ANS CNP key terms

The time required for half of the radioactive nuclei in a sample to decay. Each radionuclide has a unique half-life that determines how quickly its activity decreases and how long it remains hazardous.

The spontaneous emission of particles or electromagnetic radiation from unstable atomic nuclei as they transform to a more stable state. Measured in becquerels or curies, it indicates the number of decays per unit time.

The probability per unit time that a particular radionuclide will decay. It is related to half-life by the equation λ = ln(2)/t½ and appears in the exponential decay law.

A measure of the probability that a specific nuclear reaction (such as fission or absorption) will occur when a neutron interacts with a nucleus. Usually expressed in barns (10⁻²⁴ cm²) and depends on neutron energy.

The number of neutrons passing through a unit area per unit time, often expressed in neutrons per square centimeter per second. It determines reaction rates in a reactor when multiplied by the cross section and atom density.

The splitting of a heavy nucleus (such as uranium-235) into two lighter nuclei, accompanied by the release of neutrons and energy. This process sustains the chain reaction in a nuclear reactor.

The condition in which a nuclear chain reaction is self-sustaining, with the effective multiplication factor (k) equal to 1.0. Subcritical systems have k < 1 and supercritical systems have k > 1.

The ratio of the number of neutrons in one generation to the number in the preceding generation in a reactor. Denoted k, it determines whether the chain reaction is subcritical, critical, or supercritical.

A measure of the deviation of a reactor from criticality, often expressed in dollars or pcm. Positive reactivity makes the reactor supercritical, negative reactivity makes it subcritical, and zero reactivity means critical.

Neutrons emitted by fission products after a delay following fission, rather than promptly at the moment of fission. They constitute less than 1% of fission neutrons but are essential for controllable reactor operation.

Neutrons released almost instantaneously (within about 10⁻¹⁴ seconds) during the fission process. They make up more than 99% of fission neutrons and dominate the immediate neutron population in a reactor.

A material (such as light water, heavy water, or graphite) used in a reactor to slow down fast neutrons to thermal energies, increasing the probability of fission in uranium-235. It also helps sustain the chain reaction.

A neutron whose kinetic energy has been reduced by collisions (moderation) to approximately the average thermal energy of surrounding atoms (about 0.025 eV at room temperature). Thermal neutrons are highly effective at causing fission in U-235.

The time required for the neutron population (or reactor power) to change by a factor of e (approximately 2.718) during a constant reactivity change. A shorter period indicates a faster power change.

A rapid emergency shutdown of a nuclear reactor, typically achieved by inserting control rods to quickly reduce reactivity and stop the chain reaction. It is initiated manually or automatically by protective systems.

A movable neutron-absorbing rod (often containing boron, cadmium, or hafnium) used to control the reactivity of a reactor. Inserting rods reduces reactivity; withdrawing them increases it.

The buildup of xenon-135, a fission product with a very large neutron absorption cross section, which reduces reactor reactivity. It can cause temporary shutdowns or oscillations after power changes.

The automatic or manual insertion of control rods to shut down a reactor safely. A trip may be initiated by a protection system in response to an abnormal condition.

A safety philosophy that uses multiple independent and redundant layers of protection (physical barriers, safety systems, procedures) to prevent accidents and mitigate consequences if they occur. It is a fundamental principle in nuclear safety.

As Low As Reasonably Achievable: a principle of radiation protection that requires all exposures to be kept as low as reasonably achievable, taking into account economic and social factors. It is a regulatory requirement and a professional obligation.

The maximum permissible radiation dose to an individual (e.g., occupational worker or member of the public) as set by regulatory bodies. In the U.S., the occupational limit is 50 mSv (5 rem) per year.

The total radiation dose that will be received by an individual over a specified period (often 50 years for adults) following an intake of radioactive material. It accounts for the retention and decay of radionuclides in the body.

The assembly of characteristics and attitudes in organizations and individuals that establishes that nuclear safety issues receive the attention warranted by their significance. It includes commitment to safety, questioning attitude, and open communication.

A large water-filled basin used to store and cool irradiated nuclear fuel assemblies after removal from the reactor. The water provides shielding and removes decay heat.

Highly radioactive waste material that contains fission products and transuranic elements, typically generated from reprocessing spent fuel or from the fuel itself. It requires deep geological disposal and long-term isolation.

The heat produced by the radioactive decay of fission products and actinides after a reactor is shut down. It continues to generate significant thermal power and must be removed to prevent fuel damage.

A set of verification measures administered by the International Atomic Energy Agency to ensure that nuclear material and activities are not diverted from peaceful uses. They include material accountancy, containment, and surveillance.

The system of records, reports, and measurements used to track the quantities and locations of nuclear materials within a facility or state. It is a key component of safeguards and helps detect diversion or loss.

Measures designed to prevent unauthorized access to nuclear facilities and materials, and to detect and respond to theft, sabotage, or other malicious acts. It includes barriers, guards, and intrusion detection systems.

The prevention and detection of, and response to, theft, sabotage, unauthorized access, illegal transfer, or other malicious acts involving nuclear material or facilities. It complements safety and safeguards.

A defined area around a nuclear facility where special emergency planning measures are implemented to protect the public in case of a radiological release. It typically includes a plume exposure pathway zone and an ingestion pathway zone.

An unplanned, uncontrolled nuclear chain reaction that produces a burst of neutron and gamma radiation. It can occur when fissile material is improperly handled, such as in a geometry or mass that exceeds safe limits.

The process of increasing the proportion of the fissile isotope uranium-235 in uranium, typically from natural 0.7% to 3–5% for commercial reactor fuel. It is a key step in the nuclear fuel cycle.

The series of steps involved in producing nuclear fuel, using it in a reactor, and managing the resulting waste. It includes mining, conversion, enrichment, fabrication, irradiation, and disposal or reprocessing.

Legal and organizational measures that protect employees who report safety or regulatory violations from retaliation. It is an essential element of a strong safety culture and professional ethics.

The principles and standards of conduct expected of nuclear professionals, including integrity, objectivity, confidentiality, competence, and commitment to public safety. Ethical behavior is fundamental to maintaining trust and safety in the nuclear industry.