Plant configuration data of design, materials, manufacture and assembly of the fuel channel is basically reviewed to see the conditions that can have a significant influence on fuel channel ageing. This includes pressure tube to end fitting zero clearance and low clearance rolled joints, calandria tube design criteria, annulus spacer design criteria, and end fitting design criteria. History docket is to be reviewed to understand any design changes and modifications during manufacturing and construction. Sometimes it can be found that there have been deviation disposition requests and their mitigation of the deviation from the requirements on technical specifications during manufacturing of fuel channel.

A. II.4.3. OPERATION, INSPECTION AND MAINTENANCE ACTIVITIES

Operations, tests, inspections and maintenance activities to date on the fuel channels in field are technically reviewed. They are to be inaugural fuel channel inspections, examination of pressure tube archive samples, fuel channel elongation measurements, pressure tube diameter measurements, pressure tube wall thickness measurements, fuel channel sag measurements, gap between calandria tubes and horizontal reactivity mechanisms, pressure tube deuterium concentration sampling, pressure tube volumetric inspections for flaws, examination results of tubes removed from operation, garter spring repositioning by SLAR, and chemistry data of primary heat transport system, etc. Table A. II.3 shows an example of the inspection and maintenance history of operating fuel channels.

Table A. II.3. Example of inspection and maintenance activities for a fuel channel

A. II.4.4. ASSESSMENT OF DEGRADATION MECHANISMS

Active and plausible degradation mechanisms are identified and their impacts on future performance and life expectancy of the fuel channel components and its sub-components, in particular the pressure tube, are assessed. These assessments are based on the information generated from this programme. Engineering evaluations of ageing is required to use known knowledge and expertise for regulatory body reviews subjectively the results after evaluation. The degradation mechanisms to be addressed include irradiation induced deformation, deuterium ingress, susceptibility to delayed hydride cracking, fracture toughness reduction, service induced damage, high operating loads, and fatigue loads, etc. The deformation is found as dimensional changes, such as axial elongation, diametric expansion, wall thinning, pressure tube sag between channel annulus spacers, and fuel channel sag.

Ageing assessment is performed for the known degradation mechanisms of fuel channels and the most life limiting degradation mechanism would be channel elongation. Fuel channels of the first generation of CANDU6 reactors are expected to reach its limit of elongation before plant design life. So it might be necessary to replace the aged fuel channels for continued operation beyond design life.