Nuking someone is a definitive, MAD-triggering event. Causing a radiological disaster "accidentally" would be entirely consistent with the "stop hitting yourself" diplomacy Russia has undertaken since moving against Ukraine in 2014.
They won't blow up Chernobyl. Ukraine is a top grain exporting nation. Without wheat, Ukraine's economy would be even worse and there would be no way to feed the people.
Additionally to what was already said in other threads about this:
Why wast money?
Why do you care about saving money when you are planning on ending the world in the next hour?