Auxiliary building - A structure housing a variety of equipment and
large tanks necessary for the operation of the reactor. These include
make-up pumps, the make-up and waste gas decay tanks, and the reactor
coolant hold-up tanks.
Babcock & Wilcox (B&W) - The company that designed and supplied the
TMI-2 reactor and nuclear steam supply system.
Background radiation - Radiation arising from natural radioactive
materials always present in the environment, including solar and cosmic
radiation and radioactive elements in the upper atmosphere, the ground,
building materials, and the human body.
Beta particles - High-energy electrons; a form of ionizing radiation
that normally is stopped by the skin, or a very thin sheet of metal.
Bureau of Radiation Protection (BRP) - A division of Pennsylvania's
Department of Environmental Resources. BRP is the state's lead agency in
monitoring radiation releases from nuclear plants and advises the
Pennsylvania Emergency Management Agency during radiological emergencies.
Burns and Roe - Architectural and engineering firm responsible for the
design of TMI-2.
Candy cane - The section of pipe carrying water from the reactor to a
Chain reaction - A self-sustaining reaction; occurs in nuclear fission
when the number of neutrons released equals or exceeds the number of
neutrons absorbed plus the neutrons which escape from the reactor.
Cladding - In a nuclear reactor, the metal shell of the fuel rod in
which uranium oxide pellets are stacked.
Collective dose - The sum of the individual doses received by each
member of a certain group or population. It is calculated by multi- plying
the average dose per person by the number of persons within a specific
geographic area. Consequently, the collective dose is expressed in
person-reins. For example, a thousand people each exposed to one rem would
have a collective dose of 1,000 person-rems.
Condensate booster pumps - Three pumps located between the condensate
polisher and the main feedwater pumps.
Condensate polisher - A device that removes dissolved minerals from
the water of the feedwater system.
Condensate pumps - Three pumps in the feedwater system that pump water
from the condensers to the condensate polishers.
Condensers - Devices that cool steam to water after the steam has
passed through the turbine.
Containment building - The structure housing the nuclear reactor;
intended to contain radioactive solids, gases, and water that might be
released from the reactor vessel in an accident.
Control rod - A rod containing material that absorbs neutrons; used to
control or halt nuclear fission in a reactor.
Core - The central part of a nuclear reactor that contains the fuel
and produces the heat.
Critical - Term used to describe a nuclear reactor that is sustaining
a chain reaction.
Curie - A unit of the intensity of radioactivity in a material. A
curie is equal to 37 billion disintegrations each second.
Decay heat - Heat produced by the decay of radioactive particles; in a
nuclear reactor this heat, resulting from materials left from the fission
process, must be removed after reactor shutdown to prevent the core from
overheating. See radioactive decay.
Design basis accident (DBA) - Hypothetical accidents evaluated during
the safety review of nuclear power reactors. Plants are required to have
safeguards that will ensure that radiation releases off-site will be
within NRC limits should any of these accidents occur.
Emergency core cooling system (ECCS) - A backup system designed to
supply cooling water to the reactor core in a loss-of-coolant accident.
Emergency feedwater pumps - Backup pumps intended to supply feedwater
to the steam generators should the feedwater system fail to supply water.
Also called auxiliary feedwater pumps.
Feedwater pumps - Two large pumps capable of supplying TMI-2's two
steam generators with up to 15,500 gallons of water a minute.
Feedwater system - Water supply to the steam generators in a
pressurized water reactor that is converted to steam to drive turbines;
part of the secondary loop.
Fission - The splitting apart of a heavy atomic nucleus, into two or
more parts when a neutron strikes the nucleus. The splitting releases a
large amount of energy.
Fission products - Radioactive nuclei and elements formed by the
fission of heavy elements.
Fuel damage - The failure of fuel rods and the release of the radio-
active fission products trapped inside them. Fuel damage can occur without
a melting of the reactor's uranium.
Fuel melt - The melting .of some of the uranium oxide fuel inside a
Fuel rod - A tube containing fuel for a nuclear reactor.
Gamma rays - High-energy electromatic radiation; a form of ionizing
radiation of higher energy than X-rays, that penetrates very deep into
General emergency - Declared by the utility when an incident at a
nuclear power plant poses a potentially serious threat of radiation
releases that could affect the general public.
General Public Utilities Corporation (GPU) - A utility holding
company; parent corporation of the three companies that own TMI.
Genetic defects - Health defects inherited by a child from the mother
Half-life - The time required for half of a given radioactive
substance to decay.
Health physics - The practice of protecting humans and their
environment from the possible hazards of radiation.
High pressure injection (HPI) - A pump system, capable of pumping up
to about 1,000 gallons a minute into the reactor coolant system; part of
the emergency core cooling system.
- A radioactive form of iodine, with a half-life of 8.1 days, that can be
absorbed by the human thyroid if inhaled or ingested and cause
non-cancerous or cancerous growths.
Ionizing radiation - Radiation capable of displacing electrons from
atoms; the process produces electrically charged atoms or ions. Forms
include gamma rays, X-rays, and beta particles.
Isolation - Condition intended to contain radioactive materials
released in a nuclear accident inside the containment building.
Krypton-85 - A radioactive noble gas, with a half-life of 10.7 years,
that is not absorbed by body tissues and is soon eliminated by the body if
inhaled or ingested.
