Each nuclear plants have several square miles of space. They have a lot of land for security and other reasons. If you packed the spent fuel close together then you can stack up a year’s worth on a basketball court. Uranium is denser than lead \cite{Smith_2001}.
If you left the unburned fuel at each plant where they are currently located then in a few decades the number of molten salt reactors could be built up to use a lot of the fuel. Is there some new reactor going to come along and get all of the CO2 from coal out of the air? Or the soot and particulates out of peoples lungs? How about bringing the 4 million people per year killed early back to life?
4 million people seems like a lot. What does that number mean and how do we get to it. The air pollution in China, India and some other places is like smoking 6 cigarettes a day. What happens when you make babies, old people, asthmatics smoke 6 cigarettes a day every day?
4 million a year is close to the combat deaths for a year in World War 2. An air pollution world war that has never ended.