Nuclear Energy Glossary

50 essential terms for understanding nuclear energy, thorium technology, and reactor design.

Baseload: The minimum level of electricity demand over a 24-hour period. Nuclear power plants are ideal baseload generators because they operate continuously at high output regardless of weather or time of day.
Becquerel (Bq): The SI unit of radioactivity, defined as one nuclear decay per second. Named after Henri Becquerel, who discovered radioactivity in 1896. Used to measure the activity of a radioactive source.
Boiling Water Reactor (BWR): A type of light water reactor in which the reactor core heats water directly to steam, which drives a turbine. BWRs are simpler than pressurized water reactors but expose the turbine to potentially contaminated steam. General Electric designed the most common BWR models.
Breeder Reactor: A nuclear reactor that produces more fissile material than it consumes. The thorium fuel cycle is inherently a breeding cycle: thorium-232 absorbs neutrons and breeds uranium-233 fuel. Fast breeder reactors using the uranium-plutonium cycle have also been built, most notably in France (Superphenix) and Russia (BN-800).
CANDU: CANada Deuterium Uranium. A pressurized heavy-water reactor design developed by Atomic Energy of Canada Limited. CANDU reactors use natural (unenriched) uranium fuel and heavy water (deuterium oxide) as both moderator and coolant. They can be adapted to use thorium fuel.
Capacity Factor: The ratio of actual electricity output to the maximum possible output over a period of time. U.S. nuclear plants consistently achieve capacity factors above 90%, far exceeding wind (~35%) and solar (~25%). This makes nuclear the most reliable source of electricity generation.
Chernobyl: Site of the world's worst nuclear disaster on April 26, 1986, in Soviet Ukraine. A flawed RBMK reactor design combined with operator error caused a steam explosion and graphite fire that released massive radioactivity. The RBMK design's positive void coefficient — which made the reaction accelerate rather than slow when coolant was lost — was a fundamental design flaw not present in Western reactors or MSR designs.
Containment: The system of physical barriers designed to prevent the release of radioactive material from a nuclear reactor. Modern reactors use multiple containment layers: fuel cladding, the reactor pressure vessel, and a reinforced concrete containment building. Molten salt reactors operate at atmospheric pressure, eliminating the need for massive pressure containment.
Coolant: The fluid used to transfer heat from the reactor core to the steam generators or turbines. In light water reactors, ordinary water serves as coolant. In molten salt reactors, the salt itself acts as both coolant and fuel carrier. Other advanced designs use liquid sodium, helium gas, or lead-bismuth eutectic.
Criticality: The state in which a nuclear chain reaction is self-sustaining — each fission event produces, on average, exactly one subsequent fission event. A reactor at criticality maintains a steady power level. Supercriticality means the reaction rate is increasing; subcriticality means it is decreasing.
Decay Heat: The heat produced by the radioactive decay of fission products after a reactor has been shut down. Decay heat cannot be "turned off" and must be removed to prevent fuel damage. The Fukushima disaster was caused by the inability to remove decay heat after the tsunami destroyed backup power systems. MSR designs address this with passive drain tanks.
DOE (Department of Energy): The U.S. federal agency responsible for national energy policy, nuclear weapons stewardship, and energy research. The DOE's Office of Nuclear Energy funds advanced reactor research, including the Advanced Reactor Demonstration Program (ARDP), which supports TerraPower's Natrium and X-energy's Xe-100 projects.
Dry Cask Storage: A method of storing spent nuclear fuel in steel and concrete containers after it has cooled in a spent fuel pool for several years. Dry casks are designed to be passively cooled and are considered safe for decades to centuries. The United States currently stores approximately 90,000 metric tons of spent fuel at reactor sites in pools and dry casks.
Enrichment: The process of increasing the concentration of uranium-235 in natural uranium (which is 99.3% U-238 and only 0.7% U-235). Light water reactors require uranium enriched to 3-5% U-235. Weapons-grade uranium is enriched above 90%. The thorium fuel cycle does not require enrichment of the thorium fuel itself.
Fission: The splitting of a heavy atomic nucleus into two lighter nuclei, releasing a large amount of energy and additional neutrons. Fission of uranium-235 or uranium-233 is the energy source in nuclear reactors. Each fission event releases approximately 200 MeV of energy — roughly 50 million times more energy per atom than chemical combustion.
Fuel Cycle: The complete sequence of processes involved in using nuclear fuel: mining, conversion, enrichment (for uranium), fabrication, irradiation in a reactor, cooling, reprocessing or disposal. The thorium fuel cycle begins with thorium-232 and breeds uranium-233 in the reactor, potentially enabling a closed fuel cycle with minimal waste.
