Auxiliary feedwater meaning

In nuclear regulation and project documentation, auxiliary feedwater (AFW), also called the auxiliary feedwater system (AFWS) or, in some designs, emergency feedwater, is the back-up water system that feeds the steam generators when the main feedwater is unavailable. It is used during reactor start-up and shutdown, and under abnormal or accident conditions, to maintain decay heat removal from the reactor via the steam generators. This is an engineering term rather than a statutory definition. It is not defined in legislation, but is used across nuclear site licences, safety cases, operating rules/technical specifications, procurement and insurance contracts, and is assessed by the Office for Nuclear Regulation (ONR) against its Safety Assessment Principles and associated guidance. Legally significant features typically include independence from the main feedwater system, safety classification, redundancy and diversity (for example, separate diesel- and turbine-driven pumps to withstand station blackout), and seismic and environmental qualification, all evidenced in the licensee’s safety case and maintenance regime. Usage is broadly consistent across England & Wales, Scotland and Northern Ireland. In Ireland, which has no operating nuclear power plants, the term may still arise in regulatory policy, transboundary environmental assessment, emergency planning, and contractual documentation relating to nuclear-related services.