Take a cubic km of granite, either a cubical block, or ten square km x 100 m thick. Granite has a density of 2.75 tons/cubic meter, so a cubic km is 2.75 billion tons. Your average granite contains 4 ppm uranium, and 12 ppm thorium, so you have 44,000 tons of those radioactive elements.
Plutonium 239 is a fissionable isotope with a half life of 24,000 years. Since it is fissionable, it should be mixed into reactor fuel and burned up to get rid of it.
Uranium isn't "highly radioactive". The two main isotopes have half-lives of 700 million and 4.5 billion years. What makes used reactor fuel highly radioactive is the short-life fission products. These decay on time scales of seconds to millennia, much shorter than the original fuel. Therefore per kg they emit more radiation. If you bury them deep in a hole in a granite formation, you are not increasing the total radiation by much, and the rock provides shielding so it doesn't get out.