a) Russia plans to conquer Ukraine and use its resources. Nuclear power plants are very expensive and critical to industry. Russia wants to capture these for their own use, not blow them up and irradiate the countryside that they wish to be a part of their own country!

b) Active reactors contain very "hot" decay products that are very bad for your health if atomised by an explosion and spread around. Chernobyl is the prototypical example of this. Enriched Uranium is less radioactive than natural Uranium, that's the point! Natural Uranium would "trigger itself" prematurely due to its constant background decay radiation.