The fuel most widely used by nuclear plants for nuclear fission is Uranium. Uranium is neither renewable nor scarce however; nuclear plants use a certain kind of Uranium, U-235, because those atomic nuclei are easily split. Although Uranium is common, and about 100 times more common than silver; U-235 is relatively rare and most Uranium from the United States is mined in the Western United States. The U-235 must be extracted from the mined Uranium and processed before it can be used as a fuel. During nuclear fission, a small particle called a neutron hits the Uranium atom nucleus and splits it, releasing a great amount of energy as heat and radiation. More neutrons are also released that proceed to bombard other Uranium atoms, and the process repeats, initiating a chain reaction.
Fission: one neutron splitting the Uranium 235 atom into two elements and two neutrons; and the components of a nuclear reactor.
Nuclear fission also occurs naturally when Uranium undergoes spontaneous fission, but emitting radiation at a very slow rate, and Uranium is a superior choice for the induced
fission of nuclear power plants. Uranium is a common element on Earth and has existed since the planet formed. As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom splits). The process of capturing the neutron and splitting happens very quickly. The decay of a single U-235 atom releases approximately 200 MeV million electron volts, and there are lots of Uranium atoms in a pound (0.45 kilograms) of Uranium.
Consequently, a pound of highly enriched Uranium as used to power a nuclear submarine is equal to about a million gallons of gasoline. The splitting of an atom releases an incredible amount of heat and gamma radiation
,
or radiation made of high-energy photons. The two atoms that result from the fission later release beta radiation (superfast electrons) and gamma radiation of their own. Interdependently, scientists over-enrich a sample of Uranium to three-percent enrichment, which is sufficient for nuclear power plants, as compared to the 90 percent U-235 for weapons-grade Uranium.
Another fissionable material is Plutonium-239, created by bombarding U-238 with neutrons as commonly occurs in a nuclear reactor. Despite all the drama surrounding the word
nuclear
, power plants that depend on atomic energy operate similarly to a traditional coal-burning power plant because both heat water into pressurized steam that drives a turbine generator. The key difference between the two plants is the method of heating the water. While older plants burn fossil fuels, nuclear plants depend on the heat that develops during nuclear fission from atoms splitting and releasing energy. Nuclear reactors are machines that contain and control chain reactions, while releasing heat at a controlled rate. In electric power plants, the reactors supply the heat to turn water into steam, which drives the turbine-generators.
Two types of reactors are commissioned in the United States: boiling-water reactors (BWRs), and pressurized-water reactors (PWRs). In the BWR, the water heated by the reactor core turns directly into steam in the reactor vessel and is then used to power the turbine-generator. In a PWR, the water passing through the reactor core is kept under pressure so that it does not turn to steam but remains liquid. Steam to drive the turbine is generated in a separate piece of equipment called a steam generator, a giant cylinder with thousands of tubes through which the hot radioactive water can flow. Outside the tubes in the steam generator, nonradioactive water or clean water boils and eventually turns to steam. The clean water is replenished from one of several sources: oceans, lakes or rivers, whereas the radioactive water recirculates to the reactor core, where it is reheated and pumped back to the steam generator. Approximately seventy percent of the reactors operating in the United States are PWR.
To convert nuclear fission into electrical energy, the energy discharged by the enriched Uranium in the nuclear power plant heats water into steam. Enriched Uranium is typically formed into one-inch-long, or 2.5-centimeters-long, ceramic pellets, each with approximately the same diameter as a dime. Each fingertip-sized ceramic pellet produces the same amount of energy as 150 gallons of oil. These energy-rich pellets are stacked end-to-end in 12-foot metal fuel rods, and the rods are collected together into bundles called
fuel assemblies
. The fuel assemblies are submerged in water inside a pressure vessel where the water acts as a coolant, without which the Uranium would eventually overheat and melt. To prevent overheating,
control rods
made of a material that absorbs neutrons are inserted into the Uranium bundle with a mechanism that raises or lowers them to control the rate of nuclear reaction. Consequently, to produce more heat from the Uranium core, the control rods are lifted out of the Uranium bundle, absorbing fewer neutrons and increasing the chain reactions with more neutrons. To reverse this process and reduce heat the rods are lowered into the Uranium bundle. When the rods are completely lowered into the Uranium bundle, the reactor can be shut-down in response to an accident or to change the fuel. The Uranium bundle acts as an extremely high-energy source of heat, heating the water and producing steam. The steam rotates a turbine that spins a generator to produce electricity or electrical power. Harnessing the expansion of water into steam has been applied to significant tasks in many cultures for hundreds of years.
In some nuclear power plants, the steam from the reactor goes through a secondary, intermediate heat exchanger to exchange heat to another loop of water converting it to steam, which drives the turbine. The advantage to this design is that steam from the radioactive water/ never contacts the turbine. Also, in some reactors, the coolant fluid in contact with the reactor core is gas (carbon dioxide) or liquid metal (sodium, potassium); these types of reactors allow the core to be operated at higher temperatures. The radioactive elements inside a nuclear power plant require thicker walls than you'd find at a coal power plant, including various protective barriers containing the atomic heart of the plant.[3]