Glossary
Plain-English definitions of the 37 terms that come up most. These also appear as hover tooltips throughout the site.
- ALARA
- 'As Low As Reasonably Achievable' — the radiation-protection principle of minimizing dose, criticized for amplifying low-dose fear.
- Baseload
- Steady, always-on electricity demand — and the plants that reliably supply it around the clock.
- Becquerel
- The unit of radioactivity: one nuclear decay per second.
- Breeder reactor
- A reactor that produces more fissile fuel than it consumes by converting fertile material like U-238.
- BWR
- Boiling Water Reactor: water boils directly in the core and the steam drives the turbine.
- Capacity (MWe)(MWe, megawatt)
- Megawatts of electrical output. A large reactor is ~1,000 MWe; an SMR is under 300 MWe.
- Capacity factor
- The fraction of the time a plant actually produces its rated power. Nuclear runs at ~90%+.
- Containment
- The robust sealed structure around a reactor that traps radioactive material if something goes wrong.
- Control rods
- Neutron-absorbing rods inserted into the core to slow, speed up, or stop the chain reaction.
- Coolant
- The fluid (water, gas, liquid metal, or molten salt) that carries heat away from the reactor core.
- Decay heat
- The heat a reactor keeps producing from radioactive decay even after shutdown — why cooling must continue.
- Dry cask
- A massive sealed steel-and-concrete container that stores cooled used fuel passively, with no power or moving parts.
- Enrichment
- Raising the fraction of fissile U-235 in uranium — usually to 3–5% for power reactors.
- Fast spectrum
- A reactor that uses fast (un-moderated) neutrons, enabling fuel breeding and burning of long-lived waste.
- Firm power(dispatchable)
- Power available on demand at any moment, regardless of weather. Nuclear, gas, hydro and geothermal are firm.
- Fission
- Splitting a heavy atomic nucleus (like uranium-235) into smaller pieces, releasing energy and neutrons.
- Fusion
- Merging light nuclei (hydrogen isotopes) to release energy — the reaction that powers the Sun. Distinct from fission.
- HALEU
- High-Assay Low-Enriched Uranium: fuel enriched to just under 20% U-235, needed by many advanced reactors.
- Half-life
- The time for half of a radioactive material to decay. Shorter half-life means more intense but shorter-lived radioactivity.
- Hormesis
- The hypothesis that very low radiation doses are harmless or even mildly beneficial — disputed against LNT.
- LFTR
- Liquid-Fluoride Thorium Reactor: a molten-salt reactor concept that breeds fuel from abundant thorium.
- LNT
- Linear No-Threshold model: assumes radiation risk is proportional to dose all the way to zero, with no safe threshold. The basis of regulation.
- Meltdown
- Overheating that damages or melts a reactor core when cooling is lost. Containment is designed to trap the results.
- Microreactor
- A very small (under ~20 MW), often transportable reactor for remote sites and defense.
- Moderator
- A material (often water or graphite) that slows neutrons so they cause fission more efficiently.
- Molten salt
- A reactor design that dissolves fuel in a liquid salt, running at low pressure with strong passive safety.
- NRC
- Nuclear Regulatory Commission: the US agency that licenses and oversees commercial reactors.
- Proliferation
- The spread of nuclear-weapons capability. Commercial power fuel is a poor route to a weapon.
- PWR
- Pressurized Water Reactor: keeps water liquid under pressure and passes heat to a separate clean steam loop. The most common reactor.
- Reprocessing(recycling)
- Chemically separating usable uranium and plutonium from used fuel to make new fuel (a 'closed' fuel cycle).
- Sievert(mSv, millisievert)
- The unit of radiation dose that accounts for biological effect. Natural background is a few millisieverts per year.
- SMR
- Small Modular Reactor: a factory-built reactor under ~300 MW shipped to site.
- Thermal spectrum
- A reactor that slows neutrons with a moderator — the type used by almost all today's commercial reactors.
- TRISO
- Tristructural-isotropic fuel: tiny uranium kernels in robust ceramic coatings that retain fission products at high temperature.
- Uranium-235(U-235, U235)
- The fissile isotope of uranium that sustains a chain reaction. Only ~0.7% of natural uranium.
- Used fuel(spent fuel)
- Fuel removed from a reactor after several years of use — intensely radioactive at first, but small in volume and contained.
- Vogtle
- The Georgia plant whose Units 3 & 4 (2023–24) were the first newly built US reactors in over 30 years.