Nuclear power plants, when first introduced, were billed as the miracle energy source, that harnessed the power of the atom to generate heat. Many of the propaganda items displayed by the government were indeed true. However, nuclear power was very poorly understood. Before long, nuclear power plants were propagating across the world.
The nuclear power plant is a rather simple concept, with many different variations. The arguably the most basic is the one employed by the Russians, the RBMK type reactor. These reactors use a grid of fuel and control rods to either grow or shrink a nuclear reaction. Through this grid, simple water is pumped. When the water hits the fuel rods, blisteringly hot from nuclear fission, the water vaporizes, and is forced by its own pressure through a turbine, which in turn generates power. The RBMK reactor is the oldest operational design of any reactor. Many experimental reactors have come and gone, however the simple principle of fuel rods heating some sort of circulating fluid remains strong. Some use heavy water, H30, while others have used CO2. The loop of this circulating fluid is called the primary cooling system. The material used for these loops are kept inside of the loop as long as possible. However, to cool down the primary loop there must be a secondary loop. These consist of pumps that draw water from a nearby body of water, and cycle the water through the primary loop, and back into the body of water. This is where many problems with nuclear reactors arise.
The heated secondary coolant can destroy ecosystems, by raising the average temperature of the body of water by any number of degrees. In nature, a slight change of one degree can cause permanent damage to an ecosystem, by driving certain species to untimely extinction or altering the composition of the food web, which in turn drive certain species to extinction. In rare instances, the primary loop of heavily irradiated coolant can leak into the secondary coolant, which in turn is pumped back into the wild. This irradiated water can then contaminate the body of water, and the radioactive matter can bio accumulate into the organisms, and further damage the ecosystem.