Understanding the Operation of Nuclear Reactors

Nuclear reactors are at the heart of some of the most significant energy and scientific infrastructure on the planet. Whether generating electricity for millions of homes or producing life-saving medical radioisotopes, reactors serve purposes far beyond what most people realize. Understanding how they work – their types, their components, and their varied roles – is essential to understanding both the promise and the responsibility that comes with nuclear technology.

Types of nuclear reactors 🔗

Today, reactors derived from designs originally developed for propelling submarines and large naval ships generate about 85% of the world’s nuclear electricity. The two dominant designs are the Pressurized Water Reactor (PWR) and the Boiling Water Reactor (BWR), both classified as light water reactors (LWRs) because they use ordinary water as both coolant and neutron moderator.

Pressurized water reactors (PWRs) 🔗

PWRs use ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine. Using high pressure (around 155 bar) ensures that the water stays in a liquid state. The heated water then flows to a steam generator, where it transfers its thermal energy to the water of a secondary cycle kept at a lower pressure, which allows it to vaporize. The resulting steam then drives steam turbines linked to an electric generator.

This two-loop separation is one of the PWR’s key safety features. The PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials. PWRs are also notably stable in operation – PWR reactors are very stable due to their tendency to produce less power as temperatures increase, which makes the reactor easier to operate from a stability standpoint.

Boiling water reactors (BWRs) 🔗

The less numerous boiling water reactor (BWR) makes steam in the primary circuit above the reactor core. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam. The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit.

Unlike PWRs, the BWR uses only two separate water systems, as it has no separate steam generator system. This steam and water mixture rises to the top of the reactor and passes through two stages of moisture separation. Water droplets are then removed and steam is allowed to enter the steam line. This simpler design can reduce construction costs, but it comes with one notable trade-off: since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance.

Beyond PWRs and BWRs, other reactor types exist – including heavy water reactors (PHWRs) like the Canadian CANDU design and gas-cooled reactors – but 80% of the world’s plants are of just two similar types , making PWRs and BWRs by far the most operationally significant globally.

Core components of nuclear power plants 🔗

Regardless of reactor type, all nuclear power plants share several fundamental components that work together to generate electricity safely and efficiently. Nuclear reactors are the heart of a nuclear power plant. They contain and control nuclear chain reactions that produce heat through a physical process called fission. That heat is used to make steam that spins a turbine to create electricity.

Fuel rods and assemblies 🔗

Reactors use uranium for nuclear fuel. The uranium is processed into small ceramic pellets and stacked together into sealed metal tubes called fuel rods. Typically, more than 200 of these rods are bundled together to form a fuel assembly. A reactor core is typically made up of a couple hundred assemblies, depending on power level. During operation, some of the U-238 is changed to plutonium, and Pu-239 ends up providing about one-third of the energy from the fuel.

Control rods 🔗

Control rods are what make a nuclear reactor controllable rather than simply a runaway chain reaction. Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing many neutrons without themselves decaying.

Control rods can be inserted into the reactor core to reduce the reaction rate or withdrawn to increase it. In an emergency shutdown, the rods are dropped fully into the core. In PWRs, the control rods are held by electromagnets and fall by gravity when current is lost; full insertion safely shuts down the primary nuclear reaction.

Steam generators 🔗

Steam generators are large heat exchangers that sit between the primary and secondary cooling loops in a PWR. In typical PWR designs, the primary coolant is high-purity water, kept under high pressure so it cannot boil. This primary coolant is pumped through the reactor core where it absorbs heat from the fuel rods. It then passes through the steam generator, where it transfers its heat to lower-pressure water which is allowed to boil.

In commercial power plants, there are two to four steam generators per reactor; each steam generator can measure up to 70 feet in height and weigh as much as 800 tons. Each steam generator can contain anywhere from 3,000 to 16,000 tubes, each about 0.75 inches in diameter. These tubes serve a critical safety function as well – they physically separate the radioactive primary coolant from the non-radioactive steam that drives the turbines. Notably, unlike PWRs, boiling water reactors (BWRs) do not use steam generators. The primary coolant is allowed to boil directly in the reactor core, and the steam is simply passed through a steam turbine.

Cooling systems 🔗

Cooling is non-negotiable in a nuclear plant – without it, the reactor core would overheat rapidly. A cooling system removes heat from the reactor core and transports it to another area of the station, where the thermal energy can be harnessed to produce electricity or to do other useful work. Typically the hot coolant is used as a heat source for a boiler, and the pressurized steam from that drives one or more steam turbine driven electrical generators.

