Several years ago, I attended some lectures by geologists working to assess the Yucca Site. As I recall, there were two main issues: tectonics and groundwater. At the time, geologists suspected there was more recent seismic activity than previously reported. This was based upon a closer examination of faulting in the area. As to the hydrology, it is hard to find rock that is not fractured to some extent, and once they got underground they found a far more active groundwater system. I think they did some grouting experiments; but it has been a long time since I looked into this topic.

As for Salt Domes, they may be our best answer, but they are by no means a perfect repository for the long term. By its very nature, salt is ductile under lithostatic loading. Take a thin layer of salt, load some sediment on it, and it starts to flow; gathering together like a pool of mercury and then rising up through the overlying sediments as an inverted teardrop shaped blob. That's how the domes form in the first place. They can actually rupture onto the surface, a slowly oozing mass of salt.

The other side of the story is if that salt layer finds itself in a less arid climate. The entire area around Moab Utah (Canyonlands and Arches National Parks) is a testiment to not only the upward movement of salt but also to the giant collapse structures that develop once that salt disolves.

It's always something. When it comes to this stuff, I'd like to see it buried as deep as possible, far out of harm's way for far more than its half-life. But where?