Glossary of Terms in ARIS Reports


Advanced Gas-cooled Reactor
AGR An advanced gas-cooled reactor (AGR) is a type of nuclear reactor that uses graphite as the neutron moderator and carbon dioxide as coolant. Operating at high temperatures results in improved thermal efficiency.
Auxiliary building
Auxiliary are commonly located next to the containment structure and typically include radioactive waste systems, chemical and volume control systems, emergency cooling water storage systems.



Basic Design
When the system descriptions for all plant systems are provided; Safety analyses needed for a design approval have been completed; Licensing documents for certification is available; Procurement specifications and documentation for major components, systems, and structures have been developed; Itemized cost estimate and master schedule for the construction phase have been prepared.

Boiling Water Reactor
BWR Boiling light water cooled and moderated reactor. In a BWR, the reactor core heats water, which turns to steam and then drives a steam turbine.

Burnable Absorber, Poison
A substance with large ability to absorb neutrons. Often used to control nuclear fission in fresh fuel loads.



The maximum load a nuclear component can carry in a specified situation without exceeding limits of temperature and stress.

The amount of electric power that a generating unit can produce.

A thin metal tube, commonly made from Aluminum, stainless steel or zirconium alloys, that coat the nuclear fuel pellets and prevents corrosion of the fuel.

Commercial Operation
When the plant is first handed over by the contractors to the owner and declared officially in commercial operation. A period from the first grid connection to the Commercial Operation is called 'trial operation'.

Containment building
A hermetically sealed building which houses the nuclear reactor core and other systems that could potentially release fission products to the atmosphere during an accident.

Construction Start
First major placing of concrete, usually for the base mat of the reactor building, is done. From this date the reactor is considered to be under construction.

Control rod
A rode/plate/tube that is used to absorb the neutrons in the reactor core. These help to control the number of fission events and thus the power of the nuclear reactor.

Concept Description
When the basic idea and design goals are described; A few calculations/sketches/data are provided; Development and test needs have been identified; and rough estimates of costs and schedules are available.

Conceptual Design
When the key components and layout drawings as well as single line diagrams are available; Brief descriptions of key components and systems are provided; Identification and preliminary analysis of concept relevant incidents and accidents have been performed.

A substance that is used to remove or transfer heat from the reactor core. Common coolants include light water, heavy water, air, carbon dioxide, helium, liquid sodium, and a sodium-potassium alloy.

The central part of a reactor unit where the nuclear fuel assemblies are located and the heat for steam production is generation.

Country Nuclear Power Profiles
CNPP The CNPP compiles background information on the status and development of nuclear power programs. It consists of organizational and industrial aspects of nuclear power programs and provides information about the relevant legislative, regulatory, and international framework in each country.



Demonstration Commercial Size
Demonstration Commercial Size: to demonstrate the system's capability to operate in a utility environment.

The process at the end of a nuclear power plant's life during which the residual radioactive material is removed from the plant.

Design Net Capacity
The original Design Net Capacity (electrical power) is the unit electrical output after deducting the self-consumption power assumed by the original unit design, no matter if it has ever been routinely achieved during operation. This value does not reflect possible power changes during subsequent operation.

Design Organisation
An organisation which has done the major part of designing a specific type of nuclear reactor.

Detailed Design
When a largely completed design and a complete construction schedule are available; Manufacturing, procurement specifications are completed; Commissioning specifications are completed.

An isotope of hydrogen with one proton and one neutron in the nucleus.



Electric Capacity
The amount of electicty that a nuclear reactor is designed to generate.

Electricity Supplied
Net electrical energy produced during the reference period as measured at the unit outlet terminals, i.e. after deducting the electrical energy taken by unit auxiliaries and the losses in transformers that are considered integral parts of the unit. Also called as Net Energy Generated. Measured in MWh.

Electric power grid
An arrangement of synchronized, connected transmission and distributions line for electricity.

Experimental Reactor
Typically of up to 100 MW(th) and built to demonstrate the technology, but often including a steam plant and turbine-generators to allow operation as a small power station.



Fast neutron
A neutron with kinetic energy greater than its surroundings when released during fission.