Let-down system - A means of removing water from the reactor coolant
Loss-of-coolant accident (LOCA) - An accident involving a broken pipe,
stuck-open valve, or other leak in the reactor coolant system that results
in a loss of the water cooling the reactor core.
Make-up system - A means of adding water to the reactor coolant system
during normal operation.
Make-up Tank - A storage tank in the auxiliary building which provides
water for the make-up pumps.
Meltdown - The melting of fuel in a nuclear reactor after the loss of
coolant water. If a significant portion of the fuel should melt, the
molten fuel could melt through the reactor vessel and release large
quantities of radioactive materials into the containment building.
Metropolitan Edison Company (Met Ed) - Operator and part owner of the
Three Mile Island nuclear power plant.
Millirem - 1 one-thousandth of a rem; see rem.
Natural cooling - The circulation of water without pumping by heating
water in the core and cooling it in the steam generator.
Neutron - An uncharged particle found in the nucleus of every atom
heavier than ordinary hydrogen; neutrons sustain the fission chain
reaction in nuclear reactors.
Noble gases - Inert gases that do not react chemically and are not
absorbed by body tissues, although they may enter the blood if inhaled
into the lungs. These gases include helium, neon, krypton, xenon, and
Nuclear Regulatory Commission (NRC) - U.S. agency responsible for the
licensing and regulation of commercial, test, and research nuclear
Nucleus - The central core of an atom.
Pennsylvania Emergency Management Agency (PEMA) - Agency responsible
for the state's response to natural and human-made disasters.
Person-rems - See collective dose.
- Materials that strongly absorb neutrons; used to control or stop the
fission reaction in a nuclear reactor.
Pilot-operated relief valve (PORV) - A valve on the TMI-2 pressurizer,
designed to open when steam pressure reaches 2,255 pounds per square inch.
Potassium iodide - A chemical that readily enters the thyroid gland
when ingested. If taken in a sufficient quantity prior to exposure to
radioactive iodine, it can prevent the thyroid from absorbing any of the
potentially harmful radioactive iodine-131.
Pressure vessel - See reactor vessel.
Pressurizer - A tank that maintains the proper reactor coolant
pressure in a pressurized water reactor.
Pressurized water reactor - A nuclear reactor system in which reactor
coolant water is kept under high pressure to keep it from boiling into
Primary system - See reactor coolant system.
Radioactive decay - The spontaneous process by which an unstable
radioactive nucleus releases energy or particles to become stable.
Radioactivity - The spontaneous decay of an unstable atom. During the
decay process, ionizing radiation is usually given off.
Radiolysis - The breaking apart of a molecule by radiation, such as
the splitting of water into hydrogen and oxygen.
Reactor (nuclear) - A device in which a fission chain reaction can be
initiated, maintained, and controlled.
Reactor coolant pump - One of four large pumps used to circulate the
water cooling the core of the TMI-2 reactor.
Reactor coolant system - Water that cools the reactor core and carries
away heat. Also called the primary loop.
Reactor vessel - The steel tank containing the reactor core; also
called the pressure vessel.
Rem - A standard unit of radiation dose. Frequently radiation dose is
measured in millirems for low-level radiation; 1,000 millirems equal one
Respirator - A breathing mask that filters the air to protect against
the inhalation of radioactive materials.
Safety-related - The NRC employes several broad definitions for this
concept. By one, safety-related items are "structures, systems and
components that prevent or mitigate the consequences of postulated
accidents that could cause undue risk to the health and safety of the
public." However, the NRC has no specific list of safety-related items.
The licensee designates what in its plant is considered safety-related. If
the NRC disagrees, the question is negotiated. Safety-related items
receive closer quality control and assurance, maintenance, and NRC
Saturation temperature - The temperature at which water at a given
pressure will boil; the saturation point of water at sea-level is 212° F.
Scram - The rapid shutdown of a nuclear reactor, by dropping control
rods into the core to halt fission.
Secondary system - See feedwater system.
Site emergency - Declared by the utility when an incident at a nuclear
power plant threatens the uncontrolled release of radioactivity into the
immediate area of the plant.
Solid system - A condition in which the entire reactor coolant system,
including the pressurizer, is filled with water.
Steam generator - A heat exchanger in which reactor coolant water
flowing through tubes heats the feedwater to produce steam.
Steam table - A chart used to determine the temperature at which water
will boil at a given pressure.
Teratogenesis - The process of the development of gross abnormalities
in the developing unborn child; these abnormalities or birth defects are
Thermoluminescent dosimeter (TLD) - A device to measure nuclear
TMI - Three Mile Island; site of two nuclear power reactors operated
by Metropolitan Edison Company.
Transient - An abnormal condition or event in a nuclear power system.
Trip - A sudden shutdown of a piece of machinery.
Turbine building - A structure housing the steam turbine, generator,
and much of the feedwater system.
Uranium Oxide (UCL) - A chemical compound containing uranium and
oxygen that is used as a fuel in nuclear reactors.
Waste gas decay tank - One of two auxiliary building tanks in which
radioactive gases removed from the reactor coolant are stored.
Xenon-133 - A radioactive noble gas, with a half-life of 5.3 days,
that is not absorbed by body tissues and is soon eliminated by the body if
inhaled or ingested.
Zircaloy-4 - A zirconium alloy from which fuel rod cladding is made.