Nuclear power plant modifications may be needed for a number of different reasons. These include physical ageing of plant systems, structures and components; obsolescence in hardware and software; feedback from operating experience; and opportunities for improved plant safety, reliability or capability. However, experience has also shown that weaknesses in the design and/or implementation of modifications can present significant challenges to plant safety. They can also have a considerable impact on the commercial performance of the plant. It is therefore important that the plant modification process reflect a recognition of the potential impact of human errors and that it incorporate suitable measures to minimise the potential for such errors.

In this context, the NEA Committee on the Safety of Nuclear Installations (CSNI) and its Working Group on Human and Organisational Factors organised an international workshop in 2003 to discuss the role of human and organisational performance in the nuclear plant modification process. This technical opinion paper represents the consensus of specialists in human and organisational factors (HOF) in the NEA member countries on commendable practices and approaches to dealing with nuclear plant modifications. It considers factors that should be taken into account when developing a modification process and identifies some lessons learnt from application of the process. The paper should be of particular interest of nuclear safety regulators and nuclear power plant operators.