Energy Encyclopedia

Energy

Nuclear Energy

Commercial nuclear plants are almost all thermal-neutron reactors that slow neutrons with a moderator and remove heat with a coolant. Pressurized water reactors are the most common design worldwide. Related subjects include uranium mining and enrichment, fuel assemblies, spent-fuel storage, radioactive waste classification, and the engineered safety philosophy known as defence in depth.

Heat from splitting heavy nuclei

How it works

In a thermal reactor, neutrons split fissile nuclei. The fragments carry kinetic energy that becomes heat. A coolant carries that heat to a steam cycle (or, in some advanced concepts, another power conversion system). Control rods and the moderator help keep the chain reaction at a chosen power. Multiple physical barriers are designed to keep radioactive material contained.

Key facts

  • Fission of heavy nuclei such as uranium-235 releases heat used to raise steam.
  • Most operating power reactors are light-water designs (PWR or BWR).
  • Spent fuel remains radioactive and thermally hot; it is cooled and stored, then destined for recycling or geological disposal depending on national policy.
  • Nuclear plants do not emit carbon dioxide from the fission reaction itself.

Environment

Routine operation emits very little carbon dioxide from the fission process. Mining, construction, and decommissioning have conventional industrial impacts. Accidents, though rare, can release radionuclides; safety systems and siting exist to make that outcome unlikely. Waste isolation timescales are uniquely long for high-level waste.

Technologies

Technology

Pressurized Water Reactor Plant

The world's most common power-reactor type: water under high pressure cools the core and transfers heat to a secondary steam cycle.

Technology

Boiling Water Reactor Plant

A light-water reactor in which water boils in the core and steam goes directly to the turbine.

Technology

Small Modular Reactor

Factory-fabricated nuclear reactors of smaller output, intended to be deployed as modules.

Technology

CANDU Reactor

A pressurized heavy-water reactor with a calandria and on-power fuelling, able to use natural uranium.

nuclear power

nuclear power

Radioactivity

Radioactivity is a natural phenomenon. Unstable atom nuclei release particles: helium nuclei (alpha); electrons or positrons (beta); photons (gamma).

5 min read

nuclear power

Ionising Radiation

Ionizing radiation can be classified into alpha, beta, gamma, X-rays, and neutrons, while cosmic radiation is composed primarily of high-energy protons.

4 min read

nuclear power

Half-life

A half-life is a period after which half of the radioactive material has decayed. The half-life of isotopes is used in medicine or radiocarbon dating.

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nuclear power

Natural Sources

Natural sources of radioactivity can be found in the air, soil, cosmic rays, or humans. The annual dose received from them is called "natural background".

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nuclear power

Man-made Sources

Man-made radioactivity sources are mostly used in medicine for diagnostics and treatment. Coal and nuclear plants produce radioactive waste.

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nuclear power

Decay Series

Unstable isotopes decay in four decay series (thorium, neptunium, uranium, actinium) into other radionuclides and finally into stable atoms.

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nuclear power

Quantities and Units

The radioactivity and its effects are measured by detectors and dosimeters (activity in becquerels, dose in grays, equivalent dose in sieverts).

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nuclear power

Effects of Ionising Radiation

Ionising radiation could change material properties. In living tissue, it could damage DNA and lead to cell death. This is used in tumor treatment.

3 min read

nuclear power

Radiation Doses and Activities

A chart of the radioactivity of various objects (e.g., nuts or coal) and the doses produced by some activities (like eating bananas or visiting Chernobyl).

3 min read

nuclear power

Nuclear Power Summary

Nuclear energy originates from the makeup of the atom. Ionizing radiation caused by decay of the atom nuclei is a natural part of our environment.

17 min read

radiation

radiation

Natural and Human-Made Radiation

Background radiation is unavoidable. Medicine, industry, and nuclear energy add further doses that are managed differently.

6 min read

nuclear power industry

nuclear power industry

Fission Chain Reaction

When a neutron splits the 235U nucleus, 2-3 neutrons are released. The fission chain reaction occurs when they split other nuclei, releasing more neutrons.

3 min read

nuclear power industry

Control and Reaction States

The chain fission reaction in a nuclear reactor is controlled by the neutron absorber content. The reactor can be subcritical, critical, or supercritical.

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nuclear power industry

Principle of Operation

The binding energy that holds nucleons together is different for each isotope. When fusing light nuclei or splitting heavy nuclei, energy is released.

