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Nuclear energy holds the atoms that make up our universe together. This energy is released both when larger atoms split apart, undergoing fission, and when smaller atoms combine together, known as fusion. The amount of energy released in fission is tremendous — millions of times more than other forms of energy. Fusion releases four times as much energy as fission. Fission or fusion, all of that energy is released using reactors.
Nuclear Fission provides about 10% of the world’s electricity, powers naval vessels around the world, and has powered space missions.
The energy released by fission is a million times greater than the chemical energy released by combustion, so a small amount of nuclear fuel, usually uranium, produces an enormous amount of heat. Over four hundred nuclear power plants around the world produce 400 GW of electricity, enough for 300 million US homes.
The fission reactors at each of these nuclear power plants create steam that turns a turbine to generate electricity, just as coal and gas plants do. Find out how nuclear reactors of the present and future work.
Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR) are the most common reactors in the world today. In both PWRs and BWRs, light enriched uranium fuel, arranged in the reactor’s core, heats water. PWRs use high pressure to prevent water from turning to steam in the core; steam is created in a secondary loop using a steam generator.
BWRs generate steam directly in the core of the reactor, eliminating the need for some equipment, but resulting in radioactive steam in the turbine.
Control rods, neutron absorbing movable rods, are used in both reactors to control the chain reaction caused by neutrons from fissioning atoms being released and creating more fissions. By absorbing neutrons, the control rods sustain the chain reaction and keep it at an efficient rate.
SMRs and microreactors gained significant attention in recent years. These reactors are small in terms of size and energy production, compared to large existing reactors, so they can be constructed in a factory. When more energy production is needed, more SMRs can be added.
There are a number of significant advantages to SMRs and microreactors.
SMRs and microreactors may be small, but they can utilize a wide variety of nuclear technologies and open new markets at lower cost. The first United States SMR Design Certification is expected before the end of 2020. The first non-water based microreactor License Application was submitted in March of 2020 and is expected to be approved in 2022. SMRs and microreactors are in development or operation around the world, including floating SMRs in Russia, heat generating reactors in China, and a variety of other concepts.
Advanced reactors build on the lessons learned from LWRs and non-LWRs and offer the potential of more economic, safer, diverse, and efficient ways to generate clean energy for the future. Water based and non-water based advanced reactors are under development all over the world and significant progress towards widespread use is expected in this decade.
If the nucleus of a heavy atom–such as uranium–absorbs a neutron, the nucleus can become unstable and split. This is called nuclear fission. Fission releases energy in the form of heat. Although fission can occur naturally, fission as encountered in the modern world is usually a deliberate man-made nuclear reaction.
Typical fission events release about two hundred million eV (200 MeV) of energy. In contrast, most chemical oxidation reactions (such as burning coal) release at most a few eV per event. So, nuclear fuel contains at least ten million times more usable energy per unit mass than does chemical fuel.
Fusion is the opposite reaction of fission. In fusion, atoms are fused together.
For a fusion reaction to occur, it is necessary to bring two nuclei so close that nuclear forces become active and glue the nuclei together. Deuterium and Tritium, isotopes of hydrogen, are used in fusion reactors. Nuclear forces are small-distance forces and have to act against the electrostatic forces where positively charged nuclei repel each other. This is the reason nuclear fusion reactions occur mostly in high density, high temperature environment.
Recreating that environment is the greatest challenge to producing commercial scale fusion energy, but it’s a challenge well worth pursuing. Nuclear fusion can produce four times the amount of energy as nuclear fission.
Radiation is simply the transmission of energy from a source via waves or particles.
There are many kinds of radiation that move in waves, most of them very familiar to you, like radio waves, visible light, and x-rays. They are all part of the electromagnetic spectrum.
Radiation can also be described as non-ionizing or ionizing.
Non-ionizing radiation has enough energy to excite atoms, making them move more rapidly. Microwave ovens work by exciting water molecules, creating friction. The friction creates heat, and the heat warms the food. Other examples of non-ionizing sources include radio transmissions, cell phones, and visible light.
Ionizing radiation has enough energy to remove electrons from their orbits, creating ions. Examples of ionizing sources are high-level ultraviolet light, X-rays, and gamma rays.
Ionizing radiation happens when an unstable atom (a radioactive isotope of an element) emits particles or waves of particles to become more stable. This process is called radioactive decay.
Not all of the atoms of a radioactive isotope decay at the same time. Instead, the atoms decay at a rate that is characteristic to the isotope. The rate of decay is a fixed rate called a half-life.
The half-life of a radioactive isotope refers to the amount of time required for half of a quantity of a radioactive isotope to decay. For example, carbon-14 has a half-life of 5730 years, which means that if you take one gram of carbon-14, half of it will decay in 5730 years. Different isotopes have different half-lives.
Radioactive decay is random; we can’t tell which atoms in an isotope sample will decay. But, it is also predictable and exponential, so we can determine how long it will take for a sample to completely decay based on its half-life.
There are four basic types of ionizing radiation–alpha, beta, gamma, and neutron–and each has unique properties.
Alpha radiation happens when the unstable atom emits two protons and two neutrons—basically a helium nucleus. The original atom, with fewer protons and neutrons, becomes a different element.
Compared to other forms of ionizing radiation, alpha particles are large and heavy. They can’t travel very far, so they are useful in things like smoke detectors. They can be stopped by a piece of paper, your skin, or even just a few inches of air.
Beta radiation occurs when an atom decays by giving off a high-energy, high-speed particle that has a negative or positive charge. These “beta” particles are both smaller and more energetic than alpha particles. There are two types of beta decay. When a negatively charged particle is emitted from the nucleus, it is called beta minus decay. The negative beta particle is also called an electron, but it originates from the nucleus, not the electron cloud surrounding the atom. When a positively charged particle is emitted from the nucleus, it is called beta plus decay or positron emission, because the positively charged particles are called positrons. They have the same mass as electrons.
Beta minus decay happens when an atom has more neutrons than protons. To gain stability, a neutron becomes a proton and an electron. The proton stays in the nucleus, while the electron (beta minus particle) is emitted. Because the atom gains a proton and loses a neutron, its atomic number increases by one. The mass number stays the same because the number of nucleons stays the same.
Beta plus decay (positron emission) happens when an atom has more protons than neutrons. To become more stable, a proton becomes a neutron, and a positron is emitted. Because the nucleus loses a proton and gains a neutron, its atomic number decreases by one and the atom becomes a different element. The atom still has the same number of nucleons, though, so the mass number stays the same.
Gamma radiation and x-rays, are high-energy waves that can travel great distances at the speed of light. Both can penetrate deeply into matter.
X-rays are stopped by dense materials like bone, tumors, or lead. This makes them useful for medical diagnosis.
Gamma rays can penetrate further with higher energy. Gamma radiation can be used to precisely target and eliminate tumors; it also has a number of uses in industry, agriculture, pest-control, and more. Gamma rays be stopped by several inches of lead.
Neutron radiation is created as a result of fission reactions and happens in nuclear reactors. Neutrons are extremely high energy, so need many feet of dense materials like water or concrete to stop them. Neutron radiation can make other materials radioactive and is used to create the radioisotopes used in medical treatments.
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