Either into the sun or a black hole....of course, if a black hole then turns out to be a way to another place then we've just shifted the problem to them, but hopefully they'll have a way of dealing with it that we don't currently have.
Throwing it into the sun is a good idea...but if it then decides to throw out a large flare or something, then it's gonna really screw things up if it hits Earth.
I live just down the road from two nuke plants, I've been in both,
one of them has just been decomissioned, so eventually it's all going to get pulled down, but the reactor area itself is still going to be hot for another century, and since it'll probably be entombed in a concrete/lead shell ala Chernobyl that's no real concern.
Nuke stations, or at least Sizewell, has thousands of backups, readouts, procedures and everything else, so it's all pretty much safe....it still leaks every couple of months or so, but so far it's behaved itself.
However, nuke power is not the permenant future for earth power...powering engines for space vehicles? Great idea! (although Fusion reactors would be so much better) but for our daily needs, we need to find something just that little bit less catastrophic when it all goes wrong.