NTN REACT key terms

The ratio of neutrons produced in one generation of a reactor core to the neutrons produced in the preceding generation. A value of one means the chain reaction is exactly self-sustaining and the core is critical.

The condition in which a fissile system sustains a steady fission chain reaction, with neutron production balanced by absorption and leakage. Criticality is the normal operating state of a power reactor at power.

A state in which neutron production is less than losses, so the fission chain reaction dies away over time. Subcriticality is the required condition for shutdown, refueling, and fuel handling operations.

A measure of how far a reactor is from critical, usually expressed in pcm or dollars. Positive reactivity raises power; negative reactivity lowers it. It is calculated from the multiplication factor as (k-1)/k.

Neutrons released by certain fission-product precursors seconds to minutes after fission, rather than promptly. Although a small fraction of all neutrons, they slow power changes enough for operators and control systems to respond.

A condition in which the chain reaction is sustained by prompt neutrons alone, reached when reactivity exceeds the delayed neutron fraction. Power then rises extremely fast, beyond the ability of mechanical controls to arrest it.

The fraction of fission neutrons that are delayed, adjusted for the specific core's fuel and geometry. It sets the threshold between controllable and prompt-critical behavior and is used to express reactivity in dollars.

The change in reactivity per unit change in a core parameter such as fuel temperature, moderator temperature, or void fraction. Negative coefficients make a reactor self-limiting and are central to safe design.

The reactivity change caused by a one-degree change in moderator temperature. In most light-water reactors it is negative at operating conditions, providing a natural feedback that opposes power increases.

The reactivity change resulting from the formation of steam voids in the coolant or moderator. A negative void coefficient means boiling reduces reactivity, a key safety characteristic of most light-water reactors.

A material, such as light water, heavy water, or graphite, that slows fast neutrons to thermal energies where fission in uranium-235 is far more likely. Slowing neutrons increases the probability of further fission.

A neutron whose energy has been reduced by collisions to near the average thermal energy of the surrounding material, typically below about one electron-volt. Thermal neutrons are highly effective at causing fission in uranium-235.

A neutron born in fission with high energy, typically around two million electron-volts, before significant slowing. Fast neutrons can cause fission in fertile and fissile isotopes and must be moderated for most thermal reactors.

The splitting of a heavy nucleus into two lighter nuclei, releasing energy and several neutrons. In a reactor, fission of uranium-235 provides the heat that drives the power cycle.

An isotope created when a nucleus splits, often radioactive and a source of decay heat and radiation. Fission products also absorb neutrons, gradually reducing core reactivity as they accumulate.

Heat produced by the radioactive decay of fission products after the fission chain reaction stops. It continues for hours to days and requires forced cooling even after shutdown to prevent fuel damage.

The buildup of xenon-135, a powerful neutron absorber, after power changes or shutdown. Its delayed peak can make restart difficult for many hours and can cause reactivity swings during operation.

The amount of negative reactivity available to shut down the reactor and keep it subcritical, assuming the most reactive control element is fully withdrawn. It is a key technical specification verified before and after refueling.

The total reactivity change produced by inserting or withdrawing a control element from the core. Worth is verified periodically by measurement and must meet minimum requirements for shutdown capability.

A test in which a control element is released and allowed to fall into the core, and the resulting power decrease is used to estimate its worth. It verifies both rod function and shutdown capability.

A rapid, automatic or manual insertion of control elements to shut down the reactor. It is triggered by protective system signals and is designed to bring the core to a subcritical condition quickly.

The safety-related instrumentation and logic that automatically trips the reactor when measured parameters exceed setpoints. It is designed to fail safe and is independent of normal control systems.

The predetermined value at which a protective or control action is initiated. Setpoints are chosen with margin to safety limits and are periodically verified during surveillance testing.

A complete measurement path from sensor through signal conditioning to the device that uses the signal. Channels are duplicated or triplicated so that a single failure cannot defeat protection.

The use of more than one independent component or channel to perform a safety function, so that failure of one does not prevent the function. Redundancy is a core principle of nuclear safety design.

The use of different technologies, designs, or operating principles to perform the same safety function. Diversity guards against common-cause failures that could disable identical redundant channels simultaneously.

The design rule that a safety function must remain capable of performing its purpose despite one assumed component failure. It underpins redundancy, separation, and independence requirements in safety systems.

A single event or shared condition that causes multiple components or channels to fail at the same time. Examples include fire, flooding, or a design defect affecting identical equipment.

A scheduled check that safety equipment will perform its intended function, such as testing a trip channel or starting a standby pump. Results are documented to show continued operability and technical specification compliance.

The condition in which a system or component is capable of performing its specified safety function. Loss of operability may require compensatory measures or a shutdown under technical specification requirements.

The binding license conditions that set limits and required actions for operating a facility, including safety limits, surveillance intervals, and minimum equipment. Noncompliance requires documented action within specified times.

The minimum set of systems and components that must be operable for continued operation, together with required actions and completion times if they are not. It is part of the technical specifications.

A boundary on important process variables, such as fuel temperature or pressure, that must not be exceeded to protect the integrity of physical barriers. Safety limits are more restrictive than equipment design limits.

A layered safety approach using multiple independent barriers, systems, and procedures to prevent accidents and mitigate consequences if prevention fails. Layers include design, protection systems, containment, and emergency response.

A postulated accident that a facility must be designed and licensed to withstand without exceeding dose limits. Examples include a loss-of-coolant accident and a control rod ejection.

An accident in which a break in the primary system causes coolant inventory to fall, threatening fuel cooling. Emergency core cooling systems are designed to restore cooling and limit fuel damage.

Safety systems that inject water into the core after a loss of coolant to remove decay heat and prevent fuel melting. They typically include high- and low-pressure injection and accumulators.

A system and function that removes decay heat and sensible heat from the primary coolant after shutdown. It is required for cooling the core during cooldown and refueling.

The final physical barrier, a reinforced structure that encloses the reactor and prevents release of radioactive material during accidents. It is designed to withstand internal pressure and to be isolated on demand.

A measure of radiation effect on human tissue that weights absorbed dose by radiation type, expressed in sieverts. It allows comparison of risk from different kinds of radiation exposure.

The amount of radiation energy deposited per unit mass of material, measured in grays. It is the physical quantity from which dose equivalent and health risk estimates are derived.

The principle of keeping radiation exposure as low as reasonably achievable, considering social and economic factors. It drives work planning, shielding, and contamination control in radiation areas.

The maximum permitted radiation dose to a worker or member of the public, set by regulation. Limits are not targets; ALARA programs aim to keep exposures well below them.

Radioactive material present where it is not wanted, such as on surfaces, in water, or on personnel. Contamination is controlled by protective clothing, surveys, and decontamination procedures.

A defined area around a nuclear facility where planning and protective actions such as sheltering or evacuation may be needed after a release. Zones are typically defined as plume exposure and ingestion pathways.