Fuel Channel
A duct through a core of a reactor designed to contain or or more fuel elements or assemblies.
Fuel Element
An assembly consisting of fissile material contained in a can, the latter often having fins to improve the heat transfer between the element and the coolant. In an AGR, the term is applied to the complete assemble of fuel, fuel can, fittings and external graphite sleeve.
Fuel Stringer
An assembled column of fuel elements with tie rod(s) and top and bottom end assemblies as prepared for insertion into a fuel channel of an AGR.
Gag
A device incorporated in a reactor cooling channel to restrict the coolant flow.
Gamma-Radiation
High-energy, short wavelength electronmagnetic radiation emitted by the nuclei of many radioactive atoms during radioactive decay. Gamma-rays are highly penetrating, but are absorbed by dense materials such as lead.
Gas Baffle
An insulated metal barrier situated above the reactor core: its purpose is to allow the hot gas from the fuel channels to pass through it and to separate that gas from the cooling gas that flows through the reactor core.
Gas Circulator
Electrically driven fan that takes cool gas from the boiler outlet and forces it first through the reactor core and subsequently back through the boiler.
GCPT
Graphite Core Project Team.
Graphite
A dense, rigid, crystalline form of carbon used as a moderator in thermal reactors.
Heat Exchanger
A piece of equipment that transfers heat from one medium to another. A typical example is the boiler in gas-cooled reactors in which heat from the pressurised carbon dioxide gas coolant is used to convert water into steam and to run the turbine.
HSE
Health and Safety Executive. the public body of the Government responsible for the regulation of risks to health and safety in the UK, created as a result of the Health and Safety at Work Act 1974.
Ionising
The process by which an atom, electrically netrual under normal circumstances, becomes electrically charged by gaining or losing one or more orbital electrons.
Ionising Radiation
Any particle or radiation with sufficient energy to ionise matter through which it travels.
Irradiation
The exposure of matter to ionising radiation.
ISI
In Service Inspection. The periodic inspections carried out within the reactor pressure vessel to ensure that no changes have occurred which could threaten safety or economic operation.
Isotopes
Species of an atom with the same number of protons in their nuclei, hence having the same atomic number and belonging to the same element, but differing in the number of neutrons and this in their mass numbers. Such atoms have identical chemical properties but their nuclear characteristics may be vastly different.
Magnox Reactor
A term used for the first generation of British power reactors taken from the use of a magnesium alloy (Magnox) as the canning material. The reactors are fuelled with natural uranium, carbon dioxide cooled and graphite moderated.
Manipulator
A generic terms for any remote manipulator irrespective of the delivery system. They are often referred to colloquially as “manips”.
Moderator
A material such as heavy water, graphite or light water used in a reactor to slow down or moderate the fast neutrons produced by fission, thus increasing the likelihood of further fission.
MTR
Materials Test Reactor.
Neutron
An uncharged neutral elementary particle with a mass nearly equal to that of a proton and associated with it in the nuclei of atoms.
NII
Nuclear Installations Inspectorate. The division of the Health and Safety Executive responsible for carrying out the safety assessments in accordance with the conditions attached to nuclear site licenses.
Nuclear Reactor
A device designed and operated for the purpose of initiating and maintaining a controlled fission chain reaction in a fissile material.
Once-Through Boiler
A boiler that contains no steam/water separating drum in which no fluid recirculation takes place, the working fluid being pumped continuously through the boiler.
Plug
In fuel technology: an end cap of a can. In shielding: a moveable piece of material used to reduce the escape of radiation from an aperture such as a hole in a shield.
Pressure Vessel
A container, often made of steel or concrete, for fluids subjected to pressure above or below atmospheric. A Reactor Pressure Vessel is the vessel containing the fuel elements, moderator and coolant of a reactor. Its purpose is to enable the reactor to be operated at pressures above atmospheric in order to improve the heat transfer properties of the coolant.
Proton
An elementary particle with a positive charge equal and opposite to that of an electron. Its atmonic mass is 1,847 times that of an electron. It comprises the nucleus of an ordinary hydrogen atom, whose mas number is defined as one. It is a constituent of all nuclei.
PSD
Primary Shut Down.
PWR
Pressurised Water Reactor. A power reactor cooled and moderated by light water in a pressure vessel surrounding the core. The water is pressurised in a closed primary loop to prevent boiling, and is circulated through a heat exchanger which generates steam in a secondary loop connected to the turbine.
Radiation
The emission and propagation of energy through space or matter in the form of electromagnetic waves, such as gamma-rays, and fast moving particles.
Radioactivity
The property, possessed by some atoms, of disintegrating spontaneously with the emission of a charged particle or gamma-radiation or both.
Safety Rod
A cotrnol rod that, singly or in concert with others, provides a reserve of negative reactivity for the purpose of emergency shutdown of the reactor.
Shielding
A mass of material surrounding a source of radiation that reduces radiation intensity, for the purposes of protection of equipment, material or personnel. Dense concrete, several metres thick, is used for shielding.
Shutdown
The procedure of making a reactor sub-critical and maintaining it in that state (hold-down), thus reducing the power.
Shutdown Power
In a reactor shutdown: the (decreasing) power level due to the heat produced by fission product decay and residual fissions.
Slow Neutrons
Neutrons that have been slowed down by a moderator so as to increase the probability of their collision with a fissile nucleus and induce fission.
Standpipe
The connection between the charge face and the interior of a reactor vessel, giving access to the fuel channels, for example, for refuelling.
SSD
Secondary Shut Down.
Streaming
The undesired flow of radiation through a hole, gap, void or labyrinth.
Trepanning
A method for cutting core samples from a piece of material. A hollow cutter is used to mill a circular hole around the central sample a bit like removing an apple core. The sample can then be broken off and removed from the hole.
TRIUMPH
Television Remote Inspection Unit Multi-Purpose Head. The TV camera system used generally for inspection on AGR stations.
TTU
Trepanning Tool Unit. Consists of a fully enclosed cylindrically shaped, stainless steel tube housing a trepanning tool, drives and mechanisms used for removing samples of the graphite bricks within the reactor core.
Uranium
A heavy, slightly radioactive metallic element with an atomic number of 92. As found in nature, it is a mixture of isotopes U-235 (0.7%) and U-238 (99.3%). The artificially produced U-233 and the naturally occurring U-235 are fissile. U-238 is capable of undergoing fission with high-energy neutrons and is fertile, that is it is capable of being transformed into a fissile nuclide by neutron capture. Uranium Oxide (UO2) is used as the fuel in a reactor because of its chemical and radiation stability, good gaseous fission product retention and high melting point.
