LTO requires careful planning and scoping and several HWR utilities have already started the detailed planning of a plant LTO programme. The HWR utility would normally initiate a detailed LTO study at their particular plant many years before the end of design life in order to optimise effectiveness and cost, and to maximize asset value. The end product of this study is a business case that compares the costs of LTO for their NPP with the costs of the alternatives.

Typically, for HWRs, LTO involves a plant refurbishment, which includes pressure tube, calandria tube and partial or complete feeder replacement. Planning of these replacements is part of the LTO study work. Planning is also started for the environmental, safety and licensing issues that would need to be addressed to ensure safe and economical future operation of the unit. In addition to these studies, a systematic review of the plant SSCs is carried out to determine what other equipment refurbishment or replacement will be required due to ageing or obsolescence.

A key part of an LTO programme is to utilize the outcomes of the PLiM ageing assessments and implementation (Phase 1 and 2) in enhancing current plant programmes for extended operation. For instance, the life assessment work on the concrete containment has led to an enhanced inspection and monitoring programme at one HWR NPP. With knowledge from the containment condition assessment programme at the decommissioned Gentilly 1 plant, a detailed containment ageing management programme (including the monitoring instrumentation) for LTO was developed. The detailed life assessment work lays the foundation for the plant inspection, maintenance and operational programme enhancements to extend the life of critical equipment.

In general, ageing assessments provide the primary inputs for determining the work necessary for LTO, and for planning the optimized surveillance, maintenance, and operations programmes to achieve the utility’s targets for safety, reliability and production capacity during its extended life. Significant progress in Phase 1 and 2 of the PLiM programme provides the HWR utility with important in-depth assessments (and often with promising life prognosis) for key SSCs. These outcomes are important inputs into utility decisions to embark upon LTO.

The following list of actual recommendations was identified during one LTO study and gives an idea of the potential scope for an HWR LTO project:

• Station control computers — The control computers are obsolete. The original equipment supplier stopped making this equipment many years ago. In order to complete an additional 25 to 30 years of life, some action is needed to replace this equipment in view of the potential for declining reliability and difficulty in getting spare parts.

• Programmable digital comparators (PDCs) — The PDCs are used in the reactor shutdown systems. The issue here is quite similar to the control computers and replacement will be required to complete the refurbished life.

• Main generator — The windings of the generator have a limited life insufficient to last throughout the refurbished plant life. Several options need to be studied including replacement with a new generator and rewinding of the existing generator. Similarly, many of the generator auxiliaries will need to be addressed to ensure reliable operation after refurbishment.

• Safety improvements — In parallel with the plant physical assessment, work has been done to establish what design changes may be required to minimize regulatory concerns with future plant operation. A number of changes to improve reliability and functionality of systems and components are being studied in more depth to establish what safety benefits would come about if such modifications were made.

• Reactor component analysis — Recommendations have been made to analyse component parts of the reactor to deal with ageing issues. Parts of the calandria are to be analysed to better establish material ductility limits. Moderator nozzles are to be analysed for potential fatigue issues. Analysis is to be done to establish more clearly the source of the reactor vault leak.

• Reactor component inspection — The analyses discussed above will be supplemented by inspections during the refurbishment outage. This outage will provide a unique opportunity for these inspections, as the reactor will be without fuel, fuel channels and moderator during part of the outage.

• Balance-of-plant (BOP) components — While most of the BOP components are much easier to inspect and replace than those in the nuclear steam plant (NSP), some further investigation is underway on those BOP components whose ageing are important to long life of the NSP.

For this particular plant, the steam generators were deemed to be in good condition and have a good prognosis for 50-year life provided the detailed recommendations from the LA were implemented. Typically, additional inspections are planned on steam generators in order to gain more confidence, such as in the assessment of secondary side internals.

Some piping replacement may be necessary to address flow accelerated corrosion (FAC) issues. The ageing assessment studies for other critical systems, structures and components were completed and the results factored into the LTO programme as appropriate.