Introduction to Nuclear Power
Nuclear power plants are an important component of the global power generation mix, accounting for 10% of the world’s total power generation mix.
Nuclear reactors have many different applications, from powering submarines to producing medical isotopes for imaging, cancer treatments and research.
In this module, our focus is on generating electricity.
Overview
In a nuclear power plant, referred to as an NPP, atomic energy is converted into electrical energy.
Despite some inherent challenges, the future potential seems positive – and necessary – to meet long-term carbon reduction goals.
Extensive R&D is underway to find safer, more cost-effective plant designs.
After introducing some basic concepts, we’ll focus on the equipment and operating conditions of typical NPPs.
Topics include:
- A thermal power plant analysis framework
- The basic science of fission
- Processing uranium into fuel rod assemblies
- How a nuclear power plant actually works
- The two types of nuclear reactors
- The different equipment in a plant’s hot zone and cold zone
- Benefits and challenges of the nuclear option
- Environmental challenges, including
- Spent fuel disposition, and
- Water and air quality monitoring, and finally
- Decommissioning a plant at the end of its useful life.
A Nuclear Plant Analysis Framework
Let’s first introduce a framework that describes any thermal power plant. The essence of power generation is a dual-system process to convert a fuel to energy.
The primary system in an NPP is routinely called the hot zone, because the nuclear reaction produces radioactivity.
The primary system is always fuel-specific, and for a nuclear plant it includes:
· Fuel sourcing and containment
· The reactor, and
· The steam generator
The secondary system in an NPP is called the cold zone. The equipment in the cold zone of a nuclear plant is very similar to that in any fossil-fuel plant and includes:
· A large steam turbine,
· A generator that produces the electricity, and
· A condenser for cooling water
Before we look more closely at the hot and cold zones, it’s useful to have a basic understanding of nuclear science.
Fission Basics
Nuclear power is all about the atom.
As you might remember from secondary school… Most atoms have three different subatomic particles inside them:
· Protons
· Neutrons, and
· Electrons
The protons and neutrons are packed together into the center of the atom, called the nucleus.
The electrons, which are much smaller, whizz around the outside. Most of an atom is empty space.
The fission process is at the heart of every NPP. Fission occurs at the particle level in an atom.
Here a large, unstable uranium nucleus is struck by a low-energy thermal neutron, splitting it into two or more smaller, lighter nuclei.
The process was discovered by Enrico Fermi in 1934, earning him the 1938 Nobel Prize.
Due to its natural radioactivity, uranium is a common fuel source in an NPP after it has been processed into what is called an isotope, known as U-235.
The genius of nuclear fission is that, once initiated, it continues to fire unimpeded – a chain reaction.
Here neutrons from one atom crash into other U-235 atoms, which makes them split apart, thus releasing more spare neutrons to strike U-235 isotopes.
This fission chain reaction releases a very large amount of radioactive heat energy.
This atomic reaction cycle continues until the fission process is stopped by using reactor control rods, which we describe later.
Before fission can take place, the U-235 fuel source must be mined, processed and enriched.
Here’s a brief explanation of how natural uranium becomes a fuel source.
Uranium Processing
When mined, uranium ore naturally contains three different forms of uranium, called isotopes: U-238, U-235, and U-234.
Because its atoms split easily, U-235 is used to fuel NPPs.
However, mined uranium ore contains less than1% of U-235. The ore requires extensive refining to create a uranium product known as yellowcake.
Using a set of complex chemical and manufacturing processes:
· The yellowcake is converted to uranium hexafluoride, or UF6, informally called “hex” in the industry, and
· The UF6 is enriched, and then contains a 3 to 5% concentration of U-235 – enough to fuel a reactor.
Note: A single pound of enriched uranium contains as much energy as 1,360 metric tons of coal.
The next step in the fuel management process is to prepare the uranium for use in the NPP.
Fuel Rod Assemblies
The enriched UF6 is compressed and shaped into small pellets encased in a ceramic base mixture.
Each pellet can produce about the same amount of energy as almost 570 liters of crude oil.
The pellets are packed into long zirconium metal tubes, called fuel rods. Each fuel rod is assembled into a group of 200 or more tubes, which is called a fuel assembly or fuel bundle.
Each reactor core is made up of several hundred fuel bundles.
