds or casks. So understanding what happens to spent uranium in storage, both now and in the future, is a pressing issue for researchers. Scientists want to understand exactly how radioactive materials behave over longer periods of time. That's difficult, because we're essentially generating the data as we go — there's no precedent for humans making and trying to store nuclear waste. To get data on what happens to spent nuclear fuel over long periods of time, researchers are turning to a unique place on Earth — the oldest, and only, known natural nuclear reactor.