Fission Simple diagram of nuclear fission. In the first frame, a neutron is about to collide with the nucleus of a U-235 atom. In the second frame, the neutron has been absorbed and briefly turned the nucleus into an unstable U-236 atom. In the third frame, the U-236 atom has fissioned, resulting in two fission fragments (Ba-141 and Kr-92), three neutrons, and the release of a relatively large amount of binding energy.

PWR Power Plant Schematic

The steam that you see is from the cooling of the water after it cycles through the turbine to generate the electricity. Nuclear Reactor

Parts of the Reactor Coolant Fuel Moderator Core Heavy Water Control Rods