A plain English nuclear glossary.

As Low As Reasonably Practicable. The principle that risk must be reduced until the cost of further reduction is grossly disproportionate to the benefit gained. It underpins the structure of most safety cases in the UK regime.

A licensed individual permitted to operate reactor controls. The route to authorisation is long and plant specific, which is why operator pipelines are planned years ahead.

Continuous generation at steady output, as opposed to load following. Most large reactors are operated as baseload plant for economic and technical reasons.

A measure of energy extracted from nuclear fuel, usually expressed in gigawatt days per tonne. Higher burnup means more energy per assembly and fewer refuelling outages.

Actual output over a period divided by the theoretical maximum if the plant ran at full power continuously. A common headline measure of how well a station is being run.

Surveying a facility to establish what materials and contamination are present, and where. It determines the dismantling method, the waste route and most of the cost.

The state in which a fission chain reaction is exactly self sustaining. First criticality is the milestone at which a new reactor formally begins operating.

The process of retiring a nuclear facility, from defuelling through dismantling to site release. It routinely runs for decades after generation stops.

The body accountable for the integrity of a plant design and for approving any change to it. A senior technical function with real legal weight behind it.

The UK process by which regulators assess a reactor design before any site specific application. Usually shortened to GDA, and a major driver of hiring for new entrants.

The time taken for half the atoms in a radioactive sample to decay. It ranges from fractions of a second to millions of years, which is what makes waste categorisation necessary.

High, intermediate and low level waste. The categories determine shielding, cooling, packaging and ultimate disposal route, and they drive most of the cost in waste programmes.

Varying reactor output to match demand rather than running flat out. Technically demanding, and increasingly relevant as grids take on more intermittent renewables.

A material such as water or graphite that slows neutrons so they are more likely to cause further fission. The choice of moderator is one of the defining features of a reactor design.

A planned shutdown for refuelling, inspection and maintenance. Outage dates are fixed far in advance and are effectively immovable, which is why hiring lead time matters so much.

A structured reassessment of a plant's safety case at intervals, typically every ten years. It is a major driver of engineering and licensing work on the operating fleet.

The documented argument, supported by evidence, that a facility is acceptably safe. Whether someone authored a safety case or reviewed one is the distinction that decides most nuclear shortlists.

Small Modular Reactor. A smaller output design intended to be factory built and transported, trading unit scale for shorter build times and lower upfront capital.

A water filled pool that cools and shields fuel assemblies after removal from the reactor. Fuel typically remains there for years before dry storage or reprocessing.

A radioactive isotope of hydrogen used as fuel in deuterium tritium fusion. Handling it safely is one of the scarcest specialisms in the fusion workforce.

The conventional side of the plant, where steam drives the turbine and generator. Much of the engineering here is closer to conventional thermal power than to reactor physics.

The waste form together with its container, engineered to remain safe for as long as the disposal concept requires. Package design decisions can constrain a programme for a century.