Summary of design of nuclear vessels and piping to ASME III (NB, NC, ND) and vessels to BS 5500

Abstract

It is recognised that the general safety significance of pressurised components for nuclear power systems demands design rules beyond those for non-nuclear components. ASME III provides the most widely used design rules for nuclear components, and provides rules in three Classes to allow the rules to be matched to the safety significance of different nuclear components. ASME III Class 1 is provided for components of the highest nuclear safety significance, Class 3 for those of lowest nuclear safety significance. There is then a hierarchy of design code requirements for pressurised components, starting with non-nuclear codes as the minimum and progressing through the ASME III nuclear Classes 3, 2, 1. In establishing and assessing the safety justifications of nuclear plants it is important to have an appreciation of the gradation of requirements in the ASME III design rules and how these go beyond non-nuclear component design rules. There are two broad aspects to the structural integrity of pressurised components, namely the achievement of integrity and the demonstration of integrity. The technical requirements of design codes are associated with achieving integrity while the documentary aspects are usually associated with demonstrating integrity. In practice documents also have a part in achieving integrity in the communication of information between different organisations and personnel involved in the design process. It is not possible to assign simple numerical measures to the relative integrity afforded by non-nuclear codes and the three Classes of ASME III. Instead it is necessary to compare the different requirements of the rules for the various technical and documentary aspects. This paper summarises the most important technical and documentary aspects of the three Classes of the ASME III Code for vessels and the non-nuclear code BS 5500. A similar summary is also provided for the three Classes of ASME III rules for piping. The intention is that the paper provides a basis for appreciating the relative integrity afforded by these various rules.