Fast Reactors
Nuclear reactors are called Fast Reactors when using much higher energy neutrons that are not slowed down by a neutron moderator to cause fission.

Fuel cycle
A term for the series of steps in the mining, production and final storage of nuclear fuel.

Fuel rod
A long thin tube which are made up of pellets containing nuclear fuel. These fuel rods are put together into fuel bundles and place in the reactor core.



Gas Cooled Reactors
GCR A gas-cooled reactor is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide (helium can also be used) as coolant.

Grid Connection
When the plant is first connected to the electrical grid for the supply of power. After connecting to the grid the plant is considered in operation.



Heavy Water Reactors
HWR Heavy Water Reactors commonly use unenriched natural uranium as its fuel and heavy water (deuterium oxide D2O) as its coolant and moderator.

High-enriched uranium
HEU Uranium enriched to at least 20 percent uranium-235.



In Operation
A reactor is considered as 'operational' or 'in operation' from its first grid connection to permanent shutdown.Thus, when a reactor is temporarily not generating electricity because of outages for, e.g. refuelling, maintenance, repair, large refurbishment or political decision, the reactor is still categorized as operational. The only exception to this classification is when the reactor’s status is declared as 'long-term shutdown', then it is excluded from operational reactors even though it has not yet reached permanent shutdown. The current operational reactors and reactors under construction are listed in the IAEA's Power Reactors Information System ( ).

Integral Water Cooled Reactors
IWCR Several advanced water cooled reactor designs are called Integral designs. In these designs the whole reactor primary circuit, including, for instance, the pressurizer, coolant pumps, and steam generators are enclosed in the reactor vessel.



A company, organization or institution that has been given approval to operate a nuclear facility.

Liquid-Metal Cooled Reactors
LMR Liquid-Metal Cooled Reactors are an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation applications. They have safety advantages because the reactor doesn't need to be kept under pressure, and they allow a much higher power density than traditional coolants. Disadvantages include difficulties associated with inspection and repair of a reactor immersed in opaque molten metal, and depending on the choice of metal, corrosion and/or production of radioactive activation products may be an issue. The most common liquid metals coolants used are Mercury, Sodium and Lead.

Light water
Ordinary water (H20)

Light Water Cooled Graphite Moderated Reactor
LWGR Light water cooled graphite moderated reactor. Also known as RBMK. Using light water for cooling and graphite for moderation, it is possible to use natural uranium for fuel.



A material that is used to decrease the speed of the fast neutron produced during a fission even and thus sustaining a chain reaction.

Molten Salt Cooled Reactors
MSR Molten Salt Cooled Reactors are a class of nuclear fission reactors where a molten salt mixture is used as the coolant which at the same time can be the fuel. MSRs run at higher temperatures thus increasing the thermodynamic efficiency, while staying at low vapor pressure. They can be operated at near atmospheric pressures thus reduces the mechanical stress endured by the system and simplifying aspects of reactor design and improving safety.



Natural uranium
Uranium that can be mined with relative concentrations of isotopes: 0.7 percent of fissile uranium-235 and 99.3 percent of non-fissile uranium-238.

Non-electrical Application
Some power reactor units produce a portion of their output energy in the form of heat/steam for non-electrical applications (desalination, district heating and industrial heat). This helps to increase a NPPs efficiency by utilising the waste heat.

Nuclear Power Plant
NPP A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.

Nuclear reactor
The central part of the a nuclear power plant in which the nuclear core is located and the fission reaction takes place.

Nuclear Share
Percentage share by country of electricity generation mix for nuclear power. The ratio of the nuclear electricity production, to the total electricity production from all sources in a country.

Nuclear Steam Supply System
NSSS The NSSS consists of a nuclear reactor and all of the components necessary to produce high pressure steam, which is used to turn the turbine for the electrical generator.

Nuclear waste
The portion "used" nuclear fuel that can not be reprocessed for future use and must be stored safely.



Operational Reactor
A reactor is considered as 'operational' or 'in operation' from its first grid connection to permanent shutdown. When the reactor is temporarily not generating electricity, e.g. refuelling, maintenance, repair, large refurbishment or political decision, the reactor is still categorized as operational. The only exception to this classification is when the reactor’s status is declared as 'long-term shutdown', then it is excluded from operational reactors even though it has not yet reached permanent shutdown.

