Good luck trying to track down the thieves if they raid a couple of reactor sites and just disappear. Doubly so if it's somewhere in the wilderness. I don't even think they need to raid more than one SMR to create a pretty nasty dirty bomb...

Estimated impact varies on how much material is in an SMR and how easy it is to take out.

And a raid isn't even technically necessary if an SMR gets into the wrong hands in the first place.