After the steam passes through the turbines, it needs to be condensed back into water for reuse. Cool water, flowing through the tubes in the condenser, removes excess heat from the steam, which allows the steam to condense. The water is then pumped back to the steam generator for reuse. The heat removed at this stage is typically discharged via large cooling towers or released into a nearby body of water.

Containment structure 🔗

Surrounding all of this is the containment building – arguably the most visible feature of any nuclear plant. Every reactor is in a containment building – a reinforced steel and concrete structure that fully encloses the hot zone equipment. It is built as an emergency containment measure to block any unintentional release of radioactive material, such as steam or gas. This building is the final safety barrier that protects the public and the environment from radioactivity release.

Research reactors vs. power reactors 🔗

Not all nuclear reactors are built to generate electricity. A significant portion of the world’s reactors exist solely to advance science, medicine, and training. Understanding the distinction between research reactors and power reactors matters for appreciating the full scope of nuclear technology.

Purpose and scale 🔗

The primary use of research reactors is to provide a neutron source for research and various applications, including education and training. They are small in comparison with power reactors whose primary function is to produce electricity. Research reactor power ratings can range from zero up to 200 MW(th), compared with 3000 MW(th) for a typical large power reactor unit.

Put differently, the combined output of all the world’s research reactors is roughly equivalent to that of just one large commercial power reactor. Research reactors are, in a real sense, net energy consumers rather than energy producers – their value lies not in the electricity they could generate, but in the neutrons they release.

What research reactors actually do 🔗

Unlike the reactors in a nuclear power plant, which use the energy released when neutrons split atoms of uranium to generate electrical power, research reactors use the neutrons themselves for research, development, as well as for education and training purposes.

The applications are wide-ranging. Research reactors are designed and utilized for experiments, education and training, as well as the production of radioisotopes for medical and industrial applications. They provide a controlled environment to study and understand the behavior of materials, neutron interactions and radiation effects. One of the most medically important uses is radioisotope production – the most widely used radioisotope in medicine is technetium-99m, which comes from the radioisotope molybdenum-99 and is used for diagnostic imaging.

Research reactors also contribute directly to improving power reactor safety. Since research reactors are able to reproduce realistic conditions and study changes undergone by materials in power reactors, they provide essential support to optimize advanced reactors and to test fuels and materials for innovative reactors.

Design and operational differences 🔗

Research reactors are simpler than power reactors and operate at lower temperatures. There is also a much wider array of designs in use for research reactors than for power reactors. A common design is the pool-type reactor, where the reactor core sits submerged in a large open pool of water that serves as both coolant and radiation shield.

Their relative simplicity also makes them well-suited for training. Unlike power reactors, research reactors are well suited for education and training. Their systems and overall designs are simple and easy to access, thereby making it possible to safely simulate different reactor conditions. As the International Atomic Energy Agency (IAEA) notes, more than 220 research reactors continue to operate across 53 countries, supporting science and medicine in ways that complement but remain distinct from commercial power generation.

How it all connects 🔗

Whether it is a massive PWR generating gigawatts of electricity or a small pool-type research reactor producing cancer-diagnostic radioisotopes, nuclear reactors are engineered systems built around the same fundamental physics of neutron-driven fission. The type of reactor, its components, and its purpose all shape how that energy or those neutrons are ultimately put to use. As the U.S. Department of Energy points out, with more than 400 commercial reactors worldwide, nuclear power continues to be one of the largest sources of reliable electricity available – alongside the quieter but equally vital network of research reactors advancing medicine and science behind the scenes.

What do you think? Given that research reactors produce critical medical radioisotopes used in millions of diagnostic procedures every year, how should policymakers balance the costs and risks of maintaining these facilities against their medical benefits? And as new reactor designs like small modular reactors (SMRs) emerge, do you think the distinction between research and power reactors will remain as clear-cut as it is today?

References

1. https://nuclear.duke-energy.com/2021/03/29/what-you-need-to-know-about-the-different-types-of-nuclear-reactors
2. https://world-nuclear.org/information-library/non-power-nuclear-applications/radioisotopes-research/research-reactors
3. https://www.iaea.org/bulletin/what-are-research-reactors-how-do-they-contribute-to-sustainable-development
4. https://www.energy.gov/ne/articles/nuclear-101-how-does-nuclear-reactor-work