It is the current company that operates the nuclear power plant.

An 'outage' is defined as any status of a reactor unit, when its actual output power is lower than the reference unit power for a period of time. By this definition, the outage includes both power reduction and unit shutdown.

It is the company which owns a nuclear power plant majority.



Permanent Shutdown
When the plant is officially declared by the owner to be taken out of commercial operation and shut down permanently.

Pellet, Fuel
A small cermaic cylinder that is made up of enriched Uranium which is used as nuclear fuel.

Pressurized Heavy Water Reactor
PHWR A pressurised heavy water reactor (PHWR) is a nuclear power reactor, commonly using unenriched natural uranium as its fuel, that uses heavy water (deuterium oxide D2O) as its coolant and moderator.

Pressurised Water Reactors
PWR In a Pressurised Water Reactors, the reactor core heats water, which does not boil. This hot water then exchanges heat with a lower pressure water system, which turns to steam and drives the turbine.

Primary system
A term used to describe the reactor coolant system.

Prototype Reactor
in which much of the scaling up required for a commercial station in terms of both overall size and individual components has been incorporated.



Reactor Model
The reactor model identifies a specific reactor design series (e.g. AGR, APR, EPR, Magnox, Konvoi, VVER V-213,...) within a particular reactor type (GCR or PWR respectively). All models of the same reactor type usually have the same basic design charactersitics (moderator and coolant type and form).

Reference Energy Generation
REG Reference energy generation (MWh or GWh) for the period is the net electricity output that would be produced if a reactor unit is operated at its rated power output for the entire period.

Reference Unit Power
RUP The reference unit power expressed in units of megawatt (electrical) is the maximum (electrical) power that could be maintained continuously throughout a prolonged period of operation under reference ambient conditions. The power value is measured at the unit outlet terminals, i.e. after deducting the power taken by unit auxiliaries and the losses in the transformers that are considered integral parts of the unit.
The reference unit power is expected to remain constant unless following design changes, or a new permanent authorization, the management decides to amend the original value.



The material control and accounting program which controls the enriched nuclear material.As used by the International Atomic Energy Agency, this term also means verifying that the peaceful use commitments made in binding nonproliferation agreements, both bilateral and multilateral, are honored.

The purposeful decrease in the rate of fission in a reactor via e.g. insertions of control rods.

Site-Specific Design and Engineering
When the adjustments to adapt the design to site conditions are completed; Local cooling arrangements are finalized; Final safety analysis is completed.

Spent Fuel
Nuclear fuel that can no longer sustain a nuclear chain reaction.

Supercritical Water Cooled Reactors
SCWR Supercritical Water Cooled Reactors enables a higher thermal efficiency, as well as simplification in the balance of plant, due to the fact that there are no phase changes in the reactor. SCWRs are directly coupled to the energy conversion equipment and uses uranium oxide as a fuel. These reactors may have a thermal or fast-spectrum reactor and passive safety features are incorporated similar to those of simplified boiling water reactors.



Thermal Capacity
The Reference thermal power of the plant expressed in MW(th). The reactor thermal power is the net heat transferred from the fuel to the coolant.

Thermal Reactors
Nuclear reactors are called thermal reactors when they use low energy neutrons to sustain the chain reaction. Most current reactors are thermal reactors and a typical examples are Light Water Reactors.



Element No.92 on the periodic table. Found in natural ores and certain isotope can be used as nuclear fuel.

Under Construction
When first major placing of concrete, usually for the base mat of the reactor building, is done. From this date the reactor is considered to be under construction.

In terms of nuclear energy a unit is a single reactor at a multi-reactor nuclear power plant.



Water Cooled Reactors
WCR A Water Cooled Reactor is a type of thermal reactor that uses normal water or heavy water, as its coolant and neutron moderator and a solid compound of fissile element as its fuel. Thermal reactors are the most common type of nuclear reactor, and Water Cooled Reactors are the most common type of thermal reactor. There are three varieties of light water reactors: the pressurized water reactor (PWR), the boiling water reactor (BWR), and (most designs of) the supercritical water reactor (SCWR).