1.1 General
The purpose of this Commentary is to provide background information for certain clauses and requirements in CSA N285.0. This background information can help the user to clarify the context of the CSA N285.0 requirements. Also, this Commentary refers to sources of material that were used during the formulation of some of the requirements in CSA N285.0. With the exception of the Annexes, the main clause headings and the main clause numbers used in this Commentary correspond to those in CSA N285.0-12.
This Commentary focuses on CSA N285.0 and is organized as follows:
a)
the original intent of CSA N285.0, including
i)
historical background — how the nuclear industry developed in Canada; and
ii)
the differences between ASME Boiler and Pressure Vessel Code (BPVC), Section III, Division 1, and CSA N285.0, CANDU-specific requirements;
b)
the aim of CSA N285.0, including the provision of
i)
rules for the design of the CANDU reactors consistent with the regulatory requirements;
ii)
requirements not covered by ASME BPVC, Section III, Division 1;
iii)
classification requirements;
iv)
registration requirements, including those for welding procedures; and
v)
requirements for repairs, replacements, and modifications;
c)
the rationale for many of the requirements, which are derived from
i)
industry practice or experience;
ii)
engineering or safety analysis;
iii)
research or test programs; and
iv)
good engineering judgment; and
d)
the evolvement of CSA N285.0 through significant changes in various editions.
This Commentary addresses requirements relating to the design and construction of pressure-retaining systems and components. Standards that relate to periodic inspection of pressure-retaining components are not addressed. Also, the requirements of other interfacing Standards and ASME BPVC, Section III, Division 1 are not addressed in this Commentary. The edition of CSA N285.0 that is addressed in this Commentary is the 2012 edition, not including Updates Nos. 1 and 2. Generally, the text is presented in the same sequence used in CSA N285.0, tracing the activities during the design, fabrication and installation of an item.
Although this Commentary can be read by itself, a better understanding of the Commentary will be obtained if a copy of CSA N285.0-12 is available for reference. Many of the requirements in CSA N285.0-12 are described in this Commentary. However, the Standard must always govern since it provides the formal requirements.
Note: ASME has published a companion guide to the ASME Boiler and Pressure Vessel and Piping Codes, which provides the criteria and a commentary on selected aspects of ASME Boiler and Pressure Vessel and Piping Codes.
1.2 Historical background
The requirements in the Standard are based on provincial and federal regulations, practices developed in the design and construction of CANDU nuclear power plants, and the proven and widely adopted rules of ASME BPVC, Section III, Division 1. Their fundamental goal is to ensure public safety. Since the first Standard in the CSA N285 series was issued, regulations and best practices have evolved as the industry gained design, construction, commissioning, and operational experience. However the basic pressure boundary requirements have not changed significantly and are based on the long-standing ASME industrial practices. Where such changes have occurred, they are described in the appropriate sections.
The first CSA N285 Standard, a preliminary edition, was issued in 1975 and included both general requirements and specific requirements for Class 1, 2, and 3 components. Because provincial acts and regulations took precedence over ASME BPVC, Section III, Division 1, the 1975 edition included an appendix that listed the ASME BPVC, Section III, Division 1 paragraphs that were not applicable to nuclear power plants in Canada. These paragraphs dealt with authorization, stamping, and reports. In 1981, the content of CSA N285 were divided between two Standards, with general requirements in CSA N285.0 and requirements for Class 1, 2, and 3 systems and components in CSA N285.1.