English for Nuclear Engineering: Essential Vocabulary

"criticality"

The condition in which a nuclear chain reaction is self-sustaining, with exactly one neutron from each fission causing another fission

"The reactor reached criticality at 06:42 when the control rods were withdrawn to the prescribed position."

"neutron flux"

The number of neutrons passing through a unit area per unit time, used to measure reactor power density

"Instruments showed a neutron flux of 3×10¹³ neutrons per square centimetre per second at full power."

"reactivity coefficient"

A measure of how reactor power changes in response to a change in a physical parameter such as temperature or void fraction

"A negative temperature reactivity coefficient means the reactor self-limits if coolant temperature rises unexpectedly."

"moderation"

The process of slowing fast neutrons to thermal energies using a moderator material so they can more readily cause fission

"Light water serves a dual purpose in PWRs, providing both moderation and coolant functions."

"control rod worth"

The change in reactivity produced by fully inserting or withdrawing a control rod or bank of rods

"The shutdown margin was verified by measuring the control rod worth of all rods in the fully inserted position."

"xenon poisoning"

The accumulation of xenon-135 in the reactor core after a power reduction, temporarily suppressing the chain reaction

"The operators delayed the restart for twelve hours to allow xenon poisoning to decay sufficiently."

"delayed neutrons"

Neutrons emitted seconds to minutes after fission from radioactive decay of fission products, which allow reactor control

"Without delayed neutrons, the reactor time constant would be too short for mechanical control rod systems to respond."

"prompt criticality"

A dangerous condition in which a reactor becomes critical on prompt neutrons alone, without needing delayed neutrons

"Safety systems are designed to prevent prompt criticality under any credible accident scenario."

"defence in depth"

A safety philosophy employing multiple independent layers of protection so that no single failure can cause a radiological release

"The plant's defence in depth strategy relies on physical barriers, redundant systems, and administrative controls."

"emergency core cooling system"

A safety system that injects water into the reactor core to prevent fuel damage in the event of a loss-of-coolant accident

"Actuation of the emergency core cooling system was confirmed within seconds of detecting the break in the primary loop."

"loss-of-coolant accident"

An event in which primary coolant escapes from the reactor circuit due to a pipe break or valve failure, threatening core integrity

"The probabilistic risk assessment assigned a very low frequency to the large-break loss-of-coolant accident scenario."

"deterministic safety analysis"

A method of evaluating reactor safety by analysing defined bounding accident scenarios against prescribed acceptance criteria

"Deterministic safety analysis confirmed that peak cladding temperatures remained below the 1200 °C regulatory limit."

"safety function"

A specific action required to maintain a nuclear facility in a safe state, such as reactivity control, core cooling, or confinement

"The three fundamental safety functions — shutdown, cooling, and containment — must remain intact under all design-basis events."

"containment integrity"

The maintained structural and leak-tight condition of the containment building that prevents radioactive release to the environment

"Post-accident monitoring confirmed that containment integrity was preserved throughout the transient."

"common cause failure"

A failure mode in which a single shared event or root cause disables multiple redundant safety systems simultaneously

"The post-Fukushima review identified common cause failure of backup generators due to flooding as a key vulnerability."

"design basis accident"

A postulated accident scenario for which a nuclear facility is designed to withstand without unacceptable radiological consequences

"The large-break loss-of-coolant accident is the limiting design basis accident for most pressurised water reactors."

"uranium enrichment"

The process of increasing the proportion of the fissile isotope U-235 above the natural level of 0.7%

"Power reactor fuel typically requires uranium enrichment to between 3% and 5% U-235."

"fuel burnup"

The amount of energy produced per unit mass of nuclear fuel, expressed in megawatt-days per tonne of uranium

"Advanced fuel assemblies are licensed to achieve a fuel burnup of up to 65 GWd/tU before discharge."

"spent fuel pool"

A water-filled storage basin adjacent to the reactor building where highly radioactive used fuel assemblies are cooled

"Spent fuel pool water inventory must be maintained to ensure adequate decay heat removal."

"reprocessing"

Chemical separation of plutonium and residual uranium from spent fuel so they can be fabricated into new fuel

"France operates a large-scale reprocessing facility that recovers plutonium from spent commercial fuel."

"mixed oxide fuel"

Reactor fuel containing a blend of plutonium oxide and uranium oxide, often abbreviated MOX

"The reactor was licensed to use mixed oxide fuel assemblies in up to one third of the core."

"high-level waste"

The most radioactive category of nuclear waste, consisting primarily of fission products and actinides from spent fuel

"High-level waste requires isolation from the environment for tens of thousands of years in a geological repository."

"geological disposal"

The permanent emplacement of high-level nuclear waste in stable deep rock formations

"Finland is constructing the world's first licensed geological disposal facility for spent nuclear fuel."

