BARC OCES/DGFS key terms

Splitting of a heavy nucleus such as uranium-235 into two lighter nuclei, releasing energy and typically two or three neutrons; the chain-reaction basis of power reactors.

Joining of light nuclei such as deuterium and tritium into a heavier nucleus with release of energy; requires extremely high temperature to overcome Coulomb repulsion.

Material such as light water, heavy water, or graphite that slows fast neutrons to thermal energies by elastic scattering, increasing the probability of fission in uranium-235.

Condition in which a fissile assembly sustains a steady chain reaction because neutron production exactly balances absorption and leakage, giving an effective multiplication factor of one.

Ratio of neutrons in one generation of a reactor to those in the preceding generation; below one is subcritical, equal to one critical, above one supercritical.

Measure of how far a reactor is from critical, usually expressed as (k-1)/k in units of dollars or percent; positive values raise power, negative values lower it.

Movable rod of neutron-absorbing material such as boron, cadmium, or hafnium inserted into a core to reduce reactivity and regulate or shut down the fission chain reaction.

Rapid emergency shutdown of a reactor achieved by gravity-driven insertion of control and safety rods, terminating the chain reaction within seconds.

Neutron slowed to kinetic energy comparable with ambient thermal motion, roughly 0.025 eV at room temperature, where fission cross sections of uranium-235 are large.

Effective target area, measured in barns, expressing the probability that a given nuclear reaction such as fission or capture occurs when a neutron meets a nucleus.

Number of neutrons crossing unit area per unit time, commonly in neutrons per square centimetre per second; determines reaction rates and irradiation levels in a core.

Time required for half the atoms of a radioactive nuclide to decay; ranges from fractions of a second to billions of years and sets handling and waste requirements.

Probability per unit time that a particular radionuclide atom will decay; its reciprocal is the mean lifetime and it relates to half-life by lambda equals ln2 divided by half-life.

Spontaneous emission of alpha, beta, or gamma radiation from unstable nuclei as they transform toward a more stable configuration, measured in becquerel.

SI unit of radioactivity equal to one nuclear transformation per second; replaces the older curie, which equals 3.7 times ten to the tenth becquerel.

Energy deposited by ionising radiation per unit mass of material, measured in gray, where one gray equals one joule per kilogram.

Absorbed dose weighted by radiation type to reflect biological harm, measured in sievert; alpha particles carry a much larger weighting factor than gamma rays.

Principle of keeping radiation exposure as low as reasonably achievable, taking economic and social factors into account; a cornerstone of radiological protection practice.

Use of absorbing material such as lead, concrete, or water to reduce radiation reaching people or equipment; gamma rays need high-density material, neutrons need hydrogen-rich material.

Thickness of a specified material that reduces the intensity of a radiation beam to half its incident value; a practical design parameter for shielding calculations.

Reduction in intensity of radiation as it passes through matter due to absorption and scattering, described by an exponential law involving the linear attenuation coefficient.

Energy required to separate a nucleus into its constituent protons and neutrons; the mass defect of a nucleus corresponds to this energy through Einstein's mass-energy relation.

Difference between the sum of masses of individual nucleons and the actual mass of the assembled nucleus, appearing as binding energy when the nucleus forms.

One of several nuclides of the same element having the same number of protons but different numbers of neutrons, hence different mass numbers and often different stability.

Total count of protons and neutrons in a nucleus, written as a superscript before the element symbol; distinguishes isotopes of the same element.

Number of protons in a nucleus, which uniquely identifies the chemical element and determines its position in the periodic table.

Rule stating that a system at equilibrium shifts to oppose a change in concentration, pressure, or temperature applied to it; used to predict yield changes in reversible reactions.

Ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium; its value depends on temperature.

Negative logarithm of hydrogen ion activity in a solution; values below seven are acidic, seven neutral, and above seven basic at standard conditions.

Solution of a weak acid with its conjugate base, or a weak base with its conjugate acid, that resists changes in pH when small amounts of acid or alkali are added.

Thermodynamic quantity equal to internal energy plus pressure-volume work; change in enthalpy at constant pressure equals the heat absorbed or released by a process.

Measure of dispersal of energy or disorder in a system; total entropy of an isolated system never decreases, giving the direction of spontaneous change.

Thermodynamic potential equal to enthalpy minus temperature times entropy; a negative change indicates a spontaneous process at constant temperature and pressure.

Stress is internal force per unit area in a material; strain is the resulting fractional deformation. Their ratio in the elastic region is the modulus of elasticity.

Ratio of tensile stress to tensile strain within the linear elastic range of a material, indicating its stiffness; steel has a far higher value than aluminium.

Stress at which a material begins to deform permanently, marking the end of elastic behaviour; a key allowable-stress basis in pressure vessel and structural design.

Negative ratio of lateral strain to axial strain in a stretched or compressed material; typical metals lie near 0.3, cork near zero.

Dimensionless ratio of inertial to viscous forces in fluid flow, used to predict whether flow is laminar or turbulent in pipes and around bodies.

Statement of energy conservation along a streamline in steady, inviscid flow, relating pressure, velocity, and elevation; the basis of flow measurement devices.

Measure of a fluid's resistance to shear deformation, reflecting internal friction; it falls with rising temperature in liquids but rises in gases.

Maximum possible efficiency of a heat engine operating between two reservoirs, equal to one minus the ratio of sink to source absolute temperatures; a theoretical upper limit.

Aptitude problem presenting a sequence of numbers generated by a rule such as arithmetic progression, geometric progression, or alternating operations, requiring the next term to be found.

Reasoning item giving a group of items in which all but one share a common property; the task is to identify the exception.

Deductive reasoning problem with two or more premises and a conclusion that must be tested for validity using set relationships such as all, some, and none.

Reasoning puzzle in which family relationships are described through statements or a diagram and the candidate must determine how two people are related.