Nuclear power plants are often designed and constructed by groups of companies that come together for a single or a small number of projects. The NPPs that they construct, however, will exist for several decades.

Nuclear power plants by their nature are complex. They are composed of many components and interdependent systems that must operate in a manner that meets the design intent. Over many years of operation the plant will experience many changes, equipment will become obsolete, and physical changes in the condition of materials will occur.

It is incumbent on the operating organisation, therefore, that they main­tain the capability to objectively assess changes in plant condition and performance, appraise design changes and retain the knowledge base to do so. This capability will reside in those bodies of engineering personnel with the knowledge and experience to perform those duties and in the body of design data, drawings and materials acquired from architect engineers at the time of construction. Collectively this corporate intellectual feature is known as the Design Authority. The IAEA publication INSAG-19, Maintaining the design integrity of nuclear installations throughout their operating life, provides further detail.