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nuclear power industry

Types of Nuclear Reactions

Nuclear reactions can lead to scattering, capture, or splitting depending on neutron velocity. So-called "slow" neutrons (0.02 eV) can split 235U nuclei.

3 min read

nuclear power industry

Moderator

To increase the chance of fission, the moderator slows down neutrons by collisions. The usual moderators are water, heavy water, and graphite.

3 min read

nuclear power industry

Absorber

The absorber (boron, cadmium) captures the neutrons and thus regulates the nuclear reactor output. It can be part of the coolant or control rods.

3 min read

nuclear power industry

Coolant

The coolant removes heat from the fission reaction. The most commonly used is water, but also helium, CO2, molten sodium, lead, or fluoride salts are used.

3 min read

nuclear power industry

Fission Products

After fission, the uranium nucleus decays into two approximately equal halves. Typical products are iodine and cesium, but other ways of decay exist.

3 min read

nuclear power industry

The Largest Nuclear Power Plants

In 2012, there were 436 reactors in use worldwide. The largest nuclear power plant is Japan's Kashiwazaki Kariwa with 7,965 MW of installed power output.

3 min read

fission physics

fission physics

Coolant and Moderator

The coolant removes heat. The moderator slows neutrons. Sometimes they are the same fluid.

8 min read

nuclear fuel

nuclear fuel

Fuel Production

Uranium ore is turned into U3O8 (yellowcake) or UF6 (hex), enriched up to 3—5% of 235U, and as a UO2 compressed into pellets inserted into zircalloy tubes.

3 min read

nuclear fuel

Fuel Enrichment

Enrichment is a physical process that raises the concentration of the 235U isotope to 
3—5%. Gaseous diffusion, centrifuges, or laser excitation are used.

4 min read

nuclear fuel

Interim Storage

Spent fuel is first stored in water pools near the reactor to be cooled. Then it is moved to dry interim storage, located usually on-site.

3 min read

nuclear fuel

Reprocessing

Spent fuel can be reprocessed. Extracted uranium and plutonium can be used for MOX or uranium fuel production. The remaining waste is vitrified.

4 min read

nuclear fuel

Final Repositories

The highly radioactive vitrified waste in special leak-tight flasks will be stored for thousands of years in a deep underground final repository.

3 min read

nuclear fuel

Fuel Assembly

There are different types of nuclear fuel. A typical fuel assembly for PWR consists of fuel rods, long cylinders of zircaloy filled with UO2 pellets.

4 min read

nuclear fuel

Fuel Cycle

In a closed fuel cycle, spent nuclear fuel (used fuel removed from a reactor) is reprocessed. In an open cycle, it is stored in interim storage.

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nuclear fuel

Transportation

Fresh fuel, yellowcake or uranium ore are transported normally, while highly radioactive materials like spent fuel are transported in special flasks.

3 min read

nuclear reactors

nuclear reactors

Gas-cooled Reactor (GCR) and Advanced Gas-cooled Reactor (AGR)

Gas cooled reactors (GCR), used in Japan and the UK, use graphite as a moderator and pressurized CO2 as a coolant. It is a double-circuit power plant.

3 min read

nuclear reactors

RBMK Type Reactor

The Soviet RBMK reactor uses graphite as a moderator and light water, that boils in the channels, as a coolant. Chernobyl reactor was of RBMK type.

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nuclear reactors

High Temperature Reactor (HTGR)

In a High Temperature Gas-cooled Reactor (HTGR), the highly enriched uranium spheres are dispersed in graphite (moderator) balls. Helium is a coolant.

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nuclear reactors

Reactor Using Fast Neutrons (FR)

A reactor using fast neutron (FR) burns a mixture of uranium and plutonium. Fast breeder reactors (FBR) produce more plutonium than they consume.

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nuclear reactors

The First Reactor

The first fission reactor was Chicago Pile No. 1, constructed by Enrico Fermi (2.12.1942). The first experimental nuclear power plant was the EBR-1.

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nuclear reactors

Pressurized Water Reactor (PWR)

The pressurized water reactor (PWR) uses light water at a pressure of 15.7 MPa as a coolant. Steam in the secondary circuit propels the turbine.

3 min read

nuclear reactors

Boiling Water Reactor (BWR)

In a boiling water reactor (BWR), the water boils directly inside the reactor. Dried steam propels the turbogenerator. It is a single-circuit power plant.