Once the fuel is loaded into the nuclear plant, how does it actually work?
How Does a Nuclear Power Plant Work?
In simplest terms, the heat from atomic fission is used to heat water to create high-pressure steam.
The steam drives one or more giant turbines, tightly coupled to generators which then produce the electricity.
Here are the steps inside a nuclear plant, in the order that they occur:
- The fuel rods and cooling water are loaded into the reactor core.
- Atoms are split in a carefully controlled chain reaction, as described previously, to release thermal energy.
- Control rods in the reactor manage the atomic reactions by absorbing free neutrons.
- Water is constantly pumped through the reactor in a closed-loop system, to collect and carry the heat to a steam generator, which is a gigantic heat exchanger.
- Once the heated water enters the steam generator, available heat is transferred to a separate closed-loop system, thereby turning its water into high-pressure steam.
- High pressure steam then enters a large turbine. As the pressurized steam moves past the turbine vanes, its shaft begins to spin at high speed.
- The rotating shaft is connected to a generator which produces the electricity that flows out to the grid, and then on to homes, offices, and factories.
- A condenser and cooling water circuit is used to recover as much heat as possible before discharging access water from the facility.
Two designs account for most of global reactors: pressurized water reactors (or PWRs) and boiling water reactors (or BWRs).
Pressurized Water Reactors (PWRs)
About 70% of the world’s reactors are PWRs, 15% are BWRs, and the balance are other engineering designs.
PWRs use plain water as a coolant, and incorporate dual, independent circuits.
The primary cooling water circuit, or loop, is maintained at very high pressure to prevent it from boiling.
This loop then passes through a heat exchanger, which transfers the heat to a secondary coolant loop.
This secondary, pressurized loop spins the turbine to generate power.
Using two independent and completely unconnected loops of water ensures that the reactor-contaminated water is safely contained, and entirely separate from the cold zone plant equipment.
BWRs are very similar to PWRs.
Boiling Water Reactors (BWRs)
How do boiling water reactors differ from pressurized water reactors?
In a BWR there is only a single coolant loop that permits water flow at a lower pressure, allowing the water to boil inside the reactor core.
The steam passes through steam separators just above the reactor core, and then out to the turbine to create electricity.
Whatever the type of NPP, all plants are divided into a hot zone and a cold zone.
NPP Plant Organization
The hot zone refers to all systems located inside the containment building.
This area handles radioactive material, and thus presents an elevated risk of exposure to personnel.
The cold zone contains much less radioactive potential, and has a much lower risk of employee exposure.
In most organizations, there are duplicate and independent functional departments in each zone to minimize the risk of cross-contamination to staff and equipment.
This can mean two of everything – shift operators, maintenance departments, tool rooms, stock rooms, and equipment.
Most of the hot zone equipment at an NPP is specific to a nuclear plant, while the cold zone equipment is similar to that in any thermal power plant.
Summary
Based on our discussion this far, you should now be able to:
· Explain the science behind the fission process.
· Discuss the conversion of uranium into nuclear fuel.
· Understand the difference in the two major types of nuclear reactors.
· Outline the organizational requirements to protect staff from radiation.
Let’s now go inside a nuclear power facility starting with Hot Zone equipment, including:
Hot Zone Equipment
- The reactor
- The control rods
- The steam generator, and
- Fuel rod coolant
The Reactor
As previously discussed, the nuclear reactor contains and controls sustained fission chain reactions.
The reactor core is a cylindrical arrangement of fuel rod assemblies.
The core is about 3.5 m in diameter, over 4 m tall, and encased in a steel pressure vessel with walls that are several centimeters thick.
Once the fuel rod assemblies are placed next to each other in the core, adding water initiates the chain reaction.
The reactor core has no moving parts except for the control rods, discussed next.
Control Rods
Control rods are a critical safety feature of an NPP. In the reactor core, they regulate the output energy of the reactor and are typically in the shape of tubes or plates.
Their composition includes chemical elements such as boron or silver that can absorb many neutrons without themselves fissioning.
To reduce power and reactions, the control rods are inserted deeper into the reactor.
To increase power and reactions, the control rods are raised from the reactor core.
The depth of insertion chosen by the operation technician depends on the level of nuclear chain reaction needed.
