Alert: The second-lowest of the four emergency classifications established by the Nuclear Regulatory Commission for nuclear power plants. An alert means events are in progress or have occurred that have substantially reduced or could substantially reduce plant safety. Any radioactive releases are expected to be below Environmental Protection Agency guidelines for protection of the public. No action by the general public is required.

Alpha particles: The heaviest and least penetrating form of ionizing radiation. They can be stopped by clothing or a sheet of paper.

Atom: The basic building block of elements. Atoms consist of a nucleus, orbited by particles with a negative electrical charge (electrons). Within the nucleus are particles that have a positive electrical charge (protons) and particles that have no electrical charge (neutrons).

Background radiation: Radiation that occurs naturally in the environment, such as radon gas from the ground, cosmic rays from space and radioactive elements in the human body.

Beta particles: Small, high-energy particles of ionizing radiation. They have enough energy to penetrate skin deeply enough to damage tissue, but can be stopped by a block of wood or thin sheet of metal.

Cladding: Metal tubing, made from a special alloy, which surrounds uranium fuel pellets.

Condensate demineralizer: A large filter vessel used to remove impurities from water before it is returned to the reactor. Each unit at Susquehanna has seven available.

Condensate storage tank: A 300,000-gallon tank that holds a reserve supply of reactor water for use during normal operations, refueling or emergencies. There is one tank for each unit at Susquehanna.

Condenser: A plant system that draws steam from the turbine and, by forcing it to pass over a series of tubes filled with water, cools the steam to water for reuse in the reactor.

Containment: Physical barriers to prevent or limit the release of radiation in the event of a serious accident. Primary containment is a massive steel-reinforced concrete structure. Secondary containment has steel-reinforced concrete walls and air pressure lower than outside air pressure to prevent air leaks.

Contamination: Radioactive material deposited on a non-radioactive surface.

Control rods: Stainless steel rods, shaped like the letter “x,” filled with a material (boron carbide and hafnium) that absorbs neutrons. These rods are inserted between the fuel assemblies to control or stop the nuclear reaction.

Core: The area inside the reactor vessel where the fuel is located and where the fission process takes place. Also refers to the fuel itself.

Critical: Term used to describe the nuclear reaction inside the reactor when it is self-sustaining.

Curie: Unit used to measure the amount of radioactivity in a substance.

Decay: The process by which an atom gives off energy, in the form of radioactive particles or waves, in order to reach a stable state.

Defense in depth: Concept used in the design of nuclear power plants to improve safety. It uses multiple protective barriers and multiple backup systems to prevent or limit the release of radiation.

Emergency alert system: Radio and television stations used by county emergency management officials to broadcast official information and instructions during an emergency.

Emergency planning zone: The geographic area within 10 miles of the Susquehanna plant that includes 27 municipalities in parts of Luzerne and Columbia counties. About 70,000 people live in this area.

Enrichment: The process by which the concentration of fissionable atoms in raw uranium is increased to about 4 percent from less than 1 percent so it can be used as power plant fuel.

Fission: The splitting of atoms into smaller parts, which results in a release of energy.

Fuel assembly: An arrangement of rods containing uranium fuel. Susquehanna fuel assemblies have 91 rods in a 10-by-10 array, with a central water channel equal to a 3-by-3 array. Each reactor contains 764 fuel assemblies.

Fuel bundle: See fuel assembly.

Fuel rod: A 13.5-foot-long tube, made of a special metal alloy, that is used to hold uranium fuel pellets.

Gamma rays: Waves of ionizing radiation energetic enough to pass through a human body. It takes dense material, such as lead or several feet of concrete, to stop gamma rays.

General emergency: The highest of four emergency classifications established by the Nuclear Regulatory Commission. A general emergency means events are imminent, are in progress or have occurred involving substantial damage to the reactor core and failures to plant safety systems that are needed for public protection. Radiation releases are expected to exceed Environmental Protection Agency guidelines for protection of the public beyond plant property. The public would be asked to tune into an Emergency Alert System radio or television station for official information and instructions.

Half-life: The time it takes for a radioactive substance to lose half of its radioactivity through decay. Each radioactive substance has a unique half-life.

Low-level radioactive waste: Material that becomes contaminated through use and contact with radioactive materials. At the Susquehanna plant, this includes filter materials, protective clothing, tools, rags and other solid wastes.

Millirem: The unit used to measure exposure to radiation.

Moderator: The substance used to slow neutrons released in the fissioning process to make them more likely to split other atoms.

Neutrons: Particles within the nucleus of an atom that have no electrical charge. In a nuclear power reactor, they sustain the reaction by splitting fissionable uranium atoms.

Radiation: Electromagnetic energy in the form of particles or waves. In a nuclear power plant, the particles or waves are emitted by unstable atoms undergoing decay.

Reactor: The large metal vessel where atoms are split to create the heat needed to boil water and produce steam that turns a turbine to generate electricity.

Risk counties: Counties that have residents who live within 10 miles of the power plant and who may be asked to take protective action in the event of an emergency at the plant. The risk counties for Susquehanna are Luzerne and Columbia.

Scram: The rapid shutdown of a nuclear power reactor by the insertion of all control rods into the core to stop fission. Control rods can insert automatically or at the direction of plant operators.

Site area emergency: The second-highest of the four emergency classifications established by the Nuclear Regulatory Commission for nuclear power plants. A site area emergency means events are in progress or have occurred that have affected or are likely to affect major plant safety systems. Any radioactive releases are not expected to exceed Environmental Protection Agency guidelines for protection of the public beyond plant property. No action by the general public is required.

Spent fuel: Fuel that can no longer produce enough energy to support full-power operation of the plant. It is highly radioactive and requires special handling for safety.

Spray pond: An 8-acre, 25-million-gallon, man-made pond on Susquehanna plant property that serves as a source of cooling water for normal plant operations and emergencies. It holds enough water to meet all plant cooling needs for a minimum of 30 days.

Suppression pool: A source of nearly 1 million gallons of water for emergency cooling systems in the Susquehanna plant. There is a pool located beneath each reactor.

Transformer: A device used to increase or decrease the voltage of electricity.

Unusual event: The lowest of the four emergency classifications established by the Nuclear Regulatory Commission for nuclear power plants. An unusual event means a minor problem is in progress or has occurred that could reduce plant safety. No releases of radioactive material requiring off-site response or monitoring are expected. No action by the general public is required.

Uranium: The element used to fuel a nuclear power reactor. Uranium in nature consists mainly of two isotopes, U235 and U238. The U235 atom readily reacts with neutrons and splits into new atoms.