"conversion facility"

An industrial plant where uranium ore concentrate is chemically converted to uranium hexafluoride for enrichment

"The conversion facility processes thousands of tonnes of uranium ore concentrate annually."

"effective dose"

A measure of radiation exposure accounting for the type of radiation and the sensitivity of the tissues irradiated, expressed in sieverts

"The annual effective dose to the workforce must not exceed 20 millisieverts averaged over five years."

"ALARA principle"

As Low As Reasonably Achievable — the guiding principle that radiation doses should be kept as low as practicable

"Work planning followed the ALARA principle by rotating workers to limit individual dose accumulation."

"shielding attenuation"

The reduction in radiation intensity achieved by placing absorbing material between the source and the receptor

"Three metres of concrete provided adequate shielding attenuation to reduce gamma dose rates to acceptable levels."

"contamination survey"

A systematic measurement of radioactive contamination levels on surfaces, equipment, or personnel

"A contamination survey was performed at all exit points from the radiologically controlled area."

"personal dosimetry"

The measurement and recording of radiation dose received by individual workers using worn monitoring devices

"All workers entering controlled areas wear thermoluminescent dosimeters for personal dosimetry purposes."

"controlled area"

A designated zone where access, occupancy, and working conditions are regulated to protect workers from radiation

"Entry to the controlled area required dosimeter assignment, protective clothing, and radiation safety briefing."

"internal contamination"

Radioactive material that enters the body through inhalation, ingestion, or absorption, posing a radiation dose from within

"A whole-body count was performed to screen for internal contamination after the filter bag failure."

"half-value layer"

The thickness of a specific shielding material needed to reduce the intensity of a radiation beam by half

"Lead has a half-value layer of about 6 mm for the gamma energies emitted by cobalt-60."

"departure from nucleate boiling"

A critical heat flux condition in which a vapour film forms on the fuel cladding surface, severely reducing heat transfer

"The thermal-hydraulic design ensures that departure from nucleate boiling does not occur at 112% of rated power."

"coolant void fraction"

The fraction of the coolant volume occupied by steam bubbles or voids within the reactor core

"An increase in coolant void fraction reduces moderation and lowers core reactivity in BWR designs."

"primary loop"

The closed circuit carrying radioactive coolant directly through the reactor core and to the steam generators

"Primary loop pressure is maintained at 155 bar to prevent coolant boiling in pressurised water reactors."

"decay heat"

Residual heat generated by radioactive decay of fission products after the chain reaction has been shut down

"Decay heat removal is critical in the first hours after shutdown and requires a dedicated cooling system."

"natural circulation"

Coolant flow driven by buoyancy differences arising from density gradients between hot and cool regions, without pumps

"Passive safety systems use natural circulation to remove decay heat even during a station blackout."

"peak cladding temperature"

The maximum temperature reached by the outer surface of fuel rods, a key safety parameter in accident analyses

"Regulatory criteria require that peak cladding temperature must not exceed 1200 °C during a design basis accident."

"heat transfer coefficient"

A measure of the rate of heat exchange between a surface and a fluid per unit area per degree of temperature difference

"Turbulent flow conditions increase the heat transfer coefficient and improve fuel rod cooling efficiency."

"pressuriser"

A vessel connected to the primary loop that maintains system pressure by using heaters and spray nozzles

"The pressuriser level alarm indicated a potential leak in the primary coolant system."

"decommissioning strategy"

The chosen approach — immediate dismantlement, safe enclosure, or entombment — for retiring a nuclear facility at end of life

"The regulator approved a decommissioning strategy of immediate dismantlement to be completed within twenty years."

"characterisation survey"

A systematic radiological and physical assessment of a facility to plan decommissioning scope and waste volumes

"The characterisation survey identified that the reactor bioshield contained the highest concentration of activated material."

"licence amendment"

A formal change to an operating nuclear facility's licence, requiring regulatory review and approval

"A licence amendment was submitted to authorise the use of higher-enrichment fuel in the existing reactor core."

"probabilistic risk assessment"

A systematic analysis that quantifies the likelihood and consequences of accidents at a nuclear facility

"The probabilistic risk assessment showed a core damage frequency of less than 10⁻⁵ per reactor year."

"regulatory inspection"

A formal visit by the nuclear regulator to verify that a facility is operating in compliance with its licence and applicable standards

"The unannounced regulatory inspection found all safety systems operable and records up to date."

"clearance level"

The radiological concentration below which material can be released from regulatory control for conventional disposal or recycling

"Metal components were surveyed and confirmed below clearance levels before release to a commercial scrapyard."

"operating experience"

A systematic programme that collects, analyses, and shares lessons from events and near-misses across the nuclear industry

"The operating experience programme identified a common valve maintenance error reported at three other plants."

"independent safety review"

An assessment of a nuclear facility's safety case by experts who are independent of the design or operating organisation

"An independent safety review was commissioned before the regulator would consider extending the plant's operating licence."