Uranium-235's half-life is 700 million years. It's not even warm to the touch. Fissioning it releases much more energy than it would decaying to lead, and much, much, much faster. So that's useful for human purposes.

I wouldn't say it's a large amount of spent fuel, considering the amount of energy released. Each fuel pellet -- the size of a fingertip -- releases about as much a ton of coal.