Yellowstone is extremely far inland, on the west side of the country. A hypothetical attacker would have to cross the entire pacific ocean and somehow sneak across a third of the continent just to get to Yellowstone.

It's all but impossible to land a first strike in the center of a continent surrounded by oceans on two sides and very physically large allies on the others.

About the only options are ICBM or some type of orbital strike. We can detect pretty much any missile and nobody has orbital weapons (that we know of).

Going through the effort of attacking something in the center of the US just to take out a power station is pretty ridiculous.

At current prices, PV and many wind installations will still be cheaper than this project + transport cost. So while it could be critical for the grid, you would never rely 100% on it. The US already stores nuclear missiles not too far away from it, so I guess the military is confident they can keep that area from being attacked.