Preface 

This is the fifth edition of CSA N285.8, Technical requirements for in-service evaluation of zirconium alloy pressure tubes in CANDU reactors . It supersedes the previous editions, published in 2021, 2015, 2010, and 2005. This Standard specifies mandatory technical requirements and non-mandatory evaluation procedures for fitness-for-service assessments. Pressure tubes in Canadian CANDU® nuclear power plants are inspected in accordance with CSA N285.4, Periodic inspection of CANDU nuclear power plant components. When a detected flaw indication does not satisfy the criteria of acceptance by examination, or when pressure tube to calandria tube contact is detected or predicted, Clause 12 of CSA N285.4 permits a fitness-for-service assessment to determine acceptability. Also, Clause 12 of CSA N285.4 requires evaluation of the results of specified material property surveillance measurements. Note: CANDU (CANada Deuterium Uranium) is a registered trademark of Atomic Energy of Canada Limited. The major changes to this edition include the following: a) revisions to Clause D.5.4 to implement a revised procedure for predicting the resistance to crack initiation due to hydrided region overloads; b) guidelines for flaw evaluation for DHC initiation based on flaw-tip stress relaxation due to creep under hydride ratcheting conditions; c) revisions to Clause D.10.3 to implement a revised procedure for predicting the axial DHC growth rate; d) provided requirements for application of acceptance criteria in cases where core assessments are done probabilistically for one mechanism and deterministically for another; e) added a reference to CSA N285.6.1 for pressure tubes for use in CANDU fuel channels; f) removal of the contents of Clause C.2.3 on the probabilistic method for evaluation of service conditions for protection against fracture; g) added a new Clause 4.5.1.4 for requirements when model or methodology is determined to have a deficiency; h) general terminology cleanups to provide consistency throughout the Standard: 1) Hi, Hig, Hall-ig, and Hall terms; 2) hydrogen and deuterium concentration terminology; 3) DHC velocity; 4) changed "uptake rate" to "rate of change"; and 5) Clause A.1.4 symbols and abbreviations; i) revisions to Clause C.3.3 and Table C.1 for clarification of allowable failure frequency in reactor core assessments; and j) revisions to use of the term "degradation mechanisms" including related revisions to Clause 7.3 and Annex C. This Standard is one of a series of CSA N285 Standards that provide consistent rules for the design, fabrication, installation, inspection, and assessment of pressure-retaining systems and components in CANDU nuclear power plants. The series outlines requirements that are particularly applicable to nuclear power plants in Canada and references the appropriate requirements of the ASME Boiler and Pressure Vessel Code. Users of this Standard are reminded that the site selection, design, manufacture, construction, installation, commissioning, operation, and decommissioning of nuclear facilities in Canada are subject to the Nuclear Safety and Control Act and its Regulations. The Canadian Nuclear Safety Commission might impose additional requirements to those specified in this Standard. 

Scope 

1.1 This Standard specifies the technical requirements for the owner/operator to evaluate cold-worked Zr- 2.5 wt% Nb alloy pressure tubes in operating CANDU reactors for continued operation. Clause 13 of CSA N285.0/N285.6 Series requires that when in-service inspection results or material surveillance results do not satisfy the requirements of the original inspection program, a fitness-for-service evaluation must be performed in accordance with CSA N285.4 to demonstrate acceptance. The requirements of this Standard address the specific fitness-for-service evaluation requirements of CSA N285.4, Clause 12. 

1.2 This Standard applies only to cold-worked Zr-2.5 wt% Nb alloy pressure tubes in operating CANDU reactors and to evaluation of the volumetric inspection results, pressure tube to calandria tube contact, and material surveillance measurements listed herein. The definition of pressure tube material types within the scope of this Standard is provided in CSA N285.6.1. 

1.3 This Standard does not apply to evaluation of pressure tube dimensional changes (other than pressure tube to calandria tube contact), material property surveillance measurements beyond those defined in CSA N285.4, or other reactor types. This Standard does not apply to pressure tube materials other than cold-worked Zr-2.5 wt% Nb. 

1.4 In this Standard, "shall" is used to express a requirement, i.e., a provision that the user is obliged to satisfy in order to comply with the Standard; "should" is used to express a recommendation or that which is advised but not required; and "may" is used to express an option or that which is permissible within the limits of the Standard. Notes accompanying clauses do not include requirements or alternative requirements; the purpose of a note accompanying a clause is to separate from the text explanatory or informative material. Notes to tables and figures are considered part of the table or figure and may be written as requirements. Annexes are designated normative (mandatory) or informative (non-mandatory) to define their application.