Well, it isn't plutonium to begin. Plutonium is a product of fission but has to be processed from other heavy elements. I don't know the type of reactor in Ukraine is, but its likely a typical, run of the mill, U238 reactor. The U238 is processed into a pure form in the shape of rods that are then inserted into an assembly at specific locations around a circle or if square, in strategic locations to maximize the neutron interaction with the water. The other reason is that the other rods are made of a neutron-absorbing metal, usually Molybdenum or graphite. The reactions is controlled by how much of these neutron-absorbing metals separate and stop the interaction. This way, they can control the amount of heat generated. It is the super-heated water that is pumped through pipes into ordinary water baths, that transfer the heat to the ordinary water without radioactive contamination. That water, of course, is flashed into steam which then goes to generators (massive generators) to create the electricity.
A bomb, would not be much different than what happened to Chernobyl. Chernobyl was doing a 'spin down' test for war readiness and the reactor got out of their control. They could not control the reaction in the core chamber and when it was hot enough, it actually blew the top off the containment building. Of course, when that happened, all the liquid flash steamed and vented into the atmosphere and started rising very rapidly. The exposed core then began to meltdown and that process started vaporizing all the material around it. Cement, dirt, sand. That is what caused the majority of the contamination in the area, which exists to this day.
If the Russians bombed it, it would be an absolute catastrophe and an environmental (locally) disaster. But there would not be the big "mushroom cloud" explosion.