Design life for the plant and major components is targeted to be a certain value at the design stage (say 30 Years). Service life, during which components operate safely and reliably, may exceed the design life. This is because the actual operation in terms of say fatigue cycles, corrosion etc., for most of the components may be considerably lower than design assumptions. Thus selected components may be replaced at end of design life (say 30 years) to provide a longer service life for the plant as a whole. Therefore, unless there are critical components that cannot be physically replaced or refurbished at their end of life, the optimum life for the plant may be based on economics rather than technical issues.

The most expensive component that requires replacement in order to extend plant life is the pressure tubes. Pressure tube replacement or large scale fuel channel replacement (LSFCR) is required at about 30 years, due to ageing degradation similar to what was described in section 3. The second most expensive component to replace for LTO is the steam generators. The current generation steam generators using improved materials (alloy 800) and operational practices is to obtain high life and may not require replacement along with pressure tubes. The CANDU/ PHWR plants have all implemented steam generator life cycle management plans to inspect, monitor and mitigate steam generator degradation to achieve the design life. However, older plants may need to time Steam Generator replacement along with pressure tube replacement for LTO, so that replacement of steam generators and pressure tubes could be accomplished in the same outage.

Bulk feeder replacement is another significant activity that may be required for life extension, depending on the effectiveness of current and planned activities to mitigate feeder wall thinning due to Flow accelerated corrosion (FAC). Replacing feeders during the LSFCR may actually reduce the duration of the LSFCR by improving access to the fuel channels.

The long shut down period at LSFCR stage could also be used to take up PLiM exercise for LTO of other major SSCs like Containment, electrical systems and control and instrumentation. The PliM considerations and status with regard to such major component specific technology in CANDU/PHWRs is covered in this Chapter.

In case of India PHWRs, the following components were designated to be managed by Ageing management programme (AMP).

• Pressure tube Calandria tube & core internals

• Thermal shields, end shields

• Hanger rods

• Containment structure & Calandria vaults

• PHT headers & feeders

• Steam generator

• PHT pump body

• Bleed condenser / pressuriser / accumulators

• Shut off rod, adjuster rod & drive mechanism

• Instrumentation & control cables and connectors

• Diesel generators, motor generator sets, ACVRS

• Feed & Bleed lines & fm supply pump lines including dump & control valves, dousing system

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