19.7: Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by bombardment with neutrons has six components: nuclear fuel consisting of fissionable material, a nuclear moderator, a neutron source, control rods, reactor coolant, and a shield and containment system.

Nuclear Fuels

Nuclear fuel consists of a fissile isotope, such as uranium-235, which must be present in sufficient quantity to provide a self-sustaining chain reaction. In most pressurized water reactors, each fuel assembly consists of fuel rods that contain many thimble-sized, ceramic-encased, enriched uranium (usually UO₂) fuel pellets. Modern nuclear reactors may contain as many as 10 million fuel pellets.

Uranium-235 is a useful fuel because it produces more than one neutron per fission on average, but its natural abundance is about 0.7 percent by weight. Most power reactors require their fuel to be enriched to at least 3 to 5 percent uranium-235 by weight.

Nuclear Moderators

Neutrons produced by nuclear reactions move too fast to cause U-235 fission reliably. They must first be slowed to be absorbed by the fuel and produce additional nuclear reactions. A nuclear moderator is a substance that slows the neutrons to a speed that is low enough to cause fission. Early reactors used high-purity graphite as a moderator. Modern reactors typically use heavy water or light water as moderators.

As neutrons have a size similar to that of hydrogen nuclei, when they hit the hydrogen atoms in water molecules, they lose a substantial amount of kinetic energy. Heavy water is a better moderator, as deuterium already has a neutron and is unlikely to absorb another neutron the way that hydrogen-1 sometimes will. Moderators like water and graphite also function as a neutron reflector to keep neutrons in the core in an even distribution.

Neutron Source

Although uranium-238 and uranium-235 fission spontaneously, the process is unpredictable, and these intrinsic sources generate very few neutrons. Thus, a reactor requires a neutron emitter to initiate the fission chain reaction. A neutron source like beryllium-9 paired with an alpha emitter such as americium-249 or plutonium-239 is installed in a reactor to produce neutrons for the initiation of the chain reaction.

Control Rods

The power level of the reactor is described by the neutron multiplication factor, denoted by k. It is the ratio of the number of neutrons produced by fission in a generation to the number of neutrons produced by fission in the previous generation.

When k is less than 1, the reactor is subcritical and the energy output is decreasing; when k equals 1, the reactor is critical and the energy output is steady; and when k is greater than 1, the reactor is supercritical and the energy output is increasing.

Nuclear reactors use control rods to control the fission rate of the nuclear fuel by adjusting the number of slow neutrons present to keep the rate of the chain reaction at a safe level. Control rods are made of boron, cadmium, hafnium, or other elements that are able to absorb neutrons.

When control rod assemblies are inserted into the fuel element in the reactor core, they absorb a larger fraction of the slow neutrons, thereby slowing the rate of the fission reaction and decreasing the power produced. Conversely, if the control rods are removed, fewer neutrons are absorbed, and the fission rate and energy production increase. In an emergency, the chain reaction can be shut down by fully inserting all of the control rods into the nuclear core between the fuel rods.

Reactor Coolants

In a pressurized water reactor, the reactor coolant is used to carry the heat produced by the fission reaction to an external boiler and turbine, where it is transformed into electricity. Two heat-exchanging coolant loops are often used to prevent the transfer of contaminated coolant to the steam turbine and cooling tower. Most commonly, water is used as a coolant. Other coolants in specialized reactors include molten sodium, lead, a lead–bismuth mixture, or molten salts. A large, hyperboloid cooling tower condenses the steam in the secondary cooling circuit and is often located at some distance from the actual reactor.

Shield and Containment System

Pressurized water reactors are equipped with a containment system (or shield) that typically consists of three parts: (i) a steel shell that is 3–20 centimeters thick; the moderator within the shell absorbs much of the neutron radiation produced by the reactor; (ii) a main shield of 1–3 meters of high-density concrete that absorbs γ rays and X-rays; (iii) additional shielding to absorb incident radiation from the shielding processes of (i) and (ii). In addition, pressurized water reactors are often covered with a steel or concrete dome that is designed to contain any radioactive materials that might be released by a reactor accident.

This text is adapted from Openstax, Chemistry 2e, Section 21.4: Transmutation and Nuclear Energy.

Nuclear fission releases a great deal of thermal energy, allowing electricity generation from a steam turbine.

Nuclear fuel is typically a fissile nuclide such as uranium-235 that produces more than one neutron per fission on average. Fast neutrons released by fission must be slowed down by neutron moderators because thermal neutrons start chain reactions in fissile fuel most efficiently.

Water is a good moderator because hydrogen nuclei and neutrons have comparable sizes, ensuring that neutrons lose substantial kinetic energy in the collision. Heavy water is even better, as deuterium already has a neutron and is unlikely to absorb another.

Moderators also function as a neutron reflector to keep neutrons in the core in an even distribution.

Because the spontaneous fission of uranium-235 or 238 is unpredictable, a neutron source is installed in a reactor to ensure controlled initiation of the chain reaction.

The status of the chain reaction is described by the neutron multiplication factor, k: the ratio of the number of neutrons produced by fission in a generation to the number of neutrons produced by fission in the previous generation.

When k is less than 1, the reactor is subcritical and the energy output is decreasing. When k is 1, the reactor is critical and the energy output is steady. When k is greater than 1, the reactor is supercritical and the energy output is increasing.

The chain reaction is controlled with control rods made of neutron-absorbing materials like boron or cadmium. Fully inserted control rods absorb a large number of neutrons, keeping the reactor subcritical. Withdrawing the control rods allows more and more fissions to occur.

Coolant, such as water, transfers heat away from the reactor core to make steam for the turbine. As the reactor heats up, the neutrons move faster and are less likely to cause fissions, which helps avoid overheating.

The core is shielded by materials like water and thick concrete layers. The overall core design and containment structure both depend on the specific type of reactor.

• Nuclear Fuel - Fissionable material used in a nuclear reactor to generate power (e.g., uranium-235).
• Nuclear Moderator - Substance that slows down the neutrons in nuclear reactors (e.g., heavy water).
• Neutron Source - Emitter to initiate fission chain reactions (e.g., beryllium-9).
• Control Rods - Absorb neutrons to control fission rate and maintain safe power levels (e.g., boron).
• Reactor Coolant - Used to carry the heat produced by fission to an external boiler and turbine (e.g., water).
• Shield and containment system - Designed to contain any radioactive materials that might be released by a reactor accident.

• Define Nuclear Fuel - Explain the composition and function of nuclear fuel (e.g., uranium-235).
• Contrast Control Rods vs Reactor Coolants - Distinguish their roles in maintaining safe nuclear reactions and power generation (e.g., boron rod vs. water).
• Explore Nuclear Reactors - Describe the setup and working of nuclear reactors (e.g., sequence of components).
• Explain Uranium-235 Fission - Overview of how uranium undergoes fission in reactors to generate energy.
• Apply in Context - Illustrate how factors like control rods and coolants help maintain safe chain reactions in nuclear reactors.

• [Question 1] What is nuclear fuel and how is it used in nuclear reactors?
• [Question 2] What are control rods and how do they maintain safe fission rates?
• [Question 3] How does a moderator like heavy water contribute to nuclear reactor functionality?

• Students - Understanding key concepts about nuclear reactors can build a foundation for understanding nuclear physics and chemistry.
• Educators - Provides a clear framework to teach about nuclear energy, fission process, and components of a nuclear reactor.
• Researchers - A comprehensive understanding of nuclear reactors supports development and enhancement of nuclear technologies.
• Science Enthusiasts - Offers insights into the fascinating world of nuclear power generation and associated technologies.