Russia would not say that Ukraine was going to dirty bomb themselves unless it was what Russia is presently planning to do themselves

 Why would Russia Dirty Bomb Ukraine?

First of all Putin doesn't have any problem with turning all of Ukraine into a DMZ sort of like the one between North and South Korea with no one living there at all.

So, if Russia turned Ukraine (the whole thing) into another Chernobyl this would give them a de-militarized zone with no one living there at all. 

Think about this calculus a little and you can see why Russia might do this in actuality and then blame Ukraine for doing this.