I doubt the veracity of your claims because according to multiple sources [1][2] the only nuclear reactor in the state of Utah is located on the bottom floor of the Merrill Engineering Building on the University of Utah campus.
If a reactor were located on the BYU campus, I would expect it to be public knowledge and tracked by the IAEA.
I used to work on the top floor of MEB for many years and had the opportunity to see the glow of the core in the pool of water on multiple occasions.
Tangentially, the UofU reactor made national headlines last week when a student made a threat to blow it up if the football team lost [3].
Most of the infrastructure is likely still there as well. Most DOE facilities are also junkyards with the metals moratoriums so I could imagine this applies to BYU.
I went to BYU and while there are tunnels under campus, they're basically just for maintenance, steam and electrical conduits, etc. as far as I'm aware.
https://universe.byu.edu/2011/06/13/byu-once-housed-undergro...