causing a nuclear disaster would help with this how?

The plant is located near the front lines, along a probable route to the Sea of Azov. Blowing it up creates an obstacle, making advances and supplies more difficult.

When you hold part of your opponents country that means you’re winning.

That's a stretch. Like Germany was winning WW2 until it was kicked out of France.

True, it's probably the best Russia can achieve but far from their goals. And they've been losing ground steadily now.