Nirex has responsibilities for nuclear waste management in the UK. The company’s top level scientific objectives are to maintain technical credibility in deep disposal, to gain public acceptance for a deep geologic repository, and to provide relevant advice to customers on the safety implications of their waste packaging proposals. In compliance with these internal objectives and attending quality requirements, Nirex utilises peer reviews selectively to keep its scientific tools up to date and to periodically verify the quality of its products. In October 1998 the UK Department of Trade and Industry officially requested the OECD Nuclear Energy Agency (NEA) to undertake an international peer review, on behalf of Nirex, of an important portion of their methodology for assessing the long-term safety of a deep geologic repository. The portion to be reviewed deals with the treatment of features, events and processes (FEPs) and the construction of system evolution scenarios, as well as the use of this work in determining conceptual model adequacy and model development needs. The NEA accepted the task to perform this work on behalf of Nirex, based on an agreed upon Terms of Reference document, and formed an international review team (IRT) consisting of four internationally recognised experts plus a member of the NEA Secretariat. The review started in October 1998.