The state has had a ban on uranium mining in place since 1982, soon after the discovery of a massive uranium deposit in the state's Pittsylvania County. It's the largest known deposit in the United States and one of the largest in the world.

The owners of the deposit put its value at $6 billion and said it would be enough uranium to power all of the United States' nuclear reactors continuously for two years.

A few years after the deposit was discovered, the price of uranium plummeted and interest in mining it waned for about two decades. But after the price of uranium rebounded, the deposit's owners attempted between 2008 and 2013 to convince Virginia lawmakers to reconsider the ban. After that effort failed, they sued Virginia in federal court in 2015. The hope was that a court would invalidate the ban and clear the path for mining the uranium. Lower courts agreed with the state, however, and dismissed the lawsuit.

In asking the high court to take the case, the companies underscored the importance of uranium to the United States. Nuclear reactors powered by uranium generate about 20 percent of the electricity consumed in the United States, the companies say. Uranium also powers the nation's fleet of nuclear submarines and aircraft carriers. But 94 percent of the uranium the U.S. needs is imported, they said.

Turning the Virginia deposit into usable uranium would involve three steps. First, the uranium ore would have to be mined from the ground. The uranium would then need to be processed at a mill, where pure uranium is separated from waste rock. The waste rock, called "tailings," which remain radioactive, would then have to be securely stored.

The owners of the Virginia deposit argue that the state can regulate the uranium mining, the first step in the process, but not if the state's purpose in doing so is protecting against radiation hazards that arise from the second two steps. They say that's what motivated the state's ban. They argue the Atomic Energy Act gives federal regulators the exclusive power to regulate the radiation hazards of milling of uranium and of handling and storing the leftover tailings.