The output generated by this operation is measured in the cold zone in megawatts, MW, or kilowatts, KW.
Fuel Rod Coolant
Once installed in the reactor, all fuel assemblies are immersed in water, which acts as both a coolant and a moderator.
A moderator in an NPP is defined as the substance that slows down the neutrons to sustain the chain reaction.
Next discussed is the steam generator.
Steam Generator
It’s really nothing more than a sophisticated heat exchanger, much like the radiator commonly found in automobiles, only it works under a much higher pressure.
NPP steam generators can be 70 feet tall and weigh a staggering 680 to 725 metric tons.
Our graphics expert will adapt and build this image:
In PWRs a steam generator is placed between the primary and secondary cooling loops. Modern PWR nuclear plants typically have 2 to 4 steam generators per reactor.
Pressurized, ultra-purified water is used in the reactor to absorb heat from the fuel rod bundles.
This coolant water is then routed through the steam generator, where it transfers, or exchanges, its heat into much lower-temperature boiling water.
Note that steam generators are not used in BWRs, as the coolant boils directly in the reactor core, and the steam then passes to the turbine.
Containment Building
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, up to a pressure of 80 to 90 psi.
This building is the final safety barrier that protects the public and the environment from radioactivity release, and is considered an intrinsic part of most nuclear plant designs.
Fun Fact: In the US, the design criteria, including the thickness of the containment shell and the missile shield (the domed cap), are heavily regulated and monitored by the US Nuclear Regulatory Commission.
The regulation clearly states that the containment structure must be able to withstand the impact of a fully-loaded passenger plane without incurring a single rupture.
Having covered the hot zone equipment, let’s turn our attention to the equipment in the cold zone.
Cold Zone Equipment
Equipment typically found in a cold zone includes:
· The steam turbine
· The generator
· The condenser, and
· The cooling tower
Steam Turbine
Regardless of how steam is generated, whether in a PWR or a BWR, it serves the same purpose – to spin the turbine.
Turbines are designed to spin at high speeds.
In a typical nuclear turbine, the revolutions per minute can be anywhere between 1,800 and 2,200 RPM.
The larger a turbine is, the slower its rotation speed.
By design, turbines are durable, reliable, and can function trouble-free for over 30 years, as long as they are routinely maintained.
Generator
The spinning turbine then powers the generator rotor, which creates the electricity being sent out to the grid.
Electric generators are designed to convert rotational mechanical energy into electrical energy.
The power output of a NPP, measured in megawatts, depends on the number of reactors and generators.
For example, the Gravelines Nuclear Power Station in Nord, France, is the largest nuclear facility in Western Europe. It consists of 6 nuclear reactors of 900 MW each, capable of generating 6% of French electricity production.
Coolant water for the plant comes from the North Sea.
For more details on turbine generators, review our gas-fired power plant module.
Condenser
First a condenser is used.
It is a large heat exchanger designed to convert the exhaust steam from the turbine back into hot water.
The hot water is then returned to either the steam generator in a PWR, or to the reactor in a BWR.
Cooling water is needed from a nearby body of water, with a volume suitable for the size of the plant.
The heat is removed using a circulating coolant loop system, where a lot of it is exhausted from the cooling tower.
Note: Typically, there is a separate condenser for each steam turbine. This ensures that the condenser can maintain a vacuum in the cooling system, thus optimizing the turbine’s efficiency.
Cooling Tower
Perhaps the most recognizable elements of any NPP are the large cooling towers.
Cooling towers are an energy-efficient means of removing heat from circulating water before it returns to its source.
The white “smoke” emitted from these towers is actually steam, which is not radioactive because it has never been into contact with the radioactive material in the hot zone.
Like most forms of energy, nuclear power has both benefits and challenges.
Benefits of Nuclear Power
NPPs, unlike fossil-fuel power plants, do not produce air pollution or carbon dioxide. Since they do not create greenhouse gasses, they do not contribute to climate change.
Nuclear energy is also considered extremely reliable. Most plants, if properly maintained, can function continuously, with interruptions only for required major maintenance.
For those countries that are dependent on imports of natural resources such as coal, oil, and gas to meet their energy needs, nuclear energy provides a means of establishing a certain amount of independence.
