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Basic terminology




An atom consists of a central nucleus which is surrounded with negatively charged electrons travelling in distinct orbits. The number of protons in the nucleus is matched by the same number of orbital electrons so that the entire atom has no net electrical charge.


The medium (liquid or gas) used to transfer heat from the reactor core to the steam generators or directly to the turbines.


Condition of a nuclear reactor for being able to achieve a self-sustaining nuclear chain reaction at steady fission rate.


To clean up or reduce the radiological contamination.

Emergency protective measure

Action taken in an emergency situation during a nuclear accident to avoid or reduce the radiation dose that might otherwise be possibly received by the workers or members of the public.


A process of splitting the heavy nucleus into two or more fission fragments.


One of a group of nuclides with the same number of protons but different numbers of neutrons. (e.g. 11H, 21H, 31H are isotopes of hydrogen. 11H is called protium or light hydrogen , 21H is called deuterium (D2) or heavy hydrogen, 31H is called tritium; U-238 (consists of 92 protons and 146 neutrons) and U-235 (consists of 92 protons and 143 neutrons) are isotopes of Uranium).


A material or medium used to slow down the fast neutrons released from fission to thermal neutrons so as to induce more fission. Ordinary water, heavy water and graphite are common moderators used in nuclear reactors.

Nuclear safety

The achievement of proper operating conditions, prevention of accident or mitigation of accident consequences, resulting in protection of workers, the public and environment from undue radiation hazard.


A small, massive, positively charged structure at the centre of an atom composed of subatomic particles, namely the protons and neutrons. Protons are positively charged whereas neutrons have no charge at all.


A term used to specify a type of nuclei or atoms with reference to its nuclear properties. A nuclide is often represented by the symbol AZX, or X - A where A is the mass number equal to the numbers of protons plus the number of neutrons in the nucleus, Z is the atomic number equal to the number of protons in the nucleus, X is the chemical symbol of the atom.


Any areas outside site boundary of the nuclear power station.


All areas within the site boundary of the nuclear power station.


A continuous radiological gaseous discharge from the chimney or discharge outlets of a nuclear power station in the form of a stream of smoke or cloud shaped like a feather, which thus is known as the plume.

Refuelling outage

A temporary shutdown of the nuclear reactor unit for carrying out maintenance and overhaul of plant and equipment including the replacement of a portion of used fuel.


A process or operation to extract radioactive nuclides from spent fuel for further use.


The rapid and automatic shutting down of a nuclear reactor.

Single failure

A failure which results in the loss of capability of a component to perform its intended safety function(s), and any consequential failures which result from it.

Site area

Area managed by the nuclear power station.

Spent fuel

Fuel assemblies removed from a reactor after use. Spent fuel contains a number of highly radioactive nuclides.

Thermal neutron

A low energy neutron resulting from scattering reactions of a fast neutron with the moderator. It is called thermal neutron because the average kinetic energy of the neutron is directly proportional to the temperature.

The above information is provided by EMSD