17.2: Generating Electricity with Nuclear Energy

The Nuclear Fuel Cycle

Uranium ore must be mined, milled, and enriched to produce nuclear fuel. The nuclear fuel cycle represents the progression of nuclear fuel from creation to disposal (figure \(\PageIndex{a}\)). The first stage of the nuclear fuel cycle is uranium recovery, in which uranium ore is mined. It is then milled to produce yellowcake (uranium ore concentrate/uranium oxide/U₃O₈). Milling separates the uranium from the other parts of the ore. Each ton of mined uranium ore typically yields 1-4 pounds of yellowcake (0.05% to 0.20% yellowcake). Next, the uranium ore concentrate is converted into uranium hexafluoride (UF₆). It is then enriched to increase the concentration of uranium-235 (²³⁵U) relative to ²³⁸U. During fuel fabrication, natural and enriched UF₆ is converted into into uranium dioxide (UO₂) or uranium metal alloys for use as fuel for nuclear power plants. Disposal of spent fuel rods and other hazardous waste generated in this process are discussed in Consequences of Nuclear Energy.

Figure \(\PageIndex{a}\): The nuclear fuel cycle begins with recover (mining) of natural uranium, followed by milling, conversion, and enrichment. The enriched uranium then undergoes fuel fabrication, producing uranium dioxide (UO₂) or uranium metal alloys (MOX). The fuel then goes to a nuclear reactor at the power plant. Spent fuel rods are stored in a pool or dry cask, potentially reprocessed, and sent to disposal. Reprocessed uranium can reenter earlier steps of the fuel cycle. Wastes produced during enrichment undergo deconversion before disposal. Image by U.S. NRC (public domain)

Nuclear Reactors

The fuel, which is now in the form of cylindrical ceramic pellets are then sealed into long metal tubes called fuel rods, which are assembled in reactor cores along with control rods. Each fuel pellet, which is about 1 cm in length, stores the same amount of energy as a ton of coal. Thousands of fuel rods form the reactor core, the site of nuclear fission in a nuclear power plant (figure \(\PageIndex{b}\)).

Figure \(\PageIndex{b}\): An assembly of fuel rods containing pellets of nuclear fuel. Image by Alternative Energies and Atomic Energy Commission, France (public domain)

Heat is produced in a nuclear reactor when neutrons strike uranium atoms, causing them to split in a continuous chain reaction that releases heat energy (figure \(\PageIndex{c}\)). Specifically, fission of ²³⁵U, releases additional neutrons, which then cause the fission of nearby ²³⁵U nuclei. However, if fission occurs in too many atoms simultaneously, too much energy is released, which can cause an explosion or meltdown. This is prevented by control rods, which are made of a material such as boron that absorbs excess neutrons released in nuclear fission. When the neutron-absorbing control rods are pulled out of the core, more neutrons become available for fission, and the chain reaction speeds up, producing more heat. When they are inserted into the core, fewer neutrons are available for fission, and the chain reaction slows or stops, reducing the heat generated.

Figure \(\PageIndex{c}\): Schematic diagram of a fission chain reaction. (1) A uranium-235 atom absorbs a neutron, and fissions into two new atoms (fission fragments), releasing three new neutrons and energy. (2) One of those neutrons is absorbed by an atom of uranium-238, and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. However one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. (3) Both of those neutrons collide with uranium-235 atoms, each of which fission and release between one and three neutrons, and so on. Image and caption (modified) by Fastfission (public domain).

Nuclear reactors (figure \(\PageIndex{d}\)) contain the reactor core and the machinery needed to generate electricity from the heat released. The reactor core is submerged in water. In addition to transferring heat energy, the water also serves to slow down, or "moderate" the neutrons which is necessary for sustaining the fission reactions. Ultimately, the heat energy is used to generate high-pressure steam, which turns a turbine to generate electricity. The mechanism is similar to that of coal- or natural gas-generated electricity, but nuclear fission rather than combustion of coal is the source of heat energy.

Figure \(\PageIndex{d}\): (1) In a nuclear reactor, fuel rods full of uranium pellets are placed in water. (2) Inside the fuel rods, uranium atoms split, releasing energy. (3) This energy heats water, creating steam. (4) The steam moves through a turbine, which turns a generator to create electricity. (5) In the condensor, the steam cools back into water, which can then be used over again. At some nuclear power plants, extra heat is released from a cooling tower. Image and caption (modified) by EPA (public domain).

There are two main types of nuclear reactors: pressurized water reactors and boiling water reactors.

Pressurized Water Reactor

In a pressurized water reactor, there are three separate streams of water: the water in contact with the reactor core, the water that produces steam, and the cooling water (figure \(\PageIndex{e}\)). The reactor core is submerged in water, which is held by a steel vessel. This is surrounded by a containment structure. As the nuclear fission reaction heats the water surrounding it, the water is pumped in a cyclical stream. It transfers heat to the second stream of water, which is in a separate vessel. This second stream is kept at a lower pressure, allowing the water to boil and create steam The steam turns a turbine, generating electricity. The steam then is cooled in the condenser by a separate stream of cooling water. Because water from the reactor core does not mix with other parts of the reactor, not all of the reactor is radioactive.

Figure \(\PageIndex{e}\): There are three separate streams of water in a pressurized water reactor. The first stream is associated with the reactor core in the reactor pressure vessel. The second stream is associated with the steam generator. The third stream is from an external source and is used to condense the steam. The reactor pressure vessel contains control rods and fuel rods (not labeled). The pressure tank regulates water pressure. Hot water from the reactor moves to the steam generator, where it heats water in the second stream to produce steam. Steam from the steam generator moves through the steam line and turns a turbine, powering the electric generator. Steam cools in the condenser, and the cooled water is then pumped back into the steam generator. The reactor pressure vessel, pressure tank, and steam generator are all in a containment structure. Water from an external source is used to cool steam in the condenser. The water from the external source cools in the cone-shaped cooling tower. Image by Office of Nuclear Energy/ U.S. Department of Energy (public domain).

Boiling Water Reactor

In a boiling water reactor, there are two separate streams of water: the water in contact with the reactor core and the cooling water (figure \(\PageIndex{f}\)). The reactor core heats the water in which it is submerged. This water is held by a steel vessel that is surrounded by a containment structure. The steam produced as the reactor core heats water turns a turbine, which generates electricity. The steam then is cooled in the condenser by a separate stream of cooling water. Because water from the reactor core comes in contact with all parts of the reactor, the entire thing is radioactive.

Figure \(\PageIndex{f}\): A boiling water reactor has two streams of water. In the first stream, the same water that bathes the reactor core in the reactor pressure vessel and turns to steam and turns a turbine. The third stream is from an external source and is used to condense the steam. The reactor pressure vessel contains control rods and fuel rods (not labeled). It is within a larger containment structure. When water is heated by the reactor, it evaporates to steam. This moves through the steam line and turns a turbine, powering the electric generator. Steam cools in the condenser, and the cooled water is then pumped back into the reactor pressure vessel. Water from an external source is used to cool steam in the condenser. The water from the external source cools in the cone-shaped cooling tower. Image by Office of Nuclear Energy/ U.S. Department of Energy (public domain).

Attribution

Modified by Melissa Ha from the following sources:

• Non-renewable Energy Sources from AP Environmental Science by University of California College Prep (CC-BY). Download for free at CNX.
• Stages of the Nuclear Fuel Cycle. 2020. U.S. NRC. Accessed 01-16-2021 (public domain).
• Nuclear explained: The nuclear fuel cycle. 2020. U.S. Energy Information Administration. Accessed 01-16-2021 (public domain).