The power plant is right on the frontline. It's behind the Dnieper river, and the opposite side of it is under Ukrainian control, but Russia cannot and must not abandon the power plant unless Ukraine agrees to stop shelling it. Although considering Ukraine has been breaking the Minsk agreements for 8 years I don't think their word and signature is worth very much.

If they left the plant, they would leave it defenceless for Ukraine to either shell it or move into it. This is a war, a military occupation. There is no reason they would not secure the plant like any other installation considering at the moment it is right on the frontline, not 100km behind it.

There still needs to be people working in this plant -- it provides energy to Ukraine and Russia has not shut the plant down (doing that would kill people due to lack of electricity access). The engineers and other employees working in this plant are Ukrainian and considering what they've been saying to media, Russia has good reason to believe they cannot leave them to their own devices.

Whether Russia is using the plant as a military base is something different entirely. I have not seen any credible evidence they are actually using it to protect their troops, just hearsay.

Only thing that would save the civilians and a nuclear disaster would be to declare the plant and a buffer zone around it "sacred ground" and no military incident can take place there. But again, considering Ukraine has been shelling their own plant with their own workers in it providing electricity to their own people, I doubt such an agreement would last long.