Fukushima: The Fukushima Daiichi nuclear disaster occurred on March 11, 2011, when a magnitude 9.0 earthquake and subsequent tsunami caused station blackout at three operating reactors. Without cooling, decay heat caused fuel meltdowns and hydrogen explosions. The disaster led to worldwide reactor safety upgrades and Japan's temporary shutdown of all nuclear plants.
Fusion: The process of combining light atomic nuclei (typically hydrogen isotopes deuterium and tritium) to form heavier nuclei, releasing energy. Fusion powers the sun and stars. On Earth, achieving sustained fusion requires temperatures exceeding 100 million degrees Celsius. ITER, NIF, and private companies like Commonwealth Fusion Systems are pursuing commercial fusion power.
Generation IV: A set of six advanced nuclear reactor designs selected by the Generation IV International Forum for potential deployment by the 2030s. These include molten salt reactors (MSR), very high temperature reactors (VHTR), gas-cooled fast reactors (GFR), sodium-cooled fast reactors (SFR), lead-cooled fast reactors (LFR), and supercritical water reactors (SCWR).
Half-Life: The time required for half of a given quantity of a radioactive isotope to decay. Half-lives range from fractions of a second to billions of years. Thorium-232 has a half-life of 14.05 billion years. Uranium-233 has a half-life of 159,200 years. The fission products of the thorium cycle generally have shorter half-lives than those of the uranium-plutonium cycle.
Hanford: The Hanford Site in Washington state produced plutonium for U.S. nuclear weapons from 1943 to 1987. It is now the most contaminated nuclear site in the United States, with an ongoing cleanup effort estimated to cost over $300 billion and last until at least 2078. Hanford illustrates the legacy costs of the uranium-plutonium fuel cycle.
IAEA (International Atomic Energy Agency): The United Nations agency responsible for promoting the peaceful use of nuclear energy and preventing its use for military purposes. Based in Vienna, the IAEA conducts inspections, safeguards nuclear materials, and sets international safety standards. Established in 1957.
ITER: International Thermonuclear Experimental Reactor. A massive fusion research project under construction in Cadarache, France, funded by 35 nations. ITER aims to demonstrate the feasibility of fusion power by producing 500 MW of fusion power from 50 MW of heating input. First plasma has been repeatedly delayed and is currently projected for the early 2030s.
LCOE (Levelized Cost of Energy): A measure of the average total cost of building and operating a power plant per unit of electricity generated over its lifetime. LCOE allows comparison across energy sources. Nuclear LCOE is heavily front-loaded due to high construction costs but benefits from very low fuel costs and long operational lifetimes (60-80 years for modern plants).
LFTR (Liquid Fluoride Thorium Reactor): A specific type of molten salt reactor designed to use the thorium fuel cycle. Thorium-232 is dissolved in a fluoride salt blanket, where it breeds uranium-233. The U-233 is transferred to a separate core salt where it fissions. LFTR designs feature passive safety, atmospheric-pressure operation, and the ability to consume nearly all of their fuel. Pronounced "lifter."
Load Following: The ability of a power plant to adjust its output to match changes in electricity demand. While nuclear plants have traditionally operated at constant output (baseload), modern designs and operational procedures allow nuclear plants to load-follow. MSRs are particularly well-suited to load following due to the liquid nature of their fuel.
LWR (Light Water Reactor): The most common type of nuclear reactor worldwide, using ordinary (light) water as both coolant and neutron moderator. The two main variants are pressurized water reactors (PWR) and boiling water reactors (BWR). LWRs use enriched uranium fuel and make up about 85% of the world's operating reactors.
Moderator: A material used to slow down fast neutrons produced by fission so they are more likely to cause further fission events. In light water reactors, water serves as the moderator. In CANDU reactors, heavy water is used. In MSRs, graphite typically serves as the moderator. Some advanced fast reactors operate without a moderator.
Molten Salt Reactor (MSR): A class of nuclear reactor in which the fuel is dissolved in a molten salt mixture, typically a fluoride or chloride salt. The salt acts as both fuel carrier and primary coolant. MSRs operate at atmospheric pressure, have inherent passive safety features, and can be designed to use thorium or uranium fuel cycles. First demonstrated at Oak Ridge National Laboratory in the 1960s.
