1946
Light Water Reactor
Eugene Wigner (1902–1995), Alvin Weinberg (1915–2006)
Nuclear fuel was revolutionary. A very small amount of fuel—less than a hundred tons—could power a whole city for a year or more with no airborne pollutants. It would take 20,000 times more coal to generate the same power.
The basic idea is incredibly simple, and is the same principle used in the development of the nuclear bomb. U-235 atoms will split and generate heat when a neutron hits them. Since a splitting U-235 atom creates three new neutrons, a chain reaction can occur. Allowed to proceed uncontrolled, the chain reaction creates a nuclear bomb. If controlled, however, the U-235 becomes a powerful and consistent source of heat.
All engineers had to do was design a reactor that could safely extract the heat from the fuel without melting down or blowing up. The most popular way to solve the problem turned out to be the light water reactor. In 1946, Hungarian-American theoretical physicist Eugene Wigner and nuclear physicist Alvin Weinberg proposed and developed what would become the light water reactor as we know it.
First, engineers needed a way to hold the fuel safely, and then a way to control the rate of heat production. Enriched uranium pellets are stacked into metal tubes, which are arranged in bundles. To control the heat, control rods can be lowered between the fuel rods. The material in control rods absorbs neutrons. To shut down the reactor, all of the control rods are fully inserted.
The engineers also needed a moderator—something to slow down the neutrons enough to cause U-235 to split. They chose water for this purpose because water also extracts the heat from the reactor in the form of steam.
It seems simple enough—fuel rods, control rods, and water are the essential elements. So why are nuclear reactors so complex and expensive? It comes from the fact that if the water stops flowing, the reactor overheats and melts, probably releasing radioactivity into the environment in the process. Engineers try to build a huge amount of redundancy and safety into a nuclear power plant.
SEE ALSO Trinity Nuclear Bomb (1945), Pebble Bed Nuclear Reactor (1966), Power Plant Scrubber (1971), Chernobyl (1986), Fukushima Disaster (2011).
Pictured: Diagram of a pressurized water reactor to be used on a ship.