Though communities are often leery of accepting nuclear installations, over time they often come to be viewed as vital to local economies, relied upon for both job creation and tax revenues in addition to a steady power supply.
Challenges
Since the 2011 Fukushima disaster in Japan, public attitudes toward nuclear energy tend to be wary due to legitimate concerns about safety, security, and long-term radioactive waste management.
NPPs are costly to construct, highly regulated, and expensive to maintain. Lead times are long, and return on investment can take decades.
Finally, safety and security are of utmost importance, both from an operations standpoint and in terms of facility protection and access.
Operations and security procedures are necessarily highly regulated and rigidly enforced, but can be burdensome to maintain.
Environmental Outlook
Attitudes concerning nuclear energy vary widely across the world, particularly after the 1986 Chernobyl incident.
Many European countries have sought to either reduce or even completely abandon nuclear power.
- Germany plans to phase out its seven remaining nuclear reactors.
- Voters in Italy rejected the prospect of a renewed nuclear energy program in 2011.
- However, France is the second largest nuclear energy producer in the world, and remains largely committed to an ongoing nuclear program, although possibly at a lower level than currently in use.
Nuclear power has provided approximately 20 percent of U.S. electricity generation for some time. The current US nuclear fleet has 104 reactors.
There is substantial US interest in new nuclear construction. The Nuclear Regulatory Commission[RM1] has pending applications for 26 new units.
Actual construction commitments and timelines depend on a more favorable economic climate for the financing of such capital-intensive power projects, especially considering the current situation of abundant and inexpensive natural gas supplies.
Monitoring
All NPP operators constantly monitor the environment surrounding the facility and neighboring communities for any sign of radioactivity.
They routinely take samples of the air, water, and the soil to test for any contamination.
Sample collecting is considered an essential effort to prevent negative environmental impacts.
Additional layers of monitoring are provided by local governments and local utility companies.
Spent Fuel Disposition
A principal concern with nuclear power is radioactive waste.
Spent reactor fuel and other waste material can remain radioactive for thousands of years, with serious, long-term implications for people and the environment.
After the fuel pellets have been used, they are referred to as spent fuel. Spent fuel assemblies are extremely radioactive and must initially be stored in specially designed pools of water.
The pools provide a radiation shield and also cool the fuel. Over time, as the pools reach capacity, an alternative storage method becomes necessary.
After about one year in the pools, the radioactivity level of the spent fuel decreases, and the spent fuel can be moved into specially designed dry storage containers, called casks.
Casks are normally stored onsite in outdoor steel or concrete structures with self-contained air cooling. Cask storage is considered temporary.
Worldwide, more than a quarter million metric tons of highly radioactive waste sits in storage near NPPs and other types of nuclear facilities.
A permanent disposal facility for highly radioactive waste does not currently exist anywhere in the world, though such installations have long been under study.
The Onkalo spent-fuel repository in Finland is the only deep geological repository that has moved to the construction phase[RM2] .
Decommissioning
When a nuclear reactor has reached its end of life, it must be decommissioned, or safely taken out of service.
Decommissioning can take three forms:
· Immediate dismantlement
· Safe enclosure, which postpones decommissioning for a period of up to 60 years, or
· Entombment, which involves encasing the site in a barrier material to prevent radioactive release
The decommissioning process involves safely dismantling the reactor and all associated radioactive equipment.
The site must be left in a state that poses zero risk to the environment and surrounding life forms.
Any residual radioactive material must be cleaned up, including that found in any of the plant systems and structures.
Spent and unspent fuel must be accounted for and must be safely secured, whether stored on- or off-site.
Extremely rigid guidelines govern the decommissioning of an NPP.
To begin the decommissioning process, NPP operators must comply with national legislation, which generally requires obtaining a license from the appropriate regulatory authorities.
Summary
You should now have a good idea of how a Nuclear Power Plant works.
There are currently 440 NPPs operating around the world, with another 50 under construction.
Nuclear energy accounts for about 10% of the world’s total energy production, and though it is associated with some inherent challenges, research and innovation are ongoing.
Based on our discussion today, you should now be able to:
· Explain the difference between hot zone and cold zone equipment in an NPP.
· Identify the main equipment used in a nuclear power plant.
· Broadly outline the benefits and challenges associated with nuclear energy.
· Explain the spent-fuel disposition and the decommissioning processes.