NRC (Nuclear Regulatory Commission): The independent U.S. federal agency responsible for regulating civilian nuclear power. The NRC licenses reactor construction and operation, sets safety standards, and conducts inspections. Critics argue the NRC's regulatory framework is optimized for light water reactors and creates barriers to licensing advanced reactor designs.
Passive Safety: Safety features that function without operator action, external power, or active mechanical systems. Examples include gravity-driven cooling, negative temperature coefficients (the reaction naturally slows as temperature rises), and freeze plugs in MSRs that drain fuel into safe storage if temperature exceeds limits. Passive safety is a key advantage of Generation IV designs.
Plutonium-239: A fissile isotope produced when uranium-238 absorbs a neutron in a nuclear reactor. Pu-239 is used in nuclear weapons and accumulates in spent fuel from conventional uranium reactors. Its half-life is 24,110 years, making it a major component of the long-term hazard of nuclear waste. The thorium fuel cycle produces negligible quantities of Pu-239.
Pressurized Water Reactor (PWR): The most common type of nuclear reactor worldwide. A PWR uses pressurized water (typically at 155 bar / 2,250 psi) as both coolant and moderator. The pressurized water transfers heat to a secondary steam loop, which drives the turbine. Designed originally for U.S. Navy submarines by Westinghouse under Admiral Hyman Rickover.
Proliferation: The spread of nuclear weapons or the materials and technology to build them. Nuclear proliferation resistance is a key consideration in reactor and fuel cycle design. The thorium fuel cycle is considered more proliferation-resistant than the uranium-plutonium cycle because U-233 is contaminated with U-232, making it very difficult to use in weapons.
Rem / Sievert: Units of radiation dose equivalent, measuring the biological effect of radiation on the human body. 1 sievert (Sv) = 100 rem. Average annual background radiation exposure in the U.S. is about 3.1 millisieverts (310 millirem). A chest X-ray delivers approximately 0.02 mSv. Acute doses above 1 Sv cause radiation sickness; above 5 Sv is typically fatal.
Reprocessing: The chemical treatment of spent nuclear fuel to recover usable fissile material (plutonium and/or uranium) and separate it from fission product waste. France and Russia reprocess commercially; the U.S. banned civilian reprocessing in 1977 (later lifted but never resumed at scale). Molten salt reactors enable continuous online reprocessing of their liquid fuel.
SMR (Small Modular Reactor): A nuclear reactor with an electrical output of 300 MW or less, designed for factory fabrication and modular deployment. SMR designs include NuScale's VOYGR (light water), TerraPower's Natrium (sodium-cooled fast reactor), and X-energy's Xe-100 (high-temperature gas-cooled). SMRs aim to reduce construction costs and timelines through standardization.
Thorium-232: The naturally occurring isotope of thorium, comprising nearly 100% of natural thorium. Th-232 is fertile but not fissile — it cannot sustain a chain reaction by itself but can absorb a neutron and breed into fissile uranium-233. Three to four times more abundant than uranium in Earth's crust. Half-life: 14.05 billion years.
Three Mile Island: Site of the most serious commercial nuclear accident in U.S. history, on March 28, 1979, near Harrisburg, Pennsylvania. A cooling system malfunction and operator errors caused a partial meltdown of the TMI-2 reactor core. No deaths or significant radiation exposure resulted, but the accident severely damaged public confidence in nuclear power and led to major regulatory reforms.
Uranium-233: A fissile isotope of uranium produced by neutron bombardment of thorium-232. U-233 is the primary fuel in the thorium fuel cycle and has excellent nuclear properties for sustaining a chain reaction in thermal-spectrum reactors. It is always contaminated with U-232, which produces penetrating gamma radiation, providing inherent proliferation resistance.
Uranium-235: The only naturally occurring fissile isotope, making up approximately 0.72% of natural uranium. U-235 is the primary fuel in light water reactors (enriched to 3-5%) and the fissile material in uranium-based nuclear weapons (enriched above 90%). Its fission releases approximately 200 MeV of energy per atom.
Vitrification: The process of immobilizing high-level radioactive waste in glass. Liquid waste is mixed with glass-forming materials and heated to high temperature, then poured into stainless steel canisters where it solidifies into a durable glass form. Vitrified waste is highly resistant to leaching and is designed for long-term geological disposal.
Yucca Mountain: A proposed deep geological repository for spent nuclear fuel and high-level radioactive waste, located in Nye County, Nevada. Designated by Congress in 1987, the project was effectively defunded in 2011 due to political opposition from Nevada. The U.S. still has no permanent disposal site for its 90,000+ metric tons of spent nuclear fuel. A consent-based siting process is now being pursued.