HKIE Professional Assessment - Nuclear Discipline key terms

The ratio of the number of neutrons in one generation to the number in the preceding generation in a chain-reacting system. k = 1 denotes criticality; k < 1 subcritical; k > 1 supercritical.

A measure of the departure of a reactor from criticality, often expressed as (k-1)/k. Positive reactivity increases power; negative reactivity decreases it. Units include pcm and dollars.

Neutrons emitted by certain fission products seconds to minutes after fission, rather than promptly. They constitute less than 1% of fission neutrons but are essential for reactor controllability.

A condition where a reactor becomes critical on prompt neutrons alone, without needing delayed neutrons. It leads to a rapid, potentially explosive power increase and is avoided by design and operation.

The time required for reactor power to change by a factor of e (about 2.718) during a stable exponential transient. It is inversely related to reactivity and indicates how fast power is changing.

A reactor type using ordinary water as coolant and moderator. The primary coolant is kept at high pressure to prevent boiling, and steam for the turbine is generated in a separate secondary loop.

A reactor type in which water serves as coolant and moderator and is allowed to boil directly in the core. The resulting steam drives the turbine directly, without a separate steam generator.

The closed circuit that removes heat from the reactor core and transports it to a steam generator or turbine. It is a pressure boundary that prevents release of radioactive material.

A heat exchanger in a PWR that transfers heat from the radioactive primary coolant to non-radioactive secondary water, producing steam to drive the turbine while keeping the streams separate.

Heat produced by the radioactive decay of fission products after a reactor shuts down. It continues for a long time and must be removed to prevent core damage.

A postulated accident in which the reactor coolant system loses inventory through a break or leak, threatening core cooling. Emergency core cooling systems are designed to mitigate it.

A safety system designed to inject coolant into the reactor core during a loss-of-coolant accident to prevent overheating and fuel damage. It typically includes pumps, accumulators, and tanks.

The process and systems used to remove decay heat and sensible heat from the reactor core after shutdown, when the main heat sink is unavailable or insufficient.

The heat flux at which boiling transitions from nucleate boiling to film boiling, causing a sudden decrease in heat transfer and a rapid rise in fuel surface temperature. It is a key thermal limit.

The point at which nucleate boiling deteriorates into film boiling, leading to reduced heat transfer and potential fuel cladding failure. It is avoided by maintaining adequate thermal margins.

The difference between an operating parameter (such as power or temperature) and its safety limit, expressed as a margin to a limiting condition like DNB or fuel melt. It ensures safe operation.

The amount of energy deposited by ionizing radiation per unit mass of material, measured in gray (Gy). One gray equals one joule per kilogram.

A dose quantity that accounts for the biological effectiveness of different radiation types, calculated by multiplying absorbed dose by a radiation weighting factor. Measured in sievert (Sv).

A dose quantity that represents the overall health risk from radiation exposure, accounting for the sensitivity of different tissues. It is the sum of equivalent doses to tissues weighted by tissue factors, in sievert (Sv).

The dose received per unit time, such as microsieverts per hour. It is used to assess exposure conditions and to set radiation protection controls.

As Low As Reasonably Achievable: a principle in radiation protection to minimize doses and releases, taking into account economic and social factors. It is a regulatory requirement in many jurisdictions.

A prospective restriction on individual dose from a specific source, used in planning to ensure that overall doses remain below regulatory limits and to promote ALARA.

Material placed around a radiation source to reduce dose rates by absorbing or attenuating radiation. Common materials include lead, concrete, and water, chosen based on radiation type and energy.

The thickness of a shielding material that reduces the intensity of radiation by half. It is a useful parameter for estimating shielding effectiveness for gamma and X-rays.

A comprehensive document prepared by the licensee that demonstrates how a nuclear facility meets safety requirements. It includes descriptions of the facility, safety analyses, and operational limits.

A postulated accident that a nuclear facility must be designed to withstand without exceeding acceptable limits. It forms the basis for safety system design and regulatory acceptance.

A structured argument, supported by evidence, that demonstrates the safety of a facility or activity. It is used to justify licensing and to manage safety throughout the lifecycle.

A systematic method to estimate the likelihood and consequences of accidents using fault trees, event trees, and reliability data. It supports risk-informed decision making and safety improvements.

The estimated annual probability of severe core damage from accidents, typically calculated in a PSA. It is a key risk metric used in regulatory decisions.

A deductive technique that traces an undesired event back to combinations of component failures and human errors using logic gates. It is used in PSA to quantify system failure probabilities.

Highly radioactive waste requiring long-term isolation, typically from reprocessing spent fuel or from direct disposal of spent fuel. It generates significant decay heat and contains long-lived radionuclides.

Fuel that has been irradiated in a reactor and removed because it can no longer sustain efficient fission. It is highly radioactive and may be reprocessed or disposed of as high-level waste.

The permanent isolation of radioactive waste in a deep underground repository in a stable rock formation, using engineered and natural barriers to prevent radionuclide release.

The process of safely shutting down and dismantling a nuclear facility, including decontamination and remediation, so that the site can be released for other uses.

The process of increasing the proportion of uranium-235 in uranium, typically for use in reactor fuel. Common methods include gas centrifuge and gaseous diffusion.

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

Mixed oxide fuel containing plutonium and uranium oxides, used in some reactors as an alternative to low-enriched uranium fuel. It helps consume excess plutonium and supports resource sustainability.

A safety system that monitors reactor parameters and automatically initiates a reactor trip (scram) when limits are approached, preventing unsafe conditions. It is designed to high reliability standards.

The rapid insertion of control rods to shut down a reactor. It is triggered manually or automatically by the reactor protection system when safety limits are exceeded.

The arrangements, plans, and procedures to respond to a nuclear or radiological emergency, including notification, protective actions, and public communication, to mitigate consequences.

A defined area around a nuclear facility where special emergency planning and preparedness measures are implemented to protect the public in case of a release.

A measure taken to avoid or reduce radiation exposure during an emergency, such as sheltering, evacuation, or administration of stable iodine. It is based on dose avoidance.