3 min read

nuclear reactors

Heavy Water Reactor (PHWR)

The pressurized heavy water reactor (PHWR) uses natural uranium as fuel and heavy water as coolant and moderator. Most of them are of the CANDU type.

3 min read

nuclear reactors

Small Modular Reactors

Inside a nuclear reactor, a fission reaction takes place. Released energy is removed by coolant and used to generate electricity. A common reactor is PWR.

16 min read

nuclear reactors

The Nuclear Power Plant — How it Works

Inside the nuclear power plant reactor, a controlled fission reaction takes place. Heat removed by coolant is used to produce steam to propel the turbine.

5 min read

nuclear reactors

The Future of Fission Reactors

Several types of Generation IV reactors are under development: fast reactors cooled by lead, molten salts, or sodium; HTGR, or supercritical water reactor.

4 min read

safety

safety

INES and Nuclear Accidents

The International Nuclear Event Scale ranks events. Three Mile Island, Chernobyl, and Fukushima are the reference severe accidents.

6 min read

radioactive waste

radioactive waste

Radioactive Waste

Radioactive waste is classified as low-level, intermediate-level, or high-level; as solid, gaseous, or liquid. Each type needs different handling.

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radioactive waste

Production of Radioactive Waste

Radioactive waste, produced in medicine or industry, is low-level or intermediate-level waste. Spent nuclear fuel is high-level radioactive waste.

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radioactive waste

Types of Radioactive Waste

The radioactive waste is classified by radioisotope content, activity level (low, intermediate, high-level), or state (solid, gas, liquid).

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radioactive waste

The Processing of Radioactive Waste

Liquid radioactive waste can be evaporated, filtered, biocleaned or flocculated. Solid waste can be pressed, incinerated, vitrified or fixed in synrock.

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radioactive waste

The Disposal of Radioactive Waste

For radioactive waste with short-lived isotopes, a surface repository is sufficient, while long-lived isotopes must be isolated in a deep repository.

3 min read

nuclear plant safety

nuclear plant safety

The Safety of Nuclear Power Plants

Safety is an important aspect of nuclear power plants. Safe operation and fuel handling have to be ensured to reduce the risk of a nuclear accident.

3 min read

nuclear plant safety

Nuclear Safety

Nuclear safety is the ability of a nuclear facility to keep the fission process and resulting radioactive products always under control.

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nuclear plant safety

Safety Systems

The safety of nuclear power plants is ensured by inherent safety measures that rely on physical laws, bariers, and systems redundancy.

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nuclear plant safety

External Hazards

The nuclear power plant has to withstand terrorist attack, operator error, system malfunction, or natural disasters like floods, earthquakes, or tsunami.

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nuclear plant safety

INES Scale

INES, the international nuclear and radiological event scale from 1 to 7, is used to clearly communicate the severity of a nuclear accident.

4 min read

nuclear plant safety

Nuclear Accidents

In the nuclear power industry, only three large accidents have occurred: Three Mile Island (1979), Chernobyl (1986) and Fukushima Daiichi (2011).

15 min read

nuclear plant safety

International Nuclear Organizations

There are several organizations supervising the nuclear power industry, like the International Atomic Energy Agency or the World Nuclear Association.

3 min read

nuclear plant safety

Nuclear Power Plants and the Environment

Nuclear power plants are safe and clean. They release no CO2 or emissions. Almost all of their waste is in solid form, albeit highly radioactive.

3 min read

Learn

beginner

Nuclear Energy Courses

Ionising radiation, nuclear interactions, fuel cycles, plant operation, reactor history, PWR and BWR designs, radioactive waste, and nuclear safety — eight structured lessons.

8 lessons · 120 min

Questions

Question

How does a nuclear reactor generate electricity?

Fission heat raises steam. A turbine-generator does what it does in any thermal plant.

Question

How do nuclear plant costs compare with the electricity they produce?

Nuclear economics are dominated by construction cost and financing, not fuel. Atlas will not invent a payback multiple.

Question

What is the efficiency of different power plants?

Efficiency depends on the conversion physics. Do not use one number for every nuclear, coal, or solar plant.

Glossary

F

Fission

Splitting of a heavy nucleus.

P

Pressurized Water Reactor

Light-water reactor with a secondary steam circuit.

M

Moderator

Material that slows neutrons.

C

Containment

Structure that confines radioactive material.

S

Spent fuel

Fuel removed from a reactor after irradiation.