I find this fact both terrifying and faintly comforting at the same time. The terror comes from the emotional need to have some super special magic material keeping the nasty stuff contained. Adamantium would be ideal, if it existed.
The comfort comes from thinking through the process that must have led to the decision to use kitty litter. Imagine being the engineer with the plan to build the first nuclear power stations. You need sign off from the President, or the Secretary for Energy (Or someone important, you get the idea), and one of his advisers asks you “What are you going to do with the waste?“. You have to give the honest answer -pack it in kitty litter, put in in a barrel, and bury the barrels in an abandoned salt mine (super dry, so no chance of radioactivity spreading through water). Everyone must have been aware of the potential for humiliation if anything went wrong, but on something this important, pragmatism is everything.
By and large, kitty litter works pretty well, just don't go organic on this one. Organic kitty litter uses wheat (rather than clay) to absorb moisture. If you pack it in with decaying nuclear waste, you're effectively building a dirty bomb. This is what happens…
https://www.businessinsider.com/kitty-litter-nuclear-waste-accident-2016-8?r=UK&IR=T